Biology and epidemiology of Sclerotinia minor on peanut (Arachis hypogaea L.)

by 1979- Smith, Damon Lee

Smith, Damon Lee. Biology and Epidemiology of Sclerotinia minor on Peanut (Arachis hypogaea L.). (Under the direction of Dr. Barbara Shew) Sclerotinia blight caused by the fungus Sclerotinia minor is a serious disease of cultivated peanut (Arachis hypogaea L.) in North Carolina. Laboratory and field experiments were conducted to gain a better understanding of how environment affects sclerotial germination, mycelial growth, and plant infection in soil. Furthermore, quantitative relationships between disease incidence, severity, environmental factors, and yield loss were examined in order to improve disease forecasting systems in North Carolina. Laboratory and field experiments also were conducted to gain a better understanding of the biology of both the host and fungus. Experiments were conducted to evaluate the relative importance of tissue resistance vs. plant architecture in highperforming cultivars. In the laboratory, soil matric potential (?M) and temperature effects were measured for germination of sclerotia, mycelial expansion, and lesion expansion on detached leaflets. Temperature effect on the production of oxalic acid by mycelium of S. minor was also examined. Maximum sclerotial germination occurred at a ?M of –7.2 kPa and a temperature of 30 C. Rate of mycelial expansion and lesion development and expansion on detached leaflets were maximal at temperatures of 18-22 C. Lesions on detached leaflets developed slowly at temperatures above 26 C, and failed to develop at temperatures of 29 C or greater. Lesions formed if inoculated leaflets were moved from 29 C to a cooler temperature of 18 or 22 C. Oxalic acid production at temperatures above 29 C was negligible. In the field, incidence of Sclerotinia blight was measured at three sites in 2002 and 2003. A gradient of disease levels was established by utilizing one partially resistant cultivar, two susceptible cultivars and the fungicides fluazinam and boscalid at various rates. Weather data were collected in 2003 and modeled in both years. Disease incidence was highest on the susceptible cultivars with no fungicide treatment. Disease incidence of the partially resistant cultivar and of one susceptible cultivar in conjunction with various weather parameters were used as the dependent and independent variables, respectively, in a regression analysis. The following disease prediction model was created: Total Disease = -79.52 + 3.04 relative humidity – 0.020 relative humidity 2 – 0.47 soil temperature – 5.30 leaf wetness. In other laboratory tests, leaflets, pegs, lateral branches, and main stems of two susceptible cultivars and two partially resistant breeding lines were detached from plants grown in the greenhouse, inoculated with mycelial plugs, and placed in moisture chambers. Lesion development and severity on each part were measured for 7 days. In the field three cultivars and one breeding line were planted in replicated plots. Destructive samples of randomly selected plants were made weekly. Numbers of lesions on the four plant parts of interest were counted. Laboratory studies indicated that leaflets and pegs were the most susceptible plant tissues. Main stems and lateral branches were resistant to infection. In the field, however, lesions were found most frequently on lateral branches. Inconsistencies between laboratory and field studies indicate that there are other mechanisms of resistance operating in the field besides physiological resistance. Management strategies should focus on protecting lateral branches from infection by S. minor. Those management strategies may be improved further with the adoption of a Sclerotinia blight disease forecasting system that utilizes remote site-specific weather data with no need for onsite sensors. Biology and Epidemiology of Sclerotinia minor on Peanut (Arachis hypogaea L.) By Damon Lee Smith A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for a degree of Master of Science