Late season powdery mildew on pumpkin may severely weaken the handles and reduce marketability. Photo: Beth Gugino, Penn State
Since the pathogen (primarily Podosphaera xanthii) does not overwinter in the Northeast and mid-Atlantic regions, the pathogens move into our production fields from other nearby sources; typically moving from the southeast up along the east coast.
When it comes to cucurbit diseases, powdery mildew is unusual because disease development can be initiated at a lower relative humidity (~50% RH) and leaf dryness (over leaf wetness) favors colonization, sporulation, and dispersal of pathogen spores. Temperatures between 68 and 80°F are most favorable for disease development.
The first signs of powdery mildew are small white powdery spots most commonly seen on the underside of the leaves or within the plant canopy. When scouting, it is important to thoroughly look over the entire plant. Also, scout by cultivar to account for differences in host resistance; usually scouting the most susceptible cultivars first around the time of fruiting. If protectant fungicides are being used, sometimes the spots on the upper leaf surface are yellow or chlorotic with white powdery lesions on the corresponding underside of the leaf. Accurate diagnosis is critical because targeted conventional fungicides applied for managing powdery mildew are different than those used for downy mildew.
Although at this point in the season it is too late, host resistance is an important tool for disease management. There is a wide array of cucurbits that have been conventionally bred with resistance. Genetic resistance can often both delay the onset of powdery mildew and reduce overall disease severity. As a result, it is important to scout for powdery mildew by cultivar.
When powdery mildew occurs early in the season and is left unmanaged, it can severely reduce the photosynthetic area of the leaves needed to produce high-quality marketable fruit. On pumpkin later in the season, it can also severely damage the handles leaving them weak further reducing marketability. Fungicides are an important tool for managing powdery mildew in-season. However, resistance management is a concern. It is recommended that the most effective products are applied when symptoms are first observed (one powdery mildew spot on 45 to 50 leaves) and then later in the season when switching to a protectant spray program rather than the reverse. 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.
Annually since 2009, pumpkin powdery mildew fungicide trials have been conducted on a susceptible pumpkin cultivar such as Sorcerer and Howden at the Russell E. Larson Research and Education Center in Centre Co., PA. Products that continue to be the most effective include Torino 0.85SC (FRAC code U6), Vivando 2.5SC (U8), Luna Experience 3.34SC (3 + 7) and Quintec 2.08SC (13). These are best used when alternated with products like Fontelis 1.67SC (7), Procure 480SC (3), tebuconazole (3), Inspire Super 2.8F (3 + 9), Pristine 40WSP (11 + 7), Aprovia Top 1.62EC (3 + 11) and Rally 40WSP (3) or with micronized wettable sulfur 80W (M2). Each application should be applied tank mixed with a broad spectrum protectant fungicide to manage for fungicide resistance and always rotate between FRAC codes with each application.
The latest resistance management recommendations and efficacy ratings:
Fungicide Resistance Management Guidelines for Cucurbit Downy and Powdery Mildew Control in the Mid-Atlantic and Northeast Regions of the United States in 2018
Severe powdery mildew on a susceptible pumpkin cv. Howden at the end of August 2017. No fungicides applied. Photo: Beth Gugino, Penn State
A weekly rotation of Fontelis, Torino, and Vivando, each tank mixed with Bravo WeatherStik, 5 total applications. Photo: Beth Gugino, Penn State
Due to increasing concerns about pollinator health and their exposure to fungicides such as chlorothalonil when possible, time fungicide applications when fewer pollinators are foraging and visiting flowers and flowers are closed. In trials conducted over the past couple of years to identify alternatives to tank mixing with chlorothalonil, both Tritek (mineral oil) and Microthiol Disperss (sulfur) were determined to be equally effective tank-mix partners and pose less of a risk to bee health.
Fortunately, cucurbit powdery mildew is one of the easier diseases to manage organically and there are a number of options including copper, sulfur, oil-based products like Eco E-rase (jojoba oil), JMS Stylet oil (paraffinic oil), Trilogy (neem oil), and Organocide (sesame oil), as well as potassium bicarbonate-based products such as Kaligreen and MilStop to name a few. With these products, spray coverage is essential since they are only effective at the site of application. So apply them in a large enough volume of water at a higher pressure to move the spray and penetrate the plant canopy.