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Best time to apply foliar fungicides

serious fungal lesions on corn leaf
SCOUT EARLY: This leaf relatively high on the plant is showing symptoms of serious fungal lesions. The goal is to scout and detect small lesions so you can decide if fungicides are necessary.
Corn Illustrated: The first question is whether you need to apply fungicide at all.

Do you need foliar fungicides this year? If you do, what’s the best time for application? It will be very important to consider applying foliar fungicides this year because lots of corn was planted later than normal. Disease organisms were already “planted” in the soil from previous seasons and growing.

What does this mean? I expect leaf diseases will have an edge in attacking crops, and you must be ready to defend against them. The best offense is a good defense!

Various disease organisms become more prevalent in certain growing conditions. Northern corn leaf blight likes cool and wet conditions, as we had in early May this year. Gray leaf spot likes high humidity and high temperatures.

Disease cycle
Conservation tillage has also increased the incidence of many diseases. Some popular corn hybrids on the market have very high yield potential but are very susceptible to certain fungi that increase the probability of disease development.

These susceptible but otherwise high-yielding hybrids help in creating more disease inoculum for the following year since they’re grown on a large number of acres. Last year we had an abundance of both pathogens for northern corn leaf blight and gray leaf spot. You need to scout your fields regularly to decide if and when foliar fungicides should be sprayed.

Planting date and relative maturity of hybrids can also affect development of certain diseases. Earlier-maturity hybrids sometimes escape disease. In scouting during the last couple of years, I’ve noticed that kernels were almost dented and gray leaf spot was still below the ears. You need to protect the ear leaf and the leaves above the ears because more than 90% of yield is contributed by those leaves.

Some fungicide makers promote the use of foliar fungicides even at early vegetative stages to increase yields. Tests conducted by universities indicate that it’s not cost-effective to apply fungicides without the presence of disease.

Recall past experience
My scouting experience during the last few years has shown you don’t have to apply fungicides to all hybrids or even all fields planted on different dates, because some fields might escape disease.

Based on recent information and experience, the ideal time to apply fungicides on corn is before the ear leaf is affected by disease. The second best time is after all pollen shed is complete and silks start to turn brown. You want to protect leaves during the grain-fill period. If you wait until after tasseling, spraying fungicides before the completion of pollination can adversely affect ear development and yield.

Whatever method of foliar application you use, lesions of the fungi must be covered with fungicides to kill them or at least slow their growth. Fungicides don’t increase yield, but they can protect the yield potential of the crops in the presence of fungal diseases.  

Nanda is president of Agronomic Crops Consultants LLC. Email him at [email protected], or call him at 317-910-9876. He writes from Indianapolis.

TAGS: Fungicide
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