Click on any
image for a more
detailed view

Disease Cycle and Epidemiology

Plant diseases are often severe during periods of warm temperatures and high moisture. In the United States, rice is grown in Arkansas, California, Florida, Louisiana, Mississippi, Missouri, and Texas (Figure 13). In the United States, rice is planted mechanically (Figure 14) as seed, while in many other countries, rice seedlings are grown in beds and transplanted manually or by machine. Throughout the world, rice is normally grown partially submerged in water in paddies, although in some regions rice is grown as upland rice in much the same way as wheat. Generally, rice blast is favored by moderate temperatures (24C) and periods of high moisture that are 12 hours or longer, conditions readily attainable in flooded rice fields.

Figure 13

Figure 14

The overwintering sources of spores that comprise the primary inoculum consist of grasses, volunteer plants, infested refuse (Figure 15), and infested seed on the soil surface after mechanical planting (Figure 16). Infested seeds left on the soil surface can readily produce spores of P. oryzae for more than several weeks after planting, well after seedlings have emerged (Figure 17). In the greenhouse, the fungus also sporulates on the dead or dying coleoptiles of plants grown from infected seeds.

Figure 15

Figure 16

Figure 17

Spores produced as the primary inoculum on the overwintering tissues produce the initial infections on young seedlings when the spores that are deposited on leaves, germinate and invade leaf tissues. Disease severity is often correlated with the amount of infested material (Figure 18). For example, in Arkansas, the first visible lesions often appear approximately 45 to 55 days after planting, but the disease is much more severe in plots receiving the highest amounts of primary inoculum found on infested seeds. Lesions on the young seedlings appear within a few days after infection. These secondary lesions produce more spores and these spores are readily wind disseminated to nearby healthy leaf tissues. The secondary cycles can be repeated many times during the growing season, with the potential for very high amounts of disease within the crop. The number of cycles and the number of spores that are produced on each individual lesion can be influenced by many factors, including the temperature, rainfall, the depth of the water in the paddy, the amount of nitrogen used to fertilize the rice, and the level of genetic resistance in the cultivar that is infected (Figure 19). Generally, the leaf phase of the disease is most severe when daily temperatures are moderate, and when rice is over-fertilized, or if it is being grown in flood waters below the recommended depths. Under these conditions conducive to disease, very high incidences of disease have been recorded (Figure 19) on susceptible cultivars such as Newbonnet and RT7015 in comparison to cultivars genetically resistant to the fungus.

Figure 18

Figure 19

The amount of disease at the end of the vegetative phase of the growing season influences the amount of disease during the reproductive phase. Spores produced near the end of the growing season may infect the collar of the flag leaf producing symptoms called collar rot. They may also infect the neck when it emerges from the infected collar upon which the head will be supported to produce a condition called rotten neck or neck blast. Yield losses are significantly correlated with the extent of rotten neck. Research has shown a 0.5% loss in yield for every 1% of rotten neck. Infections of the neck are generally considered the most deleterious phase of the disease because infection at this location can reduce seed set on the entire panicle. In addition, the fungus can infect the panicle, the branches on the panicle, and the peduncles upon which the seeds are carried on these branches. Finally, new information shows that the seed also can be infected. Infections of the neck, panicle and branches of the panicle are usually relatively nondescript grayish discolorations of the tissues.

RETURN TO TOP