CLASSIFICATION OF PLANT DISEASE

• SUBMITTED BY • SOMBIR KASHYAP • MSC FINAL • 5077

• SUBMITTED TO • DR SUNDER SINGH ARYA • PROFESSOR

DEPARTMENT OF BOTANY MDU ROHTAK

DEFINITION

Plant disease – an impairment of the normal state of a plant that interrupts or modifies its vital functions.

Or “ A malfunctioning process that is caused by

continuous irritation and produce symptoms’’.The organisms that cause disease are called pathogen.E.g. bacteria, fungi, viruses and protozoa etc.The study of disease condition is called pathology, also

known as phytopathology

Difference between healthy and diseased plant

HEALTHY PLANT• Normal physiological

functions including• Normal cell division,

differentiation and development

• Absorption of water and minerals from the soil and translocation

• Photosynthesis• Reproduction

SICK /DISEASED PLANT

• A plant would not have all of these functions

CLASSIFICATION ON THE BASIS OF CAUSE

• Infectious disease – a disease that is caused by a pathogen that can spread from a diseased plant to healthy plant.

Disease is caused by living organisms.It is also known as parasitic and biotic diseaseE.g. fungi, bacteria, virus, phytoplasma,

nematodes, higher parasitic plants etc.Such as Pectobacterium atrosepticum (a potato

pathogen)

• Non-infectious disease – disease that is caused by non-living organisms.

Could not be spread to other healthy plant Such as factors Unfavourable environment e.g. frost injury, physiological wilt. Too low/high temperature Lack/excess of soil moisture Air pollution Nutrient deficiency- e.g. khaira disease in rice due deficiency of

Zinc Mineral toxicity Soil acidity or alkalinity

ON THE BASIS OF OCCURRENCE AND GEOGRAPHIC DISTRIBUTION

• Endemic – when a disease more or less constantly prevalent from year to year. These diseases are natural to one country or part of the earth. E.g. wart disease in potato in Darjeeling• Epidemic – such a disease occurs periodically but in severe state involving major area of

crop. It may be constantly present in the locality. This is because the environmental conditions favourable for rapid disease development

occur only periodically. E.g. rust, late blight, mildews.• Pandemic – E.g. late blight of potato• Sporadic – these diseases occur at very irregular intervals and locations and in relatively

few instances. A given disease may be endemic in one region and epidemic in another. E.g. leaf blight, wilt.

On the basis of natural perpetuation and mode of infection

• Soil borne – pathogen survive in soil or on infested plant debris lying in soil either as their resting spores or as mycelial strands and rhizomorphs.

E.g. Root rot , wilt and seedling blight• Air borne – the micro-organisms are spread through air

and attack the plants causing disease E.g. blight, rust, powdery mildew.• Seed borne – the micro-organisms are carried along

with seeds E.g. damping off

On the basis of symptoms

• Rust – caused by Basidiomycetes of the order Uredinales Mostly attack on leaves and stems Appear as numerous rusty, orange, yellow, or even white-

coloured spots that ruptured epidermis Some form local spots, swelling and even galls E.g. stem rust of wheat caused by Puccinia graminis, yellow

or stripe rust of wheat , rye, and barley is caused P. striiformis, leaf rust of wheat and rye by P. triticina , leaf rust of barley , crown rust of oats , corn rust , sugarcane rust , sorghum rust ,coffee leaf rust ( Hemileia vastatrix ) , cedar apple rust ( Gymnosporangium juniperi ) etc

Rust on wheat leaf, stripe,stem

Rust on sugarcane leaf

Coffee leaf rust

• Smuts – caused by a fungus of the order Ustilaginales. Many types of plant affected , but smuts are important in cereals

and other grasses. They produce a mass of black , powdery spores and no grains are

formed. The symptoms include the formation of masses of lack soot like

spores and infected plant shows some degrees of distortions. E.g. loose smut of wheat and barley caused by Ustilago nuda tritici It is world wide in distribution In India, occurs in all states where wheat is grown, losses may be

up to 40 per cent in some area.

Smut in wheat and barley

Karnal bunt of wheat –Mitra reported in 1930 and by 1985 also reported from W.B.,Gujrat, Bihar, M.P.

Pathogen- Tilletia indica The disease is usually noticed only when the partly smutted and broken kernels are

seen in threshold grain. Although infected wheat is not toxic to humans or animals, wheat grain containing

more than 3% bunted kernels is generally considered unfit for human consumption. Because the smut causes an unpleasant odour , colour and taste in wheat product. All the spikes on a plant are not infected , and within a spike only a few spikelets are

attacked . it becomes evident when the grains have developed. Grains have been partially, rarely wholly , converted into bunt balls enclosed by the

pericarp.

Karnal bunt in wheat

• Rots – diseases that infect underground plant parts. They can be caused by fungi, bacteria or soil-borne nematodes. Infections lead to disintegration of underground tissues are

difficult to manage because they are not visible. E.g. Rhizoctonia and Phytophthora are root rots. These two fungi attack the root system of many different plants. Bacteria can only enter the host tissue through wounds. Crown and collar rots occur at the soil line where the plant

emerges.

crown rot infection on Beta vulgaris

• Wilt – a wilt disease is any number of disease that affect the vascular system of plants.

Bacterial wilt of Cucurbits is caused by the bacteria Erwinia tracheiphila.

Bacteria spread through the xylem vessels from the area of infection to the main stem and the entire plant wilts and dies.

E.g. wilt disease in potato, tomato.

bacterial wilt symptoms displayed by a muskmelon plant

Wilt in tomato

• Canker – a canker is a dead area in bark or cortex of woody stems.

They are often large areas with definite margin.

Dead bark splits and falls away.An example is citrus canker (bacterial)

Citrus canker

• Powdery mildew – it is a fungal disease of foliage , stems, flowers and fruit where a superficial fungal growth covers the surface of the plant.

E.g. grapes, cucumbers, grasses, soyabean etc. Symptoms white, powdery spreading patches of fungus

on upper or lower surfaces, flowers and fruit. Sometimes tissue become stunted or distorted. Disease seldom kill their hosts but utilize their nutrients,

reduce photosynthesis , increase respiration and transpiration and reduce yields by as much as 20 to 40%.

Powdery mildew of grape and soyabean leaves

Powdery mildew of cucurbits

• Downy mildew – it is caused by family Peronosporaceae.

Many ornamental and edible plants including peas, onions, lettuce, grapevines are affected.

Main symptoms are discoloured areas on upper leaf surfaces; white, grey or purple mould below.

Timing is mainly summer and autumn, particularly in wet conditions.

E.g. rose and lettuce.

Downy mildew on rose and lettuce

On the basis of organ they attack

Root disease – Shoot disease – Fruit disease – Foliage disease –

On the basis of host plant

Cereal disease – Vegetable disease – Fruit disease – Forest disease – Ornamental disease –

On the basis of extent to which plant disease associated with plant

Localised – these disease are limited to a definite area of varying extents of an organ, or only to a particular part of the plant.

Systematic – pathogen spreads throughout the entire plant to varying extents and is associated with almost every stage of plants life cycle.

Based on pathogen generations

Monocyclic disease – those diseases which have only generation in one cropping season e.g. loose smut of wheat.

Polycyclic disease – those diseases which have more than one generation in a cropping season e.g. late blight of potato

• References • George N. Agrios , Plant pathology 5th edition• R.S. Mehrotra , Plant pathology 2nd edition