Peronospora sparsa is a plant pathogen that causes plant diseases, such as downy mildew of rose and diseases on blackberries, raspberries and other Rubus species.
Life cycle and appearance of Downy mildew
Pathogens causing downy mildew are not fungi but Oomycetes and related to Pythium and Phytophthora. They survive as oospores. From these oospores or surviving mycelium sporangia are formed, structures in which zoospores are produced. However, in downy mildews, sporangia often germinate directly and the germ tubes infect the plants either through the stomata or directly by penetrating the cuticula. For germination free water is required. For most downy mildews, the optimum temperature is around 15 °C. The closer the temperature is to these 15 °C , the shorter the leaf wetness period that is required for germination.
Inside the plant, the pathogens produce haustoria, small organs with which the pathogen can take up nutrients from the living plant cells. The pathogens continue to grow inside the leaves and after a while new spore-bearing structures are protruding from the stomata. Because there are more stomata on the underside of the leaf, this is where most fluffy symptoms are found. The pathogens are dispersed by air, water and tools. Some species, for example Hyaloperonospora parasitica and Peronospora farinose are assumed to be seed-transmitted (on the outside of the seed). Optimum temperature for germination, infection and sporulation is relatively low, in general between 10 and 20 °C. Late in the season new overwintering oospores are produced and buried in the soil with crop residues. Some species, including Peronospora sparsa overwinter in living host plants like blackberry.
Downy mildew pathogens infect mostly the leaves but occasionally also stems and fruits. They cause lesions on the upper leaf surface, bordered by the veins, which are first yellow and then turn brown. On the underside of the leaf fluffy growth appears which is first white and later turns grey-brown. This is, in fact, the sporulation of the pathogens extruding from the stomata. In lettuce, the foot is infected first and this causes the plant to fall over. In cabbage the stem is also infected in severe epidemics. In blackberry, red stripes may occur on the stems and petioles due to systemic infection. Early fruit infection may cause early reddening, after which the fruits die and shrivel. Later infections cause the berries to split and one or both parts to possibly shrivel.
How to prevent Downy mildew
- Choose resistant cultivars
- Use a wide crop rotation with non-host crops
- Prevent a humid microclimate. Achieve this by lowering crop density, good aeration during propagation and maintaining a warm and dry climate in greenhouses
- Use hygiene measures. Start clean, remove crop residues and prevent splashing water
Prevent plant diseases by optimizing plant potential and crop resilience.