Field Scouting Guide for Squash Powdery Mildew

Field Scouting Guide for Squash Powdery Mildew

When scouting pumpkin fields that are being sprayed with a protectant fungicide (i.e. chlorothalonil), keep in mind the fungicide is likely protecting the upper leaf surface, so the typical circular white powdery colonies may appear more chlorotic or yellow in color on the upper surface, but will be powdery white on the corresponding lower leaf surface. Photo by Beth K. Gugino.

Welcome to American Vegetable Grower® magazine’s first field scouting guide, this month concentrating on cucurbit powdery mildew. Each month we will bring you a different pest, ranging from weeds, to diseases, to insects, and even wildlife.

We reached out to pathologists to learn how to spot and treat powdery mildew. This month, our contributors are Beth K. Gugino, Penn State University, Debra Inglis, Washington State University, and Anthony Keinath, Clemson University in South Carolina.


Squash Powdery Mildew Basics

Scientific name: There are different strains impacting vegetable growers: Podosphaera xanthii; Golovinomyces cucurbitacearum; Leveillula taurica
Common name: Powdery mildew; squash powdery mildew
Crops affected: All cucurbits are susceptible, although many commercial cultivars have resistance.
Geographical range: Powdery mildew on pumpkin and other cucurbits has a worldwide distribution, including all of the U.S.

The Economic Impact of Powdery Mildew

Powdery mildew can reduce watermelon yields directly by causing the plants to produce fewer fruit, and indirectly by killing leaves that shade the fruit, leading to sunburn. The losses can be up to $1,500 per acre, or 75% of the value of the crop. Powdery mildew also can cause pumpkins to set fewer fruit, delay fruit ripening, and cause the handles to be soft, so they don’t hold up when the fruit are sold.

Cosmetic issues can have an impact as well, with sunscald giving fruit a less than desirable rind color. For pumpkins, handles/stems can be an important factor in purchasing decisions for consumers.

How to ID Powdery Mildew

Gugino: Fortunately, powdery mildew is fairly easy to identify. However, it requires scouting not only the upper leaf surface, but the lower leaf surface, petioles, and crown. Powdery mildew is favored by dense foliage and lower light intensity so the microclimate within the plant canopy is more favorable for disease development. Therefore, powdery mildew tends to first develop on the underside of older leaves within the canopy.

Inglis: Powdery mildew on cucurbit leaves is usually quite distinctive in that symptoms first appear as superficial white spots, which expand into powdery, white masses of fungal mycelium and spores on infected plants. The powdery masses sometimes have tiny, black structures, which are the fruiting bodies of the fungus.

Keinath: Scouts should look for the characteristic powdery white fungus growth on the bottom of the leaf, directly below the yellow spots.

Field Scouting Guide for Squash Powdery MildewHow to Distinguish Powdery Mildew from Other Problems

Inglis: Sometimes powdery mildews are confused with downy mildews. Although the names are similar, downy mildews often produce a gray/brown (not powdery), felt-like growth of spores and mycelium on leaf undersides, and are caused by a different group of plant pathogens. Also, any leaves severely affected by powdery mildew that turn brown and die, might be mistaken for drought stress or Verticillium wilt, but I think in most cases that would be unlikely.

Keinath: The pale yellow spots of powdery mildew on the upper leaf surface can be mistaken for downy mildew or possibly for spider mite damage, although spider mite damage on watermelon normally is a brighter yellow than powdery mildew, and the yellowing is clustered in the center of the leaf.


Field Scouting Guide for Squash Powdery Mildew 

Cultural Management

Inglis: Planting resistant or tolerant cultivars whenever they are available will help to slow down disease progress. Different races of cucurbit powdery mildews have been identified in some cucurbit growing regions of the U.S., but not in western Washington, thus growers here often need to base their cultivar choices on past experience. Also, using crop rotation, avoiding shading, and not planting dense stands can be useful cultural control measures in addition to fungicide applications.

Keinath: Growers can use resistant cultivars plus organic fungicides, then switch to traditional chemistry if powdery mildew continues to develop.

Known Resistances

Podosphaera xanthii is resistant to azoxystrobin and pyraclostrobin, Keinath says. Resistance to other fungicide active ingredients, such as boscalid and quinoxyfen, has been found in various parts of the eastern U.S., so fungicides with these active ingredients may not work in all areas.

Guginio’s Tips to Get the Most from Fungicides

Fungicides, when combined with host resistance, can be very effective for managing powdery mildew on pumpkin, Gugino says.

Single-site mode-of-action products that are specific for powdery mildew tend to be most effective. Many of these also have translaminar or locally systemic activity, meaning that when they are applied to the upper surface of the leaf they will move through the leaf tissue and protect the underside of the leaf.

Field Scouting Guide for Squash Powdery Mildew

The escaped ornamental Verbena bonariensis is also susceptible to Podosphaera xanthii, so if this plant is nearby, and has powdery mildew, spores have been blown into the area. Photo by Frank Wouters

This is in comparison to protectant-type fungicides such as chlorothalonil, which will only protect the plant surface the product was applied to, so adequate coverage is essential. It is important to include protectants in with a powdery mildew fungicide program for resistance management so that the pathogen population is exposed to more than one fungicide active ingredient at a time.

Gugino has been conducting pumpkin powdery mildew product efficacy trials at the Russell E. Larson Research and Education Center in Centre Co., PA, annually since 2009. There are a lot of products registered for managing powdery mildew, so it is very important to pay attention to their Fungicide Resistance Action Committee (FRAC) codes when developing a fungicide program.

She suggests initiating a program with one of the more effective products such as Torino (Gowan Company; FRAC U6), Vivando (BASF; FRAC U8) or Quintec (Dow AgroSciences; FRAC 13) and then rotating them with products that have different modes of action such as Fontelis DuPont; (FRAC 7) or Pristine (BASF; FRAC 11 and 7) or a FRAC 3 fungicide like Procure (Arysta LifeScience) or Tebuconazole (Albaugh), Inspire Super (Syngenta; FRAC 3 and 9) and Aprovia Top (Syngenta; FRAC 7 and 3).

It is important to always read the fungicide label because not all cucurbits are on all labels and the label is the federal law.

Powdery mildew is one of the easier diseases to manage organically on cucurbits and there are a number of options including copper, sulfur, oils like Eco E-rase from IJO Products (jojoba oil), JMS Stylet oil from JMS Flower Farms (paraffinic oil), Trilogy from Certis USA (neem oil) and Organocide from Organic Laboratories (sesame oil), as well as potassium bicarbonate based products such as Kaligreen (Arysta LifeScience)and MilStop (BioWorks) to name a few.

5 Tips to Managing Powdery Mildew

Gugino says there are five things for growers to keep in mind when trying to manage powdery mildew in their pumpkin fields:

  1. Select resistant varieties whenever possible. Host resistance is always the first line of defense.
  2. Before the season begins, have a powdery mildew fungicide program in place. To manage for fungicide resistance, know which products you are going to apply and in what sequence based on the FRAC code numbers. Having this plan in advance means less work during the production season.
  3. Obtaining good fungicide coverage is as important as developing a fungicide spray program. This can be challenging as the crop canopy expands. Use water-sensitive paper within the crop canopy to gain a sense of coverage and adjust/calibrate equipment as needed.
  4. If planting multiple successions, avoid planting down wind of the earlier crop and disk under the crop residue once with harvest to reduce inoculum.
  5. When scouting, scout by variety and keep good field notes on where and when powdery mildew was first observed and how it is progressing during the season. Evaluate your spray program to determine if it is providing adequate disease control.