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Rice blast 
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.
Sheath blight
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.
 Kernel smut
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.
Brown leaf spot or brown spot
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
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.
Straighthead
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 ³parrot beaked,² which is diagnostic for straighthead. But the
symptoms may be uncommon on certain varieties.
Panicle blight
The disease, caused by Pseudomonas glumae bacterium, is characterized by
brown or straw-colored discoloration of florets on a panicle branch.
Lesions are not apparent below the grain, and panicle branches remain green.
In some cases, there is some browning of the sheath, which may be confused
with sheath rot. The grain stops developing and the florets turn gray. It is
more severe in late-planted rice where the rice is maturing and late night
temperatures are high.
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