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Weekly vegetable update 8/10/2023

Authors: Natalie Hoidal, Marissa Schuh, Anthony Adams

We're reaching the summer tipping point where most people have planted their last successions of major crops like broccoli, carrots, beets, etc., and growers are shifting into a slightly different workload that's more focused on harvest and maintenance. That said, growers who focus on winter markets may be starting to ramp up seeding for winter high tunnel crops. As summer produce continues to ripen, we're seeing some common problems with fruit set, mostly related to heat and disease.

Crop updates

Cucumbers: Vines are producing a good amount of fruit on most farms. We are seeing some spots, high tunnels especially, where fruit are small and malformed. The most common cause of this is poor pollination. If you are in a high tunnel and working to exclude cucumber beetles, you may need to add additional pollinators if you are seeing lots of small, misshaped, or shriveled fruit.

Pollination issues cause smaller, weirdly shaped cucumbers. Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo,

These fruit can sometimes fall victim to secondary infections. If you are seeing dense matts of purply, highly visible fuzz, choanephora rot may be the cause of your issues.

Choanephora rots in cucumbers (it can also affect other vine crops). Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo,

Garlic: We’ve received a few photos of funky garlic bulbs. As a first step for figuring out what’s going on with your garlic, check out the brand new Growing Garlic in Minnesota Guide. You are always welcome to reach out to us for diagnostic help as well!

Sweet corn:
Corn earworm trap catches spiked in Blue Earth county over the last week. Traps in Rosemount, Lamberton, Crookston, and Owatonna have caught few moths. Placing a trap on your own farm is the best way to get an idea of what is going on with corn earworm populations, which fluctuate a lot with the weather and location. We haven’t seen a whole lot of zippering, but we also would not be surprised if some of you are dealing with it since it is exacerbated by drought and heat, and sweet corn is often not irrigated.

Zipper pattern in sweet corn from heat and lack of soil moisture. Photo: RL Nielsen

Lettuce: We’re seeing some bottom rot in lettuce. Bottom Rot is caused but a Fungus Rhizoctonia solani. The infection occurs during the development of the lettuce head. You will find brown lesions at the site of infection, often close to the soil. The infection then spreads inside the lettuce head. Since this pathogen feeds on decaying organic matter, it can be especially hard to manage in soils where a lot of compost has been recently applied. It also has a wide host range and can infect carrots, beets, Brassicas, and more, so rotating has little impact. Improving drainage and airflow are two important strategies for managing this disease. Growers have also reported varietal differences in tolerance, so pay close attention to which varieties are holding up best if you are dealing with this problem. Two solutions for this pathogen include anaerobic soil disinfestation (see below), and the use of Trichoderma containing products like Root Shield.

Penn State Department of Plant Pathology & Environmental Microbiology Archives , Penn State University,

Potatoes: As potato plants start to die back, begin cutting back on irrigation. Potatoes are susceptible to soft rot during harvest, especially during hot weather. Take extra care as you harvest to avoid scrapes and bruises, and if possible, take some extra time to carefully sort out any tubers / bulbs that have obvious damage. They can be field cured if there is no rain in the forecast; simply use an undercutter bar or harvest forks to loosen the soil and let the potatoes sit for a few days before harvesting (while avoiding direct light).

Starting to plan for winter crops

Are you thinking of growing greens in your high tunnel this winter? Don’t let the growing season get ahead of you: planting will need to happen in just a few weeks. Johnnys has a great chart for helping growers decide when to plant various winter-hardy vegetables in a high tunnel to allow for winter harvest. All of these dates are in relation to the “persephone period” where we have less than 10 horus of sunlight per day, at which point plants slow down significantly.

For example. Kale should be planted between 15-13 weeks before the persephone period (ideally in succession with 1 planting each week during this period). Persephone periods in Minnesota include:

  • Rushford: November 7 - February 4
  • St. Cloud: November 4 - February 6
  • Duluth: November 2 - February 8
  • Thief River Falls: October 31 - February 10

So, if you wanted to harvest winter kale in St. Cloud, you would plant 15, 14, and 13 weeks before November 7: this means 2 weeks ago, 1 week ago, and this week! Thankfully there are plenty of shorter season options: Spinach (baby leaf) and baby leaf Brassicas can be planted just 5-6 weeks before the persephone period, and full leaf spinach, baby cale, and claytonia can be planted 7-8 weeks before.

What if you have plants in the greenhouse until mid to late October? You can start your winter crops early and transplant later. Often this doesn’t work out well with tomatoes, but it can work well to follow a shorter season crop like cucumbers with a winter spinach crop.

Check out the full chart from Johnnys here.

New Xerces pollinator-friendly cover cropping for vegetable growers publication

Have you thought about adding cover crops to your operation with the goal of supporting pollinators? You’re in luck! The Xerces Society and the Grossman Lab have been working on a multi-year research project to study which cover crops or cover crop mixes best support pollinators in the Upper Midwest. This research was done at University of Minnesota research stations as well as on farms in Minnesota. The team found that for spring planted cover crops, buckwheat and phacelia supported the greatest abundance and diversity of pollinators and other beneficial insects. In the summer, buckwheat was the top pollinator supporter, followed by sunflowers. The insects most commonly found in cover crops included wasps, minute pirate bugs, soldier beetles, sweat bees, fireflies, bumblebees, ladybeetles, butterflies, mining bees, and syrphid flies.

You can read the full publication here, which includes tips for seeding rates and establishment.

Photo: Julie Weisenhorn


Grasshopper populations that have been brewing all year are coming to a boil as adults reach their full size. Grasshoppers have one generation a year, so the small grasshoppers you might have noticed in June are now the full sized ones hopping around. Grasshopper populations spike in hot, dry years, as the main controller of grasshopper populations is a fungal pathogen that attacks eggs during periods of wet weather.

Grasshopper feeding is often primarily cosmetic. There are no organic control options.
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