If your cotton develops leafspot, don't blame the resulting yield loss just on the disease. Potassium deficiency is the major culprit.
One solution: Foliar-applied fungicides - if potassium (K) deficiency is detected by the fourth week of bloom, says Glen Harris, University of Georgia extension agronomist.
Potassium adds strength to leaf cells. The lack of it makes plants susceptible to fungal infections such as leafspot. And leafspot diseases have been more frequent, more severe and have appeared much earlier in the growing season over the past few years, says Harris.
"In some cases, cotton was totally defoliated soon after the fourth week of bloom.
"If K deficiency is detected around the fourth week of bloom and is not severe, foliar K sprays may increase yields."
But if severe K deficiency occurs late, around the sixth week of bloom, foliar applications won't help, Harris adds.
Leafspot is caused by a variety of fungi. Cercospora and alternaria are the most common, but stemphylium is spreading. Harris estimates that 2,000 acres of Georgia cotton were infected by stemphylium in 1995. By 1996, 20,000 acres were infected.
Leafspot usually is considered a secondary factor in yield loss, says Harris.
"In almost every case where leafspot was a factor in yield loss, low soil K, low plant tissue K and/or low petiole K were present.
"Most of the time, K deficiency was discovered on full-season varieties under irrigation around the fourth week of bloom with heavy fruit set."
That's when there's a heavy demand for K, he explains. Cotton roots begin to decline because they're competing with developing bolls for carbohydrates and can't take up soil K.
University of Tennessee researchers studied both soil and foliar K applications on cotton in conventional and no-till systems, says agronomist Don Howard.
Yields from both tillage systems were increased by soil-applied K, he says. But foliar K brought a better response in no-till than in conventional.
Howard's conclusion: applying K to the soil or leaves can reduce leafspot severity.
If you've experienced leafspot defoliation, consider these management practices this year or next:
1) Soil test and keep K levels in the medium-to-high range.
2) On sandy soils, apply half the K at planting and half at sidedressing or first square.
3) Test petioles. Even when soil K levels are medium to high, foliar applications can increase yields in certain situations. A complete petiole test can detect the need for foliar K two weeks before leafspot symptoms appear.
4) Use in-furrow fungicides to reduce root rots and seedling diseases, making root systems healthier for K uptake.