Phytophthora: Not Just a Bean Disease

Mention the word “Phytophthora” and many readers will think of damping off in soybean fields. They think of massive seedling death due to fungal infection. However, this disease is not only a scourge in grain production. Phytophthora can present a serious threat in peppers, tomatoes, and eggplants. In our area, it is well known for sometimes causing devastating losses in pumpkins.

The water-loving fungus responsible for causing this disease is not the same species that plagues soybean fields, but it is related. Beans are attacked by Phytophthora sojae, while pumpkins are attacked by Phytophthora capsici. The names may be different, but the lifecycles of both fungi are similar. Phytophthora fungi can infect plants throughout the growing season killing seedling plants, causing crown collapse in more mature plants, forming leaf and/or stem lesions, and infecting fruit both pre- and post-harvest.

Phytophthora capsici persists in the soil and/or within debris as thick walled spores, termed oospores, that tend to be very hardy. Oospores are the “offspring” of “mating” by strands of fungal hair (mycelia). Given warm, wet conditions – oospores produce “bag-like” structures called sporangia (singular “sporangium”). Eventually, zoospores (swimming spores that actually have “tails” – correctly named “flagella”) are produced. Given time and wet conditions, these swimming spores come into contact with and eventually enter the plant. Additional sporangia are produced from this infection, and those sporangia release additional zoospores. The fungus itself may be observed as a white mold growing on plant tissue.

Symptoms of infection vary. Small dark lesions may develop in proximity and a few inches above the soil line as plants are eventually killed by the “damping off” version of this disease. Crown rot may eventually produce dark brown lesions that girdle the stem, killing everything above. Leaf symptoms may eventually cause foliage to brown. Fruit can become covered with white, moldy lesions. Lesions often start on the portion of the fruit in contact with the soil, but they can also appear in proximity to the fruit stem. Yield losses may approach 100 percent in some cases.

Phytophthora management in beans and Phytophthora management in pumpkins have some similarities. However, there is one significant difference between management in pumpkins and management in beans.

In beans, the use of resistant varieties is widespread. Soybean growers are familiar with resistant traits such as Rps1k, Rps1c, and Rps1a, and they often use these traits in the field. Unfortunately, resistance has not proven to be an option in pumpkin production. Instead, pumpkin growers rely upon a host of cultural and chemical management techniques.

On the cultural side, growers are encouraged to rotate pumpkin production from field to field to reduce the chances of encountering Phytophthora blight. They are also encouraged to stay away from fields that were previously in other Phytophthora host crops (peppers, tomatoes, etc.).

Growers also try to encourage a less-favorable environment for the disease by planting the crop where water does not tend to stand. In a similar fashion, growers are also encouraged to maintain good drainage. Both practices eliminate water from the Phytophthora-equation making it difficult for zoopsores to reach healthy plant tissue. Minimizing the spread of this disease is also important. The University of Illinois discourages growers from working fields when wet, encourages growers to clean equipment – when possible – before moving from field to field, and emphasizes that diseased fruit/plant material should be removed from the field as soon as possible.

Growers are encouraged to harvest ripe fruit as soon as possible to reduce any potential exposure to the fungus and/or time for the disease to develop post-infection. Chemical recommendations typically revolve around the use of mefenoxam (an RNA inhibiting seed treatment) and mixtures of dimethomorph (a post-emergence, sterol-inhibiting fungicide) and copper compounds. The former provides a few weeks of protection following emergence.

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