Different techniques for detection of plant pathogens

Zohaib ul Hassan Plant pathology (zuhaib1144@gmail.com)

Plant pathogen“An entity that can incite the disease in plant”. Disturb the normal physiological functioning of

plant. Take its nutrient from plant.

Plant pathology It is the branch of Agriculture , which deals with

The study of plant diseases. It includes: importance, occurrence, symptoms,

cause, etiology, disease cycle and management of diseases.

Major plant pathogensFungiBacteriaNematodes viruses

ISOLATION & DETECTION TECHNIQUES FOR PLANT PATHOGENS

Koch,s postulates: (1881-1882)I. AssociationII. Isolation , PurificationIII.Re-inoculation

Compare the symptoms of diseased and healthy plants.

Modification's in postulatesE.F. Smith : (1903)Re-isolation , was added by E.F. Smith.For obligate parasites some modifications

also done.1.Study of bio-chemical properties for the particular virus.2.Study of intrinsic properties.

TECHNIQUES USED FOR DETECTION OF PATHOGENS

Histopathological: ( on the basis of infected tissues) , mostly done in cases of fungi, bacteria and nematodes.

Serological (interaction between antigen and antibody)

Molecular techniques ( Nucleic acid based methods)

Serological and NA based techniques are mostly used for viruses.

HISTOPATHOLOGICAL TECHNIQUES SymptomologyVisual observation through naked eye.All plant pathogens can detect through

characteristics symptoms, which they have produced in different plant parts.

Symptomology (fungal diseases)

FUNGAL DISEASESPOWDERY MILDEW: WILT:

Identification of BacteriaOn the basis of symptoms. (ooze production is the

characteristics symptom of plant pathogenic bacteria)

Characteristics symptom of bacteria.

SYMPTOMS OF VIRUSDwarfing & stuntingMosaicChlorosisNecrosisRing spotEnation or TumorVariegation

CULTURE GROWTH/ COLONY MORPHOLOGYFor Bacteria and Fungi.Growth on culture media will give the

information about the pathogen. e.g. Xanthomonas axonopodis pv. citri gives

yellowish colour growth , when streaked on growth media,

(nutrient agar).In fungi: Rhizopus spp. Gives the cottony

growth on PDA , while Geotrichum candidum gives the condensed growth.

Geotrichum spp. Rhizopus spp.

xanthomonas

MOUNTSTemporary & Permanent.

Used for fungi, nematodes, bacteria.

Can detect the pathogen to observe it under microscope or stereoscope.

MICROSCOPYTo detect the pathogen on the basis of

morphology.

In case of fungi: reproductive structures, vegetative structures, spores can be seen under microscope. From which we can identify the fungus up to genera level.

We can detect the nematodes after preparing slide from suspension, and visualized it under microscope.

MICROSCOPY

STAININGBacterial cells, fungi and nematodes can be

colored with a stain.Provide contrast with the background or to make

cellular organelles visible.SIMPLE STAINS: methylene blue, basic fuchsin,

and crystal violet.

BACTERIAL STAINNING

Staining a bacterial smear. (a) Pass the slide, smear side up, through a flame three times. (b) Flood the smear with a stain and let stand one minute. (c) Rinse the slide gently with water, making sure the stream of water does not strike the smear directly. (d) Carefully blot the slide dry.

Gram StainThe Gram stain separates almost all bacteria into

two large groups: the Gram-positive bacteria, which stain blue, and the Gram-negative bacteria, which stain pink.

Gram staining

Gram-positive bacteria Gram-negative bacteria.

Differentiation on basis of bio-chemical properties

A more precise method is to determine whether or not the bacteria utilize a particular biochemical pathway.

Many bacteria use carbohydrates as energy sources.

Starch hydrolysis. Some bacteria do not hydrolyze starch (left), while others do (right), leaving a clear ring in the agar around the bacterial culture.

Biochemical propertiesAbility of different bacteria toferment the carbohydrate dextrose.Some bacteria produce acids as end products of dextrose fermentation(center). Others produce both acid and gaseous end products (left). Some cannot ferment dextrose at all (right).