Growth, sporulation, and pathogenicity of the raspberry pathogen Phytophthora rubi under different temperature and moisture regimes.

Phytophthora root rot of raspberry, caused mostly by Phytophthora rubi, is a significant issue for the Washington State red raspberry industry. Long considered to be a cool weather pathogen, it is often assumed that this pathogen is most active and infective during the cool, wet winters of the region when soil temperatures range from 5 to 10°C; however, there is little data to support this view. In addition, more recent research found that late summer symptoms of root disease were strongly associated with P. rubi. Experiments were therefore conducted at four temperatures from 5 to 20°C to evaluate the effect of temperature on P. rubi mycelial growth and sporulation and the effect of both temperature and soil moisture on the pathogenicity of P. rubi to red raspberry. Phytophthora rubi grew fastest and sporulated the most heavily at 20°C. However, disease was most severe at both 15 and 20°C. The soil moisture parameters tested did not affect pathogenicity results. These results show that P. rubi is more likely to infect during the spring and summer months from May through September, when soil temperatures are consistently in the 15 to 20°C range.