A good boll load and high yield potential doesn't necessarily exempt a cotton field from having seed rot. In fact, the problem tends to favor such conditions.
“We don't see much seed rot in fields that don't have a high boll load,” says Mike Jones, Clemson University Extension cotton specialist. “Fields with seed rot tend to appear normal with a good boll load. Bolls appear normal on the outside, and the fields usually are green and loaded up.”
Seed rot was detected in South Carolina in both 1999 and 2000, says Jones, and growers have lost as much as 10 to 15 percent of their potential yield to the problem. Surveys found seed rot in all 22 cotton-growing counties of the state, in irrigated and non-irrigated fields, in early and late plantings and in all varieties, he adds.
Number of problems
South Carolina cotton producers, like others in the lower Southeast, have been plagued in recent years by a number of production problems, notes Jones.
“Our yields have suffered in the past two years, with statewide averages of 428 pounds per acre in 1999 and 604 pounds per acre in 2000. Drought and weather problems have contributed to these low yields.
“Also, a high percentage of our acres are planted in Bt varieties, and this has caused problems with secondary pests, especially stink bugs. This past year, our growers sprayed two or three times for stink bugs in Bt cotton fields.
“We've also had a real problem getting our crop out of the fields, and we've seen a lot of hard locked cotton, particularly on the bottom of the plant. This probably is due to a combination of seed rot and drought,” he says.
Seed rot was first detected in South Carolina in July 1999 in Hamptom County, says Jones. It was discovered by a consultant and a grower who were cutting bolls to determine stink bug damage and boll maturity.
“If you cut and take a cross section on an affected boll, you'll see extensive seed rot inside. Many of the seeds will be dark and rotten. We've discovered the problem in more than 45 varieties over two years, and this includes conventional and transgenic. We've found it in Bt, Roundup Ready, stacked gene and BXN cotton varieties,” he says.
Seed rot has been seen in cotton bolls with and without supernumerary carpels, he continues. “A supernumerary carpel is actually a boll that's growing inside of another boll. Some of the cases in the literature describe symptoms similar to what we've found. The inside boll opens the tips of the outer bolls, allowing water and organisms to get inside and cause problems. We're also interested in insect feeding and damage because symptoms are similar to seed rot.”
Seed rot has been found to be more severe in older bolls, says Jones, and in larger bolls at the bottom of the plant canopy or fruiting positions closer to the main stem.
“We tried to isolate any pathogens that were inside the boll. Basically, all we found were bacteria — we found numerous species of bacteria inside the bolls. We didn't find a lot of fungi or other organisms.
“We don't know if the problem is weather related. We brought in physiologists to help determine if pollination and the fertilization process were affected in these bolls by drought and high temperatures. These bolls do not show symptoms until they're at least three to four weeks old.
“If there were problems with pollination or fertilization, you'd expect that the seed would not be developed. This is not typical soft rot where everything turns to mush. These seed are discolored with a wet appearance.”
Dead, hollow and discolored seed all can be symptoms of seed rot, says Jones. And, he adds, these seed have developed for about five weeks.
“If there was a problem with the physiology of the plant, you wouldn't expect the seed to develop for such a long period inside the boll before being aborted.”
When bolls affected by seed rot open, the cotton does not “fluff,” says Jones. “The picker won't pick this cotton. If it could pick it, the cotton would be of extremely poor quality.”
Seed rot, he says, can look very similar to stink bug damage in cotton. The difference, he adds, is that with stink bug damage, the rot is localized — only the affected seed will die. Also, you usually can see stink bug punctures.
Researchers, says Jones, have found no direct correlation between bacteria found in seed at planting and the bacteria found in bolls affected by seed rot.
“When we first started looking at this problem, we talked about the stress of drought and high temperatures on the cotton plant, and what effect this might have on seed rot. Whenever you have a disease, you must have the right environmental conditions for the disease to occur and attack hosts.
“Environment definitely plays a role in seed rot. We don't know if it's a direct cause of the problem, or if it just sets things in motion. We were in a severe drought when we first found the problem in 1999, and we were in even worse shape in 2000. We were in a severe drought during most of the boll development period.”