Phytopthera often a consequence of flood irrigation

Many farmers who flood-irrigate their crops tend to either use highly contaminated water supplies or, as is most often the case, flood irrigate for excessive periods of time retaining water in the channels for more than one day.

Phytopthera is a fungal pathogen endemic in most areas and following extended periods of flood irrigation infect and cause “green-wilt” in chilli pepper crops. This wilting is most often seen within 48 hours of an extended period of flood irrigation. Having infected the crop and causing wilt, farmers tend to think that the wilting is a result of lack of water, particularly during periods of hot weather, and flood irrigate their crops more. All this may happen within a single week which adds complications as to the original cause of the wilting. Pathologists and extension officers may visit the field some two or more weeks after the initial wilting symptoms and conclude that the now “plant deaths” are a result of bacterial wilt (Ralstonia solanacearum). This conclusion, supported by the presence of bacterial ooze emanating from cut sections (see protocol “Ooze test”) of the lower stem of plants which may also be confirmed by plating out on TZC media (see protocol “Screening for bacterial wilt in tomato”), is misleading and derails many a breeding program. In fact bacterial wilt is in the vast majority of cases a secondary infection. Most varieties of chilli pepper are indeed resistant to bacterial wilt but following infection by phytopthera succumb to the infection due to the inactivation of the resistance mechanism in the plant.

So the real truth here is that breeding programs should be looking to breed for resistance to phytopthera while confirming for resistance to bacterial wilt. It may be that the pepper variety is also susceptible to bacterial wilt (often the case with blocky peppers). Also be mindful of the scale of pathogenicity of the many isolates of bacterial wilt with those found in Indonesia breaking bacterial wilt resistance in varieties resistant in other parts of Asia. Screening for both bacterial wilt and phytopthera at the seedling stage is the best method of screening and is both simple and straight forward if you have a suitably equipped lab.