First Report of Cylindrocladium Black Rot Caused by Cylindrocladium parasiticum on Peanut in Texas

Abstract

During August 2004, Cylindrocladium parasiticum Crous, M.J. Wingf., & Alfenas (teleomorph Calonectria ilicicola Boedijin & Reitsma) was isolated from peanut (Arachis hypogaea L.) cv. NC 7 taken from an irrigated field in Terry County, TX. On 24 September, the mean length of patches with symptoms of dead plants and confirmed for the presence of C. parasiticum was 40 cm (averaged over 52 patches). Pods, pegs, roots, and lower stems of affected plants had decayed black lesions. No perithecia were observed. Roots were plated on water agar, and mycelia growth was transferred to potato dextrose agar (PDA). Conidia and microsclerotia typical of C. parasiticum had formed at 14 days. After 21 days on PDA, the following mean measurements were recorded: macroconidia 57 × 7 μm (usually two septations); stipe 115 × 4 μm; and sphaeropedunculate vesicle 15 × 10 μm. These are within reported ranges for C. parasiticum (1). Four-week-old cultures were comminuted in tap water and added (mycelium and microsclerotia) to potting medium (noninfested nonsterile peanut field soil/peat moss potting mix, 1:1). Peanut-specific Bradyrhizobium sp. was applied before planting. Three 3-day-old peanut cv. Tamrun 88 seedlings were transplanted in infested soil (four replicate pots, five isolates). Root, hypocotyl, pod, peg, and lower stem lesions were evident after 8 (Trial 1, one isolate) and 16 weeks (Trial 2, four isolates). Virulence varied among isolates (3). C. parasiticum was reisolated on acidified PDA after both trials. In Trial 3, six plant species (three replicate pots, five plants per pot) were challenged with one isolate by drenching a tap water suspension of comminuted microsclerotia and mycelium from 4-week-old PDA cultures into potting medium and transplanting peanut seedlings or planting seeds for other hosts. Root rot severity (0 to 5 index) (3) and reisolation percentages were cv. Tamrun 88 peanut 2.0, 100%; cv. Tamrun 96 peanut 2.0, 92%; cv. Hutcheson soybean (Glycine max (L.) Merr.) 3.3, 64%; cv. Blue Lake 274 green bean (Phaseolus vulgaris L.) 2.7, 40%; cv. California Blackeye 8046 southern pea (Vigna unguiculata (L.) Walp. 2.0, 70%; and cv. Jubilee Hybrid sweet corn (Zea mays L.) 0.0, 100%. Noninoculated peanut controls in all trials had no black decayed root lesions and attempts to isolate the fungus were unsuccessful. Peanut seeds planted in the production field were obtained from the southeastern United States. This field had been scheduled for seed production until C. parasiticum was identified. Temperatures and rainfall amounts during the 2004 growing season were less and greater than long-term averages, respectively. No other infested peanut fields have been confirmed or suspected in the region or state. There is continued risk of contamination from shipments of infected seeds from infested areas (2). This pathogen may pose a serious threat to >100,000 ha of peanut production in western Texas and eastern New Mexico. References: (1) P. W. Crous. Taxonomy and Pathology of Cylindrocladium (Calonectria) and Allied Genera. The American Phytopathological Society, St. Paul, MN, 2002. (2) B. L. Randall-Schadel et al. Plant Dis. 85:362, 2001. (3) R. C. Rowe and M. K. Beute. Phytopathology 65:422, 1975.