Diurnal effects on sporangium and zoospore production by Phytophthora ramorum on Rhododendron ‘Cunningham’s White’

Abstract

ABSTRACT We evaluated sporangium and zoospore production by three isolates of Phytophthora ramorum on Rhododendron ‘Cunningham’s White’ leaves under light and dark conditions at both variable and constant (14 C) temperature. P. ramorum–infected leaves were detached and placed in funnels inside of a 62-L plastic storage container located in a growth chamber. Cool mist was introduced to the container to create a high-humidity environment. Sporangia and zoospores were collected over a 4-day period by misting leaves with 5 mL of distilled water, which was collected in conical test tubes that also contained runoff from the misting. Spores were collected daily just before a 13-h light period and again just before an 11-h dark period. Sporangia and zoospores in the collection tubes were counted using a dissecting microscope following staining with lactoglycerin/aniline blue. Large differences in sporangium and zoospore numbers observed for the dark versus light periods were observed on days 2, 3, and 4. A diurnal effect has been observed for production of propagules of other oomycetes, but such effects have not been previously reported for P. ramorum. This information will help provide a better understanding of patterns of inoculum production by P. ramorum and resulting fluctuations in inoculum density that will influence sudden oak death epidemics in forest ecosystems in the United States and other countries where it occurs.