Introduction to Plant Introduction to Plant PathologyPathology

AND environmental impact

Disease = disturbance from plant Disease = disturbance from plant pathogen or environmental factor that pathogen or environmental factor that interferes with plant physiologyinterferes with plant physiologyCauses changes in plant appearance or

yield lossDisease results from:Direct damage to cellsToxins, growth regulators, or other

byproducts that affect metabolismUse of nutrients and water or interference

with their uptake

Disease PyramidDisease Pyramid

The interaction of components of plant disease can be expanded to include time and humans.

Time is often considered as the fourth component of plant disease development.

The four components together can quantify the amount of disease.

The human equation can affect the three components of the disease triangle and should be considered as a fifth component in disease development.

Host FactorsHost Factors

All plants can be considered hostsDegree of genetic uniformity – crop plants – inbred

linesAge – affects disease development depending on

plant-pathogen interactionThere are different levels of susceptibility, which

include:◦ Immune - cannot be infected. ◦ Susceptible - can be infected. ◦ Resistant - may or may not be infected, and is the plant able

to prevent the pathogen from killing it. ie. defense compounds

Pathogen FactorsPathogen Factors

Amount of inoculumPathogen geneticsVirulence of the pathogenType of reproduction:

◦Monocyclic◦polycyclic

Ecology and mode of spread◦Air◦Soil◦Seed◦Vector dependency

Environmental FactorsEnvironmental Factors

MoistureTemperatureEffect of human culture practice

◦Monoculture◦Amount of inoculum: seed quality, disease residues, rotation, alternate host

◦Introduction of new pathogens

Disease DevelopmentDisease Development

Every infectious disease requires a series of sequential events in order for disease to develop.

Specific characteristics are unique for each disease.

General events are:1. dispersal of the pathogen to the host2. penetration and infection of the host3. invasion and colonization of the host4. reproduction of the pathogen5. pathogen dispersal6. pathogen survival between growing seasons

and/or in the absence of a host

FungiFungi

Diverse and widespreadFilamentous (hyphae) form a network of

mycelium (lots of hyphae)Recognized by reproductive structures

(mushrooms, rusts, conks, etc.)Most of the 100,000 spp. are saprophytes

◦Live on dead organic matterApproximately 8,000 species attack plants

◦Plant pathogens

Fungal DiseasesFungal Diseases

Reproduction by sexual and asexual means

Spread through a variety of methods◦ wind/water blown spores◦ rhizomorphs◦ Sclerotia (overwintering)

Include organisms from Kingdom Protista, that are now classified outside the Kingdom Fungi:◦ Downy mildews◦ Pythium◦ Phytophthora◦ Clubroots

SymptomsSymptoms

Initially, similar to drought & starvation:◦Plants appear off-colour◦Weakened & susceptible to attack◦Wilting and dieback occur later◦Younger plants usually killed rapidly◦Older plants decline over time (years)◦Roots have brownish streaks

BacteriaBacteria

Prokaryotic microscopic organisms◦ Free living single cells, or◦ Filamentous colonies

Reproduce via binary fission◦ 2 daughter cells are identical to mother cell

Don’t usually produce resistant resting spores◦ Need host or growth medium to survive

For rapid spread, plant infecting bacteria usually require:

◦ Warmth◦ Moist conditions

Bacterial DiseasesBacterial Diseases

Less common than fungal or viral diseasesThey can be either:

◦parasites, saprophytes, or autotrophsSymptoms include:

◦Cankers, Wilts, Shoot Blights, Leaf Spots,Scabs, Soft Rots, & Galls

Generally, cannot invade healthy tissue; need wound or opening to infect.

Control methods usually cultural in nature (don’t use antibiotics on large scale)

Bacterial DiseasesBacterial Diseases

Bacterial galls: In some cases, toxic materials are produced that cause plant tissues of roots, stems or leaves to grow abnormally as in crown gall.

Bacterial leaf spot disease: The bacteria usually enter through leaf stomata. Symptoms include water-soaking, slimy texture, fishy or rotten odor, confined initially between leaf veins resulting in discrete spots that have straight sides and appear angular.

Gene on gene action!Gene on gene action!

 Evolution of the plant–bacterial pathogen interaction. (a) Plants have evolved receptors that could recognize PAMPs and triggers basal defence.  (b) Bacterium injects effector protein through type III secretion system (T3SS) to interfere with defence signalling or response. (c) Plant responds to infection by generation of immune receptors encoding for nucleotide-binding (NB), MAP kinase, leucine-rich-repeat (LRR) R-proteins that recognizes effector protein and triggers an acute defence response usually involving hypersensitive response (HR) and programmed cell death

Disease DevelopmentDisease Development

Infections occur through leaf scars and wounds. These give rise to small cankers in which the bacteria survive the winter.

Rain or water splash, and pruning tools spread the bacterium.

Bacteria overwinter in active cankers, in infected buds, and on the surface of infected and healthy trees and weeds.

The bacterium reproduces best between 21ºC and 25ºC.

The disease seems to be more severe after cold winters and prolonged spring rains.

VirusesViruses

Viruses are "submicroscopic" entities that infect

individual host plant cells. Viruses are obligate parasites: They can only

replicate themselves within a host's cell. In the virus infected plant, production of

chlorophyll may cease (chlorosis, necrosis)Cells may either grow and divide rapidly or may

grow very slowly and be unable to divide

Viral DiseasesViral Diseases

> 400 viruses infect plants; few are economically important pathogens

The infection remains foreverViruses are transmitted from plant to plant

by living factors: insects, mites, fungi and nematodes

Or non-living factors: rubbing, abrasion or other mechanical means (including grafting or other forms of vegetative propagation)

Occasionally transmitted in seed.

Virus Disease SymptomsVirus Disease Symptoms

The symptoms of most virus diseases can be put into four categories:

1. Lack of chlorophyll formation in normally green organs

2. Stunting or other growth inhibition 3. Distortions 4. Necrotic areas or lesions

NematodesNematodesMicroscopic roundworms◦ Barely visible with naked eye◦ No segments

Up to 4mm longClear or transparentFeed with stylet

◦ Pierce plants (pests)◦ Kill arthropods (beneficials)

Nematode DiseasesNematode Diseases

Plant pathogenic nematodes = pests◦ Infect roots & bulbs (below-ground)

◦ Foliar nematodes (above-ground)

◦ Also vectors of plant viruses

As they feed, they weaken & stress plants – also predispose to other problems

Causes bulb & root decline, and root knotsSpread by splashing water, and infested soil &

plant parts

Shoot NematodesShoot Nematodes ((AphelenchoidesAphelenchoides spp.) spp.)

Foliar nematodes feed inside leaves between

major veins causing chlorosis and necrosis.

Injury is most often seen at the base of older

foliage.

When plants with a net-like pattern of veins

become infested with foliar nematodes, the

tissues collapse in wedge-shaped areas and then

change color.

Root NematodesRoot Nematodes

Moisture and nutrient stress symptoms and general stunting are common (by killing meristem tissue)

Root lesion nematodes (Pratylenchus spp.)Burrowing nematodes (Radopholus similis)

destroy root cortex tissues as they feedRoot-knot nematodes (Meloidogyne spp.)

inject growth-regulating substances into root tissues as they feed, stimulating growths called galls or knots

Environmental and cultural Environmental and cultural factors affecting buildup of factors affecting buildup of fungal and bacterial plant fungal and bacterial plant pathogenspathogens

MoistureTemperatureDispersal agentsSoil pHOther

MoistureMoisture

Activates resting stagesAffects germination of spores and

penetration into hostWater on leavesHumiditySplashing water distributes inoculumLeaf wetness = best indicator but

difficult to measure

MoistureMoisture

Activates resting stagesAffects germination of spores and penetration

into hostWater on leavesHumiditySplashing water distributes inoculumLeaf wetness = best indicator but difficult to

measure

Rainy, cloudy conditions = important for spread and growth of many diseases

TemperatureTemperature

Affects growth ratesSome pathogens adapted to certain temp.

rangesRefrigeration = important for

management

Dispersal AgentsDispersal Agents

Bacteria, fungi are limited in mobility, need to be moved by:

WaterWindPeople, machineryInsects, other animals

Soil pHSoil pH

specific requirements for many soil-borne pathogens

Other

Widespread planting of genetically homogeneous crops can favor epidemic

Management of Plant Management of Plant Disease –Disease –StrategiesStrategies

Eliminate or reduce initial inoculum, or delay its introduction (preventive)

Slow the rate of increase, shorten exposure to favorable conditions

Management of Plant DiseaseManagement of Plant Disease

SanitationFungicidesHost plant resistanceCrop rotationCultural practicesTemperatureBiological controlOrganic amendmentsImproved plant health and nutrition

Sanitation (aimed at excluding Sanitation (aimed at excluding pest)pest)

Avoid infested sitesClean soil, planting material, tools, etc.Inspection and quarantineRemove infected debrisTissue culture can provide disease-free

planting material

FungicidesFungicides

Bactericides, if target is bacteriaDusts, sprays, fumigants, etc. Foliar, soil, seed, wound, or post-plant

applicationPreventative – slows rate of increaseInsecticides may also be useful for

managing insect vectors

Host Plant ResistanceHost Plant Resistance

Caution: pathogens can have multiple isolates

Vertical resistance – against some genotypes of a pathogen

Horizontal resistance – not limited to certain genotypes, across all isolates

Host genetic diversity is important to slow epidemics

Crop RotationCrop Rotation

Useful vs soil-borne diseases

Residues of some plants (e.g., cabbage family) may be toxic to some pathogens

Cultural Practices to Cultural Practices to Minimize Spread of DiseaseMinimize Spread of Disease

Favorable irrigation practices (drip vs overhead)

Timing of PlantingWider row spacingsEradicate alternate hosts for viruses

Important to minimize water and humidity to limit disease spread

Moisture management

TemperatureTemperature

Heat for soil sterilizationHot water treatment of planting materialSolarizationRefrigeration to slow disease progress in

harvested material

Management of Plant DiseaseManagement of Plant Disease

Sanitation Fungicides Host plant resistance Crop rotation Cultural practices Temperature

Biological control – Rhizobacteria may interfere with colonization of plant roots by fungi and bacteria

Organic amendments (avoid diseased plants in mulch, etc.)

Improved plant health and nutrition

The End!The End!

Any Questions?