The Effect of Matric and Osmotic Potential of Soil on Phytophthora Root Disease of Persea indica

STERNE, R. E. , G. A. ZENTMYER, and M. R. KAUFMANN. 1977. The effect of matric and osmotic potential of soil on Phytophthora root disease of Persea indica. Phytopathology 67:1491-1494. The influence of the matric and osmotic potential of soil on the root system of P. indica seedlings with black lesions, and root disease caused by chlamydospores of Phytophthora infection was verified by plating roots on PIoVP medium. cinnamomi was examined by growing seedlings of Persea Over the range of potentials tested, disease severity was a indica (a species related to commercial avocado) in infested function of matric potential but not of osmotic potential. The soil with different combinations of matric and osmotic percentage of diseased roots averaged 80 to 90%, 50 to 90%, potential. Matric potential in a range from 0 to -0.25 bar was and 10 to 50% at matric potentials of zero, -0.05, and -0.10 controlled by ceramic tension plates. Osmotic potential of bar, respectively. At -0.25 bar, only a few lesions occurred. soil was adjusted with NaCl or CaC12 and was in a range Disease was not significantly different over the range of (-0.37 to -2.37 bar) found in the saline soils in avocado osmotic potentials imposed with these respective matric groves in California where root rot caused by P. cinnamomi potentials. is a problem. Disease was rated by determining the portion of In avocado (Persea americana Mill.) plantings in MATERIALS AND METHODS California, the severity of root rot caused by Phytophthora cinnamomi Rand. increases when the Inoculum. --Isolate Pc 40, ATCC 32992, water content of soil remains near saturation (zero water (Phytophthora Culture Collection, Department of Plant potential) because of overirrigation or poor drainage (15). Pathology, University of California, Riverside) of P. Yet, it is reported that the fungus grows rapidly at quite cinnamomi was used in all experiments. Suspensions of low water potentials (1, 6, 12). In sandy soil with water chlamydospores were obtained from mycelial mats of 30potential adjusted osmotically, mycelial growth rate of P. to 40-day-old cultures grown in the dark at 24 C in 250-ml cinnamomi was highest at -10 to -15 bars and was onebottles containing 25 ml of V-8 juice broth (100 ml V-8 half optimum at -20 to -25 bars (1). With matricjuice, 2 g CaCO3, and 900 ml demineralized water). The controlled water potential, optimum and one-half mats were washed thoroughly with sterile deionized optimum growth rate occurred at -5 to -10 bars and -15 water, and the chlamydospores were separated from to -20 bars, respectively (1). The results indicated that P. mycelium by comminuting the mats on a Vortex mixer at cinnamomi tolerated low water potentials and was less speed 7 for 30 sec. Chlamydospores were separated from tolerant to stress associated with low matric potential hyphae and damaged spores, and the germinability and than with low osmotic potential. viability of the spores were tested as described (10). The Since damage to avocado roots by P. cinnamomi is concentration of spores in a stock suspension was greatest in the field when the water potential of soil is determined with a 1-ml capacity eelworm slide (German considerably higher than the potentials reported to Hawksley Ltd., Lancing, England). The stock reduce mycelial growth rate (1), we felt that an evaluation chlamydospore suspension was stored at 8 ± I C but never of matric and osmotic effects on Phytophthora root longer than 16 hr prior to use. disease at high soil water potentials was in order. This Control of matric and osmotic potential. -The soil paper describes a system in which plants were grown in matric potential was controlled by applying tension to a soil containing P. cinnamomi; the matric potential of the layer of soil on a tension plate (8, 9). Soil was placed as a soil was controlled over a range from zero to slurry on the unglazed upper surface of a hollow, porous approximately -0.3 bar. We used the system to examine ceramic plate. The lower surface and sides of the plate had the influence of matric and osmotic components of water been sealed by glazing. A plexiglass cylinder was sealed to potential on root disease in soil infested with the edges of the unglazed surface and provided an inner chlamydospores of P. cinnamomi. surface diameter of 8.5 cm. The cylinder was 8.5 cm tall and allowed adequate room for a layer of soil 1.5 cm in Copyright © 1977 The American Phytopathological Society, 3340 depth and air space for shoots of seedlings used in the Pilot Knob Road, St. Paul, MN 55121. All rights reserved, tests. The tops of the cylinders were covered with