Effect of Container Size, Inoculum Density, and Test Duration on Detecting Resistance to Cylindrocladium Black Rot of Peanut
Abstract Cylindrocladium black rot (CBR), caused by the fungal pathogen Cylindrocladium parasiticum Crous, Wingfield, and Alfenas, is an important peanut (Arachis hypogaea L.) disease in the Virginia-Carolina and Southeastern U.S. production regions and was first confirmed in Texas in 2004. We refined a screening technique and compared disease assessment methods for CBR using peanut germplasm with known resistance levels. Resistant genotype ‘NC 3033’ and susceptible ‘NC 7’ were compared in a growth chamber using 66 and 164 cm3 container sizes, 15 and 25 microsclerotia/g soil inoculum densities, and 4, 5, and 6 week durations. Root rot ratings (0 to 5 index), percent taproot necrosis and percent secondary root necrosis were estimated. Taproot necrosis was the most reliable method for differentiating between the resistant and susceptible genotype, followed by root rot ratings. The use of secondary root necrosis ratings was less consistent for detecting differences than taproot ratings. Large containers (164 cm3) inoculated at the 25 microsclerotia/g soil density was the most reliable combination for attaining significant genotype differences for taproot necrosis and root rot ratings. The 5 week duration provided the most consistent results for taproot necrosis, while 6 weeks was the most reliable duration for root rot ratings. Percent taproot necrosis is a valid, more objective alternative to root rot index ratings.