Powdery mildew is identified by examining the underside of the plant’s fully expanded leaves. An infected leaf will have powdery white mycelia.

By Chris Crawford

Powdery mildew again has reared its ugly head this spring planting season, and it continues to be the No. 1 disease affecting cucurbit growers.

Cucurbit growers have described this year’s prevalence of powdery mildew as moderate to severe, said Gene McAvoy, University of Florida’s Institute of Food and Agricultural Sciences county extension director and agent for Hendry County. This is an increase from spring 2005, when the powdery mildew level was described as low to moderate. 

Mark Verbeck, independent crop consultant and owner of Red Gator Con-sulting, Lehigh Acres, agrees. “This is the worst year I’ve seen in four years,” he said. “Out of a five rating, I give it a four.”

Verbeck, who works in fields south and east of Immokalee, Fla., attributes this season’s abundance of powdery mildew to wetter conditions.

The disease is present almost every year in squash and cantaloupe. But in watermelon, it might hit hard one year and be nonexistent the next, said Don Hopkins, professor and center director of the UF/IFAS Mid-Florida Research and Education Center in Apopka.

Powdery mildew doesn’t kill crops like an untreated case of downey mildew can — but powdery mildew can severely damage the yield, McAvoy said.

Scouting and conditions

Identifying powdery mildew should start with examining the underside of the plant’s fully expanded leaves, said Ken Pernezny, professor of plant pathology at the UF/IFAS Everglades Research and Education Center in Belle Glade.

Often, the first noticeable symptoms are yellow spots on the upper side of the plants leaves, McAvoy said. When the leaf is turned over, the powdery white mycelia are noticed. He said that on watermelon, powdery mildew is not as distinctive.

Verbeck said he usually sees little white spots on the lower leaves first, then one to two weeks later, all of the leaves turn white.

As the disease spreads, the spots grow into one white mass that looks like talcum powder on the upper surface of older leaves or other plant parts. A large portion of the talc-like powder mass on the leaf surface is composed of spores, which are easily blown by wind to nearby susceptible plants. Heavily infected leaves turn yellow before becoming dry and brown. Plants under nutritional stress usually will develop powdery mildew much sooner than plants of the same age that are grown with good nutrition, according to UF/IFAS document PP-14.

Powdery mildew is most dangerous during the cool months of the winter and spring, from Thanksgiving to May 1, Pernezny said.

“Every squash grower will deal with powdery mildew — it’s part of the business,” he said. “You just have to be vigilant, especially past Thanksgiving Day.”

The fungus that causes the disease on cucurbits can reproduce in relatively dry conditions. Increased humidity intensifies the severity of the disease, and the infection is enhanced during periods of heavy dew.

Unlike downy mildew, powdery mildew becomes severe during periods of low rainfall in the winter and spring months. Researchers think the fungus survives year-round on wild cucurbits and other weeds, according to the UF/IFAS document.

About 90 percent of Florida’s squash fields are affected by powdery mildew annually.

Damaging disease

About 90 percent of Florida’s squash fields are affected by powdery mildew annually, McAvoy said. After the second or third pick, the plants have less natural resistance and thus are more susceptible to the disease. McAvoy also estimates that 50 percent of cucumber fields and 30 percent of watermelon fields are affected each year.

Some years, powdery mildew might cause a 25 percent to 30 percent loss of squash production, while other years it might cause only a 5 percent loss, Hopkins said, adding that on cantalopue, these numbers are much lower.

Yields of many infected vegetables are reduced because of premature foliage loss. Premature loss of leaves can result in reduced market quality because the fruit becomes sunburned or has low sugar content caused by premature or incomplete ripening, said Meg McGrath, vegetable pathologist at the Department of Plant Pathology at Cornell University in Riverhead, N.Y. Extensive premature defoliation of the older leaves may ensue if the disease is not controlled.

Verbeck said the yields of squash he scouts in the field haven’t been severely affected by powdery mildew, but treating the fields to control the disease costs an extra $60 to $80 per acre.

Powdery mildew affects the various cucurbit varieties differently. On summer squash, generally just the leaves and the stems shrivel and die from infection. But on cantaloupe, the fruit can be forced to ripen early, compromising taste and composition, Hopkins said. Watermelons are more resistant to powdery mildew. When infected, the yield usually is reduced slightly only by a shortened growing season.

In a few crops, direct damage to the marketable produce occurs. On honeydew and muskmelon, severe leaf infection usually results in lower sugar content and reduction of fruit quality. It also can lead to imperfections on fruit rind, such as speckling and edema.

Powdery mildew infection also predisposes plants to other diseases, including gummy stem blight and black rot, McGrath said.

Controlling powdery mildew

It is important to check with seed companies to learn which varieties are most mildew resistant, Pernezny said. Many cucumber varieties have very good resistance, whereas other cucurbit varieties do not.

Crop rotation and other cultural practices have little effect on powdery mildew incidence and development because spores are produced in large quantities and are spread widely by wind, affecting much of the acreage devoted to cucurbit crops.

Pamela Roberts, associate professor of plant pathology at the UF/IFAS Southwest Florida Research and Education Center in Immokalee, recommends using resistant cultivars whenever possible.

But using cultivars alone probably isn’t enough to fend off powdery mildew. Chemical control is strongly recommended, too. 

McAvoy said that in a climate such as south Florida, preventative spraying is essential.

Pernezny agrees. “Preemptive treatment is the best strategy to fight powdery mildew,” he said.

Weekly rotation of strobilurin fungicides with different chemistries, such as Quadris and Cabrio, will enhance resistance to powdery mildew, he said. Coverage of the treatment, especially to the underside of leaves, is essential to successful application.

Verbeck said Red Gator Consulting rotates Novas and strobilurins, but he has heard that other growers have found success using Topsin mixed with phosphoric acid. Verbeck usually sprays the plants right out of the ground and then every 14 to 20 days after.

If powdery mildew already has made its way into the fields, Hopkins said similar treatment with fungicides is necessary. He suggests beginning the fungicide rotation with Bravo and mancozebs such as Manzate, Dithane or Penncozeb.

Spray and rotate these  fungicides, which are less expensive than strobilurins, once a week. These types of fungicides are met with less resistance, especially against downey mildew and other diseases. Then rotate strobilurins in for the last sprays of the year.

Organic growers still can use a couple of fungicides to deal with powdery mildew, such as copper and sulfur, Roberts said. McAvoy said he’s seen good results rotating strobilurin fungicides and sulfur.



Watermelon line may help combat disease

U.S. Department of Agriculture Agricultural Research Service researchers and cooperators are introducing watermelon stock that may help breeders combat powdery mildew, a disease that threatens watermelon yields and quality in several states, according to an agency news release.

Recently, two races of powdery mildew have been reported on watermelon. Existing watermelon lines, which were thought to be resistant, were found to be susceptible. But ARS researchers and colleagues discovered the first documented resistance to race 1 powdery mildew in a ARS germplasm collection.

The scientists developed the new watermelon line, PI 525088-PMR, by repeatedly selecting the most resistant plants from the line PI 525088.

Watermelon historically has been resistant to powdery mildew, but the disease has become widespread during the past few years.

The new watermelon line could prove useful for introducing resistance to race 1 powdery mildew into commercial watermelon cultivars. Ultimately, it may also reduce the amount of fungicide needed to control the disease.