The Problem With Powdery Mildew In Pumpkin

 Powdery mildew, as shown on the pumpkin leaf here, can reduce the marketability of crops it affects. Photo credit: Beth Krueger Gugino, Penn State
Powdery mildew, as shown on the pumpkin leaf here, can reduce the marketability of crops it affects.
Photo credit: Beth Krueger Gugino, Penn State

Powdery mildew can be a problem, especially for pumpkins. Resistant varieties are available, but it’s important to note that none are immune. It’s also critical to make a correct diagnosis when the disease shows up and to know which crop protectants to use.

I caught up with Beth Gugino, an Associate Professor in the Vegetable Pathology Department of Plant Pathology and Environmental Micro-biology College of Agricultural Sciences at Penn State, to find out more on keeping this pathogen out of your pumpkin fields.

Q1 Powdery mildew typically shows up first as white powdery spots on the leaves’ undersides. Why is it so important to scout the entire plant for powdery mildew?
Gugino: The microclimate within the plant canopy tends to be more favorable for disease development, so it is important to not only look at the upper surface of the leaves, but also the lower surface as well as the petioles and crown of the plant. Often powdery mildew will first be seen on the older leaves. Powdery mildew is favored by dense foliage and lower light intensity. Infection is favored by the higher relative humidity that can be found within the crop canopy. However, unlike many other pathogens, infection also can occur at a relative humidity less than 50%.

Q2 Why is accurate diagnosis so critical?
Gugino: It is very important to be able to distinguish between the symptoms of powdery mildew and downy mildew because the fungicides that are effective for powdery mildew are not effective for managing downy mildew and vice versa. They are very different pathogens despite both having mildew as part of their common name.

Q3 When do symptoms typically begin to show up? What environmental conditions are best for the disease?
Gugino: Powdery mildew typically develops around the onset of fruiting. Drier conditions favor pathogen colonization, sporulation, and spread, so more frequent rain events tend to slow down powdery mildew development during the season. Powdery mildew also is favored by temperatures from 68°F to 81°F. Disease development essentially stops once temperatures get more than 100°F.

Q4 What fungicides do you recommend? Should growers use preventive products, too?
Gugino: Fungicides, when combined with host resistance, can be very effective for managing powdery mildew. Single site mode of action products specific for powdery mildew tend to be most effective. Many of these also have translaminar or locally systemic activity meaning when they are applied to the upper surface of the leaf, it then will move through the leaf tissue and protect the underside of the leaf. This is in comparison with protectant type fungicides, such as chlorothalonil, which will only protect the plant surface to which the product was applied so adequate coverage is essential.

It is important to include protectants in with a powdery mildew fungicide program for resistance management so the pathogen population is exposed to more than one fungicide active ingredient at a time.
I have been conducting pumpkin powdery mildew product efficacy trials at the Russell E. Larson Research and Education Center in Centre County, 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 deciding on a fungicide program.

I suggest initiating a program with one of the more effective products such as Torino (Gowan Co., FRAC code U6), Vivando (BASF Ag Products, FRAC code U8), or Quintec (Dow AgroSciences, FRAC code 13) and then rotating them with products that have different modes of action such as Fontelis (DuPont Crop Protection, FRAC code 7) or Pristine (BASF Ag Products, FRAC code 11 + 7) or a FRAC code 3 fungicide like Procure (Arysta LifeScience North America) or tebuconazole, Inspire Super (Syngenta, FRAC code 3 + 9), and Aprovia Top (Syngenta, FRAC code 7 + 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 (jojoba oil), JMS Stylet oil (JMS Flower Farms Inc., paraffinic oil), Trilogy (Certis USA, neem oil), and Organocide (sesame oil), as well as potassium bicarbonate-based products such as Kaligreen (Arysta LifeScience North America) and MilStop (BioWorks) to name a few.

Q5 What are the dangers of an early outbreak of powdery mildew that is left unmanaged?
Gugino: Powdery mildew is primarily a foliar disease that — if left unmanaged — can reduce the photosynthetic capacity of the plant thus reducing both fruit yield and quality. It also can lead to defoliation and sunscald on immature fruit. The disease can severely reduce the quality of the handles on ornamental pumpkins and other cucurbits grown for the fall harvest season.

Q6 Is powdery mildew on pumpkin as big of an issue as it was 15 years ago? How have resistant varieties helped keep this disease at bay?
Gugino: Powdery mildew continues to occur annually, typically moving across production regions following the succession of cucurbit production along the East Coast during the season. Overwintering inoculum is not a common source of the pathogen so disease management strategies such as crop rotation are less important for a disease like powdery mildew. Resistant varieties are important because they delay the onset of the disease, which in turn reduces the amount of potential inoculum available to spread to other cucurbit fields.

Q7 There are varieties highly resistant and some that are intermediately resistant or tolerant. Can you explain the difference?
Gugino: The terminology used to describe powdery mildew resistance varies. The important point to keep in mind is that none of the varieties are immune. The selection and planting of resistant varieties will typically delay the onset of the disease and will reduce overall disease severity.

Often, varieties that are designated as highly resistant obtained a copy of the powdery mildew resistance allele from each parent through conventional breeding methods compared to those designated as intermediately resistant or tolerant, which only received one copy from one parent. The resistance is most effective when it is from both parents (homozygous resistance) compared to one parent (heterozygous resistance).

Q8 What advice do you have on avoiding fungicide resistance?
Gugino: Fungicides are an effective tool for managing powdery mildew; however, there is considerable concern over the development of fungicide resistance. For resistance management, it is best to start applying the most effective products when you first start to see symptoms (one lesion on 45 to 50 leaves) and then switch to a protectant spray program later in the season. In the long run, this will reduce the selection pressure for powdery mildew spores that are resistant to the fungicide because fewer spores are exposed to the active ingredient when disease severity is low.

When applying powdery mildew-specific fungicides, each application should be tank mixed with a protectant fungicide again to manage for fungicide resistance. However, due to increasing concerns about pollinator health and the use of fungicides such as cholorothalonil, when possible, time fungicide applications when fewer pollinators are foraging and visiting flowers.

One resource developed for growers in the Mid-Atlantic region is the “Fungicide Resistance Management Guidelines for Vegetable Crops in the Mid-Atlantic Region” (http://is.gd/PSU_Guides).
This guide complements the “Mid-Atlantic Commercial Vegetable Production Recommendations” summary and provides a chart for each crop, which details information about FRAC codes and fungicide resistance management for recommended products and diseases. Our growers have found this to be a tremendous resource for making fungicide selection decisions.

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