Peronospora belbahrii
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Author(s): Bruce Watt, University of Maine
Source: IPM Images
Peronospora belbahrii Thines 2009 - (basil downy mildew)
The oomycete causes downy mildew of basil (Ocimum) in southern Europe and North America. It can spread rapidly in plantings during humid and warm conditions and can cause total crop losses. Symptoms include chlorotic leaf lesions and the appearance of dark purplish brown sporangia on the underside of the leaves. These spread easily through the air and the pathogen can also disperse through seeds or infected plants. It does not survive the winter in cooler regions but can still grow at 15°C. The risk of epidemics can be reduced by avoiding moist conditions (increased row width, drip irrigation). Resistance to the disease is not known.
Propagation is mainly asexually with sporophores emerging from leaf stomata and producing spores. The sporangia are ovoid to subglobose, and typically 25-30 x 20-25 µm large. The formation of durable oospores requires the contact of both mating types. Oospores are apparently not common and are brown and spherical, about 40-50 µm in diameter.
For a review see Wyenandt et al., 2015.
- Other images of Peronospora belbahrii (IPM Images - click to enlarge)
purplish sporangia on the undersurface of Ocimum leaf
symptoms on Ocimum leaves
sporangiophores
sporangiophores and sporangia
sporangia
