Evaluating cotton for resistance to ascochyta blight
Prolonged cool, humid conditions on the High Plains of Texas cause disease symptoms in cotton similar to Ascochyta blight (Ascochyta gossypii Sydow). Different sizes of lesions with dark pycnidia in the centers develop on cotton leaves and purpling is sometimes exhibited on the edges of these lesions. This research was designed to determine what is causing these disease symptoms being observed on the High Plains of Texas. Lesions from field samples were cultured and Phoma spp. and Alternaria spp. were predominantly isolated whereas A. gossypii was never isolated. Cultivars inoculated with Phoma spp. and incubated under greenhouse conditions developed very few lesions. This can possibly be explained by fluctuating temperatures not allowing optimum fungal growth. Cultivars were also exposed to four inoculation treatments consisting of Phoma spp., Alternaria spp., a combination of the two fungi, and a control (uninoculated). They were incubated in a Stultz Scientific Seed Germinator to maintain a constant temperature with a high relative humidity. These cultivars experienced a rapid browning process which began on the edges of leaves and continued until defoliation occurred. There were no significant differences between inoculation treatments which suggests either the environmental conditions to which the plants were exposed or insects transferring fungi from plant to plant were causing the rapid browning process. Cultivars were also inoculated with A. gossypii to determine if this fungus was causing the lesions seen in the field and allowing Phoma spp. and Alternaria spp. to become established as a source of secondary infection. These cultivars were incubated in the same seed germinator after it had been sterilized and all of the control plants were screened before the inoculated plants to prevent insects from causing contamination. Again, a rapid browning process occurred with both inoculated and uninoculated cultivars and there were no significant differences between these two inoculation treatments. This supports the hypothesis that the rapid browning process is probably caused by environmental conditions and not by fungi. Disease symptoms similar to those being observed in the field have yet to appear under controlled conditions. Further research involving fluctuating environments and possibly more fungal inoculations are needed to determine the actual cause of the disease symptoms being observed on the High Plains of Texas.