Several species of Phytophthora can affect tomatoes ( damping-off, Phytophthora root rot ) by causing symptoms on the roots and crown , sometimes on the foliage and on the fruits in contact with the soil .

Root contaminations frequently take place early, often in the nursery or after planting, both in soil and above ground. They cause symptoms that appear before those caused by other root necrosis agents. Whatever species is prevalent, light brown lesions appear on the young roots, gradually extend to the entire root system and lead to more or less rotting of the latter. Rot sometimes spreads to the crown which takes on a brown to black tint (figure 1). Plants can wilt and dry out.

Among the Phytophthora spp. root and / or crown, it is worth mentioning:
- P. nicotianae (figure 1), widely distributed in the world and better known in France under the name “ P. parasitica ”. It attacks nursery seedlings as well as the roots, crowns and fruits of adult plants. On roots, it causes wet and brownish lesions; the root system eventually turns completely brown and rots. The roots are sometimes surrounded locally by alterations which quickly lead to the death of their distal part. This chromist can reach the neck and cause a diffuse, brownish lesion. In France, these “telluric symptoms” are observed above all on plants in nurseries or in the weeks following planting in the ground. It affects all crops, amateur gardens, intensive crops in the open or under cover, tomatoes intended for industry. Rootstocks resistant to the main soil-borne pests are also affected. It is found in soilless cultures, introduced through plants; its incidence is limited there, in no way comparable to that of P. cryptogea ;
- P. cryptogea , which also rages in the weeks following planting, causes a brown lesion at the neck and gradually surrounds it. The root system also rots . Eventually the leaves turn yellow, wilt and dry out. This Phytophthora is described in several countries on the roots and in nutrient solutions of soilless crops. In France, we have observed it occasionally in rare soilless crops, causing significant root loss and mortality of adult plants;
- P. arecae was reported in the Netherlands in the 1960s on tomatoes grown in poorly drained and sometimes over-fertilized soils. The symptoms caused by this fungus appeared in the weeks following planting and were mainly localized on the roots of the plants. The latter presented a more or less extensive rot leading to rapid wilting of the plants;
- P. capsici , very common on vegetables, causes damping off, damp, soft and black-brown cankers on the crown, and root rot causing them to break down. Nearby vascular tissue may take on a brownish tint (observed above ground in South Africa);
- P. citricola was considered responsible for crown rot and wilting on seedlings in Italy in 1960;
- P. mexicana causes damping-off and increased seedling mortality. It mainly causes lesions on the fruits;
- P. erythroseptica has been implicated in root and crown rots occurring in tomatoes produced in Australia. The plants also showed reduced growth and wilting. It is also reported in soilless crops in Bulgaria.

Note that P. cactorum (Lebert & Cohn) J. Schröt. and P. drechsleri Tucker have been very occasionally reported on tomatoes.

For additional information on these Oomycetes, you can consult the sheet Miscellaneous Oomycetes .