Hyaloperonospora camelinae on Camelina sativa in Washington State: Detection, Seed Transmission, and Chemical Control.

Camelina (Camelina sativa) plants with symptoms of downy mildew were obtained from three different locations in Washington State. Based on polymerase chain reaction (PCR) and sequencing of the internal transcribed spacer (ITS)1-5.8S-ITS2 region, the causal pathogen was identified as Hyaloperonospora camelinae. The PCR primers consistently amplified 699-bp bands from the infected plants but not from the asymptomatic plants. A comparison of the sequences with those in GenBank revealed 100% sequence similarity to H. camelinae. Growth and development of the H. camelinae was observed in different tissues using light microscopy and scanning electron microscopy (SEM). Light microscopic observation revealed the presence of oospores in the infected leaves and SEM revealed the presence of conidia and conidiophores on the seed surface. To determine whether H. camelinae is a seed-transmitted pathogen, seed collected from infected plants were planted in Sunshine professional growing mix maintained in a growth chamber. Disease symptoms were observed in 96% of the seedlings compared with 3% of the seedlings grown from seed from asymptomatic plants, which indicates that H. camelinae is a seed-transmitted pathogen. Seed treated with mefenoxam, a fungicide specific for Oomycetes, significantly reduced the incidence of the disease.