Cotton environmental stress during the South Carolina growing season

A cotton seed rot developed during the 1999 growing season in South Carolina. Since the severity varied across the state and the state was in the midst of a drought, questions arose about what impact weather may have had upon the disease. Since crops never grow according to a strict calendar schedule, a climatology produced using phenological dates as the time variable will be indicative of weather conditions that the crop actually experiences during each growth phase.

Cotton phenological dates were compiled from USDA-NASS SC Agricultural Statistics Service weekly reports for each year from 1979 through 1999. This weekly information was transformed to a standard calendar week that started on 1 March each year in order to compare years.

Environmental stress on growing plants combines the effects of moisture short fall and of sensible heat load. A checkbook method in which precipitation furnished the input and open pan evaporation weighted by percent available moisture determined the loss was used to calculate soil moisture. In this study, cotton was assumed to be under moisture stress when the available soil moisture dropped below 50 percent of the maximum amount. A simple variable for indicating sensible heat stress and the magnitude of stress is found by calculating the number of degrees that the air temperature is above a threshold level. Accumulating (summing) this difference over time during which a crop is under moisture stress should be an indication of environmental stress effects upon the plant.

South Carolina's 1999 cotton crop was growing under deficit soil moisture from planting until it started to square. Timely rainfall in some sections of the state relieved the moisture stress at the beginning of the flowering stage. High daytime temperatures, however, returned during flowering as rainfall quit. Thus from about mid flowering onward, the crop was growing under both moisture and sensible heat stress conditions.

Similar weather conditions have occurred in prior years. The timing or onset of stress was different in 1999 when compared to the previous 20 years. In 1999, heat and moisture stress were concentrated during the flowering stage and were not seen during the earlier part of the flower development stage. Thus environmental stress resulted in cotton yields well below average.