Hardlock has proven to be a complex cotton disease with several possible causes, including stink bug damage, seed rot and the fungus Fusarium verticillioides.
"Hardlock of cotton can be considered a subset of more general boll rots," says Jim Marois, University of Florida Extension plant pathologist. "The major difference is that with hardlock, the fiber doesn’t fluff out when the boll opens. But the boll itself is not decayed as in traditional boll rots."
It’s impossible, adds Marois, to predict if a boll will fluff out normally before opening. "But we feel that Fusarium hardlock, as part of the hardlock complex, can be controlled with appropriate fungicide applications."
Hardlock, he explains, occurs when seed cotton adheres in compact wedges within the individual partitions — or locules — of matured, opened cotton bolls. In hardlocked bolls, the lint has failed to fluff out. The individual fibers are not loose and white but interlaced in discolored, locule-shaped crescents.
"Although the property of the fiber may not be grossly affected, the consistency of the seed cotton mass is such that conventional spindle picking equipment cannot extract the fiber from the locules. In conventional mechanical harvesting, hardlocked cotton often is knocked to the ground or strung out of the boll, giving the appearance of poor harvesting procedures," says Marois.
The incidence of hardlock, he continues, has been associated with the use of high rates of nitrogen fertilizer, high plant densities, high temperatures and humidity during the growing season, insect damage, and — in South Carolina — with seed rot.
Studies were conducted at the North Florida Research and Education Center in Quincy in 2001 and 2002 to determine the cause of hardlock and to evaluate possible control strategies.
In 2001, the cotton varieties Suregrow 501BR and DPL 458B/RR were used. In 2002, DPL 555B/RR was used. The crops were maintained according to University of Florida recommendations.
Orthene and methyl parathion were used when needed to control Southern green stinkbugs and brown stinkbugs. Cotton was harvested with a spindle plot picker.
In 2001, 20 bolls were picked from the first fruiting branch at weekly intervals for eight weeks. These bolls were cut transversely and examined for seed rot and other symptoms.
In 2001 and 2002, the potential of Fusarium verticillioides to incite disease was tested by inoculating stems and flowers of cotton plants with five isolates of the fungi.
Also in 2001 and 2002, trials were conducted to determine the effect of fungicides on Fusarium hardlock. Researchers also looked at the impact of weather on hardlock of cotton. Linear regression analysis of disease severity to temperature and relative humidity was conducted to help quantify the potential of weather conditions to affect the severity of hardlock.
Marois reports that in the first phase of the experiment, there was no direct relationship between the frequency of microorganisms isolated and the appearance of visibly diseased tissue in the seed and peduncle during the season. Overall, more bacteria were isolated from the seeds, and more fungi isolated from the peduncles. The most prevalent fungus was Fusarium spp., accounting for more than 80 percent of the fungi isolated. Further analysis showed that more than 80 to 85 percent of the isolates of Fusarium were Fusarium verticillioides.
In the inoculation portion of the study, the severity of hardlock increased significantly in cotton bolls that were inoculated with five isolates of Fusarium verticillioides.
In the fungicide portion of the trial, natural disease pressure was greater in 2002 than in 2001, says Marois. In 2001, the severity of hardlock was not affected by fungicide treatments and ranged from 10 to 20 percent in the plots. Yields of seed cotton in the 2001 test plots were significantly affected by the rate of nitrogen and the application of benomyl.
In 2002, he says, natural disease pressure was very high, and the application of thiophanate-methyl significantly reduced the severity of hardlock and increased yield. Fungicide applications made during the eight weeks of bloom were the most effective at controlling hardlock and resulted in an 87-percent yield increase over the unsprayed plots. Applications made during boll opening increased yield over the control by approximately 30 percent.
Fungicide applications made both at bloom and boll opening did not increase yield significantly over those achieved with only bloom applications. They did, however, further decrease disease.
Across all treatments, the severity of hardlock was indirectly correlated with yield. In addition, cotton seed weight was increased significantly with applications of fungicide during the bloom period.
In testing the impact of weather on disease development, hardlock severity ranged from 40 to 95 percent from flowers tagged during the eight-week period. There was a significant correlation between disease severity and temperature and relative humidity.
The study, says Marois, confirmed that Fusarium verticillioides was a fungal pathogen infecting cotton flowers and developing further in the boll as it matures. The fungus, he adds, was able to further infect the cotton seed, reducing seed weight and germination. It also infected peduncles, sometimes leading to discoloration.
"Although the pathogen is seed-borne, it is not obvious that this is important in the epidemiology of the disease, as the pathogen is widespread in the environment and often is associated with decaying plant material."
The importance of hardlock in reducing yield was clearly shown in 2002, says Marois. Hardlock was very severe in the Florida Panhandle in 2002, and the average yield was reduced significantly. When the hardlock cotton that was knocked to the ground was collected, ginned and weighed, it was found to account for most of the yield reduction.
Flower thrips, he says, also may play a role in the development of hardlock. Thrips numbers were reduced by traditional insecticide programs, but the potential role of thrips in damaging cotton flowers or serving as vectors of pathogens has not been investigated.
High numbers of thrips were observed in cotton flowers during the Florida experiments, he adds, and they may have the potential of carrying pathogens into the flower.
The high correlation between hardlock severity and the atmospheric conditions on the day of bloom further indicates the importance of flower infections in the epidemiology of the disease, says Marois.
"It may be possible to develop a disease forecast model that will help growers determine when fungicides need to be applied to manage the disease effectively."
Further studies are needed, he says, to determine the timing and rates of different fungicides for optimum management of hardlock disease.
"In the Southeast, cotton blooms produce bolls that are capable of maturing for six to 10 weeks. So, there is a potential need to protect flowers from infection over a long period. Accordingly, the best strategy may be the application of systemic compounds or systemic resistance-inducing compounds. It may be possible to focus on the earlier flowers or first four weeks of bloom, since these contribute the most to yield in some production systems."