- Other plant pests & diseases: Pests & diseases
The word 'phytophthora' comes from the Greek phytón (plant) and phthorá (destruction), so it means 'plant destroyer.' When Phythophthera is raging through your potatoes or tomatoes, you'll remember why this name was given to the disease.
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Phytophthora is caused by a genus of Oomycetes[1] (water molds). Oomycetes are very similar to fungi but are in fact not and so are called pseudo-fungi. Approximately 100 species of Phytophthora have been described but there exist many more.
When you are an organic gardener, there are a few things that you can do against this disease. For tomatoes, don't plant them outside but in a greenhouse and never spray water over the plants but only on the soil underneath. Some varieties of potatoes are largely resistant against phytophthora. Apply a rigid scheme of crop rotation. The spores can survive in the soil for at least three years, which means that you can grow potatoes on the same field only in the fourth year.
Infections occur when the crop remains wet for at least four to eight hours, at a relative humidity of more than 95 %. Phytophthora usually strikes only when the weather gets hotter, which means that potato species that grow early in the season can be harvested before the plants are infected. But those early potato species are also usually more sensitive, which is why i plant several species of potatoes, some for early harvest and others, which are more resistant against phytophthora, for winter storage.
The weakest plants will be affected before the stronger and healthy plants. Therefore, make sure you grow potatoes and tomatoes on the best soil you have.
Remove diseased foliage and tubers and preferably destroy them rather than putting them on the compost heap.
Don't add too much lime to your soil for crops that are sensitive to Phytophthera.
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