Guy Padgett, Colyer, Patrick D., Whitam, Kenneth
The cotton disease called “bronze wilt” (it gives the leaves a copper color and they wilt) was first observed in Louisiana and Mississippi in 1995. While losses in Mississippi were minimal, the disease caused yield reduction in some Louisiana cotton fields.
Since 1995, bronze wilt has been reported in most cotton-producing states, but its occurrence and impact on cotton varies considerably. The variation is due in part to the cotton varieties planted and severity of the disease. LSU AgCenter research indicates several varieties are susceptible – Hartz 1215, 1220, 1244; Paymaster 1560, 1218 B/R; Stoneville 132, 373, 5599; and Suregrow 125. Furthermore, this condition is most severe in short-season varieties. Most of the susceptible varieties are no longer marketed in the state.
The cause of bronze wilt has not been determined with certainty. It has been suggested drought stress, fertility and planting date may be factors that affect bronze will development.
Bronze wilt is also referred to as “copper top,” “sudden wilt” and “phloem wilt.” Symptoms have been observed on juvenile plants as early as late June, but symptoms are usually observed during fruit development. Initial symptoms include bronzing and wilting of the foliage near the top of the plant, and the terminal of infected plants is warmer than non-affected plants. As the condition progresses, stems of affected plants turn from bronze to red, and the entire plant wilts. Most yield losses result from the shedding of golf ball-sized bolls. In rare instances, reports of dead tissue where secondary branches attach to the main stem have been reported.
Symptoms of bronze wilt are often confused with mite injury, early maturity and potassium or magnesium deficiency. Therefore, it is crucial to identify this condition to avoid implementing unnecessary management practices. Since the etiology of the disease is unknown, management strategies have not been developed.
The best strategy is to avoid planting cultivars with a history of bronze wilt.
Tracking the disease
Experiments were conducted from 2000 to 2003 to monitor bronze wilt at the Macon Ridge Research Station near Winnsboro, La. The varieties Stoneville 5599 B/R, Stoneville 373 and Paymaster 1218 B/R were evaluated in 2001; Stoneville 5599 B/R in 2002; and Stoneville 373 in 2000 and 2003. Plants were monitored weekly. When bronze wilt symptoms appeared on a plant, the plant was flagged with the date. Spatial distribution of bronze wilt was recorded at the end of the growing season after plants were defoliated.
Spatial distribution varied considerably in both tests in 2001. In one test, incidence was lowest in the center of the plot and greatest adjacent to the center and on one end. Incidence in a second test was greatest 5 to 15 feet into the plot and lowest near the center. In 2002, symptomatic plants were more uniformly distributed throughout the plot compared to distribution in 2001. Spatial distribution of symptomatic plants varied dramatically in 2003 ranging from zero plants 15 feet into the plot to one plant on the plot ends. No established trends in spatial distribution were noticed within years; however, when data were summarized across years, there was a slight trend toward increased incidence on the plot end.
Bronze wilt increased over time in all years and spatial distribution varied considerably. Planting date may have impacted incidence in 2000, but not in 2001. In 2001, final incidence was similar in both tests, but epidemics progressed faster in the late-planted cotton. No “edge-effect” was noticed. However, this may differ in years when incidence is severe. These data suggest that this disorder is randomly distributed in fields. Therefore, scouting for this problem cannot focus on specific areas in the field. Since incidence was low in all years, additional research is necessary to fully understand the epidemiology of bronze wilt.
Other studies were conducted to determine the effect of planting date and nitrogen fertilization on bronze wilt incidence. Planting date studies were conducted over a two-year period. Bronze wilt was not affected by planting date. Two studies were conducted evaluating the effects of nitrogen rates on bronze wilt incidence. Nitrogen rates did not influence the disease.
A test was conducted at the Red River Research Station in Bossier City, La., on the effect of sulfur and phosphorus nutrition on the severity of bronze wilt. There was no difference in incidence of bronze wilt among treatments, but incidence was very low, so it was not possible to make valid conclusions.