Distribution and seasonal population dynamics of Phytophthora citrophthora and P. parasitica in Arizona citrus orchards and effect of fungicides on tree health

The distribution and seasonal population dynamics of Phytophthora citrophthora and P. parasitica within citrus orchards in southwestern and central Arizona were determined over a multiple-year period. In central Arizona, P. citrophthora alone, P. parasitica alone, or both pathogens together were recovered from 7, 37, and 41% of sampled orchards, respectively, whereas in the southwestern production area, the same pathogens alone or in combination were recovered from 17, 50, and 17% of sampled orchards, respectively. For a 6-year period, the average population density of P. parasitica in southwestern Arizona was 16.7 propagules/g of dry soil. For 2 of 3 years, the population density of P. citrophthora at the 10-cm soil depth was significantly higher in the spring than in the preceding winter or the following autumn season. There were no significant seasonal multiple-year differences in population levels of P. parasitica. Propagule densities of both pathogens, as well as root densities, generally decreased as soil depth increased from 10 to 60 cm. No consistent significant correlation was detected between propagule density of either pathogen and soil temperature or soil moisture at the time of collection. A multiple-year treatment program with fosetyl-Al or metalaxyl resulted in significantly healthier tree canopies and higher root densities compared to nontreated trees; however, population densities of P. citrophthora and P. parasitica did not differ significantly when nontreated trees were compared to those receiving fungicide treatments.