- classification : Chromista, Oomycota, Oomycetes, Peronosporomycetidae, Peronosporales, Peronosporaceae

- Distribution and damage

This fungus , a systemic pathogen of lettuce, is fairly confidential; it was mainly observed on the roots of lettuce cultivated in soilless (NFT) in Virginia. It is one of the rare mildews, along with that of sunflower, able to sporulate on the roots of plants.


- Symptoms

This terrestrial downy mildew causes necrosis on the roots and forms numerous sporangiophores there bearing sporangia at the end of sterigmas. Zoospores are released from an apical pore. It seems unable to sporulate on lettuce leaves. Affected salads show reduced size.


- Elements of biology

Its mode of conservation is not known. The oospores present in the roots should be able to contribute. Once encysted, the zoospores germinate and directly penetrate the roots. The fungus has intercellular development. Subsequently, sporangiophores are produced on the surface of the roots. The zoospores released contribute to the spread of the disease. The oospores aplérotiques also form in the tissues. This fungus appears to be perfectly suited to the conditions of NFT lettuce crops. Its optimum temperature ranges between 22 and 28 ° C . It does not appear to cause damage at temperatures below 18 ° C.

- Protection

The control methods that can be used to control it are identical to those recommended for Oomycetes . It should be noted that some cultivars are resistant to this new root blight of lettuce.


NB: The legislation on pesticides evolving very quickly, we advise you to consult the e-phy site of the Ministry of Agriculture and Fisheries which is an online catalog of plant protection products and their uses, fertilizers and culture media approved in France . This also applies to all biological products based on microorganisms or natural substances.