Initial powdery mildew infestation
Heavy powdery mildew infestation
This article was co-written with Dr. Sally Miller, Emeritus OSU Plant Pathology, updated from 2023 post.
The article is detailed and thorough as OSU has had an active powdery mildew evaluation program for over two decades. Here are the key messages to focus on; greater details of each are further into the text:
1. Mid-July is the time to start scouting for PM; when the first colonies are found, begin treatment.
2. Treat every 7-14 days using hollow cone nozzles at 35-50 GPA with a target of 65 PSI.
3. Alternate FRAC codes (letter/number on top of label) when spraying to reduce PM fungicide resistance. For example spray 1 Quintec (FRAC 13), spray 2 Procure (FRAC 3), spray 3 Quintec, spray 4 Procure, etc. Adding a protectant (FRAC M) like Manzate Pro or Chlorothalonil (Bravo) can be done with every spray and enhances overall PM control; it’s the one exception to the rotation rule.
4. There are many great to good products available for PM control; at least 3 current sources are listed in this article to find products; Sally Miller and Jim Jasinski’s trial results listed below and the Midwest Vegetable Production Guide.
5. Some cost of application data is provided toward the end of the article so growers can pair efficacy with economics to see which suits their production goals best.
General PM Biology
Powdery mildew usually appears on pumpkins and other cucurbits in Ohio beginning in early to mid July. While it has not been seen in South Charleston at the research station yet (Clark County), it has been reported in Wayne County. The pathogen, Psudoperonospora cubensis, does not overwinter in Ohio; infections result from spores blown into the area on the wind. Powdery mildew is favored by moderate to high temperatures and high humidity. However, unlike most other fungal plant pathogens, it is inhibited by free moisture on the leaf surface.
Signs of infection are small circular powdery growths on either side of the leaf, typically the lower leaf surface first. These spots enlarge and can eventually cover most of the leaf surface and kill the leaves. Stems and leaf petioles are also susceptible, but the disease is not observed on fruit. In pumpkins, powdery mildew may also attack the “handles”, which can be further damaged by secondary pathogens.
Powdery mildew is managed using disease-resistant varieties (powdery mildew tolerant and resistant hybrids) and fungicides. Pumpkin and squash varieties vary in resistance to powdery mildew; in general, the more susceptible the variety, the more fungicide needed. The choice of fungicide is important because insensitivity to overused fungicides is common. It is critical that a fungicide resistance management program is followed. Alternate fungicides in different FRAC (Fungicide Resistance Action Committee) groups, indicating different modes of action against the fungus. Fungicide applications should begin when the disease first appears and incidence is low (rule of thumb: at least one leaf of 50 scouted). Fungicides that are labeled for use against cucurbit powdery mildew can be found in the searchable Midwest Vegetable Production Guide for Commercial Growers. In the Cucurbit chapter, a fungicide table lists recommended products and their efficacy against powdery mildew.
Product Efficacy Trials
Field exposed greenhouse based studies at OSU of efficacy of powdery mildew fungicides in Ohio in 2021 indicated that Aprovia Top, Luna Experience, Inspire Super, Rally, Miravis Prime, Luna Sensation, Microthiol Disperss, Vivando and Procure provided very good control of powdery mildew on pumpkins (see table in color below). Velum Prime, Cevya, Prolivo and Gatten provided good control of powdery mildew on upper leaf surfaces but poor control on the lower surfaces.
Quintec provided good control in 2021 but in other years and other states has failed due to resistance. Fontelis, Bravo Weather Stik, Merivon Xemium, Pristine, and Torino have been shown to provide poor or variable control in Ohio or other states and are not recommended.
Table 1. Field exposed and greenhouse incubated powdery mildew fungicide results.
Jim Jasinski, OSU Extension, has been running field trials in southwest Ohio at the Western Ag Research Station since 2014 to assess fungicide efficacy against powdery mildew on pumpkins. Yearly results of these trials (2014 – 2023) can be found here toward the middle of the page.
Some effective season long fungicide combinations, based on 2021-2023 data, are shown in the second table below. Some of these fungicides were not effective in the Field/Greenhouse bioassays (table in color), but when paired with a broad spectrum protectant like Manzate, provided more effective control than when applied alone. Most experts suggest adding a broad spectrum protectant fungicide like Manzate to more powdery mildew-targeted fungicides to reduce the risk of fungicide development, boost fungicide efficacy and protect the crop from other diseases. However, programs based solely on protective products such as Manzate or Bravo, do not provide good season long protection so addition of other fungicides is necessary.
Always check the label for the full list of allowed crops, use recommendations and restrictions.
Table 2. Field Based Season Long Effective Fungicide Treatments to Control PM in Pumpkin in Ohio in 2021 – 2023.
*sequential applications for research only, must be rotated per label for grower use.
Spray Costs
While much time and energy is spent determining the most effective fungicides to control powdery mildew, less effort has been focused on the cost of the fungicide program, which is a huge consideration for growers. Below are listed some price estimates for a handful of fungicides. Prices extended to growers hinge on many factors so if the prices at your dealer vary compared to what is shown below, that is normal and to be expected.
Cevya – $5.92/oz (ca. $17.75 per application)
Gatten – $4.03/oz (ca. $24.18 per application)
Procure – $3.75/oz (ca. $30 per application)
SiGuard – $13/qt (ca. $13 per application)
Manzate Pro Stick – $3.60/lb (ca. $9 per application)
Quintec – $3.80/oz (ca. $22.80 per application)
Vacciplant – $1/oz (ca. $14 per application)
Vivando – $2.30/oz (ca. $35.42 per application)
Luna Experience – $5.15/oz (ca. $30.9 per application)
Microthiol Disperss – $1.85/lb (ca. $7.40 per application)