Sheath blight, caused by the fungus Rhizoctonia solani AG-1-IA, is the most important and widespread disease in Mid-South and Delta rice. Environmental conditions favoring development are hot, damp weather (temperatures 80 degrees to 90 degrees Fahrenheit), nighttime temperatures of 74 degrees to 82 degrees F and high humidity (95 percent). Sheath blight symptoms usually appear on the sheath of the first leaf above the waterline about the time of internode elongation. An oblong, water-soaked lesion on the leaf sheath first appears at or near the waterline. The chlorophyll (green color) fades, and with time, the lesion becomes about 1 inch long and develops a grayish-white center surrounded by a dark purplish or reddish-brown margin. Similar lesions may appear on the sheaths of upper leaves in three or four days when conditions are favorable. Lesions often develop on the leaf blades as well, unlike other sheath spot diseases.
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Small oval to circular brown spots are the first symptoms of this disease caused by the fungus Cochiobolus miyabeanus. The disease may also be called Helminthosporium leaf spot. During the early development stages, brown leaf spot may resemble early blast or chemical damage. Spots vary in size and shape, ranging from minute dark spots to large ovals or circles. The smaller spots are dark brown to reddish-brown. The larger spots have a dark brown margin and a light, reddish-brown or gray center. Spots on the leaf sheath and hulls are similar to those on the leaves. Though the disease may be found on younger plants, it is more prevalent as plants approach maturity and leaves begin to senesce. The disease is common on plants near and around cold water levees, on plants suffering from nutritional stress and on drought-stressed plants.
False smut, caused by the fungus Ustilaginoidea virens, has traditionally been a minor disease in the United States. Large orange to brown-green fruiting structures on one or more grains of the mature panicle characterize this disease. Over time, surface spores turn from orange to dark olive-green and spread the disease to immature panicles. The "smut balls" survive between crops in the soil.
Rice blast, caused by the fungus Pyricularia grisea, can result in severe losses to susceptible varieties when environmental conditions such as warm, moist weather favor disease development. The blast fungus causes leaf symptoms on young plants and panicle blast and rotten neck symptoms later in the growing season. Leaf lesions are spindle-shaped and elongated with brown to purple-brown borders and grayish centers. The rotten neck phase of the disease is most damaging. With rotten neck, a brownish lesion on the internode at the base of the panicle often prevents the grains from filling or weakens the neck of the panicle so that filled heads break off before harvest.
Kernel smut, caused by the fungus Neovossia horrida (Tilletia barclayana), causes the endosperm of the rice grain to be replaced partially or completely by a black mass of smut spores. Usually only one to five grains per panicle are infected, but up to 15 percent of the kernels may be infected on highly susceptible varieties. Disease symptoms usually occur when rice approaches maturity as black spore masses form in place of all the kernel or a portion of it. The disease is easily observed in the morning when the smut spores absorb water. Once expanded by the absorption of water, the spore masses push out of the hull to become visible. As the day progresses, the spore masses dry down to a powder and can easily be rubbed off on fingers. The black spores survive in the soil between crops.
A physiological disorder, straighthead is associated with sandy, sandy loam and some silt loam soils; fields with arsenic residues; fields with high organic matter such as those recently cleared of timber, those with a long history of fish production or pasture; or fields with large amounts of plant residue incorporated into the soil before flooding. Under certain conditions, arsenic levels can increase in these soils where cotton has been grown and MSMA or other arsenical pesticides used. Panicles are upright at maturity because the grain does not fill or panicles do not emerge from the flag leaf sheath. Especially in long-grain varieties, the rice hulls may be distorted or "parrot beaked," which is diagnostic for straighthead. But the symptoms may be uncommon on certain varieties.