Alison Robertson, Department of Plant Pathology
The early application of foliar fungicides to corn (V4-V7) is currently being encouraged by chemical companies across the corn belt. The idea is to tank-mix the fungicide with the post-emergence herbicide application and save on application costs. Furthermore, for additional cost savings, half or lower rates of fungicides are being marketed. It is my understanding that this early application of fungicide will not necessarily replace the VT/R1 application of foliar fungicide, rather it is an addition to the tassel/silking application.
Since the primary action of a fungicide is to control disease, what diseases might be targeted with this early application? Usually the only foliar disease present at V4-V7 is anthracnose leaf blight. Symptoms of this disease are more common in corn-following-corn fields and can be seen as early as V2-V3 on the lowest leaves of the corn plant. Thus, a V4-V7 application of a fungicide would be too late to completely control this disease. Once corn starts growing rapidly, it usually outgrows the disease. There have been suggestions that leaf blight could increase the risk of anthracnose stalk rot later in the season. Preliminary data from trials done in Iowa in the past three years indicate there is no relationship between anthracnose leaf blight and stalk rot.
Another early season foliar disease that can occur is eyespot. Eyespot is favored by cool, wet conditions, so in normal growing seasons this disease does not progress and become the problem it was in north and central Iowa in 2009.
Inoculum of other common foliar disease (gray leaf spot, common rust and northern corn leaf blight) may be present in corn fields at this time, and yes, an application of fungicide at V4-V7 would certainly prevent infection on the leaves exposed to the fungicide. Remember, however, that new leaves, that have not been exposed to the fungicide, are emerging every 3 to 4 days (depending on the weather), and inoculum of foliar diseases is being produced 24/7. Fungicides are not translocated from one leaf to another. Thus, early season infection of unprotected leaves is still possible, and may still lead to epidemic levels of disease later in the growing season.
Limited research has been done to support the practice of early applications of fungicides to corn. In 2009, we evaluated applications of Headline at early growth stages at two locations in Iowa. At Crawfordsville, we looked at the effect of Headline alone or in combination with Headline AMP at various growth stages on disease severity, lodging, yield and moisture (Table 1). In terms of yield, we saw little justification for the use of an application of foliar fungicide at V6, either alone or in combination with a VT/R1 application.
At the second trial in Ames, the effect of Headline (6 oz/A) at multiple timings on disease severity, lodging yield and grain moisture was assessed (Table 2). Similarly, there was little justification, in terms of yield, for early applications of fungicides on corn.
Carl Bradley recently summarized the few replicated research trials done in five Midwestern states that evaluated the effect of early applications of foliar fungicide on corn yields. His summary includes data from the two Iowa sites.
This growing season we will, once more, be partnering with outlying ISU research farms to evaluate the effect of various foliar fungicide applications on corn. This year we will incorporate the V6 application timing in these trials.
Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Robertson may be reached at (515) 294-6708 or by email at firstname.lastname@example.org.