Southern rust of corn is caused by the fungus Puccinia polysora. Yield loss due to this disease can be severe when environmental conditions favorable for development persist. Losses of 45% have been recorded, with up to 65% lodging of field plants.
In 2015, sweet corn growers around Belle Glade, FL, saw an unusually early outbreak of southern corn rust. This disease is considered to be tropical or subtropical in distribution and typically does not show up around Belle Glade until the late spring, but the early disease development being observed is likely the result of relatively warm conditions that have prevailed this winter coupled with heavy fogs, which have blanketed the area recently.
The symptoms of southern rust are orange to brown masses of spores (urediospores) that erupt through the upper leaf surface. Leaves, stalks, and the husks on ears may be infected. Southern rust typically sporulates profusely on the upper leaf surface and only sparsely on the lower leaf surface. In contrast, common rust (caused by Puccinia sorghi) produces spores on both surfaces.
Common rust tends to produce elongated pustules whereas southern corn rust has somewhat rounded pustules.
Identification can be done quickly with a microscope. The rounded urediospores of common rust tend to be uniform in diameter whereas those of southern corn rust are oblong in shape.
Unlike common rust, which typically abates as plants tassel and reach maturity, southern rust can continue to develop and infect plants beyond tasselling.
Survival And Spread
Southern corn rust tends to occur more frequently during the late spring and fall in South Florida and during the summer months in North Florida, particularly with late-planted or double-cropped corn. The optimum temperatures for the development of southern corn rust are higher than that for common rust.
The optimum conditions for southern rust development are temperatures from 80°F to 90°F and high humidity. Leaf wetness periods of eight hours will support infection at near optimal temperatures. As temperatures deviate above or below the optimum, leaf-wetness periods up to 16 hours may be required for infection.
Many hybrids, which have resistance to common rust, have no resistance to southern rust. Super-sweet (SH-2) varieties are particularly susceptible.
When sequential plantings are made, earlier plantings should be downwind in relation to predominant winds. For example, in South Florida, east winds predominate during the sweet corn season; hence the corn should be planted from west to east.
Fungicides are the major means used to control rust in sweet corn, particularly in peninsular Florida.
Spray programs should begin at the first sign of rust. Strobilurin and triazole fungicides work well and should be used in a program with the broad-spectrum protectant mancozeb. Several sprays may be required. Use of a spreader-sticker, particularly when the plants are young and have waxy leaves, may assist in obtaining good coverage.