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.
|Leaf Blight||Exserohilum turcicum||Large elongated spots with gray centers and tan-to-reddish borders|
|Target leaf spot||Bipolaris sorghicola||Round-to-elliptical spots with reddish purple centers and tan margins.|
|Anthracnose||Colletotrichum graminicola||Elliptical-shaped spots that are 1/8″-7/8″ in diameter. Tan-to-red with distinct margins.|
|Gray leaf spot||Cercospora sorghi||Dark purple spots having a grayish cast when pathoggen is producing spores. Elongate to round, 1/4″ and larger.|
|Zonate leaf spot||Gleocercospora sorghi||Large, irregular-shaped spots having a bullseye appearance.|
|Rough||Ascochyta sorghina||Grayish spots that are rough to leaf spot the touch because of raised black fruiting bodies.|
|Sooty stripe||Ramulispora sorghi||Elongate spots having a sooty appearance because of sclerotia.|
Bacterial Stripe (bacterium – Pseudomonas andropogoni): This is the most common bacterial disease of sorghum. The disease is characterized by long narrow stripes that vary from red to black depending on the type of sorghum. These stripes are confined between the veins and may have a crusty surface when the bacterial slime dries on the surface. This disease has not been a serious enough problem to warrant specific control in Texas.
Bacterial Streak (bacterium – Xanthomonas holcicola): Streak first appears as dark-green watersoaked tissue between veins that later turns brown with red margins. Control measures have not been warranted.
Bacterial Spot (bacterium – Pseudomonas syringae): Spots first appear as watersoaked, green areas on lower leaves before infection spreads to upper leaves. Spots later turn tan with reddish borders. Small lesions are often red throughout. Control measures are not usually necessary.
Anthracnose (fungus – Colletotrichum graminicola): The anthracnose fungus damages foliage and stems of grain sorghum. On susceptible hybrids, the stem holding the head (peduncle) becomes infected and a brown sunken area with distinct margins develops. Figure 4. When infected stems are cut lenghtwise with a knife, one can see that the fungus has penetrated the soft pith tissue and caused brick-red discolorations. This peduncle infection inhibits the flow of water and nutrients to the grain causing poor grain development. The fungus also invades individual grains and the small branches of the panicle. Rapid and severe yield loss can result from panicle and peduncle infections. Leaf lesions are small, elliptical to circular, usually less than 3/8-inch in diameter. These spots develop small, circular, straw-colored centers with wide margins that may vary in color from reddish to tan to blackish purple. The spots may coalesce to form larger areas of infected tissue. In Texas, anthracnose is restricted mostly to the gulf coast areas. The use of resistant hybrids and good management of crop residue are effective control measures.
Rust (fungus – Puccinia purpurea): Rust appears on leaves as small raised pustules or blisters that rupture and release many reddish-brown spores. These pustules occur on both the upper and lower leaf surfaces. This disease usually appears when plants near maturity and infection is confined primarily to mature leaves. Grain yield losses are usually not serious and occurrence of the disease is sporadic. Forage sorghum yields may be affected most. The rust fungus also attacks Johnsongrass and overwinters on this host in southern production areas.
Charcoal Rot (fungus – Macrophomina phaseolina): Grain sorghum plants affected by the charcoal rot fungus fail to fill grain properly and may lodge in the latter part of the season. Infected stalks show an internal shredding at and above the ground line. This can be observed by splitting the stalk and noting the deteriorated soft pith tissue leaving the tougher vascular strands. Figure 5. Fungal structures (sclerotia) can be observed in the affected tissue which appears as though it has been dusted with black pepper. Another type of stalk rot (Pythium sp. and Fusarium sp.) may show the shredded condition but the black specks (sclerotia) will be lacking. Conditions under which charcoal rot is favored include stressful hot soil tempertures and low soil moisture during the postflowering period. Host plants are usually in the early-milk to late-dough stage when infection occurs. The fungus is common and widely distributed in nature. Avoiding moisture stress, proper management of crop residue, crop rotation, avoiding excessive plant populations, balancing nitrogen and potassium fertility levels, and growing drought-tolerant, lodging-resistant hybrids represent the best means of control.
Fusarium Stalk Rot (fungus – Fusarium moniliforme): Like charcoal rot, Fusarium stalk rot usually develops on mature to nearly mature plants that have been subjected to some form of stress. Infection takes place at the base of the plant and produces discoloration in the stalk. When shredding of the vascular area occurs from this organism, black fungal bodies (sclerotia) are not present as they are with charcoal rot. Avoiding stress problems by proper use of cultural practices is the best approach to control.
Root Rot Complex (fungi – Fusarium moniliforme, Pythium sp. and others): Several fungi are involved in producing a root rot condition of grain sorghum. One or more of the causal fungi may be involved depending on conditions and organisms present in the soil. Each organism produces distinct symptoms, but identification becomes more complex when other factors are involved. Rotation with non-related crops will lower the population of pathogenic organisms present in the soil.
NOTE: Texas AgriLife Extension Service publication B-6004, “Disease Response of Grain Sorghum Hybrids” is available for helping to choose disease resistant commercial grain sorghum hybrids.