Control of Phytophthora Root Rot of Processing Tomato with Ethazol and Metalaxyl. N. Ioannou, Department of Plant Pathology, University of California, Davis 95616. R. G. Grogan, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 68:429-435. Accepted for publication 13 December 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-429.

Metalaxyl at low concentrations in vitro inhibited mycelial growth and sporangium formation of Phytophthora parasitica, the main causal agent of root and crown rot of tomato in California. Germination of chlamydospores and zoospores was relatively unaffected by metalaxyl, but germ tube growth of both was inhibited. Ethazol was more toxic than metalaxyl to mycelial growth and sporangium formation and also inhibited zoospore and chlamydospore germination. Both fungicides at high concentrations reduced duration of zoospore motility, but neither affected zoospore formation and release. In the greenhouse, a single preplant soil treatment with metalaxyl 5G or 50WP at 2–10 μg a.i./ml soil protected tomato from Phytophthora root rot for 7 wk. Higher rates (50–250 μg a.i./ml soil) were phytotoxic. At these rates, metalaxyl was active systemically in leaves but not in stems. Tomato plants grown for 1 mo in soil treated with metalaxyl (100 μg a.i./ml soil), then transplanted in infested, untreated soil, were partially protected against root rot for only 1 wk, whereas lower concentrations were ineffective. Seed treatments with metalaxyl 50WP (7.5–30 g a.i./kg seed) were as effective against root rot as soil treatments, without being phytotoxic. Ethazol was 50–100 times less effective than metalaxyl when applied as soil or seed treatments. Both chemicals, at rates effective for disease control, acted as fungistats rather than fungicides. In the field, preplant soil treatments (30-cm bands) with metalaxyl 5G (0.3–1.5 kg a.i./ha) or ethazol (1.5–15 kg a.i./ha) and seed treatments with metalaxyl 50WP (7.5–30 g a.i./kg seed) provided partial control of Phytophthora root rot during earlier stages of growth, and total yield was increased. Yield increases generally were proportional to the level of apparent disease control. Soil treatments with metalaxyl 5G (0.75–1.5 kg a.i./ha) were the most effective for level and duration of protection and yield increase.