Conditions Ripe for “Plant Destroyer”

Summer squash showing symptoms of P. capsici. Photo: G.J. Holmes

Anyone who has ever seen a crop collapse in just a day or two due to Phytophthora will understand why its name means “plant destroyer” (from the Greek phyton (plant) and phthora (destruction). The Irish in the mid-1800s who watched their potato fields turn black and rot from P. infestans would no doubt agree.

While there are many species of Phytophthora, the primary species affecting cucurbits is P. capsici. It causes damping off, leaf spots, stem rot, crown rot, and fruit rot. Where crown rot occurs, the plant will collapse and die quickly. More often for cucurbit growers, however, fruit rot is the main concern.

“By the time fruit rot occurs, you’ve already put a high investment of time and money into the crop,” says Shine Taylor, field development representative for DuPont Crop Protection. In the case of Pcap, you have to be preventative, not curative. It develops so quickly that if you try to spray once you see it, you will be too late.”


Warm Weather, Wet Conditions

Phytophthora is a member of a group of fungi known as water molds; the scientific name is Oomycetes. It needs wet conditions and warm air and soil temperatures to spread. Ideal conditions are air temperatures between 75° and 80°F and soil temperatures above 65°F. It can reproduce asexually via sporangia, which grow into plant tissues directly, or produce tiny zoospores with flagella, or tails, that can actually swim in standing water to other plants. Phytophthora can also produce oospores via sexual reproduction. Oospores have thick walls and can survive for several years in the soil, coming to life when conditions are right.

“We’re in the perfect temperature range for Phytophthora right now,” Taylor says. “We’ve had a significant warmup along the eastern seaboard. When nights are warm and there’s high humidity, that’s when it becomes problematic.”

Fruit often, but not always, becomes infected on the side that has contact with the ground. An infected plant can also transmit the fungus systemically. Once infected, the fruit develops soft lesions and a noticeable whitish growth over the surface, and will become soft and mushy. If Phytophthora is identified within a field, it is especially important to inspect fruit for a week after harvest to make sure symptoms have not developed.


Common Sense Cultural Practices Combined With Chemical Control

Phytophthora-resistant genes have been identified in wild cucurbits, but resistant cultivars for commercial production are not yet available. All commercial cucurbits are susceptible, including squash, cucumber, pumpkin, watermelon and cantaloupe. A program that combines cultural controls with chemicals is the best way for farmers to avoid crop damage.

“Avoid growing cucurbits in poorly drained fields,” Taylor says. “If a field has a history of Pcap infection, avoid growing susceptible crops in it, because the oospores can last for several years in the soil. If you are overhead watering, do it in the morning so foliage and fruit can dry off during the day. Growing on plastic is another way to keep Phytophthora spores from splashing onto plants, and it keeps fruit from contact with the soil.”

In addition, follow sanitary practices such as disinfecting equipment, shoes, and tools when traveling between fields, Taylor says. If you have an infected field, make sure the water drains away from irrigation ponds and other growing areas.

Taylor recommends growers apply fungicide weekly beginning when the first foliage is set and continue until harvest.

“Fortunately, most cucurbits, especially summer squash, are short-season crops so there aren’t that many applications,” he says. “When conditions are especially ripe for infection, you may want to step up the frequency. You do have to follow the label carefully with regard to worker reentry intervals, pre-harvest intervals and maximum residue levels, especially if you are spraying during the fruiting stage.”

Like all spray programs, it’s important to rotate between FRAC (Fungicide Resistance Action Committee) groups to keep the Phytophthora from developing resistance. For help in choosing the appropriate product for control of Pcap, please check your local Extension/research publications in your area or call your local agent or DuPont Crop Protection contact for more information.

“With a short-season crop, you will probably be rotating anyway, because you will be trying to control downy mildew and powdery mildew at the same time,” Taylor says. “Many Phytophthora products are strictly for Phytophthora, but some do overlap.

“If you are growing cucurbits, you need to be thinking about Phytophthora capsici. Powdery mildew and downy mildew primarily affect foliage, but Pcap really impacts fruit. Make sure you spray the right material at the right time, which requires scouting, knowledge of when and where conditions might be favorable and then choosing the right product.”