Provado is also effective against sap-feeding larvae, but the range of STLM stages present at one time by late summer means that a significant portion of the pest population escapes control when Provado is used. But remember that control is unlikely to pay off unless there are an average of 3 to 5 new mines per leaf. In northern Illinois, rust pustules are appearing on leaves of susceptible sweet corn varieties. Common rust occurs when sweet corn is grown near field corn. Yield losses in commercial sweet corn can range between 0 and 50 percent, depending on environmental factors and resistance of the corn variety or hybrid grown.
Common rust is caused by the fungus Puccinia sorghi.
Rust development and spread are favored when temperatures are cool and relative humidity is high. This fungus stops growing when the weather gets hot, but when cooler temperatures return, it will start growing again. The circular to elongate pustules of this fungus are initially golden or reddish brown to cinnamon brown in color and become brownish black as the plant matures. These pustules contain the fungal spores and appear on any above ground part of the corn plant, however, they are most abundant on the upper and lower leaf surfaces. Older tissue on the corn plant is generally more resistant to the fungus.