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Plant & Pest Advisory – July 8, 2025 – Andy Wyenandt
Cucurbit powdery mildew (CPM), caused by Podosphaera xanthii, is one the most important diseases of
cucurbit crops throughout the world. The pathogen is an obligate parasite, just like cucurbit downy mildew, meaning it needs a living host in order to survive. In northern regions that have a killing frost in the fall the pathogen will die out when the crop freezes. Not being able to overwinter, the pathogen must be re-introduced each spring or summer in the mid-Atlantic region. The pathogen accomplishes this by re-infecting cucurbit crops in the spring as they are planted up the east coast starting in Florida, then the Carolina’s, Virginia, and so forth. By late May, as soon as cucurbit crops begin to germinate in the mid-Atlantic region, the potential threat for potential powdery mildew infections begin.
The first step in mitigating CPM begins with planting powdery mildew tolerant (PMT) or resistant (PMR)
cultivars if they meet your needs. It is important to remember that these cultivars are not “immune” to
CPM; they will become infected at some point in the growing season depending on disease pressure.
Hopefully, this will occur later in the season when compared to CPM susceptible cultivars. Organic
growers hoping to mitigate losses to powdery mildew should always chose CPM tolerant or resistant
cucurbit cultivars first. There are a number of OMRI-approved fungicides labeled to help suppress CPM
development, these should always be used in concert with CPM tolerant or resistant cultivars and a
preventative fungicide program. Cultural practices such as increasing in-row plant spacing to improve air
flow cultivation to keep weeds to a minimum and reduce use of overhead irrigation will help reduce
disease pressure from another important pathogen, cucurbit downy mildew. Thus, growing cucurbits on
a mulch with drip irrigation has its advantages, but also increases costs.
Over the past 15 years, there have been a number of new fungicides released with new modes of action
(i.e., new FRAC groups) for CPM control in cucurbit crops. Unfortunately, all have a moderate to high-risk
for resistance development because of their specific modes of action. The good news are these new
fungicide chemistries have less effects on humans, non-target organisms, and the environment.
These fungicides offer new strategies when it comes to controlling and mitigating losses to CPM. Instead
of rotating two fungicides with a moderate to high-risk for resistance development every other week
( A – B – A – B), growers now have option to reduce the total number of times any single fungicide might
be applied during the production season; further reducing the risk for resistance development to any one mode of action. For example, in pumpkin, a new CPM preventative fungicide program may look like this:
A – B – C – D – E – A – B – C – D – E
Where each letter above equals a fungicide from a different FRAC group.
A protectant fungicide such as chlorothalonil or mancozeb should be added to the tank mix with each
high-risk fungicide to reduce selection pressure and to help control other important diseases such as anthracnose and Plectosporium blight.
In this type of CPM preventative program any one high-risk fungicide would only be applied twice per
growing season and 5 weeks apart greatly reducing the risk for fungicide resistance development. Importantly, for cucurbit growers, the easiest method to mitigate the potential for fungicide resistance development are to reduce the total number of applications of any one high-risk fungicide during the production season.
Dr. Sally Miller, from The Ohio State University and Dr. Meg McGrath, from Cornell University have written excellent articles for controlling CPM in 2022. ‘Preparing for cucurbit powdery mildew’ by Dr. Sally Miller, OSU ‘Cucurbit powdery mildew’ by Dr. Meg McGrath, Cornell University
For more information on fungicide use, FRAC groups, and specific control recommendations please see
the 2024/2025 Mid-Atlantic Commercial Vegetable Production Recommendation Guide.