Sporangia and zoospores are the primary and most abundant propagules produced by P. ramorum that can infect susceptible hosts. These propagules can only develop and disperse under wet conditions when temperatures are suitable.
In laboratory tests, California P. ramorum isolates formed sporangia at temperatures between 10 and 30 °C (50 and 86 °F), with peak production occurring at temperatures between 16 and 22 °C (61-72 °F) (Davidson and others 2005, Englander and others 2006). In forests, temperature and moisture levels fluctuate: UC Berkeley researchers showed that 12 hours of wetness at temperatures greater than 19 °C (66 °F ) are needed for abundant sporangium production (M. Garbelotto, personal communication).
Like spore production, the infection process also requires wet conditions. For example, P. ramorum requires 6 to 12 hours of continuously wet conditions to infect California bay leaves (Garbelotto and others 2003 ). Even under wet conditions, infection by P. ramorum can be limited by low temperatures. Under laboratory conditions, wet California bay leaves were much less likely to become infected at 12 °C (54 °F) than at 18 °C (65 °F) (Garbelotto and others 2003).
The combination of relatively warm temperatures and extended periods of wetness are especially favorable for P. ramorum infection. Infection of oaks and tanoaks is greatest in years with frequent late spring rains, and disease potential may be especially severe when these conditions occur two or more years in a row (Swiecki and Bernhardt 2008a). In contrast, new infections are uncommon after a dry spring, especially in inland areas (Swiecki and Bernhardt 2008a). In coastal areas, foliage can be wetted for extended periods due to fog drip, so new infections can develop even in years during which rainfall is not optimum for disease spread.
Greater P. ramorum infection levels and higher spore production develop on California bay leaves in shady sites with high humidity and prolonged leaf wetness periods. Hence, California bay leaf infections are usually more severe in shaded drainages and in stands on north- and east-facing slopes. High infection levels also occur more commonly in the lower, shaded leaves of the inner canopy. Leaves in the upper canopy, where lower humidity and more intense sunlight lead to shorter leaf wetness periods, develop fewer foliar P. ramorum infections (Swiecki and Bernhardt 2007, 2008b). These microclimate effects are more pronounced in drier climates away from coastal fog.