Temperature Effects on the Onset of Sporulation by Phytophthora ramorum on Rhododendron ‘Cunningham's White’

Abstract The effect of temperature and moist period on the onset of sporangia production by Phytophthora ramorum on Rhododendron ‘Cunningham's White’ was examined with misted detached leaves held in humid chambers. Following wound inoculation with sporangia, leaves were pre‐incubated at 20°C for either 24 or 72 h prior to placement at six different temperatures (4, 10, 15, 20, 25 and 30°C). The overall mean moist period required for first occurrence of sporulation over all six temperatures was 3.24 days with the 24‐h pre‐incubation time, compared with 1.49 days for the 72‐h pre‐incubation time. Following 24 h pre‐incubation at 20°C and at an incubation temperature of 15°C, sporangia were first collected from leaves following a 24 h incubation. At 10 and 20°C, sporangia were first collected after 48 h, whereas at 4, 25 and 30°C, sporangia were first collected after 3 days. Following 72 h pre‐incubation at 20°C, sporulation generally occurred within 1 day, even at temperatures such at 4 and 30°C that are suboptimal for sporulation. The highest levels of P. ramorum sporulation were observed at 20°C. P. ramorum formed sporangia on host tissue under moist conditions within the same time frame reported for P. phaseoli , P. palmivora and P. nicotianae , but substantially more slowly than certain other species such as P. infestans . Quantifying moisture and temperature conditions for initiation of sporangia production provides knowledge which leads to a greater understanding of the epidemic potential of P. ramorum .