Now is not the time to stop scouting. It may not be pleasant out there in the field (think pollen and hot temperature), but the weather the past couple of weeks has been favorable for gray leaf spot development. Gray leaf spot development is favored by mean daily temperatures between 72 degrees F and 85 degrees F, and high humidity (higher than 90%). "I have had several reports this past week of fields in which the gray leaf spot has developed up to the ear leaf," says Iowa State University Extension plant pathologist Alison Robertson. "Northern leaf blight development has also been progressing in several fields in central Iowa."
Robertson adds, "I have been scouting my field plots the past couple of days and also have noticed that gray leaf spot and northern leaf blight have developed rapidly in the past two weeks. One thing I noted is that the disease severity has been hybrid specific. I have several different corn hybrids in my plots, but only one or two may be at threshold for a fungicide application. That is, the disease is present on the third leaf below the ear leaf or higher. The affected hybrids always are rated more susceptible to disease."
Can a fungicide application be made after brown silk?
Yes. Most of the fungicides used on corn (Headline, Headline AMP, Stratego YLD and Quilt Xcel) have a pre-harvest interval of 7 days (Headline) or 30 days (other products), which means in theory a product could be applied up to R5 (dent). "We have some data from 2007 through 2009 for foliar applications after R2 (blister; around brown silk), which is summarized in Table 1," says Robertson. "Although the yield response with an R3/R4 application of fungicide was not as high as the other timings, there was low disease pressure in these trials. In general, yield responses to a fungicide application are greater when disease is present in the field."
If disease occurs after brown silk, would a fungicide protect yield?
"That is a good question," says Robertson. "There are few data I am aware of. Consider that:
• The reproductive growth period in corn (VT to R6) typically lasts approximately 64 to 65 days.
• Dry matter accumulation starts at R2 growth stage of corn and rapidly increases through approximately R5.75 (which is three-fourths milk line).
• And 55% of dry matter accumulating after the R5 growth stage.
If the disease threshold is met at R3 or R4 growth stage of corn, could a fungicide application protect yield, bearing in mind dry matter accumulation is already 20% to 30% complete?
Robertson answers: Two researchers Harkin and Arkridge (2009) evaluated one (R4), two (R4 and R3), three (R2, R3 and R4) and four (VT, R2, R3 and R4) applications of Headline fungicide on two corn hybrids in double-crop corn in Alabama. On one hybrid, northern leaf blight was predominant, while on the second hybrid southern rust was predominant. A single application of Headline at R4 significantly reduced rust severity and, although yield was higher (95.8 bu/acre), it was not significantly different at the 5% level from the unsprayed check (88.7 bu/acre). Two, three and four applications reduced rust severity and resulted in higher yields (104.9-119.1 bu/acre).
Similarly, with northern leaf blight, a single application of Headline at R4 reduced disease but multiple applications were more effective. Yields did not differ between treatments.
"These data suggest that a fungicide application after R2 will slow disease development, although this may not always result in an increase in yield," says Robertson. "However, reducing leaf disease could reduce stalk rot severity and therefore contribute to better standability."