Cultivars and fungicides affect phytophthora root rot in processing tomatoes

Field trials tested different processing tomato (Lycopersicon esculentum Miller) cultivars and the effect of fungicides in a site artificially infested with Phytophthora nicotianae Breda de Hann, the cause of Phytophthora root rot. In a trial screening nine cultivars, with and without phosphorous acid (PA) sprays, cultivar L343 showed more root rot at harvest than all the other cultivars, whether sprayed or unsprayed. The remaining cultivars showed little difference in root rot. Cultivars K7201, Hypeel696, Alta and Brigade (without PA sprays) yielded up to 56% more than an industry standard UC82B. Treatment of all cultivars with PA at 1 L a.i./100 L once before transplanting and three times during the growing season, increased yields of three cultivars (by up to 98% for the cultivar L343) above those of unsprayed plants. However, these treatments were phytotoxic to all cultivars. Yield differences between PA-treated cultivars were not statistically significant (P < 0.05). A second trial, with the cultivar UC82B, showed that soil treatments of metalaxyl (a total of 3 kg a.i./ha) following a preplant spray with PA at 1.0 L a.i./100 L reduced root rot and increased yield by 79%. One preplant spray of PA (as above) and three sprays during the growing season at 0.5 L a.i./100 L failed to reduce root rot at harvest but increased yield by up to 54%. PA sprays at 1.0 or 2.0 L a.i./100 L were phytotoxic and plants yielded less than those treated at the lowest rate, although the 2.0 L rate reduced root rot. The addition of a wetting agent did not affect the efficacy or phytotoxicity of PA spray treatments. These trials have shown the potential for fungicides and tolerant cultivars for management of Phytophthora root rot of tomato.