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 5 no decay to 5 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.

Evaluation of Resistance to Cylindrocladium Parasiticum of Runner-Type Peanut in The Greenhouse and Field

“…Greenhouse evaluations provide the faster results, but results were not well correlated with those from the field. Recent work on virginiatype peanuts showed that germplasm reactions from greenhouse inoculations were significantly correlated with field results (Hollowell et al, 2008). Therefore specific inoculation techniques may affect results and should be verified with runnertype peanuts to facilitate germplasm evaluation.…”

Reaction of the Core Collection of Peanut Germplasm to Sclerotinia Blight and Pepper Spot1

“…Both Tamspan 90 and Southwest Runner are consistently among the most resistant cultivars in field evaluations. Ratings from assays designed to identify physiologic resistance on whole or detached plant parts are generally not well correlated with field screenings (1,15,20,21) Tamspan 90 was grown on nearly all of the problem fields in Oklahoma during the 1990's, but susceptibility to web blotch (caused by Phoma arachidicola Marassas, G.D. Pauer & Boerema) and reduced demand for spanish peanut have contributed to the increase in acreage planted to runner cultivars. While Southwest Runner is partially resistant to Sclerotinia blight, it was never accepted by the shelling industry because of its small seed size.…”

“…Partial resistance in these market types has two forms. Physiologic resistance is expressed as reduced lesion expansion over time as measured on inoculated seedlings or detached plant parts following incubation under artificial conditions favorable for disease development (1,4,15,20,21). Architectural or morphological resistance is expressed in some genotypes with an erect growth habit and sparse plant canopy that results in reduced contact of main and lateral stems with mycelium from the germinating sclerotia or adjacent infections; and/or reduced periods of microclimate favorable for disease development compared to spreading genotypes with a dense canopy (1,4,6).…”

Three Inoculation Methods for Evaluating Sclerotinia Blight Resistance in Peanut

“…Laboratory-based resistance assays can bypass many 35 disadvantages of field tests, but laboratory assays also have unique limitations. Most 36 significantly, laboratory resistance assays can only measure physiological resistance and do 37 not assess the significant effect of plant canopy architecture on disease resistance 38 (Brenneman et al, 1988; Bailey and Brune, 1997; Hollowell et al, 2003Hollowell et al, , 2008. However, 39 markers associated with physiological resistance are generally the primary interest for 40 phenotyping.…”