Screening of Virginia-Type Peanut Breeding Lines for Resistance to Cylindrocladium Black Rot and Sclerotinia Blight in the Greenhouse

Abstract Cylindrocladium black rot (CBR) caused by Cylindrocladium parasiticum and Sclerotinia blight caused by Sclerotinia minor are two economically important diseases of peanut (Arachis hypogaea) in the Virginia-Carolina production area. Developing cultivars with resistance to both diseases requires screening of new peanut breeding lines for resistance. Because field evaluations of resistance to these diseases often fail to produce usable results, greenhouse protocols were used to screen breeding lines and cultivars for resistance. For CBR, two seeds of a genotype were planted in a “cone-tainer” filled with a planting medium artificially infested with 25 microsclerotia of C. parasiticum per g of medium. After approximately 8 wk, the roots were washed and rated for degree of decay on a 0–5 proportional scale (0  =  no decay to 5  =  completely decayed). For Sclerotinia blight, plants were inoculated at 6 wk after planting by pushing a plug of potato dextrose agar (PDA) colonized by S. minor and protected from desiccation in a BEEM embedding capsule onto a freshly cut petiole on the main stem of the plant. Inoculated plants were placed in a mist chamber to maintain the high humidity necessary for infection. Lesion lengths were measured 4, 5, 6, and 7 days after inoculation, and areas under the disease progress curves (AUDPC) were calculated. All tests were conducted as incomplete block designs with six replications for CBR tests and four replications for Sclerotinia blight tests. Adjusted entry means were computed from each year's tests and used in summary analyses. Of the 125 breeding lines and checks tested at least once from 2003 through 2006, 51 were tested in at least two years, 34 in at least three years, and 15 lines were tested in all four years. Of the 15 lines tested in all four years, registered germplasm line N96076L had the lowest AUDPC for Sclerotinia blight (58 mm days), but had the greatest CBR root decay score (4.1 decay rating units). Several closely related breeding lines descended from a cross of N96076L and NC 12C were not significantly different from the most resistant line for either disease with scores ranging from 2.2–3.0 decay rating units for CBR and 63–99 mm days for Sclerotinia blight. Correlations of multiple-year greenhouse assay means with field disease incidence means were 0.83 for CBR and 0.35 for Sclerotinia blight. The greenhouse assay for CBR was a reasonably good predictor of field performance, but the assay for Sclerotinia blight was less reliable as a predictor.