“We all know what normal cutout looks like in cotton — it's a gradual process, usually working from the bottom to the top of the plant, from the older to the younger leaves. Normally, there's a good mixture of red and yellow colors in with the green.” — Glen Harris, University of Georgia Extension soil scientist
“Why is my cotton turning red?” Puzzled growers throughout Georgia were asking this question late last summer, says Glen Harris, University of Georgia Extension soil scientist. The reason for this phenomenon, he says, was premature cutout. The causes are less certain.
“We all know what normal cutout looks like in cotton — it's a gradual process, usually working from the bottom to the top of the plant, from the older to the younger leaves,” says Harris. “Normally, there's a good mixture of red and yellow colors in with the green.”
This past year, however, at about mid-August, growers began seeing a rapid cutout, he says. In some cases, it occurred almost overnight, he adds.
“We considered several causes for this premature cutout, including potassium deficiency, bronze wilt, fusarium wilt and nematodes. And, because some of the cutout appeared soon after a rain, some thought it might be leaf scald, or even the result of lightning strikes,” notes Harris.
The problem, he says, probably isn't so much premature cutout as it is accelerated leaf maturation or leaf damage. “The leaf is there to produce carbohydrates and photosynthesis. The ability to photosynthesize declines after the leaf turns about 25 days old. Any stress on the plant will accelerate that process. This includes stress due to heat and drought.
“Anything that stresses the plant can cause this process to accelerate. After looking at a number of fields, I'm thoroughly convinced that the primary culprit to this premature cutout was heat and drought. That's not to say that other factors didn't contribute. In many cases, they did,” says Harris.
From 1997 to 2000, during the period covering from May through August, Georgia experienced the most severe water deficit on record in the state, he continues. The winter months during this time also were relatively dry, he says, and temperatures during the summer were hotter than normal, contributing to the drought.
“Added together, all of these factors contributed to the premature cutout phenomenon. To avoid this problem in the future, we must do the best job possible managing water, and that's easier said than done if you're in a dryland situation. But if we manage water properly, we'll relieve stress on the plant.
“We also need to take care of things like nitrogen and potassium, and make sure that a deficiency in these areas doesn't add to the stress. Maintaining a proper pH level and managing nematodes also are important.”
As growers prepare for the 2001 crop, it's important to consider the nitrogen requirements of cotton, says Harris. “From a leaf physiology standpoint, nitrogen is important in determining leaf size and in the vegetative reproductive balance of the plant.
Reasons behind suggestions
“That's why we don't recommend putting out all of our nitrogen at planting. Applying all of the nitrogen on the front end can make those bottom leaves very large, shading out the crop. This results in fruiting problems.”
Rising fuel prices make it even more important that growers properly manage nitrogen, says Harris. Producers need to focus on making split applications of nitrogen and on basing their nitrogen rates on yield goals, he says.
In Georgia, a yield goal of 750 pounds of lint per acre requires a recommended nitrogen rate of 60 pounds per acre. A 1,000-pound yield goal requires 75 pounds of nitrogen per acre while a 1,250-pound goal requires 90 pounds of nitrogen. A 1,500-pound yield goal has a recommended nitrogen rate of 105 pounds per acre.
These rates, says Harris, should be adjusted according to other factors. For example, a grower should increase his N rate by 25 percent if he is planting in deep, sandy soil; if he is planting cotton following cotton; or if the field has a history of inadequate growth.
“Anything that stresses the plant can cause this process to accelerate. After looking at a number of fields, I'm thoroughly convinced that the primary culprit to this premature cutout was heat and drought. That's not to say that other factors didn't contribute. In many cases, they did.”
Growers should decrease the nitrogen rate by 25 percent if he is planting cotton following peanuts or soybeans; if the cotton is following good stands of winter legumes such as clover or vetch; or if the field has a history of rank or excessive growth.
Potassium also is an important factor in the ability of the plant to photosynthesize, says Harris. In addition, potassium contributes to lint quality, he adds.
“We need to go back to putting out the recommended rates of potash at planting, based on soil testing. We also need to take a look at foliar feeding. In fact, a good strategy might be putting out half of the potassium at planting, plus a foliar application.”
Recent field trials in Georgia indicate that timely foliar K applications made at about peak bloom may be more beneficial than split applications of soil-applied potassium, reports Harris.
“We don't need to spray potassium nitrate on every acre of cotton. But you might want to look at in high-yield situations, or where you've seen potassium deficiencies, or in deep sands where you might have lost some potassium.”
It's also important, says Harris, that cotton producers maintain soil pH. “Proper pH is important not only for nutrient availability, but proper levels also prevent nutrient toxicity. Toxic elements such as aluminum are kept unavailable when pH is above 5.5.”
Last year, many growers wanted to know if the drought could have prevented lime from reacting, resulting in low pH levels, says the soil scientist.
“That's very possible, but we need to look back to the last time we applied lime. If you limed last spring, and you didn't receive much rain on that field, it's possible that the lime still is sitting there. If you have a low pH, and you haven't limed in two or three years, you probably need to lime.
Searching for cause
Researchers still are searching for the cause of the black root problem in Georgia cotton, says Harris. The problem was first detected in 1997, and it appears to be limited to the poorly drained, flat-wood soils found in the southeast corner of the state.
“We haven't seen any cases in Coastal Plain soils. We originally thought it was a disease organism. It looked very much like charcoal rot. Our plant pathologist tried but could not isolate an organism.”
Researchers also thought that the problem might be due to chloride toxicity, he says. When a similar problem was found in soybeans in the 1970s and 1980s, it was found that certain varieties could exclude the uptake of chloride and they were not damaged.
But Georgia researchers looked at 50 cotton varieties, and all varieties showed signs of black root, says Harris. In another experiment, however, looking at the effect of different soil amendments, less black root was seen where poultry litter was used.
“The best treatment was poultry litter. Where there was no poultry litter, we made 400 pounds of lint. Where we applied two tons of litter, we made 600 pounds of lint. And, where we applied four tons of litter, we made 800 pounds of lint. We're not sure why, but we'll continue looking at it.”