Management
A combination of chemical, genetic, and cultural methods are used to control sorghum downy mildew. A number of single-gene sources of resistance have been found, and many resistant varieties are commercially available. However, new pathotypes of the fungus continue to evolve to overcome different sources of host plant resistance. Seed treatments with the systemic fungicides metalaxyl and mefenoxam have been widely used to prevent systemic infections of Sorghum Downy Mildew. Metalaxyl, which inhibits protein synthesis in the pathogen, can also be applied as a foliar spray. In Texas, use of these two fungicides, in conjunction with the use of resistant hybrids, made the disease a minor problem until one of the pathotypes endemic in the region evolved resistance to both chemicals.
A variety of cultural controls are effective against sorghum downy mildew. In areas where oospores are the principal source of inoculum, such as in the United States, crop rotation is an effective strategy, as oospores will be stimulated to germinate by both host and non-host crops, but will not be able to infect plants apart from corn, sorghum, and Johnson grass. Flax (Linum usitatissimum) is an example of a trap crop that is used to reduce the amount of oospore inoculum in the soil, before planting a susceptible crop like sorghum or maize. Deep tillage also reduces the amount of oospores surviving in the soil and therefore the incidence of the disease.
In areas where conidia are the principal source of inoculum, such as in most of the tropics, early planting can be effective in avoiding secondary infections. This is because an early crop will be able to avoid infection from conidia produced by later infected crops in the region. For inoculum that survives in seeds, drying to less than 20% moisture is effective in killing the pathogen.
Read more about this topic: Peronosclerospora Sorghi
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