Influence of Soil Temperature and Moisture on the Severity of Cylindrocladium Black Rot in Peanut

PHIPPS, P. M., and M. K. BEUTE. 1977. Influence of soil temperature and moisture on the severity of Cylindrocladium black rot in peanut. Phytopathology 67:1104-1107. A soil temperature of 25 C and moisture content near field temperature of 35 C. A lower degree of root infection and rot capacity were most conducive for infection and rot of peanut resulted at 20, 25, and 30 C when soil was irrigated only after roots by Cylindrocladium crotalariae. Less root infection the moisture content reached a level near the wilting point of and rot occurred in soil at 20 C and 30 C, and no measurable plants. In experiments conducted with artificially-infested disease resulted in soil at 35 C. The response of plants in soil field soil, the survival of C. crotalariae microsclerotia in soil at 35 C for 9 hr during daylight, then 25 C for the remainder was not affected either by the irrigation regime or by of each day was similar to that obtained at a constant soil temperature. Additional key words: Arachis hypogaea 'Florigiant', Calonectria crotalariae. Cylindrocladium black rot of peanut is caused by the determined by counting the number of ms in six, 1-ml soilborne fungus, Cylindrocladium crotalariae (Loos) samples. Bell and Sobers (3). Microsclerotia (ms) of C. crotalariae Soil (fine sandy loam) was collected from the A horizon -are thought to be the primary propagule that enables of a peanut field where Cylindrocladium black rot had long-term survival in naturally-infested soil (6, 11). The never been observed and C. crotalariae was not detected annual increase in incidence of this disease in peanut in soil assays by the elutriation method (6). Quantities of fields in North Carolina and Virginia has necessitated an soil weighing 1,700 g (1,618 g on a dry wt basis) were intensive effort to develop strategies for disease control. infested with 16,180 ms from the standardized suspension The absence of effective agricultural chemicals for disease to provide an inoculum density of 10 ms/g soil. Each control (2, 10) has increased efforts to identify resistant portion of soil was mixed in polyethylene bags for 5 min cultivars of peanut (5, 12). before being dispensed into 15-cm-diameter plastic pots Effective screening of plants in a breeding program and with sealed bottoms. subsequent selection of resistant genotypes requires Control of soil temperature and moisture.-Pots were knowledge of conditions conducive for disease placed in separate water-bath tanks equipped to maintain development. The current investigation was made to soil temperatures of 20, 25, 30, and 35 C (± 1.5 C). determine the optimum soil temperature and moisture for Ambient air temperatures, which ranged from 25 C to 40 development of Cylindrocladium black rot in peanut C, were recorded daily with a maximum-minimum under greenhouse conditions, thermometer. Soil moisture content at field capacity (FC) was 9% (w/w) as determined by the method described by MATERIALS AND METHODS Daubenmire (4). Based on results in a preliminary study, a soil moisture content of 3% was determined to be near Inoculum preparation and infestation of the wilting point (WP) of peanuts. Moisture soil.--Microsclerotia of five pathogenic isolates of C. determinations of soil at pressures of 0.1, 0.33, and 15.0 crotalariae were produced in a liquid medium as bars were 11.97, 5.10, and 2.49% moisture content, described by Rowe et al. (11). After 8-10 wk of respectively, with the pressure membrane apparatus incubation, ms larger than 74 /m were separated from method (7). fungal mycelium and suspended in water as previously Two soil moisture regimes were maintained at each soil reported (6). The density 9 f ms in suspension was then temperature. The first involved irrigating soil to FC (9% w/w) once daily during the first 4 wk of the experiment, Copyright © 1977 The American Phytopathological Society, 3340 then twice daily during the remainder of the experiment. Pilot Knob Road, St. Paul, MN 55121. All rights reserved. In the second regime, soil was irrigated to field capacity