Browsing by Author "H. David Shew, Committee Member"

Now showing 1 - 3 of 3

Biology and Epidemiology of Sclerotinia minor on Peanut (Arachis hypogaea L.)
(2004-03-19) Smith, Damon Lee; Barbara B. Shew, Committee Chair; Marc A. Cubeta, Committee Co-Chair; H. David Shew, Committee Member; Thomas G. Isleib, Committee Member
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 (ΨM) 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 ΨM 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.
Population structure of Sclerotinia homoeocarpa from turfgrass
(2010-04-29) Taylor, Troy Ashton; H. David Shew, Committee Member; Ignazio Carbone, Committee Co-Chair; Lane Tredway, Committee Co-Chair
The purpose of this research was to determine population structure of Sclerotinia homoeocarpa, the causal organism of dollar spot, from turfgrass worldwide. Sclerotinia homoeocarpa is prevalent throughout the entire world and more money is spent controlling dollar spot than any other turf disease. The lack of disease cycle knowledge coupled with increased levels of fungicide resistance make understanding the genetic diversity of this fungus vital to ensure acceptable levels of control in the future. One hundred and nine isolates of S. homoeocarpa were obtained from the United States, Dominican Republic, Chile, Italy, United Kingdom and Japan. Isolates were analyzed using nuclear DNA from the ITS, Î²-tubulin, Calmodulin and TEF1-Î± loci. The nuclear DNA data suggests strong subdivision of S. homoeocarpa isolates based on host species, with isolates from warm season hosts and isolates from cool season hosts being genetically distinct. Vegetative compatibility tests for the 109 isolates were also utilized to determine the ability of individuals within the population to form heterokaryons with other individuals. Isolates in the study were compared to previously determine vegetative compatibility groups (VCGs) as well as tested with one another. The VCG work resulted in 9 previously unidentified VCGs. Isolates from warm season host species and isolates from cool season host species belonged to separate VCGs, supporting the subdivision seen in the nuclear DNA data. This work has shed new light on the population structure of Sclerotinia homoeocarpa and could potentially lead to alterations in control recommendations of dollar spot.
Screening tobacco germplasm for resistance to diseases affecting transplant production
(2004-02-06) Elliott, Patsy Elizabeth; H. David Shew, Committee Member; Verne Sisson, Committee Member; Jennifer Levin, Committee Chair
Rhizoctonia solani causes stem rot and target spot of greenhouse-produced tobacco seedlings. No fungicides are registered for control of these diseases, so sanitation is the primary disease management strategy. Seedling resistance to R. solani has not been characterized in current tobacco germplasm. The objective of this study was to screen seedlings of a diverse array of accessions, including several classes of tobacco cultivars and related Nicotiana species for resistance to a stem rot (AG-4) and a target spot isolate (AG-3) of R. solani. Further studies were conducted to determine if the resistance identified is heritable. Experiments were conducted in environmentally controlled growth chambers at the NCSU phytotron. Tobacco seedlings were grown in polystyrene trays floating on a nutrient solution to replicate greenhouse growth conditions. Approximately two weeks after germination, rice grains colonized by R. solani were placed on the surface of the growth medium to infest the medium. Symptoms, including death, stem lesions, and target spot lesions, were observed for 42 days after infesting the soil for stem rot and 56 days for target spot. Data were analyzed using a GLM procedure in SAS (SAS Institute, Cary, NC). Significant differences were observed among the accessions in level of resistance to stem rot and target spot. Disease incidence ranged from 12.5 to 100% for stem rot and 6.2 to 97.9% for target spot. This wide range of disease incidence observed among accessions for both diseases indicates that useful levels of resistance may exist to both diseases and may be useful in future breeding efforts.