Grain Sorghum Disease Sorghum vulgare Seed Rots and Seedling Diseases (fungi – Rhizoctonia solani, Fusarium sp., Pythium sp. and others): Seed rots and seedling diseases are more prevalent when seed is planted in cool, wet soils, and especially when packing rains seal the soil surface. Seedling emergence and growth is slowed under such conditions, and several soil- borne fungi are able to infect the tender seedlings. Other factors such as poor seed quality or improperly placed fertilizer or herbicide often contribute to the problem. Continuous growth of one crop over a period of years may favor buildup of organisms that cause seedling disease. Use of high quality seed treated with a recommended seed protectant is important. Precision planting to insure proper depth in a well-prepared seedbed is very helpful. Sorghum Downy Mildew (fungus – Peronosclerospora sorghi ): Young, systemically infected plants have light green to yellowish stripes lengthwise in the leaves often with a grayish-white downy fungal growth consisting of numerous tiny spores on the lower surface of the leaf opposite the pale striped areas. Figure 1. Soil-borne spores cause systemic infection of the young seedlings. These systemically infected plants will not produce heads. Later infections may parially or completely inhibit grain formation. Plants can also be infected by air-borne spores from the underside of infected leaves. These infections result in localized leaf lesions. Grain sorghum hybrids vary considerably in their susceptibility. Growers should select adapted, high producing hybrids that have resistance to this fungus. Growing highly susceptible grain or forage sorghum builds up populations of the organism in soil, making the problem more severe even in resistant hybrids. Use seed treated with a systemic fungicide containing metalaxyl and resistant hybrids to control this disease. Crazy Top Downy Mildew (fungus – Sclerophthora macrospora ): This fungal disease can be troublesome in low lying areas that become flooded. Infected plants have thick, stiff, twisted, pale green leaves with bumpy surfaces. The leaves often turn downward, and the plants produce many shoots or suckers giving a bunchy appearance. Infected plants do not produce heads or produce a proliferation of leafy tissue in place of the head. Wild and cultivated grasses can serve as sources of inoculum. There are differences in susceptibility among grain sorghum hybrids but these differences are not significant. Installing drainage structures and diverting water movement to avoid flooding is the most reasonable suggestion for control at this time. Maize Dwarf Mosaic Virus (MDMV): maize dwarf mosaic is a virus disease that occurs over all the sorghum producing areas of Texas. Its ability to cause damage is dependent on the presence of an overwintering virus host (mainly Johnsongrass), aphid populations to facilitate virus transmission and the susceptibility of the hybrid being grown. Affected plants have mottled (light green blotchiness) terminal leaves. Figure 2.These alternate light- and darker-green areas in the leaf can be more easily seen when held between the viewer and a light source. Observers should always look at the newest leaves for the most severe symptoms. Highly susceptible hybrids are stunted with chlorotic symptoms in the upper leaves and suffer significant yield losses. Some hybrids produce a red leaf symptom when plants are infected and when night temperatures are below 55 degrees F. Use tolerant hybrids and control Johnsongrass in and around the field to manage this disease. Head Smut (fungus – Sporisorium reilianum ): This disease is characterized by the large, dark-brown smut galls that emerge in place of the panicle. Figure 3.The gall is first covered with a whitish membrane which soon breaks and allows spores to be scattered by the wind. Plants become infected while in the seedling stage but evidence of infection is not apparent until heading time. The smut gall produces thousands of spores which become soil-borne and initiate systemic infection of seedlings in subsequent years. Different races of the fungus exist which may result in a sorghum hybrid being resistant in one area but not another. New sources of resistance have been found and growers should utilize resistant hybrids to avoid losses from this disease. Covered Kernel Smut (fungus – Sporisorium sorghi ): This smut disease was once quite destructive but is seldom seen now because most seed is chemically treated. The disease destroys all kernels in a head and replaces them with a cone- shaped gall or may affect only portions of a panicle. At harvest time, these galls are broken and spores contaminate the outer surface of other kernels. This disease is controlled by use of chemical seed treatment, use of clean seed and planting resistant hybrids. Loose Kernel Smut (fungus – Sporisorium cruenta ): Galls formed by loose kernel smut are long and pointed and the thin membrane covering them usually breaks soon after galls reach full size. This disease presents no immediate problem because the control measures mentioned for covered kernel smut have virtually eliminated occurrence of this disease. Foliage Diseases Caused by Fungi: A number of fungal organisms cause foliage infection and may become severe under certain conditions. Occurrence has seldom been consistent or damaging enough to warrant the development of specific control practices. No fungicides have been cleared for use on sorghum foliage. Hybrids vary in their susceptibility to these diseases and resistance is available if their occurrence becomes a problem. The following information will aid in the identification of specific foliage diseases.
Posted on: Fri, 03 Oct 2014 10:22:02 +0000
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