Colonization of citrus roots by Phytophthora citrophthora and P. parasitica in daily soil temperature fluctuations between favorable and inhibitory levels

Studies compared colonization of citrus rootlets by Phytophthora citrophthora or P. parasitica under constant favorable or inhibitory temperatures to root colonization under various daily combinations of favorable and inhibitory temperatures. Colonization of rough lemon rootlets after incubation for 96 h in the presence of soil naturally infested with P. citrophthora was detected at 9 through 27°C; however, the extent of colonization detected at 27°C was significantly lower than that observed at the other temperatures. In comparison, colonization of rootlets in the presence of soil naturally infested with P. parasitica was detected at constant incubation temperatures ranging from 12 to 33°C; however, the extent of colonization at 33°C was significantly lower than that observed at the other temperatures. Critical threshold temperatures, defined as thermal values at or above which colonization of rootlets was significantly restricted or prevented, were 27°C for P. citrophthora and 33°C for P. parasitica. After four consecutive 24-h periods, the magnitude of rootlet colonization by both pathogens was significantly less under incubation that included a daily time of at least 2.5 h at or above the threshold temperature when compared to rootlet colonization for 96 h at a constant favorable temperature. Significantly fewer sporangia were produced by P. citrophthora or P. parasitica at 24 h than after 48 and 72 h. The extent of root infection caused by P. citrophthora and P. parasitica at 24 or 30°C, respectively, was significantly lower at incubation periods of 4, 8, and 16 h than at periods of 24, 48, and 72 h. A fivefold increase in duration of zoospore motility was observed for P. citrophthora at 24°C than at 30°C, temperatures that respectively favor and prevent rootlet colonization; while an 11-fold increase was detected for zoospores of P. parasitica at favorable compared to inhibitory temperatures of 30 and 36°C, respectively. Temperature periods partially as well as entirely at or above the critical threshold values may reduce the degree of citrus rootlet colonization by P. citrophthora and P. parasitica by retarding the rate of sporangium formation and zoospore production and the duration of zoospore motility, compared to periods of equal duration that are entirely favorable for rootlet colonization. More efficient use of fungicides for control of Phytophthora root rot of citrus could be possible by application only when soil temperatures favor disease development.