Two rusts can impact Kentucky’s sweet corn crop. Common rust is caused by the fungus Puccinia sorghi. This rust begins as oval to elongate cinnamon brown pustules scattered over both surfaces of the leaves. The pustules rupture and expose dusty red spores and later black spores. The red spores are spread by wind and can infect corn leaves directly; the black spores are overwintering spores which germinate and indirectly infect Oxalis, an alternate host for the corn rust fungus. In southern areas, red spores persist on corn from season to season, thus initiating early-season disease development. In northern areas, disease initiation depends on reintroduction of wind-borne red spores from southern areas or on development of special spores on the alternate host, Oxalis. The
disease is promoted by cool temperatures (61 to 70*F) and very high relative humidity.
Southern corn rust, caused by the fungus Puccinia polysora, is generally confined to the southeastern area in the United States. Southern rust pustules are lighter in color, smaller, more circular and do not break open as early as common rust pustules. The southern rust fungus overwinters as spores on infected corn plants and in residue from diseased plants. Southern rust if favored by high temperatures (80*F) and high relative humidity.
Cultural: Plant one of the several rust resistant cultivars available. Consider planting resistant cultivars for all crops.
Monitoring: Rust is mainly a problem on later plantings, so watch early plantings closely to determine risk on later plantings. In most seasons, fungicides are not needed. But if warranted, apply one of the fungicides listed
under leaf blights. Early application may be necessary on late plantings to avoid serious crop damage in hot, humid seasons.
Application: None available.
Smut causes large, fleshy galls (pustules) filled with black spore masses on leaves, stems, tassels and ears. Immature galls are white and spongy; mature galls turn brown and contain dark powdery spores. The smut
fungus overwinters in soil. Disease potential is greatest in hot (79 to 94*F) and dry weather as well as following stress, especially cultivation, hail and insects. Smutted tissues are edible, containing mainly fungus. For those
wanting to encourage smut development for commercial reason, the chances of infection are increased by spraying smutted material from the previous season with vegetable oil and water so that the contaminant can
enter injuries made to the plant by hail, cultivation or insects. In one study a grower wanting to achieve a very high incidence of smut had success by applying the mixture as a course spray at 100 gallons per acre within
hours of a hail storm.
Cultural: Plant one of the few smut tolerant hybrids such as Apache, Bellringer, Commanche, Comet, Gold Cup and Merit. Practice crop rotation.
Chemical: None available.