Fungicide manufacturers claim a significant yield boost from use of foliar-applied fungicides, applied at or near cotton bloom.

BASF, manufacturers of Headline fungicide contend in one season their product increased cotton yields in 77 on-farm trials by an average of 65 pounds of lint per acre with early or mid-bloom applications and 94 pounds of lint per acre with sequential bloom applications.

Quadris, another fungicide labeled for foliar application at or near cotton bloom has shown in research in Arizona to increase boll number on some nodes, but the increase did not affect cotton yield.

Researchers attributed the lack of yield increase to the long growing season in Arizona, which makes it possible for cotton plants to compensate minor changes in boll setting during the main fruiting cycle.

In Virginia and North Carolina a large part of the cotton crop was planted in cold, damp soil that went from nighttime temperatures for a couple of weeks after planting in the 40s to nighttime temperatures that remained in the 70s for a month after planting, combined with daytime temperatures pushing 100 degrees F.

There is some concern that slow developing and late-planted cotton in Virginia and the northern end of North Carolina may not have time to compensate for changes in boll set           

Despite a planting season and early growth period that seems ideal for stress, University of Tennessee Researcher Chris Main says there isn’t enough evidence of yield increase to advise cotton growers to use a foliar fungicide.

Later planting dates may not be the best solution to managing boll rot, which will be the primary disease concern for growers in the Southeast.    

The best way to manage boll rot is to prevent the plants from becoming rank by lowering nitrogen rates and correctly using plant growth regulators, he says.

“Since a late planted crop will have lower yield potential, resist the temptation to use products that are not proven effective in university trials to save on input costs helping improve your profitability at the end of the year,” Main advises.      

As part of a Beltwide fungicide test program, North Carolina State University Cotton Specialist Keith Edmisten conducted a series of tests in 2010 using Headline and Qaudris — two of the most commonly used fungicides labeled for use on cotton.                                   

“One of the first things we learned from these studies is that in North Carolina foliar diseases of cotton are frequently linked to potassium deficiency.

“Dry weather and other stresses may contribute to the potassium shortage, but most of the disease problems we’ve seen in the past few years were in some way associated with a lack of potassium or plant use of potassium,” he says.   

The need for K rises dramatically when bolls are set on plants, because developing bolls have a high K requirement. It is crucial that potassium be available when the plant is setting fruit on the first position of the first several branches, because 70 to 75 percent of the total yield occurs from first position bolls on the first 7 or 8 fruiting branches.