(Oomycota: Class Oomycetes: Order Pythiales: Fam. Pythiaceae)
Life cycle
Phytophthora infestans is the best known Phytophthora species. This fungus is related to other Phytophthora and Pythium species, which belong to the Oomycetes class. Phytophthora infestans is commonly known as the dreaded potato blight.
The fungus survives as mycelium in tubers and grows on into the stem. Once it has reached the parts above the soil surface, sporangiophores are produced, which project from the stomata in the stems and leaves. Here, sporangia appear that are released and land on leaves. The sporangia either germinate directly, infecting the plant, or produce zoospores that germinate and then infect the plant. The fungus grows in the foliage between the plant cells and produces haustoria, feeding organs used to 'suck' nutrients out of the plant cells. The plant cells consequently die and the fungus continues to grow, producing new sporangiophores that emerge from the stomata. Sporangia are dispersed by wind and rain. This enables the fungus to seriously attack crops in very little time, causing severe losses. Rain-borne zoospores released from sporangia that land on the soil infect the tubers via lenticels and eyes. Tubers can also be infected if they come into contact with the fungus during harvesting.
Since the 1980s, the sexual cycle, during which oospores are formed, has also been found in the Netherlands. Oospores can also serve as primary source of infection. The shorter generation times and increased sporulation observed indicate that the fungus is becoming increasingly aggressive. This makes the disease more and more difficult to control. In tomatoes, the fungus is seed-borne, which is how seedlings are infected.
Host plants
Potato, tomato.
Symptoms
The fungus causes watery blotches, not clearly defined, with a layer of white mould, mostly on the underside of the leaves. Watery blotches appear on the top of the leaves. The centre of the blotch turns greyish brown and new white mould forms at the edge. A pale green zone is often visible round the edge of the lesion. Elongated brown blotches develop on the plant stems, usually on all sides.
Conditions
Moderate temperatures between 10 °C and 25 °C and leaf wetness or relative humidity >90% are required for sporulation. Sporangia are released at varying relative humidities. Growth stops below 2 °C and above 27 °C. In favourable conditions, the time between infection and the appearance of new sporangia is just three or four days.
