Phytophthora species can attack many crops, from vegetables and fruit to ornamental plants and forest trees. All Phytophthora species are water molds, and as such thrive in wet conditions, spreading in water itself or in moist soils. Responsible for the Irish potato famine, tomato late blight, sudden oak death and other economically important plant diseases, Phytophthora pathogens can be cause economically devastating crop losses.

These pathogenic species are prolific, long-lived without a host in a soil-borne spore form and technically are oomycytes, which were once considered a type of fungi. Upwards of 200 species (perhaps even more than 500) of Phytophthora are thought to exist, but fewer than 100 have been identified and studied.

P. capsici in squash

Phytophthora capsici can infect eggplant, melons, snap beans, tomatoes, cucumbers and squash. All cucurbits are susceptible to the pathogen. With survival spores that can live in the soil for 10 years without a host plant, avoiding the subsequent planting of susceptible crops via long rotation schedules is a cumbersome but needed strategy.

“When you have crops that it will feed on, it will not allow the reduction of those (spore) populations in soils,” Dr. Mary Hausbeck, Michigan State University vegetable specialist, said during the recent virtual Great Lakes Expo.

P. capsici causes both crown and fruit rots. This is particularly important for growers of squash, as both the plant itself and the fruits have contact with the soil, and the crop has a long maturity time. This soil contact – as well as splashing soil during driving rains – allows the survival spore stage to rapidly spread throughout a field.

Other stages of growth in the P. capsici lifecycle like water too. The sporangia, produced in very high numbers by infected plants, can move onto new plants via water splash, while zoospores can swim through surface water or soil drainage to spread the disease.

Because of their long maturation time, squash plants have ample opportunity to become infected, and to succumb to that infection. P. capsici can infect the vines themselves. Even if a grower is able to keep the vines alive, the fruits are then susceptible to infection throughout their lifetime.

Signs of P. capsici include a watery mark and “powdered sugar”-type growth on fruits. The fruits may look fine from above but show signs of contamination on the bottom sides, where it may (at first) be difficult to spot. Infections occur even on parts of the fruit that have no soil contact, due to soil spores being splashed up.

Treatment options

P. capsici is able to develop resistance to fungicides. Rotating fungicides with differing FRAC codes and modes of action is important. There are a variety of fungicides that are effective against the pathogen. However, fungicide applications for both plant and fruit are needed, and must completely cover the crown area and the fruit itself. Due to the foliar growth of squash vines, complete coverage of fruits is difficult to achieve.

“A long maturity time means an extended exposure, and many, many fungicide applications, which is expensive,” Hausbeck said. Reliance on fungicides alone is not an effective strategy.

Cultivar studies have shown that there are some squash varieties that have much less susceptibility to P. capsici than others. Hausbeck studied a dozen varieties, some Cucurbita moschata and some C. maxima, to determine which proved most resistant to both crown and fruit rot. Individual plants were inoculated at the soil line with P. capsici at 50 days old. The plants were grown in rows in raised beds covered with black plastic.

The cultivars fell into three categories: low, mid and high susceptibility to crown rot. The low susceptibility group included all four of the C. moschata varieties trialed: New England Cheese, Butternut Ultra, Buckskin and Dickinson. None of the C. maxima varieties were in the low susceptibility group. Sweet Mama, Golden Delicious and NK580 had the least resistance, with very high rates of plant death.

Researchers added soil-applied fungicides to highly susceptible cultivars to determine effectiveness. When fungicides were applied to the soil at the base of the plant via a soil drench, in order to target the crown, they were able to reduce plant death to 40% (from 100% of the untreated cultivar). Foliar sprays alone left 80% of the plants dead.

“You have to get the soil and crowns. You have to get those areas,” Hausbeck emphasized. “Dead plants don’t yield anything. Getting to the finish line on hard squash is a two-step process. First of all, you’ve got to keep the plants alive.”

The second step is keeping fruit from becoming infected. This is where foliar sprays are needed, particularly when growing cultivars most susceptible to P. capsici fruit rot.

Researchers inoculated young fruit directly and found there were several levels of fruit rot resistance among the tested cultivars. Young fruit, at about seven days post-pollination, were found to be extremely susceptible to fruit rot across all cultivars. At 14 days maturity, New England Cheddar and Buckskin showed some resistance. By 21 days, the four C. moschata squash varieties showed very good resistance, but cultivars in the C. maxima family did not.

When growing more susceptible cultivars, “you’ll have to be really diligent on your fungicide selection and your fungicide placement,” Hausbeck said. “There are some cultivars that become a bit more resistant as they get older. These same cultivars also have … intermediate resistance to crown rot. One thing that can help you, to build a foundation, is cultivar selection.”

Fungicides are important to maximize yield even with resistant varieties, and must be applied before symptoms develop. Avoid planting squash in low-lying fields or in areas of poor drainage. Wet and rainy conditions require more frequent fungicide applications.

Keeping plants and fruit off the soil can also be beneficial. Growing squash in raised beds, covered with black plastic, offers some protection. But vines will sprawl between rows. Planting a cover crop, which can then be terminated to form a mat, protecting vines and fruits from soil contact, is a good choice, particularly when fungicides are not an option.

Another proactive method of protection is to harvest squash early in their window of maturity, to decrease the risk of developing an infection. Doing so is a “really recommended, common-sense approach,” especially in fields with a history of the disease, she said.

When moving equipment through fields, treat the most infected fields last, and immediately power wash equipment to remove soil and lingering spores so they are not transported to other fields.

Biofumigation studies have shown that some mustard crops seem to enhance P. capsici populations, indicating that those mustards are serving as hosts. In other studies, biofumigation had no effect on levels of field inoculation of P. capsici.

With 40,000 acres of squash grown in the Midwest alone, preventing P. capsici from devastating the harvest requires multiple approaches, including targeted use of approved fungicides and careful cultivar selection.