Location: Foreign Disease-Weed Science ResearchTitle: The effect of relative humidity on germination of Sporangia of Phytophthora ramorum Author
Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2016
Publication Date: N/A
Interpretive Summary: We studied the effects of different relative humidity levels on germination of spores of an important fungus-like pathogen of forest trees and nursery plants. The name of the pathogen is Phytophthora ramorum, and it causes diseases known as sudden oak death and ramorum blight. We controlled relative humidity values within a closed chamber in which the experiments were performed. Spores were placed on small nylon squares inside the humidity chambers and left for exposure periods ranging from 1 to 24 hours, and then germination of the spores was assessed. Spores germinated best at high (100%) relative humidity, with an average of 88% germination. However, germination declined over time to 18% germination after 24 hours at 100% relative humidity. At lower relative humidity levels, correspondingly fewer spores germinated, and also showed a decline over time. For the lowest humidity settings of 80 and 85% relative humidity, germination was only observed at a 1 hour exposure and not for longer exposures, indicating that these low humidity levels were lethal to the spores. Knowledge of humidity levels over which spores of P. ramorum can germinate, provides information that can be used to predict when and where disease epidemics may occur.
Technical Abstract: Sporangia of three isolates of P. ramorum representing three different clonal lineages were subjected to relative humidity (RH) levels between 80 and 100% for exposure periods ranging from 1 to 24 h at 20°C in darkness. Airtight snap-lid plastic containers (21.5 x 14.5 x 5 cm) were used as humidity chambers with 130 ml of glycerin solution added to each container. Glycerin concentrations corresponded to 100, 95, 90, 85, and 80% RH based on refractive index measurements. Sporangia suspensions were pipeted onto nitrile mesh squares (1.5 x 1.5 cm, 15 micron pore size) which were placed in the humidity chambers which were incubated at 20°C in darkness. Following exposure periods of 1, 2, 4, 8, 12, and 24 h, mesh squares were inverted onto Petri dishes of PARP selective medium and sporangia germination assessed after 24 and 48 h. At 100% RH, we observed a mean value of 88% germination at 1 h exposure declining to 18% germination following 24 h incubation. At 95% RH, a steeper decline in germination was noted, with means ranging from 79% at 1 h to less than 1% at 24 h exposure. At 90% RH, no germination was noted after 8 or more h exposure, and values were 57%, 22%, and 3% germination for the 1, 2, and 4 h exposures, respectively. Germination was only observed at 1 h exposure for both the 85% RH treatment (52% germination) and the 80% RH treatment (38% germination). Longer exposures at these two RH values showed no germination. The three isolates responded similarly over the range of RH values tested. We found that the germination response of P. ramorum sporangia to RH values between 80% and 100% was comparable to that reported for other Phytophthora species. Knowledge of conditions that affect P. ramorum sporangia germination can shed light on pathogenesis and epidemic potential and lead to improved control recommendations.