Most plant diseases are infectious. They result from the interaction of two organisms, the host plant which is the suscept of the disease and the patho gen which is the causal agent of the disease. A wide array of pathogens cause disease. These include many fungi, bacteria, viruses, mycoplasma, nematodes, and parasitic phanerogams. Several abiotic environmental factors can also cause plant disease. Disease resistance may be partial or nearly complete. Complete susceptibility of a plant to a pathogen is rare. In such hosts, nothing impedes pathogen development throughout the disease cycle. A large proportion of pathogen propagules reach the host and infect it. Pathogen development is rapid and abundant in the host. The pathogen reproduces rapidly and over a prolonged period. If the pathogen has an efficient means of dispersal, it spreads rapidly to new infection courts. Under favorable en vironmental conditions an explosive epiphytotic results. In the resistant plant, host and pathogen are in conflict; pathogen growth and development are suboptimal. The resistance can take various forms and numerous genetic systems are known to result in host resistance. Genetic variation within the pathogen is of extreme importance in disease resistance studies and in disease control by means of host resistance (1-4). Some forms of host resistance function against certain races or biotypes of a pathogen but not all against others. In contrast, other forms of resistance function against all biotypes of a pathogen.
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Hooker et al. (1971) studied this question.