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PLANT DISEASES

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For most people, the subject of plant diseases is probably the most daunting topic covered during

Master Gardener training. To begin with, disease problems tend to strike fear in the hearts of

gardeners, because they seem to come from nowhere and often lead to the death of a treasured plant.

Furthermore, unlike insect pests, the organisms that cause plant diseases typically cannot be seen

with the naked eye. This seems to make plant diseases all the more mysterious and certainly makes

them extremely difficult to identify correctly. Often knowledge of the exact organism that causes a

disease is necessary to determine an appropriate control.

The goal of this unit of Master Gardener training is to provide you with some basic background

information on plant diseases, the organisms that cause these diseases, and methods for plant disease

control. Hopefully, by the time you have completed this unit, you will also find out that plant

disease-causing organisms are pretty amazing and actually kind of interesting!

THE DISEASE TRIANGLE

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A convenient way of thinking about plant diseases is by using a concept called the "Disease

Triangle".

Disease represents the interaction between three factors (the three corners of the triangle): a

susceptible hot, a pathogen (disease causing organism) and a favorable environment. If all of these

factors are present, disease results; if one or more of the factors is not present, then disease does not

occur.

Methods of disease control (which we'll discuss in greater detail later) can be thought of as

modifying the disease triangle by reducing or eliminating one of the corners of the triangle. For

example, if you use resistant ornamental varieties in your garden (e.g., a powdery mildew resistant

phlox variety), you are eliminating the "susceptible host" and can thus reduce or prevent disease.

Similarly, for some diseases, by removing diseased plant material, you can reduce or eliminate

disease because you are eliminating the pathogen. Finally, you can reduce or eliminate a "favorable

environment" for disease by doing something as simple as not over-watering in your garden.

CAUSES OF PLANT DISEASES

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Causes of plant diseases can be divided into two major categories: BIOTIC and ABIOTIC.

BIOTIC causes of plant diseases are those that are biological in origin. There are five major biotic

causes of plant diseases. These are the fungi, bacteria, viruses, nematodes and phytoplasmas.

ABIOTIC causes of plant diseases are non-biological in origin. Examples include things like adverse

weather conditions or environmental pollutants. Often the affects of abiotic factors on plants are

referred to as INJURIES.

We'll now proceed to discuss in more detail each of the five biotic causes of plant diseases, as well as

biotic causes.

CHARACTERISTICS OF FUNGI

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Fungi are by far the most important group of plant pathogens. There are over 200,000 known species

of fungi and over 8,000 species can cause plant diseases. Note that the majority of fungi are NOT

plant pathogens. These fungi, which primarily survive on non-living organic material, are called

SAPROPHYTES.

Many fungi (including some plant pathogens) can be seen with the naked eye. Typically however,

you are not seeing the major "body" of the fungi, but simply a reproductive structure (e.g., a

mushroom).

The non-reproductive "body" of most fungi (called the vegetative phase) is composed of a mass of a

fungal threads called HYPHAE (singular = HYPHA). This mass of hyphae is called a MYCELIUM

(plural = MYCELIA). While an individual hypha is microscopic, mycelia are often visible with the

naked eye.

Fungi reproduce by production of spores that can be a variety of shapes and colors. They are

produced in a wide variety of reproductive structures that, in general, are referred to as FRUITING

BODIES. Most fungal plant pathogens (except for a few that produce mushrooms) form fruiting

bodies that are microscopic in size.

Because characteristics of fungal spores and fruiting bodies are what help us identify fungi,

accurately diagnosing a fungal disease without a microscope can be difficult, if not impossible.

CHARACTERISTICS OF FUNGI: HYPHAE

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HYPHAE (singular = HYPHA) are the threadlike structures that comprise the major body of a

fungus. You can think of a hypha as a long tube. Many hyphae in a mass are called MYCELIUM

(plural = MYCELIA).

Most fungi produce hyphae, as the one shown here, that have CROSS WALLS (partitions dividing

the tubes into individual cells). Noting the presence or absence of cross walls can be important in

distinguishing and identifying some plant pathogenic fungi.

The location of the cross walls can also be diagnostic. In this example the cross walls at a branch in

the hyphae are offset a bit (see the arrow) giving the resulting cell a T-shape. This configuration of

cross walls is characteristic of the fungus Rhizoctonia, which is a common root rot fungus.

CHARACTERISTICS OF FUNGI: ASEXUAL REPRODUCTIVE STRUCTURES

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Fungi produce a wide range of reproductive structures (generically called FRUITING BODIES). The

small black blobs in this photo (see the arrow) are fruiting bodies (called pycnidia in this case) of the

fungus Septoria, which can cause spots on variety of different plants, including (in this photo)

cucumber.

Within fruiting bodies, a fungus produces spores. Some fruiting bodies are asexual in nature. The

spores produced in these fruiting bodies are NOT the result of recombination of genes from more

than one fungal "parent". The spores are produced by a single fungal "parent" and thus when these

spores germinate, they produce "offspring" that are genetically identical to the "parent" fungus.

CHARACTERISTICS OF FUNGI: SEXUAL REPRODUCTIVE STRUCTURES

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While some fruiting bodies are asexual in nature, others are produced sexually. The fruiting body in

this photo (see the black arrow) is called a cleistothecium, and is the sexual fruiting body of a

powdery mildew fungus called Microsphaera. The spores (see the blue arrow) produced in sexual

fruiting bodies result from the recombination of genes from two fungal "parents". When these

spores germinate, they produce "offspring" that are genetically different from both "parents".

Note that there are a couple of interesting characteristics of powdery mildew cleistothecia. Spores in

this fruiting body are produced in sacks (see the red arrow). Also, the fruiting bodies have nifty arm-

like attachments (see green arrow), called appendages. The number of sacks of spores in the

cleistothecium (in this example, more than 1), and the shape of the appendages (in this example, they

look like antlers) are characteristics used to classify a powdery mildew fungus into the appropriate

genus.

CHARACTERISTICS OF FUNGI: SPORES

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In addition to being interested in the kind of fruiting bodies produced by plant pathogenic fungi,

diagnosticians are also very interested in the types of SPORES that are produced in these fruiting

bodies. Fungal spores can be thought of as the "seeds" of the fungus, although spores are much less

complex that true seeds.

Fungal spores come in all shapes and sizes. Some, like these spores of the fungus Fusarium, are

multi-cellular (have more than one cell) and are colorless. The canoe or banana shape of these cells

is also characteristic of this particular fungus.

Often I refer to Fusarium as the "Cheshire cat" fungus, because the spores remind me of the Cheshire

cat's grin from "Alice in Wonderland".


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Other fungi, such as the fungus Thielaviopsis in this photo, produce spores that are multi-cellular, but

darkly pigmented. Dark pigmentation is thought to enhance the ability of spores to survive exposure

to adverse environmental conditions, such as exposure to ultraviolet light, and to ward off attack by

other parasitic soil fungi and bacteria.

I refer to Thielaviopsis as the "Tootsie Roll®" fungus, because the spores that it produces, while

multi-cellular, break into individual cells, just like the sections of a Tootsie-Roll®.


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Spores typically function as a means of reproduction and dispersal for a fungus. They can also serve

as a means of survival during periods of unfavorable environmental conditions. The spores (called

oospores) in this photo (see arrow) are those of a common root rot fungus called Pythium. Note that

these spores have very thick walls that help prevent loss of water, and serve as a barrier to prevent

attack by other microorganisms. Spores of this type have been reported to survive for 40 years in

soil.

FUNGAL DISEASES THAT ARE EASY TO IDENTIFY

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As noted previously, in order to accurately identify most fungal plant diseases, you need to look for

and identify the spores and/or fruiting bodies produced by the pathogen. Some fungal diseases

however, can be identified by eye, without the aid of a microscope.

One such disease is powdery mildew (shown in this photo on African violet). If you notice a white

coating (see arrow) on the surface of the leaves or flowers of a plant that looks as if someone

sprinkled the surface with powdered sugar or talcum powder, then powdery mildew is the most likely

problem. The white that you see is a combination of hyphae and asexual spores.

The name powdery mildew actually describes a group of diseases that occur on a wide range of

plants, and that are caused by several closely related fungi. Powdery mildew fungi tend to be

relatively host specific (i.e., the fungus that causes powdery mildew on African violet is NOT exactly

the same fungus that causes powdery mildew on phlox). However, if you see powdery mildew on

one plant, look for it on other plants. The presence of powdery mildew on one plant indicates that

environmental conditions (high humidity) are favorable for it to occur on other plants as well.

FUNGAL DISEASES THAT ARE EASY TO IDENTIFY

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Like powdery mildews, rusts (rose rust is shown in this photo) are easily identified by eye. The

rusty-orange, powdery PUSTULES (see arrow) that are produced by rusts are distinct and

characteristic. The spores of rust fungi are easily removed from the leaf surface by gentle rubbing.

Rust fungi are particularly incredible fungi. In addition to their typically orange spores (called

urediospores), these fungi can produce up to four additional, distinct spore types. In addition, some

rusts have alternate hosts, spending some of their life cycle on one host and the rest on a second host.

A classic example is the cedar-apple rust fungus, which spends a portion of its life cycle on red

cedar, the rest on apple. Between the two hosts this rust fungus produces four different types of

spores.

SYMPTOMS OF FUNGAL DISEASES

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We are able to identify powdery mildew and rusts, in part, because the pathogens produce distinctive

fungal structures (generically called SIGNS) that we can easily identify. Most diseases will not be

this cooperative.

For most fungal diseases, we must rely on disease SYMPTOMS to help us to determine the cause of

the disease. Symptoms are simply abnormalities in a plant that arise during disease development.

Most of the time symptoms will NOT definitively tell us the exact disease problem, but often

symptoms can give us a sense of whether a disease is caused by a fungus or some other pathogen.

WHAT KINDS OF SYMPTOMS ARE CAUSED BY FUNGAL PATHOGENS?

SPOTS are a common symptom of fungal pathogens. A spot is simply a roughly circular

NECROTIC (dead) area, typically found on a leaf. Larger, more irregularly shaped spots are

sometimes referred to as LESIONS. Sometimes fungal spots or lesions have yellow haloes.

This slide shows black spot of elm caused by the fungus Gnomonia ulmea.


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When spots or lesions enlarge and merge (see arrow), so that large areas of a leaf are necrotic and

diseased, then we call this a BLIGHT.

Perhaps the most familiar blights that homeowners encounter are those that occur on tomatoes.

Alternaria leaf blight caused by Alternaria solani (left) and Septoria leaf blight caused by Septoria

lycopersici (right) are common in home vegetable gardens, causing browning and loss of leaves from

the base of the tomato plant, up.


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Lesions that occur on the stems of herbaceous plants or on the trunks and branches of woody plants

are called CANKERS. Often cankers are sunken and discolored, or, as in this example of a canker

disease on honey locust, the outer bark layer has totally fallen away.


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Fungi cause a wide variety of ROTS. Rots involve the wholesale destruction and disintegration of a

large portion (if not all) of a plant. Typically, affected plant parts become discolored and soft.

Common fungal rots include crown rots (shown in this photo), where the crown of the plant is

affected, and root rots, where root tissue is infected and degraded.

The disease shown in this picture is southern blight caused by the fungus Sclerotium rolfsii. This

disease is one that has traditionally been associated with warmer climates and the fungus is not

thought to survive harsh Wisconsin winters. However, the Plant Disease Diagnostics Clinic at the

UW-Madison has seen an increase in the number of cases of this disease during the 1998 and 1999

growing seasons. In some cases, the pathogen appears to have been introduced into Wisconsin on

ornamentals that were produced in the south. In other situations, there is some evidence suggesting

that the pathogen may have been introduced in mulch. Finally, there is also some evidence to

suggest that the pathogen may have survived at some sites in Wisconsin during the mild winters of

1997/1998 and 1998/1999. To identify this disease, look for white mycelia on the surface of

unthrifty plants near the base. Imbedded in the mycelia, you will find small tannish to reddish seed-

like objects (see white arrow). These seed-like objects, called SCLEROTIA (singular =

SCLEROTIUM), are masses of tightly packed hyphae that serve as resting/survival structures for the

fungus.


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FRUIT ROTS are another type of rot associated with fungi. Often these rots start off as "dry" rots.

After the fungus invades however, other microorganisms (particularly bacteria) follow, and it is these

"secondary" microorganisms that lead to the soft, slimy, disgusting symptoms that people typically

associate with rots.

This slide shows Phytophthora fruit rot of pumpkin.


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WILTS are another common symptom associated with fungal infections. Wilts basically indicate

that a plant is not getting enough water. Thus leaves (and often the entire plant) lose their turgidity

and the plant has a "droopy" appearance.

Whenever you see a wilted plant, you should ask, "Why isn't this plant getting enough water?".

Perhaps the plant just hasn't been watered for awhile and thus there is not enough soil moisture to

support the plant's needs.

Alternatively, maybe the plant has a ROOT ROT. For example, the Cineraria in this photo has a

root rot caused by the fungus Pythium. In this situation, there is plenty of water in the soil, but the

root system has been decayed to the point that it can no longer absorb enough water to support the

plant. If you see a plant like this, check with the owner. If the owner of the plant has been faithfully

watering (or more likely over-watering) the plant and it is wilted, then check the root system for

decay. A root rot is a good guess in such a situation.

Finally, maybe the plant has a VASCULAR WILT. Vascular wilt pathogens invade the plant's

water-conducting tissue (called the XYLEM). These pathogens either cause blockages in the xylem

themselves, or the plant itself blocks off the xylem in an effort to localize the pathogen. In either

case, blockage of the xylem leads to a reduction in water moving from the roots to the above-ground

parts of the plants, and can lead to wilt symptoms. If you have a plant that has been watered

adequately (but not over-watered), and that has a root system that is not decayed, then a vascular wilt

is a strong possibility.


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If you suspect that a plant has a vascular wilt, then another symptom you can look for is

VASCULAR DISCOLORATION or BROWNING. As a reaction to the infection, the xylem

changes from its normal whitish color to a brown, green or gray color (see arrow). Not all vascular

wilts lead to vascular discoloration, but when it does occur, vascular discoloration can be an

important clue as to the cause of a wilt problem.

In this photo, vascular browning induced by the fungus Verticillium (cause of Verticillium wilt) is

shown on one of this pathogen's major hosts, maple.

CHARACTERISTICS OF BACTERIA

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Bacteria are arguably the second most important group of plant pathogens. There are approximately

200 pathogenic species. Unlike fungi, bacteria are not visible to the naked eye and can only be seen

using a microscope. Also, compared to fungi, bacteria are extremely simple. They are unicellular

(single-celled), although certain bacteria have cells arranged in structures that are physically (but not

functionally) similar to fungal hyphae. Reproductively, bacteria are also very simple. They do not

form complex fruiting bodies, but reproduce by the process of BINARY FISSION. In this simple

process, existing cells enlarge, then divide in half. The two cells that result from this process are

genetically identical to each other and to the parental cell from which they have arisen.

Bacterial diseases are virtually impossible to identify definitively based on symptoms. Typically, in

order to identify bacterial pathogens, the bacterium must be isolated from diseased tissue and then

grown on specialized media. Alternatively, bacteria can also be identified using immunological

techniques.

CHARACTERISTICS OF BACTERIA: BACTERIAL CELLS

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Bacterial cells are extremely small (see arrow). Even when magnified to 400 times

their normal size (as they are in this photo) they are difficult to distinguish as

individual objects. Often one must stain the cells (as in this photo) to see them. In

general, bacterial cells tend to be either spherical or rod-shaped (as these are). Most

plant pathogenic bacteria produce rod-shaped cells.

SYMPTOMS OF BACTERIAL DISEASES

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Plant pathogenic bacteria produce a wide range of SPOT symptoms. Often spots produced by

bacteria have a water-soaked appearance and a well-defined yellow halo (see arrow). However, be

cautious when attempting to distinguish bacterial and fungal spots. Spots produced by bacteria and

fungi are typically very similar.


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Another characteristic that can sometimes be used to help distinguish bacterial spots from fungal

spots is the shape of the spot. Bacterial lesions are often delimited (bordered) by veins (see arrow)

and thus often have an angular appearance. While fungal lesions can also be delimited by veins,

more often they are not and thus have a more irregular shape.

SIGNS OF BACTERIA

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If you suspect that a spot or lesion has been caused by a bacterial infection, and you have access to a

microscope, then you can confirm your suspicion by looking for BACTERIAL STREAMING. Cut

the lesion from the leaf and place it on a microscope slide in a drop of water. Slice through the lesion

with a razor blade. If bacteria are present they will "stream" out from the cut edges of the lesion. In

this photo, the grainy material in the upper half the photo is a mass of bacterial cells that have

streamed from a lesion (the dark mass at the bottom of the slide). Often large numbers of streaming

cells will be particularly visible where veins have been cut. While this technique will not identify

exactly which bacterium is involved in the disease, it can provide a useful clue that the disease is

bacterial in nature.


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When bacterial diseases affect large sections of an entire leaf or plant, BLIGHT results. This photo

shows bacterial blight of geranium caused by the bacterium Xanthomonas campestris pv. pelargonii.

Because bacterial pathogens tend to be restricted in their movement by veins, often blighted areas on

leaves caused by bacteria have a wedge shape. The widest area of the wedge is at the edge of the

leaf, the narrowest area near the petiole.


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Like fungi, bacteria can cause ROTS and soft rot of potato (shown in this photo) is a classic example.

The bacterium that causes this disease, Erwinia carotovora, produces an enzyme that degrades

pectin, the substance that serves as the "glue" that holds plant cells together. Once pectin is

degraded, the potato tuber becomes a squishy, disgusting mess. Interestingly, the foul smell that

people typically associate with rotted potatoes is not due to Erwinia carotovora. The chemicals that

give rotten potatoes their awful smell are produced by other bacteria that invade the rotted tissue after

the pathogen has done its work.


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Several classic VASCULAR WILTS are caused by bacterial pathogens. A prime example is

bacterial wilt of cucurbits (cucumbers, melons, pumpkins, squash, etc.). As with fungal vascular

pathogens, the bacterium that causes this disease invades the water conducting tissue (xylem) of the

plant and leads to a blockage of the tissue. Classic wilt symptoms result. Interestingly, this pathogen

is carried around by the cucumber beetle, which introduces the bacterium into the plant when it

feeds.

An insect (or any other organism) that carries around a pathogen is called a VECTOR.

SIGNS OF BACTERIA

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Bacterial wilt of cucurbits can be relatively easily diagnosed in the field. Cut a wilted vine near the

crown of the plant. Hold the two cut pieces apart for several seconds, then rub the cut ends together.

Next slowly draw the two pieces apart. If the bacterial wilt pathogen is present, you will see slimy

strands (they look like spider silk) stretching between the two vine pieces (see arrow). These stands

are composed of plant sap and bacterial cells, held together by a sticky substance (called LPS) that is

produced by and serves as a protective coat for the bacterium.


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Another symptom associated with some bacterial infections is the production of GALLS (see arrow).

Galls are composed of plant tissue where there is an abnormal overproduction of cells and where

cells typically grow abnormally large. While some fungi can cause galls, the classic gall disease is

crown gall (shown in this photo) caused by the bacterium Agrobacterium tumefaciens. This

bacterium induces gall formation by injecting a portion of its own DNA into a plant cell. This DNA

hijacks the cell's metabolism and gall formation results.

Agrobacterium tumefaciens has revolutionized the area of plant genetic engineering. Scientists

discovered that they could insert genes of interest into that portion of the bacterium's DNA that is

injected into plant cells. Once genes are inserted, scientists inoculate plants with the altered

Agrobacterium tumefaciens and the bacterium goes through its normal infection process. It injects its

DNA (along with the extra DNA) into the plant and the inserted genes are expressed. This technique

provides a novel and relatively easy way of incorporating genes from one plant into another plant,

even if the two plants are not closely related and would not normally breed with one another.

CHARACTERISTICS OF VIRUSES

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Viruses are another very common and very important cause of plant diseases. While bacteria are

small, viruses are even smaller. Bacteria can be seen with a light microscope, but viruses can

typically only be seen using an electron microscope.

Viruses are also much simpler structurally than bacteria. Bacteria have true cells. Viruses do not.

Most viruses are composed of a piece of genetic material with a protein coat. Viral genetic material

can be either deoxyribonucleic acid or DNA (the same kind of genetic material found in human

cells), or ribonucleic acid or RNA. Most known plant viruses contain RNA.

Because viruses are so simple, they do not have the "machinery" necessary to reproduce. Thus after

infecting a cell, they "hijack" the cell and force it to produce viral particles.

Viruses can be transmitted (moved from plant to plant) in several ways. Many are mechanically

transmitted. These viruses can be moved about by a grower handling an infected plant, then handling

a healthy plant, or simply by two plants rubbing together. Other viruses are transmitted by insects

(these insects are called VECTORS). These viruses are picked up by the insects as they feed and

then are moved from plant to plant as the insects move from plant to plant. Some insect-transmitted

viruses actually reproduce in the insect vector as well as in the plant host.

SIGNS OF VIRUSES

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As noted previously, viruses can only really be seen using an electron microscope. This photo

(electron micrograph) shows viral particles magnified about 100,000 times. These particles are

spherical in shape (actually icosahedral). Plant viruses can also be rigid or flexuous (flexible) rods.

ECONOMICALLY IMPORTANT VIRUSES

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Not all viral infections are necessarily bad, at least from a human standpoint. In fact, some viral

infections in tulips are highly desirable, because (as in this photo) they lead to interesting flower

colorings.

SYMPTOMS OF VIRAL DISEASES

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A classic symptom of a viral infection is called MOSAIC. Mosaic is the production of a blotchy

light and dark green coloring in foliage. Many viruses induce mosaic and many of these viruses have

the word mosaic in their name [tobacco mosaic virus, cucumber mosaic virus, poinsettia mosaic virus

(shown in this photo), etc.].


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Viruses can also induce a variety of interesting line patterns in leaves (see arrow). This photo shows

typical symptoms of rose mosaic virus on rose.


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When blotchy coloring occurs on flowers (or other colored plant parts), the symptom is referred to as

MOTTLE rather than mosaic.


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Viruses can cause to a variety of LEAF and FRUIT DISTORTIONS as well. This photo shows the

fruit and leaves of a zucchini that has a viral infection. Note that in addition to having a mosaic

pattern, the leaves also have narrow, "STRAPPY" leaf blades that are CURLED and TWISTED.

The leaves also are thicker and more leathery than normal. The fruit is exhibiting mosaic/mottling

symptoms and has WARTS (the yellow raised bumps). All of these symptoms are very typical of

viral infections.


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Some viruses induce production of concentric ring patterns on fruit or leaves. This is called

RINGSPOT. Many viruses carry the word ringspot in their names [e.g, tobacco ringspot virus,

papaya ringspot (shown in this photo), etc.].


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In the previous example, ringspot symptoms were simply a discoloration of the plant tissue. No real

tissue NECROSIS (death) occurred. In other viral infections, such as infection by impatiens necrotic

spot virus or INSV (shown in this photo), both ringspotting and necrosis result (see arrow).

INSV is a very important virus in production of ornamental plants. It can infect a wide range of

ornamentals. Once infected with INSV, plants should be destroyed. INSV is transmitted by thrips, a

very common insect pest in greenhouses. If thrips are not kept under control, an entire greenhouse

full of plants can very rapidly become infected with INSV.

CHARACTERISTICS OF PHYTOPLASMAS

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At one time, phytoplasmas were called mycoplasma-like organisms (MLOs) because they resembled

organisms (mycoplasmas) that cause diseases in animals. Recently the name phytoplasma was

coined to describe mycoplasmas that specifically cause plant diseases.

Phytoplasmas are rather odd bacteria-like organisms. They differ from true bacteria in that they are

much smaller, and in that they lack a cell wall. Because of their small size, diseases caused by

phytoplasmas (often called "yellows") were, for a long time, considered to be viral diseases.

Phytoplasmas live and reproduce in the plant's food-conducting tissue (called the PHLOEM). As is

the case with bacteria, phytoplasmas reproduce by binary fission. Phytoplasmas are transmitted by

insects, in particular, leafhoppers. These insects are phloem-feeders.

SIGNS OF PHYTOPLASMAS

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Phytoplasmas are smaller than ordinary bacteria and so, like viruses, they are most easily seen using

an electron microscope. However, phytoplasmas also can be seen in the phloem using light

microscopy, although this requires use of fluorescent dyes. These dyes bind to DNA (the genetic

material that phytoplasmas possess) and glow when exposed to UV light. Phloem cells do not

contain DNA under normal circumstances.

This photo shows phytoplasma cells (see arrow) in a phloem cell. The phytoplasma cells have been

stained and then exposed to UV light.

SYMPTOMS OF PHYTOPLASMA DISEASES

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Symptoms associated with phytoplasma infections are truly bizarre. One of the most common

symptoms is VIRESCENCE, where normally colored plant parts (such as flowers) are green in color.

Another common symptom is BROOMING, where plant parts become overly branched and bushy.

The coneflower on the right in this photo shows both symptoms.


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BROOMING is also a common symptom of ash yellows. The brooming in this example is extremely

severe.


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Another symptom often associated with phytoplasma infections is FASCIATION or a flattening of

plant parts. This photo shows a fasciated maple branch.


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These Cosmos plants, suffering from aster yellows, are showing virescence symptoms. In addition,

the TWISTING and DISTORTION of the plants is also characteristic of a phytoplasma infection.

CHARACTERISTICS OF NEMATODES

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Nematodes represent the final, major group of plant pathogens, and these organisms rival, if not

exceed, fungi in their complexity. Nematodes are very large compared to other pathogens and can be

readily seen with a light microscope and on occasion with the naked eye. They have sensory,

digestive, and reproductive organs. Nematodes reproduce through production of eggs, and go

through several juvenile phases prior to becoming adults.

SIGNS OF NEMATODES

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With proper training, nematodes can be easily identified once they have been extracted from soil or

infected plant material. The major problem in identification is that plant pathogenic nematodes are

often intermixed with non-plant pathogenic nematodes. Typically, non-pathogenic nematodes

dramatically outnumber pathogenic nematodes in such a mix. In order to help distinguish pathogenic

from non-pathogenic nematodes, look for a STYLET (see arrow). A stylet is a modified tooth that

helps a nematode puncture and feed on plant tissue.

SYMPTOMS OF NEMATODE INFECTIONS

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A classic symptom associated with infection by some nematodes is the formation of root GALLS

(see arrow). Root knot nematode (Meloidogyne sp.) causes this kind of symptom. The nematode

tunnels into the root and sets up housekeeping. As the nematode feeds, its saliva stimulates the plant

root cells to divide excessively and expand to larger than normal size. This leads to gall formation.

In addition, growth of abnormal cells can lead to collapse of the plant's food and water-conducting

tissue (i.e., phloem and xylem). This in turn can lead to wilt symptoms and discolorations of foliage

that are typical of nutrient deficiencies (e.g., yellowing or reddening).

SYMPTOMS OF NEMATODE DISEASES

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While most nematodes cause root symptoms (either galling or general tissue necrosis), a few

nematodes can cause foliar symptoms. An example that can have an impact in the ornamental

industry is the nematode Aphelenchoides. This nematode can lead to NECROSIS (tissue death) or

DISCOLORATIONS including yellowing or reddening. Often LESIONS (dead areas) are relatively

large and delimited (bordered) by veins. This combination of characteristics tends to distinguish

Aphelenchoides-induced lesions from those caused by bacteria or fungi. However, actual extraction

and identification of the nematode under a microscope is necessary for confirmation.

This photo shows an infection of Aphelenchoides on mum.

ABIOTIC CAUSES OF DISEASE

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Some plant diseases or abnormalities are not caused by biological agents such as fungi, bacteria,

viruses, phytoplasmas or nematodes. These diseases can be caused by a variety of ABIOTIC (non-

biological) factors. Often plant abnormalities caused by abiotic factors are called INJURIES.

Common abiotic factors that can influence plant health include nutritional problems (including

nutrient deficiencies or toxicities), pesticide exposure, environmental pollutants, and adverse

weather.

SYMPTOMS OF NUTRIENT ABNORMALITIES

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Often nutrient abnormalities show up as DISCOLORATIONS of foliage. Common discoloration

symptoms include YELLOWING or CHLOROSIS, and REDDENING. Noting the pattern of

yellowing or other discoloration can sometimes provide clues as to the type of abnormality. In this

photo of a poinsettia, yellowing has occurred along the margins of the leaf and is due to a lack of

molybdenum, a minor plant nutrient. General yellowing of foliage may indicate a lack of nitrogen.

If leaf veins remain green, and leaf blades turn yellow, then an iron or manganese deficiency may be

the problem. Reddening of foliage may indicate a phosphorus deficiency.

SYMPTOMS OF NUTRIENT ABNORMALITIES

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While yellowing is often a symptom of a nutrient deficiency, it can also indicate an overabundance

of a particular nutrient. The ivy in this photo has received too much copper, which has proved toxic

and led to yellowing.

SYMPTOMS OF DAMAGE DUE TO PESTICIDE EXPOSURE

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With an increase in the number of herbicides being used on lawns by homeowners and lawn care

professionals, the incidence of accidental exposure of non-target plants to these herbicides has also

increased. Common symptoms of herbicide exposure include CURLING and CUPPING. This

geranium plant was exposed to the herbicide 2,4-D.


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Tomatoes (as well as grapes) are particularly sensitive to low levels of herbicide exposure. Another

common symptom of herbicide exposure is TWISTING, as shown in this tomato exposed to 2,4-D.


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DISTORTION of flowers is also a common symptom of herbicide exposure as shown in this photo of

a rose exposed to a phenoxy-type herbicide.

Note that symptoms of herbicide exposure are often very similar to symptoms caused by viruses or

phytoplasmas. Often distinguishing between these three causes is very difficult. Information on

recent pesticide applications, the number and types of plants affected, the location of the affected

plants, and the presence or absence of possible pathogen vectors may help you sort out whether

biotic or abiotic factors are involved, and further pinpoint the exact cause of the problem.


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In addition to improper use of herbicides, improper use of fungicides can lead to plant injury. In this

example, MARGINAL BURNING on the leaves of this geranium is due to application of four times

the labeled rate of a common root rot fungicide called Banrot.

When using pesticides, be sure to read and follow all of the instructions on the pesticide label,

including the recommended rates. Using excess pesticide or applying the pesticide more frequently

that is listed on the label is a criminal offense, punishable by fine or imprisonment.

SYMPTOMS OF DAMAGE DUE TO ENVIRONMENTAL POLLUTANTS

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Contaminants in water or the air can also affect plant health. These pollutants can cause a wide

range of symptoms, many of which may look like damage caused by nutrient abnormalities or even

insect damage. In this photo, the MARGINAL BRONZING symptoms of this Schefflera plant are

due to an exposure to sulfur dioxide. High levels of sulfur dioxide can sometimes be found near

plants that burn coal.


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High levels of fluoride in water or fluorine gas in the air can cause symptoms like those in this

Dracena. Note the papery thin, almost transparent characteristic of these LESIONS, compared to

those that are typically caused by fungi or bacteria. Also note that damage is concentrated towards

the margins of the leaf where fluorine tends to accumulate.


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Some environmental pollutants can have a biological origin. The browning in the arborvitae is due to

dog urine.

SYMPTOMS OF DAMAGE DUE TO ADVERSE WEATHER CONDITIONS

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Extremes of weather can also lead to plant injury. Common weather-related injuries include COLD

INJURY shown in this photo on spruce. As the new branch tips (candles) began to expand, the tree

experienced a period of cold that was severe enough to kill the growing tips. To correctly diagnose

this kind of injury, you must remember to ask a grower when during the season symptoms first

appeared and whether any unusual weather occurred just before the onset of symptoms.


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On the other end of the spectrum are HEAT INJURIES. In this case the temperature in a ginseng

garden got high enough to lead to tissue damage. The papery, almost transparent appearance of the

damaged tissue of these plants suggests that a pathogen is NOT involved. As with COLD INJURY,

knowing the weather conditions prior to the onset of symptoms can be critical in identifying the

cause of this disorder.


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Often lack of water is a common cause of injury to plants. In this photo, the yew is suffering from a

disorder caused WINTER INJURY. Damage of this sort typically occurs in the early spring when air

temperatures rise, but soil temperatures remain cold, often due to a layer of snow. As the air

temperature rises, the foliage of the shrub becomes active and begins to use water. Unfortunately,

the root system of the shrub, which is experiencing colder temperatures, does not become active.

Thus there is no way to replace the water that is being used by the foliage, which becomes desiccated

and dies.


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Lack of water can also be due to DROUGHT STRESS during periods when rain is not plentiful. The

REDDENING of spruce branches in this photo is a typical symptom of drought stress in spruce.

DISEASE CONTROL

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Thus far, we have discussed the various biotic and abiotic causes of plant disease. In the next section

we will discuss methods for disease control.

DISEASE CONTROL - CULTURAL CONTROL

Typically the most common way in which homeowners attempt to control plant diseases is through

the use of CULTURAL CONTROLS. Cultural controls involve ways in which you can modify how

you grow your plants in order to reduce or eliminate disease. One general area of cultural control is

to avoid the pathogen.

AVOIDING THE PATHOGEN.

Pathogen avoidance may seem difficult, but there are several straightforward ways in which you can

accomplish this.

BUY SEED FROM A REPUTABLE SOURCE.

Many plant pathogens, particularly viral and bacterial pathogens are carried on seed. Therefore

using clean seed to start can help minimize disease problems later on. If possible, buy seed that is

certified as disease or pathogen-free. Certification doesn't guarantee that there are absolutely no

pathogens in the seed, but does indicate that the seed has been produced in a manner that will

minimize the likelihood of pathogens being present. Often certified seed is produced in dry areas

(e.g., in the western USA) where pathogens typically don't grow well. Certified seed is also tested

for pathogens during the certification process and meets some set of standards for the percentage of

seeds that may be infected with a given pathogen. If you buy certified seed, you should ask for

information on where the seed was produced, what pathogens it has been tested for and what the

testing standards are.


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AVOID CONDUCIVE ENVIRONMENTAL CONDITIONS

Recall that a favorable environment is one of the three sides of the disease triangle. Eliminating a

favorable environment can eliminate or dramatically reduce disease. Once again, you may not feel

that you have much control over environmental conditions. However there are several, easy

techniques that you can use to modify the environment in your garden so that it is less favorable for

disease development.

MODERATE SOIL MOISTURE

Perhaps the most common disease problem seen in the Plant Disease Diagnostics Clinic is that of

root rot. While you can't stop the rain from coming, you do have control over how often and how

much you water during dry periods. DON'T OVER-WATER. Provide enough moisture to prevent

your plants from experiencing water stress, by don't continue to water once plants have received

what they need. In addition, be careful not to over-mulch. Mulch tends to help retain water and a

layer greater than three inches thick is not needed or recommended, and can be detrimental.

REDUCE HUMIDITY

Infection by many plant pathogens is favored during periods of high humidity, which in turn leads to

extended periods when leaves are wet. The longer the leaf wetness period, the more likely infection

will occur. Moderation of soil moisture is one way of reducing humidity. Less water in the soil

means less water evaporating and contributing to air humidity. Also, don't plant ornamentals too

close together. Crowded plants tend to reduce air movement, which in turn tends to lead to higher

humidity and longer periods of leaf wetness.

FERTILIZE PLANTS PROPERLY


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SANITATION

Keeping your garden, work areas and tools clean is a fundamental and important means of

pathogen/disease control.

DISINFEST TOOLS

In general, you should make a habit of cleaning and disinfesting your gardening tools when moving

from area to area in your garden. This can be particularly critical when you have been working in an

area where you have historically had disease problems. It is also important to disinfect pruning

shears as you move from tree to tree or from shrub to shrub. Sometimes you may even want to

disinfest pruning shears between cuts on the same tree or shrub. By disinfesting your tools, you

reduce the chance of moving pathogens from place to place in your garden.

To disinfest tools, use a 10% bleach solution, alcohol or another disinfectant. Often people ask

whether Lysol spray will work. It should because the major ingredient is alcohol.

ADEQUATELY CLEAN POTS AND BENCHES

Dirty pots and workbenches can serve as reservoirs of plant pathogens. Wash bench tops thoroughly,

then disinfest with a 10% bleach solution or another disinfectant. To disinfest clay pots, wash them

thoroughly, then soak them for a minimum of 20 minutes in a 10% bleach solution. Be cautious

when reusing plastic pots. Some research indicates that some root rot pathogens (e.g., Pythium and

Phytophthora) can survive in plastic, even after soaking in 10% bleach.

USE PASTEURIZED SOIL

DISEASE CONTROL - CHEMICAL CONTROL

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Another method of controlling plant diseases is by use of chemical pesticides such as fungicides (for

control of fungi) and bactericides (for control of bacteria) and nematicides (for control of

nematodes). Viruses and phytoplasmas cannot currently be controlled using chemical pesticides.

Fungicides and bactericides fall into two major groups.

PROTECTANT fungicides are those that are applied to the surface of a plant and must coat the

surface to be effective. Protectant fungicides are NOT absorbed by the plant. They form a chemical

barrier between the pathogen and the plant. Good coverage is critical for protectant fungicides to be

effective.

SYSTEMIC fungicides are absorbed by the plant and redistributed internally. Thus even coverage is

not as critical as with protectant fungicides. Systemic fungicides tend to remain effective for a

longer period than protectant fungicides, because they are less likely to wear or wash off the plant,

and have less exposure to environmental factors that may lead to degradation.

FUMIGANTS are another type of compound used for control of pathogens, particularly pathogens

found in soil. Fumigants are general biocides. They are toxic to all kinds of biological organisms

including pathogens, non-pathogens, insects, weeds, and even desirable plants and animals. I do not

recommend use of fumigants in urban settings.

DISEASE CONTROL - CHEMICAL CONTROL

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Very few fungicides and bactericides are available for use by homeowners. When choosing a

pesticide for home use, be sure that the plant to which the product is to be applied is listed on the

pesticide label. If the plant is not listed, then use of the pesticide on that plant is illegal and

punishable by fine or imprisonment.

Once you decide to use a pesticide, be sure to use that product safely. Read the entire pesticide label

before using it and be sure to follow the instructions on the label. The label will provide detailed

information on the kinds of plants that can be treated, how much of the product and how often the

product can be used, the type of clothing that must be worn when using the pesticide and the length

of time you must wait after applying the pesticide before you can again work in or eat produce from

your garden.

Pesticide applicator training is available for anyone who is contemplating working with or applying

pesticides. While training is designed primarily for people who want to become professional

pesticide applicators, I also recommend it for homeowners. For a nominal fee, the training provides

extensive, very easy to understand information on pesticides and their use.

DISEASE CONTROL - BIOLOGICAL CONTROL

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The third major category of plant disease control is biological control.

DISEASE RESISTANCE

The classical form of biological control is use of disease resistant plants. While common in

agricultural settings, disease resistant plants are less common in ornamental crops. When disease

resistant varieties are available, choose them carefully. Ask questions.

To which diseases is this variety resistant? You want to choose varieties that are resistant to the

diseases common in your area.

How was the resistance tested? You should look for disease resistance that has been tested in several

sites, over several years and under a variety of weather conditions. Plants tested in this manner are

more likely to be resistant in your garden, under your environmental conditions, than those tested for

a single year and in a single site.

Keep in mind that disease resistance is relative. Some plants that are resistant under certain

environmental conditions will not be resistant under other, more extreme environments. Prime

examples are powdery mildew resistant Phlox varieties. In a normal year, these varieties exhibit a

high level of powdery mildew resistance. However, during extremely wet years, these varieties may

not perform that differently from powdery mildew susceptible varieties.

Also keep in mind that resistance is not forever. Varieties that are resistant today, may not (and

probably will not) be resistant many years down the road. Pathogens are very plastic. Once resistant

varieties are introduced, these varieties tend to select for variants (sub-populations) of the pathogen

that can overcome the resistance. Fighting diseases using resistant varieties is an ongoing battle.

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