Producers attempting to understand phytophthora

By MICHEL ELBEN
Staff Reporter

HARRINGTON, Del. — Making an effort to manage a disease is a struggle, but eradicating a disease from your crops can be an all-out war.
Phytophthora capsici is a fungus known to infect many vegetables such as peppers, tomatoes, melons, cucumbers and squash.
Preventive methods can keep the fungus from causing severe losses.
Phytophthora capsici likes wet weather. The fungus grows under small water-soaked areas on the underside of leaves.
“Phytophthora can happen from overwatering,” said Dr. Mary Hausbeck, Michigan State University.
Irregular spots of various shapes and sizes form infected leaves are often wilted and turn light green or grayish-green.
They later become tan or white.
“That’s the white moldy stuff you see on the fruit,” Hausbeck said.
The fungus moves into stems from infected leaves and sections of the plant are killed. Fungus that develops at the soil line or that affects major branches cause the plant to wilt or die.
Root rots can be severe and stunt the plant or cause plant decline. Fruit rots are irregular in shape and olive green or light green with water soaked borders.
Rots expand rapidly and fruits can be completely diseased and desiccated, causing the formation of “mummified” fruits. Infected seeds are brown and shriveled.
Phytophthora capsici produces microscopic, asexual spores called sporangia. Sporangia are pear shaped, pointed at the tip. In water, sporangia form and release several swimming spores called zoospores. Sporangia also germinate directly by producing several germ tubes that begin new fungal colonies.
In some fields, the fungus is soil borne and initial infections of roots, collars, and lower leaves occur.
The fungus grows within the host and produces sporangia on the surface of diseased tissue, especially leaves. Sporangia are spread by splashing water from irrigation or rain.
“They form swimming spores,” Hausbeck said. “That multiplies from one to 20 to 40.”
With moisture present, zoospores are formed and released. These zoospores swim for a few minutes to more than an hour before encysting.
“The fruit can break down during transit or on grocers’ shelves, it’s a big problem,” Hausbeck said. “It’s a double whammy.”
Environmental conditions such as water temperature, nutrition, pH, and other factors determine the length of time zoospores continue to swim.
Sporangia are also moved within the field by contact with field equipment, clothing, gloves, tools, etc.
Movement of soil from one field to another on equipment or boots will move this pathogen.
The fungus survives in the soil and host debris. Roots, stems, and mummified fruits left in the field after harvest, harbor the pathogen for months.
Rotation with non-susceptible crops will reduce the amount of Phytophthora capsici surviving in soil. Fresh, clean seeds should be planted to establish healthy transplants.
Some plants are more tolerant and have better resistance to the fungus.
Cucumber vines seem to tolerate phytophthora, but the vines of pumpkins, yellow squash, zucchini and hard squash are very susceptible.
“Crop rotation isn’t the answer to this,” Hausbeck said.
Hausbeck said the chemical companies “were getting somewhere” with drip irrigation but they did not yet entirely understand the fungus.
“Drip application looks the most promising but it’s so hard because the fruits are all different,” she said.