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Sclerotinia blight, caused by Sclerotinia minor Jagger, has become one of the major limiting factors in peanut (Arachis hypogaea L.) production. The objectives of this research were to evaluate the effects of plant spacing on disease incidence and severity of Sclerotinia blight in peanut research plots, to measure the level of apparent resistance at different seeding rates, and to determine which methods would produce clearest selection criteria in space-planted breeding plots. Four peanut cultivars, Tamspan 90, Southwest Runner, Okrun, and Flavor Runner 458, were evaluated in field plots at four plant spacings (6, 15, 30, and 46 cm) in 2003 and 2004. Increased plant spacing improved sensitivity of disease incidence based determination of cultivar resistance but did not increase mean incidence significantly. Disease severity reached the highest level at the widest plant spacing. Final disease incidence provided excellent differentiation of genotypes with different levels of resistance and required the least amount of labor as compared with other methods of disease assessment. THERE ARE many constraints to peanut production, including a wide array of insects, diseases, and abiotic stresses. Sclerotinia blight has become one of the major limiting factors in peanut production (Melouk and Shokes, 1995). The first report of Sclerotinia blight affecting peanuts in the USA was in Virginia in 1971. In recent years, the disease has become more severe and spread to North Carolina, Oklahoma, New Mexico, Louisiana, and Texas (Smith et al., 1991a; Wildman et al., 1992).Yield losses of 10%arenot uncommon; however in cases of severe infection, yield losses of up to 50% may occur in a single field (Melouk and Shokes, 1995). Sclerotinia minor will attack all tissues of the peanut plant, but stem infections are the most economically important because reproductive pegs are attached to the stems (Chappell et al., 1995). Temperature, relative humidity, and soil moisture play vital roles in the infection and colonization of plant tissues by S. minor. Sclerotinia minor is a soil-borne pathogen. The disease is most severe during cool, wet weather, with an optimum growth range of 15 to 258C and a relative humidity approaching saturation (95–100%). High humidity promotes myceliogenic germination of sclerotia and is positively correlated with disease development (Dow et al., 1988a, 1988b). Disease development in the field is low when plants are small and without a dense canopy or complete ground cover. Outbreak of Sclerotinia blight is most often observed after vines are within 15 cm of touching or after vines lap between rows (Dow et al., 1988b; Phipps, 1995). Sclerotinia blight development is greatest as the plants reach maturity in September and October because of cooler night time temperatures and higher relative humidities normally associated with fall climate changes. During this time the plant canopies expand, contributing to the maintenance of higher humidity close to the ground (Dow et al., 1988b). Current Sclerotinia blight management recommendations include planting resistant cultivars, avoiding high seeding rates, cultivating before 15 June or eliminating cultivation entirely by integrated pest management to reduce the negative effects of nontarget fungicide applications, weekly field scouting for early detection, and fungicide treatments (Brenneman et al., 1988). “Omega 500F” (SCP 71512–1B-1000 0503 126357, Syngenta, Greensboro, NC), a new generation fluazinam (Smith et al., 1991a), has been effective for control of Sclerotinia blight in peanut; however, treatments are costly, particularly with the reduced prices associated with the elimination of the peanut quota system (K.E. Dashiell, personal communications, 2004). Sclerotina minor has a wide range of hosts including 21 families, 66 genera, and 94 species of both cultivated and wild plants, surviving 3 to 8 yr in the soil as sclerotia without a host (Abawi et al., 1985; Goldman et al., 1995; Melzer et al., 1997). Wide host ranges and sclerotial longevity limit the effectiveness of crop rotation as a means of control for Sclerotinia blight (Goldman et al., 1995). Use of host plant resistance is generally included as the primary means of mitigating production losses in grower fields, particularly when the commodity prices are low (Jordan et al., 1999). A single study published in 1992, utilized area under the disease progress curve (AUDPC) of disease severity to study resistance heritability. This study indicated that while broad sense heritability was high (41–50.3%) narrow sense heritability was low (14–23%) (Wildman et al., 1992). There seem to be multiple mechanisms of resistance that control infection by S. minor. These factors include avoidance of disease due to architecture, maturity, and/or greater resistance of the plant tissue (Chappell et al., 1995). Genotypes with more prostrate growth habits exhibit more susceptibility to disease than those with a more upright growth habit because of increased plant canopy moisture and reduced temperatures. Detached-shoot tests have demonstrated that there is also an additional physiological form of resistance of an unknown form (Akem et al., 1992). Peanut breeding lines with spanish ancestry appear to be more resistant to S. minor than other market classes because of the more upright architecture which A.L. Maas, USDA-ARS, Coastal Plain Exp. Sta., P.O. Box 748, Tifton GA 31794; K.E. Dashiell, USDA-ARS, Northern Grain Insects Res. Lab., 2923 Medary Ave., Brookings, SD 57006; H.A. Melouk, USDAARS, 127 NRC, Oklahoma State University, Stillwater, OK 74078. This manuscript is a portion of the senior author’s dissertation required for the Ph.D. degree at Oklahoma State University. All programs and services of the USDA and Oklahoma State University are offered on a nondiscriminatory basis without regard to race, color, national origin, religion, sex, age, marital status, or handicap. Received 7 Oct. 2005. *Corresponding author ([email protected]). Published in Crop Sci. 46:1341–1345 (2006). Crop Breeding & Genetics doi:10.2135/cropsci2005.10-0335 a Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA R e p ro d u c e d fr o m C ro p S c ie n c e . P u b lis h e d b y C ro p S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 1341 Published online April 25, 2006