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 and cultivation to keep weeds to a minimum will also be advantageous. Avoiding the 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.

In the past, a typical conventional fungicide program consisted of rotating two different FRAC group fungicides every other week, such that the pattern looked like:

A – B – A – B – A – B

Often a protectant fungicide such as chlorothalonil or mancozeb is added to the tank mix on a weekly basis to 1) help control other important fungal diseases, such as anthracnose or gummy stem blight and 2) to help reduce selection pressure on the high-risk fungicide that was being applied. This type of preventative program was used for many years, because, in most cases there were just a few effective fungicides available for CPM control depending on the crop. An example of this would be:

A = (azoxystrobin [FRAC group 11] + chlorothalonil (MO5) rotated weekly with B = (myclobutanil [FRAC group 3] + chlorothalonil (MO5)

This type of control strategy worked extremely well as long as the pathogen didn’t develop resistance to either the FRAC group 11 (azoxystrobin) or FRAC group 3 (myclobutanil) fungicide. To better understand modes of action and how fungicide resistance develops in FRAC group 11 and FRAC group 3 fungicides please click here. Unfortunately, because of fungicide resistance development this type of program is no longer effective and is no longer recommended for CPM control.

Over the past 10 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 include:

• FRAC group 13 (quinoxyfen)
• FRAC group 39 (fenazaquin)
• FRAC group 50 (metrafenone)
• FRAC group U06 (cyflufenamid)
• FRAC group U013 (flutianil)

Not all of the fungicides listed above are labeled for all cucurbit crops. Growers will need to refer to local recommendations and the label for crop specifics. Remember, the label is the law.

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 A=(FRAC group 3);B=(FRAC group 13); C=(FRAC group 50); D=(FRAC group U013); E=(FRAC group 11)

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.

When to start spraying for CPM

Initiating a preventative spray programs begins with paying attention to Extension reports, scouting, and when the crop was seeded. If the crop is seeded the early-spring (i.e., early to late May) there is a very good chance CPM is not present in the mid-Atlantic region. If CPM is not present, there is no need to initiate a spray program using high-risk fungicides. In this instance, general protectant fungicides such as chlorothalonil will help mitigate other foliar diseases. As cucurbit crops are seeded into early to mid-June (and afterward) the risk for CPM development will rise in the mid-Atlantic region. This is when scouting and paying close attention to Extension reports becomes important. The first application should be done when CPM has been detected in the immediate region or when it is detected by scouting (e.g., with one lesion found on the underside of 45 mature leaves per acre). This will help reduce the use of unwarranted high-risk fungicide applications early in the production season. Importantly, the use of PMR or PMT cucurbit varieties will also help delay the onset of CPM development as well. Once CPM preventative fungicide programs are initiated, applications need to occur at every 7 to 10 days (at the latest) for as long as you expect to harvest (e.g., summer squash) or hold the crop (e.g., pumpkin and winter squash). During harvest, growers need to pay careful attention to pre-harvest intervals because they may vary significantly between different FRAC groups or fungicides within the same FRAC group (a good example are fungicides in FRAC group 3). Once harvest is complete, those blocks or fields need to be destroyed immediately to help reduce the spread of CPM to other blocks or fields that are scheduled to be harvested later in the production season. This is especially important for other diseases such as cucurbit downy mildew.

In some instances, rotating between many different FRAC group fungicides are not an option because the chemistries aren’t available for use. An example would be leaf spot control in spinach, where FRAC groups (7, 11, 7 + 11, 7 + 12, and 9 + 12) are available. In this example, options for control might look like this:

A – B – C – D

Where A=(FRAC group 7); B=(FRAC group 9 + 12); C=(FRAC group 11); D=(FRAC group 7 + 12)

Here, we have maximized the use of as many different FRAC groups as possible and spread their use as far apart as we can during the production season. Its important to remember that fungicides with more than one active ingredient (e.g., 7 + 11) should also be rotated as far apart as possible with fungicides that contain the single active ingredient (e.g., FRAC group 7 or FRAC group 11).

Monitoring fungicide efficacy

With the use of high-risk fungicides, all growers need to monitor fungicide efficacy accordingly. Once the lack of efficacy is detected there is a chance that fungicide resistance might be present. Importantly, the lack of efficacy should not be misconstrued with poor applications or waiting too long between fungicide applications. Reports of poor efficacy from Extension personnel from one region may not reflect fungicide efficacy in another region. Therefore, fungicide efficacy needs to be done at the farm level and the only way to accomplish this is to scout your fields and know what is and isn’t working for you.

The principles mentioned above also extend to other important diseases in vegetable production where there are multiple FRAC groups with high-risk fungicides available to control specific diseases. As a general rule, growers need to rotate as many different modes-of-action (i.e., fungicides from different FRAC groups) as possible during the production season to help mitigate fungicide resistance development in conjunction with best management practices.

For more information on fungicide use, FRAC groups, and specific control recommendations please see the 2024/2025 Mid-Atlantic Commercial Vegetable Production Recommendation Guide.