Stemphylium leaf spot, caused by Stemphylium solani, was first recognized in Georgia’s cotton fields in 1995. Like Cercospora and Alternaria leaf spot diseases that were identified earlier in the state, Stemphylium was found to be linked to potassium deficiencies in the cotton plant. In severe cases, premature defoliation and significant yield losses are associated with Stemphylium leaf spot.
During the reign of DP 555, symptoms of potassium deficiency were less evident, and some believe that Stemphylium leaf spot was less problematic. However, there is concern that due to the introduction of new shorter-season, faster-fruiting replacement varieties, increased drought conditions and rising potassium fertilizer prices, Stemphylium leaf spot may become a more widespread problem once again.
Beginning in about 2005, county agents and consultants began to notice a severe leaf spot that seemed unrelated to potassium deficiency. Unlike Stemphylium leaf spot, which seemed to hit the entire plant all at once, the new leaf spot disease developed in the lower canopy first and spread into the middle of the canopy.
The new disease was not associated with the yellowing and bronzed foliage that occurs with potassium deficiency. Eventually, it was determined that this new disease was Corynespora or “target” leaf spot, which is a more typical, traditional disease tied to a pathogen and weather conditions.
Prevent By Good Fertility
Because Stemphylium was recognized long before Corynespora leaf spot, more is known about this disease. A good soil fertility program, where potassium is not allowed to become deficient, will minimize this type of leaf spot problem.
“These leaf spot occurrences are based on a deficiency of potassium,” says Glen Harris, University of Georgia Extension agronomist. “If the plant is low in potassium, the leaf cells are weak and susceptible to infection.”
He says to put all potash out at planting because sidedress applications do not work as well.
“If you keep potassium deficiency away, you should prevent Stemphy-lium leaf spot,” Harris says.
Fungicides Can Improve Lint Yield
Bob Kemerait, University of Georgia Extension plant pathologist, says that researchers now know that the disease Corynespora leaf spot, which starts at the bottom of the plant and moves up from there, can affect the plant regardless of potassium level.
Through trials over the past several years, Kemerait says research shows that the use of well-timed fungicides can increase lint yield.
“We know from looking at trials that if the disease does not develop because the weather conditions are too hot and dry, spraying because you see Cory-nespora at the bottom of the plant may not result in any profit to the grower,” he says.
“But, where the disease does develop, we are seeing a positive economic response to a fungicide application.”
Look For This Leaf Spot
Kemerait says that the fungicide application helps to protect the plant and can prevent premature defoliation by at least four or five weeks.
“This gives the plant time enough for the bolls to mature and to open properly,” Kemerait says. “Often when I see Corynespora leaf spot that is causing significant defoliation and damage, it is in some of the best-growth cotton under a thick canopy. You likely will not see the defoliation driving by; you have to get out and look in the field.”
Identification Is Key
“The bottom line is that producers have to know which leaf spot is in their field,” says Harris. “If it’s Stemphy-lium, it’s a potassium issue, and a fungicide application won’t likely work; if it’s Corynespora, it’s not a potassium problem, and a fungicide application might be needed.
“Just like you have to know which stink bug you are trying to control, now you have to know which leaf spot problem you are dealing with, too.”
Contact Amanda Huber at (352) 486-7006 or firstname.lastname@example.org.
Stemphylium Leaf Spot:
• Related to potassium deficiency.
• Yellowing and small spots with purple border appear throughout the plant, even at the top.
• Fungicides have little or no effect on disease.
Corynespora Leaf Spot:
• Typical disease tied to pathogen and weather pattern.
• Not related to nutrient deficiency.
• Occurs predominantly in the lower canopy.
• Well-timed fungicide can increase lint yield.