It shouldn't be much of a surprise that we've picked up some cases of downy mildew, Peronospora sparsa, in a variety of proprietary blackberry up and down the Central Coast. In both cases, the blackberries were uncovered during the rainstorms of March. Both situations have since been covered with plastic for tunnels, and symptoms have no longer been observed to be advancing.
An important note here is that for positive identification, since fungal structures of downy mildew generally become visible after 5 to 11 days at 65 o F, this growth was induced by placing symptomatic leaves in a “moist chamber” – really just a vegetable crisper with a wet paper towel on the bottom (see picture below) and leaving them there for several days. In our case, the downy mildew grew out adequately after this time and a positive identification could be made under the microscope.
There is more information on downy mildew in blackberry on the UC IPM Caneberry site:
https://ipm.ucanr.edu/agriculture/caneberries/downy-mildew/
Further information on this disease can be found on this same blog also:
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=1053