Control of plant diseases-some possible approaches

All the different approaches to control disease losses can be fitted into the broad categorizations of Van Der Plank, viz., vertical and horizontal resistances. While in the former we follow the strategy of matching each change in the pathogen with a change in the variety of the host plant, in the latter we follow the strategy of stopping the pathogen from gaining any advantage from a change of race, i.e., making changes of races irrelevant to control.

We still do not know all about horizontal resistance. Till we know enough about it, and manipulate it to an enduring and lasting advantage we should continue our efforts to exploit vertical resistance as much as possible. Increasing resistance by incorporating more resistant factors into a common agronomic base as suggested by Watson and Singh (1952), Schafer et al. (1963) or developing multilineal varieties with lines carrying as many diverse genes for resistance as possible, as suggested by Jensen (1952) and Borlaug (1958) are some of these lines in which our efforts should continue to be based. Vertical resistance can still be useful for situations where there has been a record of varietal stability or where the disease multiplies slowly. We may continue to add vertical resistance to the new varieties at the same time incorporating as much horizontal resistance as possible into them. Our attention should be concentrated more on horizontal resistance for situations where there is a record of very frequent varietal changes.

We should continue our efforts to know more about resistance and tolerance in plants and also about the potentialities for virulence of the pathogen. Perhaps, even if we succeed in splitting resistance into its various components as is often done for yield, it may be difficult to combine all those environmentally influenced characters into a single variety. Mter all, disease is only subsidiary to yield and quality, for which a crop is grown. Vavilov (1951) lists about 46 characters which go to make an ideal wheat and jfwe add to this already complex set of characters the components of resistance or tolerance, the breeding work may become more complex. Van Der Plank while talking about the primary aims (yield and quality) and the subsidiary aims (disease resistance etc.) in plant breeding righdy cautions that only optimal amount of attention should be given to disease in plant breeding, “neither too little nor too much”.

Finally, international cooperation in free exchange of ideas, materials, testing of materials for stability of resistance under different conditions etc., would hasten the rate of progress in tackling the disease problem in crop plants.