Seed and soil transmission of downy mildew (Perofascia lepidii) in seed producing garden cress

Downy mildew on garden cress caused by Perofascia lepidii has become prevalent in organic seed producing areas of Germany resulting in limited seed production and high yield losses. Downy mildew occurrence in young plants in fields that have never been cultivated with garden cress raised questions about infested seeds and soils as sources of the primary inoculum and the role of other members of the plant family Brassicaceae as hosts of P. lepidii. To address these questions 31 seed lots from nine garden cress fields were examined for contamination using P. lepidii species-specific PCR assays. DNA of P. lepidii was detected in 17 seed lots out of 31. In bioassays no disease was observed on plants grown from infested seed lots but in a few plants grown from seeds. The significance of soilborne inoculum was investigated by assessing disease incidence in plants grown in soil samples. Garden cress with downy mildew symptoms were recorded when plants were grown in pathogen infested soil samples. The results show that P. lepidii is able to survive in the soil and can be a primary inoculum source for downy mildew disease. Oospores formed in infected plants underpin the role of these survival structures that get incorporated into soil after harvesting. In a host specificity-test none of the tested species Brassica juncea; Brassica napus; Brassica rapa subsp. pekinensis; Brassica rapa silvestris; Raphanus sativus var. oleiformis; Raphanus sativus var. sativus; Sinapis alba; Sinapis nigra; Arabidopsis thaliana; Capsella bursa-pastoris; Cardamine pratense; Cardaria draba; Nasturtium officinale were infected by P. lepidii.