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Disease Management

The successful management of rice blast results from a comprehensive series of recommendations that employ several different management strategies involving techniques within each strategy as outlined below.

Figure 20

Figure 21

Cultural strategies to manage this disease include useful techniques that rice producers are urged to follow. Crop rotation is one simple and effective technique that is highly recommended simply because it provides a mechanism that separates viable spores in crop residue (Figure 15) from the newly emerging seedlings. A second technique that rice producers are urged to consider is to fertilize each of the different cultivars properly. Overuse of nitrogen fertilizers, as shown above, increases the amount of rice blast in their fields while often failing to significantly increase yields. A third technique that is often overlooked or difficult to employ in some fields is maintaining a proper flood level for the rice to grow (Figure 20). Rice blast is known to be more severe in fields or parts of fields in which the water in paddies falls below recommended levels. Thus, rice blast is often severe in fields that were not rotated, are under-irrigated, and are over-fertilized (Figure 21). Finally, using high quality and disease-free seed is always highly recommended because infested seeds left on the soil surface provide inoculum from which epidemics develop.

Genetic resistance is a strategy that has long been the mainstay of successful rice production in the United States, even though the fungus has shown genetic variability for virulence. In the United States, many rice cultivars contain genes that confer resistance to one or more of the individual races of the fungus found regionally, and many also contain a high level of resistance to the fungus that is conferred by many other genes. The major difficulty in controlling rice blast with genetic resistance is that there are many races of the pathogen, and cultivars containing a single gene conferring resistance to a specific race of the pathogen often become susceptible over time with the development of new races that can infect plants with that specific resistance gene. For example, a mutation of an ‘avirulence’ gene in the blast fungus that produces an elicitor that induced the expression of resistance in the host with the Pi-ta gene for resistance has led to the formation of a new race capable of infecting plants that carry that important resistance gene, simply because the plant did not recognize the new race of the pathogen. In this way, the new race of the fungus has eluded resistance, which means that the cultivars containing this important resistance gene are very susceptible to the new race, even though they remain resistant to the other races of the fungus.

Figure 22

Figure 23

A third strategy that has long been viewed as a last resort for rice blast is the use of chemical fungicides to control the disease. There are two basic techniques that can be used to manage diseases with the chemical fungicide strategy. The first technique uses seed treatments to prevent infection of the seedlings after germination. The second technique uses fungicides to prevent infection of leaves and panicles during the growing season by making one or two applications of fungicides to the foliage to protect the panicles when they are emerging from the boot. This technique attempts to reduce the incidence of rice blast on the panicle necks and panicles. Both techniques have been used to manage rice blast but neither one is considered to be highly successful. In addition, the use of foliar fungicides applied by airplanes (Figure 24) can be very expensive.

Figure 24

Managing rice blast requires vigilance and careful integration of many strategies and techniques learned in all parts of the world by the individual rice producer. A single mistake can result in crop failure from this one important plant disease.

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