Molecular characterisation of downy mildew caused by Perofascia lepidii on garden cress and conditions favouring disease development

Abstract

Seed propagation of garden cress (Lepidium sativum) on organic farms can be compromised by downy mildew disease, which can develop rapidly, especially when relative humidity levels are high. The causative pathogen is thought to be Perofascia lepidii, particularly in Germany, although similar symptoms can be induced by the other oomycetes Hyaloperonospora sp. and Albugo lepidii. Here, a species-specific PCR assay was developed to allow the rapid discrimination of P. lepidii from these other oomycete pathogens. To control the downy mildew disease on garden cress, knowledge about conditions favouring the disease development is required. Thus, we studied the effect of temperature on sporangia germination in vitro and evaluated the impact of inoculum density, the effect of post-inoculation temperature and the duration of leaf wetness on disease development. P. lepidii sporangia germinated most freely in vitro when air temperatures were in the range of 5–10 °C, while a post-inoculation temperature range of 15–25 °C was optimal for disease development. A positive relationship between the duration of leaf wetness and disease severity was observed within less than 1 h after inoculation.