Powdery mildew and other mid-season cucurbit management

Midsummer is when we start to see all kinds of problems in cucurbit crops. This article provides a brief overview of potential problems and interventions that growers can address at this point in the season.

Disease pressure

While there are whole host of pathogens that affect cucurbits this time of year, the most important disease in Minnesota is likely powdery mildew (primarily Podosphaera xanthii). This fungus produces asexual spores which can spread quickly and easily on the wind. Powdery mildew is rather unique in that it does not need humid conditions to spread, so once it's present, it can profilerate no matter the conditions (though initial infection typically occurs in humid weather). There are many powdery mildew resistant and tolerant cucurbits, but if you are growing a variety with limited tolerance, spraying is pretty much your only management option at this point in the season.

Meg McGrath, Extension Vegetable Pathologist at Cornell, conducts annual trials of fungicides (incluing OMRI approved) on powdery mildew. See her most recent report with recommendations here. Meg also joined us for the Great Lakes Vegetable Growers Network lunch-break webinar a couple of weeks ago to talk about powdery mildew management. You can listen here or on iTunes, Spotify, or wherever you listen to podcasts.  
 

Other pathogens we've been seeing this summer include downy mildew (growers across the Midwest are seeing downy mildew, but so far we have no reports in Minnesota), angular leaf spot, Alternaria leaf blight, and Squash Mosaic Virus. Our Extenison website has a complete list of additional pathogens that can affect cucurbit crops in Minnesota.
 

Insect pressure

Cucumber beetle pressure remains high throughout the state. Cucumber beetles can cause feeding damage to flowers and fruit at this period of the season, and can also transmit bacterial wilt and squash mosaic virus. At this point in the season, 25% defoliation is the threshold for spraying. However, if you're seeing virus symptoms or damage to the fruit that will make it unmarketable, consider doing so earlier. Kaolin clay is a relatively effective physical barrier for cucumber beetles in both organic and conventional systems. Pyrethrin and neem products are recommended for organic systems; for conventional systems there are many products available, which are outlined in the Midwest Vegetable Production Guide for Commercial Growers.
 

CB adult feeding on a pumpkin (E.C. Burkness, UMN)

Squash bugs
Squash bugs (Anasa tristis) are primarily an issue for cucumbers and squash. Most squash beetle eggs are laid by midsummer, meaning nymphs are now emerging and developing. If your squash or pumpkins are wilting, check for squash bugs at the base of plants or the undersides of leaves. If you plan to spray, now is the most effective window for management, when the nymphs are still relatively young. As they develop and become adults, insecticides will become less effective. See the Midwest Guide (linked above) for insecticide recommendations.

Squash vine borer
I typically think of sqash vine borer (Melitta curcurbitae) as an issue in home gardens. They may infect a plant or two in a field, but they rarely do enough damage for commercial growers to notice. However, I've received more reports of squash vine borer this year than normal. The good news is, there is only one generation per year. The bad news is, most of the damage has already been done. Adult vine borers lay their eggs until around midsummer, and then their larvae tunnel through the stems of squash, pumpkins, zucchini, and summer squash. While it is possible to physically cut them out, it would likely be time-prohibitive to do so. These larvae will overwinter the soil, so if you're seeing high populations this year, anticipate that you may see high populations again in the year to come.
 

Squash vine borer adults UMN Extension

Pollination

Cucurbits rely on insect pollination, which can be very difficult to manage. High temperatures at this point in the season can cause flower and fruit abortion, and there's little that growers can do to remedy the situation. However, good management practices like proper spacing, sufficient (but not too much) fertilization, and creating abundant habitat for pollinators can improve your odds of good pollination. For an in-depth discussion of pollination in cucurbits, listen to our recent What's Killing my Kale episode with cucurbit breeder Dr. Brent Loy.