Authors: Natalie Hoidal and Marissa Schuh, Extension Educators, University of Minnesota Extension
Our winter / spring vegetable series focused on helping you anticipate and prepare for the growing season’s potential problems continues. This week we're talking about pumpkins and squash. Read on to learn more about variety selection, nutrient management, powdery mildew, and resources for anticipating common insect pests.
Variety selection
Storage
We’re seeing more and more farmers add fall and winter CSAs, meaning fall storage crops are increasingly important. Johnnys has a great guide to the storage lives of different pumpkins and squash. Pumpkins and squash have different storage requirements than pretty much any other vegetables; they require cool, dry conditions (50–60°F with 50-70% relative humidity), whereas many other storage crops require colder, moire humid conditions. Think about how much space you have available for storage, and whether you’re able to keep it at the proper temperature and humidity before figuring out which varieties you’d like to grow for fall sales. If you are not able to provide the proper storage conditions, shelf life will decrease substantially.
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| Squash and pumpkins from the arboretum display trials. Photo: MN Landscape Arboretum |
Resistance to insects and diseases
Insect and disease pressure in pumpkins and squash differs substantially from farm to farm across the state. Variety selection can help you to avoid or tolerate common insects and pathogens, but there’s no one-variety-fits-all solution. The following varieties tend to be more tolerant to the following issues:- Squash vine borer: Varieties under the species umbrella of Cucurbita moschata tend to resist squash vine borer better than other species
- Squash bug: Butternut squash, royal acorn squash, and sweet cheese pumpkins tend to show some resistance to squash bug damage
- Powdery mildew: Many varieties have powdery mildew tolerance and resistance: see a full list here.
Local performance
It’s always a good idea to talk with your neighbors and nearby farmers to see which varieties are doing well in your immediate area. Extension educator Annie Klodd and Rod Elmstrand did a pumpkin variety trial in 2019 at Rod’s farm near North Branch Minnesota; this is the most recent Minnesota specific pumpkin variety trial
Nutrient management
Timing your split application
Every year we get a couple of calls from growers with beautiful green squash plants that are just not setting fruit. While cucurbit pollination is complicated and influenced by a number of factors, one thing that can prevent fruit set is too much nitrogen at the wrong time. In most cases, pumpkins and squash should be fertilized with a split application, with half of your fertilizer applied at planting, and half as the vines begin to run. This is especially important if you’re using a quicker release fertilizer, or growing on sandy or low organic matter soil.
By fertilizing too late, you can send a confusing signal to your plants, and essentially tell them to remain in its vegetative stage of development. This can also happen if there is simply too much nitrogen available to your plants, regardless of timing. See the Nutrient Management Guide for Commercial Growers in Minnesota for fertilizer rates based on your soil test.
Calculating an N credit from your cover crop
Pumpkins and squash are an ideal crop to plant following an overwintered cover crop, or even a short lived spring cover crop. They can be planted later in the season than many vegetables, allowing plenty of time for incorporation and breakdown. If you have cover crops in your fields that you plan to terminate this spring, make sure you’re calculating the nitrogen credit from those cover crops. Since pumpkins and squash are quite sensitive to too much N, you’ll need to lower your rate of fertilizer or compost application to account for the nitrogen provided by your cover crop (even if it’s not a legume!). This video walks you through a quick and easy way to figure out how much N your cover crop will provide.
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| Graduate student Naomy Candelaria taking a biomass sample from a Phacelia cover crop to calculate the nitrogen contribution |
Powdery Mildew
The Basics
- Cucurbit powdery mildew is a fungal disease that attacks foliage and handles of pumpkins (and other vine crops)
- Powdery mildew causes leaves to dry out and wither prematurely, causing pumpkins to ripen too early and exposing them to the sun
- Unlike many plant diseases, powdery mildew doesn’t need moisture to get established or spread (though wet weather and high humidity can speed its spread)
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| Yellow spots and white spores of powdery mildew on a butternut squash. Photo: Edward Sikora, Auburn University, Bugwood.org |
Starting the Year off Right
While the mid-July arrival of powdery mildew arriving is months off, the decisions you make before planting will be the basis of powdery mildew control. Cornell collects varieties advertised with powdery mildew control. Resistant varieties may still need sprays to meet the expectations of your market.
Summarizing variety trials with powdery mildew resistant varieties broadly is difficult because the pumpkin market is so diverse (growing for consistent size to fill a bin vs growing novelty varieties for a roadside stand). I recommend checking the varieties you are looking at growing against variety trials and the powdery mildew list, and thinking about what you and your farm find important in a variety.
Finding Powdery Mildew when it Arrives
It is important to find powdery mildew before it becomes widespread. We typically see it arrive in Minnesota in mid-July, and plants that are starting to set fruit are more vulnerable. When scouting for powdery mildew, check the underside of leaves. It is typically first seen in leaves in the crown, which are older and shaded. Powdery mildew still has that characteristic white, powdered donut appearance, which when found on the underside of the leaf may have a corresponding yellow area on the upper side of the leaf.
Managing with Sprays
The generally recommended threshold for treatment is 1 out of every 50 inspected leaves has powdery mildew symptoms.
If you are choosing to treat powdery mildew, keep in mind that when spraying for diseases, all we can do is prevent. Products do not truly cure the plant. This makes early detection key in managing powdery mildew -- sprays done early in the season when the disease is first found are going to offer the plants more protection. Sprays meant to rescue heavily infested plants closer to harvest will be much less effective and will promote the development of fungicide resistance.
Powdery mildew is a constantly evolving disease, so it is important to rotate by FRAC code to assure products in your program are attacking the fungus in different ways.
For a list of products suggested in our region, check out the Midwest Vegetable Production Guide.
Cornell has rounded up organic products labelled for powdery mildew control.
Pumpkin and squash insects
Insects are best managed preventatively. Knowing which insects to anticipate in your pumpkins can help you to identify them early, and allows you to use preventative management techniques. We have a new pumpkin cropping calendar outlining the most common pumpkin and squash pests, when to look for them, and suggestions for management.
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| New illustrated cropping calendars display the lifecycles of common insect pests along with management recommendations. Illustrations by Eleanor Jensen. |
For a complete list of all of the insects and diseases we see in Minnesota pumpkins, see the VegEdge pumpkin page.
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