Disease control by Physical, Chemical and Biological methods, Resistant varieties
Plant Disease control by Physical methods:-
> The hot water treatment method of Jensen was developed in 1887 which was used to control loose smut disease of wheat, barley and Oats. Until the development of systemic fungicide hot water treatment was the only method to control loose smut. Hot water treatment is also effective in the control of nematodes.
> Solar energy treatment to control loose smut was first developed by Lutlzra. In this method seeds are first rinsed or soaked in water for 4-5 hrs. before drying them in scorching sun.
> Hot air treatment for the control of virus in propagating stocks was first developed by Kunkal in Peach yellow.
Plant Disease control by Chemical methods:-
> Chemical barrier to protect the host plant and/or eradicate an existing infection.
> Pesticides typically cannot “cure” heavily diseased plants.
> Types of pesticides:- fungicides, bactericides, nematicides, insecticides, biocides.
> Contact fungicide:- effective only at the site of application (protectant) must be applied before pathogen infects the plant; new growth emerging after application is not protected. examples: mancozeb, coppers, chlorothalonil, captan.
> Systemic fungicide:- absorbed & translocated (moved from application site) by the plant locally systemic = moves short distances (towards leaf margin) within the plant from the site of application (e.g., benomyl, triforine) systemic = moves further within the plant from the site of application (e.g., metalaxyl moves from roots up to shoots and foliage).
Methods:-
a. Seed treatment:- With fungicide before transplanting.
i. Soil treating chemicals:- It is used for controlling such soil borne diseases which attack on seeds or seedlings. The examples of such chemicals are – Formaldehyde, Captan, Thiram, Zineb, Organo-mercurials, PCNB, Ethylene dibromide, vapam etc.
ii. For Externally seed borne diseases, chemicals such as formalin, copper carbonate, captan, organo-mercurials (Agrosan GN and Ceresan) are used for seed treatment.
iii. For Internally seed borne diseases (i.e. loose smut), hot water treatment and solar treatment are used.
iv. Systemic Organic Compounds are effective chemicals for controlling both externally and internally seed borne diseases eg. Oxanthin derivatives (Plantvax and Vitavax), Benlate, Bavistin, Demosan.
v. For controlling air borne diseases, foliar application of chemicals is more effective.
vi. The common copper fungicides are: Perenox, Perelan, Blitox, Cuprokyt, Cuprosanand Fytolan. Its use is comparatively better than that of Bordeaux mixture.
b. Seed dressing:- With organomercurials and systemic fungicides.
Plant Disease control by Biological methods:- Biological control involves the use of one living organism to control another.
Biological Control Agents:-
a Parasitoids
b. Pathogens
c. Predators
d. Antagonists of Plant Diseases
a Parasitoids:-
> Each parasitoids requires only one host, which it kills for its development into a free living adult.
> Parasitoids are of the same size as the hosts , or sometimes even smaller.
> Mechanism:-
- The Female parasitoid lays eggs inside or on an insect host.
- Legless larva on hatching feed on the hosts internally or externally.
- Many adult parasitoids feed on nectar or pollen and serve as important pollinators.
> Examples:- Trichogramma,Ichneumoid,Scelionoid,Braconid, Encryrtid,Eulophid,Chalcid wasps and Tachinid Flies.
> Revolution of Trichogramma chilonis:-
- Trichogramma chilonis has been successfully released in Jammu & Kashmir, Himachal Pradesh, Punjab, Haryana, Gujrat , Uttrakhand , Uttar Pradesh, Bihar , Karnataka, Tamil Nadu ,Maharashtra and few other states.
- It has given 56-82% protection against the sugarcane tissue borers.
b. Pathogens:-
> Pathogens are disease causing organisms e.g. Bacteria, Fungi, Viruses ,Protozoa and Nematodes in Insect Pests which kill their host or debilitate the future generation.
> Mechanism:-
- The infected insects are unable to feed properly ,remain stunted, lose their body colour and get paralysed.
- Dead insects are often found hanging on the plants, or a fungal growth on the body of dried insect could be easily located in the field.
- Caterpillar, Pseudoplusia includens , killed by entomopathogenic fungus Nomuraea rileyi , white-disease in soybean crop.
c. Predators:-
> These are free living and larger in size than their prey, requiring several preys to complete their life cycle.
> Some smaller predators release a powerful poison, use a trap or hunt in groups to be more effective.
> Mechanism:-
- Green lacewings (very common and important predator of crop pests).
- The female lays small pale green, oval shaped eggs at the end of long silken stalks, which ultimately turns grey.
- The larva have well developed legs and pincer like jaws with which they suck the body fluids from the prey.
d. Antagonists of Plant Diseases:- The microorganisms used in biological suppression of plant diseases which grow in association with plant diseases are termed as antagonists.
An antagonist microorganism adversely affects the disease causing organism by following:-
i. Competition:- The competition which takes place between two organisms in terms of nutrients etc.
ii. Antibiosis:- An association between organisms that is injurious to one of them. Antibiosis, is provided by marigold (Tagetes species) roots, which release terthienyls, chemicals that are toxic to several species of nematodes and fungi.
iii. Hyperparastism:- The parasitic habit of one species upon another parasitic species which is harmful to one of them.
iv. Mycoparasitism:- A parasitism of a fungus(host) by another fungus(mycoparasite). Trichoderma harzianum, used as seed treatment against pathogenic fungus(Sclerotium rolfsii) on chickpea and sugarbeet.
Plant Disease control by Resistant varieties:- Using resistant varieties is the most efficient way of controlling vegetable diseases. Make an effort to buy resistant varieties when they are available.
> The following terms are important in a discussion of crop resistance:
i. Immunity
ii. Resistance
iii. Tolerance
> Immunity is the rule in the plant kingdom; most plants are immune to most pathogens. Therefore, one does not have to worry that the black spot on roses will appear next year on the garden tomatoes.
> Resistance, strictly speaking, refers to the lower disease levels seen in some cultivars or varieties of a particular crop species compared to other susceptible cultivars. Always choose resistant varieties when available.
> Tolerance refers to varieties or cultivars that appear to be just as heavily diseased as standard varieties, but which manage to be more productive or vigorous.