Plant Disease Inciting Agents

The plants in nature are exposed to a number of parasites and pathogens. The distinction of parasite and pathogen must be very clear in mind before distinguishing disease causing organisms from other micro organisms living on and getting nutrition from the host. 

Grass smut grown on MEA medium

The surface of plant is vulnerable to both disease causing pathogens and other environmental conditions. But the pathogens can be classified into one of the following groups.

i) Biotic Agents (Bacteria, Mollicutes, Fungi, Algae, Nematodes, Mycoplasmas etc)

ii) Abiotic agents (Environmental factors like Temperature, Rainfall, Humidity, High or low pH etc)

iii) Mesobiotic agents (These are Viruses and Viroids)

1 . BIOTIC AGENTS

A) FUNGI

Alternaria sp.

Fungi are small, generally microscopic, eukaryotic, usually filamentous, branched, spore-bearing organisms that lack chlorophyll. Fungi have cell walls that contain chitin and glucans (but no cellulose) as the skeletal components.

Classification  of fungi are

                  a. Oomycota

                  b. Zygomycota  

                  c. Basidiomycota

                  d. Dueteromycota

SYMPTOMS

The various symptoms caused by fungi in plants include, leaf spots, blight, canker, dieback, root rot, damping off, basal stem rot, anthracnose, scab, decline,  clubroot, galls, warts, withes-broom, leaf curls, wilts, rusts, smuts, mildews, are common diseases caused by fungi.

EXAMPLES

Late blight of potato, loose smut of wheat, kernel bunt, rust of wheat, powdery mildews of various crops and plants.

B) BACTERIA

Bacteria of different shapes

These are generally single-celled prokaryotic microorganisms whose genetic material (DNA) is not bound by a membrane and therefore is not organized into a nucleus. Their cells consist of cytoplasm containing DNA and small (70 S) ribosomes.

SYMPTOMS 

The symptoms caused are similar to those caused by fungi because they infect as fungi do. These include root rot, leaf curl, scab, canker, soft rots of fruits of fruits.

EXAMPLES 

Bacterial leaf spots and blight of cereals and grasses, bacterial spots of pepper and tomato and bacterial vascular wilts.

C) MOLLICUTES

Mollicutes are the class of bacteria wtihout cell walls. A few of them have helical structure and are called Spiroplasmas where as most of them has  rounded to elongated structures and are now called Phytoplasmas. 

SYMPTOMS 

The main characteristics of yellows-type diseases are a more or less gradual, uniform yellowing or reddening of the leaves, smaller leaves, shortening of the internodes and stunting of the plant, excessive proliferation of shoots and formation of witches�-brooms, greening or sterility of flowers, reduced yields, and, finally, a more or less rapid dieback, decline, and death of the plant. Root abnormalities and necrosis often precede the aboveground symptoms.  

EXAMPLES 

More than 200 different diseases are caused by them on different plants among these are included some of the most destructive diseases of plants, like Pear decline, grape yellows, coconut lethal yellow, apple proliferation, aster yellow of vegetables and ornamental plants and stolbur of tomato. 

D) PARASITIC HIGHER PLANTS

Dodder plant

More than 2500 species of higher plant are found living parasitically on other plants. Their main common characteristic is that these parasites are vascular plants that have developed specialized organs which penetrate the tissues of other (host) vascular plants, establish connections to the host plant vascular elements, and absorb nutrients from them. These parasitic plants produce flowers and seeds and belong to several widely separated botanical families. They vary greatly in their dependence on their host plants. Some, e.g., mistletoes, have chlorophyll but no roots so they depend on their hosts only for water and minerals.

Others, e.g., dodder have little or no chlorophyll and no true roots so they depend entirely on their hosts for their existence. Relatively few of the known parasitic higher plants cause important diseases on agricultural crops or forest trees.

EXAMPLES 

Dodder, witchweed, broomrapes, dwarf mistletoes of conifers, kudzu vine etc.

E) PARASITIC GREEN ALGAE

Cephaleuros virescens

Green algae are single-celled organisms that form colonies, or multicellular, free-living organisms, all of which have chlorophyll b. Algae are the organisms, often microorganisms, other than typical land plants, that can carry on photosynthesis. Algae are sometimes considered as protists with chloroplasts.

Algae are the main producers of photosynthetic materials in aquatic ecosystems, including unstable areas such as muds, sands, and intertidal aquatic habitats.

EXAMPLES 

Cephaleuros which is an intercellular parasite in the leaves of a number of angiosperms such as Magnolia, Rhododendron, Camellia sinensis (Tea) and Piper nigrum (Pepper). Till now much work has been done on the �red rust of tea� caused by Cephaleuros virescens.

F) NEMATODES

Meloidogyne sp. female 

The nematodes are roundworms which live in soil or water. Many of them are found are free-living and other are parasitic on animals and plants. They are the only plant parasite belonging to the animal kingdom studied in plant pathology.

EXAMPLES 

Meloidogyne (root-knot nematode), Heterodera (cyst nematode), Globodera (cyst nematode).

2. ABIOTIC AGENTS

Disease may be incited by 

• Saline Soil
• high or low termperature, 
• unfavourable oxygen relations
• accumulation of injurious impurities
• lightening injuries
• mineral deficiencies or excess
• injurious atmospheric gases 
• toxicitiy of pesticides  
• improper cultural practices
• soil acidity and soil alkalinity etc.

Characteristics of diseases caused by abiotic agents

• They are caused by excess or loss of something that support life.
• They cannot be transferred from diseased to healthy plant since they occur in absence of pathogen.
• Infect plant in all stages of their lives (e.g., seed, seedling, mature plant, or fruit).
• Symptoms vary from slight to severe and plants may even die.
• Symptoms vary in kind and severity depending upon the particular environmental factor involved and with the degree of deviation from normal.
• The diseases may be caused due to chemical injury, nutritive deficiency, freeze damage, russetting of fruit, wilting due to water stress, flooding etc.

3. MESOBIOTIC AGENTS

A) VIRUSES

Cauliflower Mosaic Virus (CaMV)

A virus nucleoproteins that multiplies only inside living cells and have ability to cause disease.

SYMPTOMS 

Symptoms are usually produced on leaves but sometimes on fruits, stem and roots as well. The viral infection always including dwarfing or stunting of entire plant and reduction in total yield. The effects may be slight or severe. The common symptoms are local lesions, ring spots and mosaics, chlorosis, vein clearing, vein banding, bunchy top, hypertrophy, atrophy, curling, crinckling of leaves. 

EXAMPLES 

Tobacco mosaic virus, Cotton leaf curl virus, rose mosaic virus, peppr leaf curl virus, potato virus X and Y etc.

B) VIROIDS

Potato Spindle Tuber Viroid

Virioids are low molecular weight, small, ribonucleic acids that can infect plant cells, replicate themselve and cause diseases.

Viroids differ from viruses in two characteristics: 

(1) Their RNA is very small consisting of 250 - 370 bases as compared to viral RNA which is range from 4 to 20 kilobases. 

(2) The viroid RNA lies naked i.e., without any protein coat which is present over the RNA of viruses.

EXAMPLES 

At least 40 plant diseases have been reported to be caused by virioids which include; potato spindle tuber, citrus exocortis, apple scar skin, chrysanthemum stunt, tomato apical stunt viroid etc.