July 21, 2010

Gray leaf spot lesions seen on Indiana corn hybrids

Purdue University experts are seeing gray leaf spot lesions on some susceptible corn hybrids in Indiana and they are encouraging farmers to scout fields to assess the level of disease.

The fungus that causes gray leaf spot infects the corn plant during prolonged periods of warm weather between 75-80 degrees and at more than 90% relative humidity.

"Spores can cease development during low humidity periods and then resume the infection process once humidity rises," said Kiersten Wise, Purdue Extension plant pathologist.

Wise explained that each lesion can produce many spores, which are splashed or blown to the upper leaves or to other plants where they can survive until conditions are favorable for infection.

Lesions on susceptible corn hybrids expand parallel to leaf veins and may grow to 1.5-2 inches long. Due to the infection process, symptoms may not be noticeable for up to two weeks after infection.

Farmers can lessen the economic blow of gray leaf spot by implementing preventative management strategies, such as fungicide application. Wise said because fungicides are an additional cost to corn producers, growers need to consider economic factors and other disease factors before deciding whether or not to apply them.

Indiana research has shown strobilurin and strobilurin/triazole premix fungicides to be most effective at preventing yield loss when applied in response to disease presence and at the tasseling to early silking growth stage.

Wise noted that corn growers should consider a fungicide application if the hybrid they planted is rated as susceptible or moderately susceptible and 50% of the plants in a field have disease lesions present on the third leaf below the ear leaf or prior to tasseling.

Susceptible hybrids planted in no-till or reduced-till fields are at high risk for developing gray leaf spot, but weather is the primary factor in disease development.