Resistance to Potassium Phosphite in Phytophthora Species Causing Citrus Brown Rot and Integrated Practices for Management of Resistant Isolates

Phytophthora citrophthora, P. syringae, P. nicotianae, and less commonly P. hibernalis are causal agents of brown rot of citrus fruit in California. The chronic disease occurs during the winter season, requires annual management, and has limited California citrus exports because of quarantines in some markets. Potassium phosphite (KPO₃) is registered as a pre- and postharvest fungicide in the United States to manage Phytophthora brown rot. We evaluated the in vitro toxicity of KPO₃ to 65, 60, and 38 isolates of P. citrophthora, P. syringae, and P. nicotianae, respectively, that were obtained from major growing regions of California. Frequency distributions of effective concentrations to inhibit mycelial growth by 50% (EC₅₀ values) were not normally distributed, with skewness values of 1.84, 1.60, and −0.51 for each species, respectively. Isolates considered sensitive (EC₅₀ values <25 µg/ml), moderately resistant (EC₅₀ values from 25 to 75 µg/ml), or resistant (EC₅₀ values >75 µg/ml) were identified for each species. The majority of P. citrophthora (83.1%) and P. syringae (78.3%) isolates were sensitive, whereas most P. nicotianae isolates (86.8%) were moderately resistant or resistant. Resistance factors were calculated as 65, 19, and 10 for the three species, respectively. In preharvest field trials, KPO₃ (2,280 g/ha) applications were not effective in reducing citrus brown rot incidence when orange fruit were inoculated with a resistant (EC₅₀ = 161.9 µg/ml) isolate of P. citrophthora, demonstrating the potential for field resistance. Oxathiapiprolin (32.6 g/ha), however, was highly effective, indicating the absence of multidrug resistance. Postharvest treatments with KPO₃ were only effective in reducing brown rot caused by the resistant isolate of P. citrophthora to a low incidence when high rates (8,000 µg/ml) were used in heated (54°C) applications. The sensitive and moderately resistant isolates were managed using rates of 4,000 µg/ml, but heated treatments at this rate were needed to reduce brown rot to commercially acceptable levels when decay was caused by a moderately resistant isolate.