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Weekly vegetable update 6/15/2023

Authors: Natalie Hoidal, Shane Bugeja, Claire LaCanne , Anthony Adams, Quincy Sadowski,

This has been a difficult week for farming with unhealthy air quality impacting most of the state for the last few days, and drought conditions getting worse, particularly in East Central MN. Air quality conditions are expected to improve over the next couple of days. In this week’s update, we discuss ozone damage symptoms, whether or not to use shade cloth in high tunnels, and a variety of crop updates.

Crop updates

Garlic: Garlic is producing scapes this week. Growers in Southern MN are likely already harvesting, and garlic in Aitkin county looked like it would be producing full scapes within days. Some reminders about garlic scape harvest:

  • Accidentally removing even one leaf can reduce bulb size, so cutting by hand vs. mowing them off is usually worth the extra effort
  • Snap the scapes by hand about 1 inch above the top leaf
  • Remember to keep irrigating for a week or two after scape harvest, as the bulb needs soil moisture to size up
Photo: Natalie Hoidal


Fruiting solanaceous vegetables: Around this time of year we start to get a lot of questions about the three lined potato beetle, which is primarily a pest of tomatillo, but can show up on any of the solanaceous crops. This insect is distinct from the Colorado Potato Beetle, but has a nearly identical lifecycle. Check out the post from last week about potato beetle management to learn more about managing both Colorado and Three lined potato beetles.  

Photo: Jeff Hahn

Potato: Potato scab is a common bacterial disease that causes corky spots on the skin of the potato. It is unique because it is more likely to form when the soil is dry during early tuber development. While not visible until harvest, symptoms do not affect the yield of the crop. The lesions may affect the marketability, but the potatoes are edible (though with reduced shelf life). While the soil should not be wet, irrigate enough so that the soil does not dry out completely. This is especially important over the next few weeks as tubers begin to form. In the long-term, use disease free seed potatoes, select resistant varieties, and have a crop rotation system with crops that are not susceptible to potato scab.

Peas: Summer dryness and heat has taken hold of most of the state. However, the cool and wet spring swung open the door to root rot infection. Some pathogens could be hitting legumes now when a healthy root system is critical. Aphanomyces is one such disease damaging to peas—causing wilting, stunting, and poor nodule growth. This water mold (oomycete) is known to be associated with short rotations (<7 years), heavy soils, and can persist for a long time (perhaps decades) in spots. When designing rotations to avoid pathogen buildup, always account for cover crops. Peas are frequently added to green manures and seed mixes, and can serve as interim hosts for diseases such as aphanomyces to persist on your farm. North Dakota State University Extension has an excellent field guide that can help you quickly narrow down your list of suspects if you notice sudden wilting in your peas. Root rot symptoms can be hard to tell apart, but this guide is filled with detailed pictures and straightforward information.

Strawberries: We’re seeing plenty of strawberry leaf blight, leaf spot, and leaf scorch in addition to the fruit issues we discussed in last week’s update. Renovating strawberries after harvest and removing old, diseased leaves is the primary way we recommend managing these pathogens. Read more about them here.

Photo: Leaf spot on strawberry. Michelle Grabowski

Cucurbits: Take some time this week to prune high tunnel cucumbers if you have not done so already. Pruning is critical for good air flow, disease prevention, and easy harvesting. Ideally, use a shears rather than doing this by hand, and make sure to sanitize your pruning shears after every few plants to avoid spreading diseases.

Brassicas: We continue to hear that flea beetles are giving farmers grief. Remember to keep using row cover and trap crops for new successions of Brassicas, and review last week’s flea beetle article for a more complete overview of flea beetle management strategies.

We are entering the period of the year where we could see Swede Midge damage. Up to this point, this invasive insect has only been detected/doing damage in St. Paul. Keep an eye out for plants with unusual growth patterns and brown corky scarring. For more information on this pest and the damage it causes, see the Swede Midge web page.

Swede Midge damage, Image: Julie Kikkert, Cornell Cooperative Extension,

Sweet corn: Migrating armyworm moths have been moving into Minnesota since early May, with large numbers of armyworm larvae (caterpillars) currently being found in areas of southern Minnesota. Producers who have small grains, grass pastures, grass cover crops, or corn should be on the lookout for armyworms. Corn following a cereal rye cover crop is a particularly high risk situation. Read here for more information about scouting for and managing armyworms.

Should you use shade cloth on your high tunnel this summer?

Anyone who has spent more than a few minutes in a high tunnel in the middle of the summer knows that it can be pretty miserably hot inside tunnels. There was an email thread on a farmer listserv a couple of weeks ago about using shade cloth to make the heat more manageable, and what the impacts to crops would be. Vegetable specialists from around the region weighed in, and shared that in slightly warmer climates (e.g. Indiana, Illinois, Iowa), it’s very common to use 30-50% shade cloth. Shade cloth can help to reduce both plant and human stress in the hottest parts of the summer.

There was a study in Iowa about shade cloth and bell pepper yields published in 2020, where researchers found that light-reducing shade cloth reduced sunscald incidence, but that 50% light-reducing shade cloth decreased marketable number of fruit and fruit weight. They concluded that growers should not exceed 30% light-reducing shadecloth, at least in Iowa.

Another way to help reduce temperatures in high tunnels is with gable vents. I’ve seen hundreds of high tunnels in Minnesota, and very few have proper ventilation at the top of the tunnel. Simply raising the sides of the tunnel does not provide sufficient airflow to vent excess heat out of the tunnel. Since hot air rises, installing a basic gable vent at the top end(s) of your tunnel gives the hot air a place to go. There are fancy gable vent systems that open and close automatically based on temperature, but a basic vent that can be manually opened and closed also works well. 

High tunnel ventilation with a gable vent. Photo: UMN high tunnel manual

Ozone impacts to vegetable crops

We’ve been writing about wildfire smoke for the past couple of weeks, but there’s an additional pollutant in the forecast this week: ozone. Ozone damage can show up as white, bronze, tan, or reddish speckling, or even as interveinal chlorosis, and some studies have shown that repeated ozone exposure can cause yield reductions by as much as 5-15% in some vegetable crops. While there is little we can do to prevent ozone injury to crops (aside from bigger picture changes in pollution reduction), it’s important to know what it looks like so that you don’t mistake it for a nutrient deficiency or insect damage. Read more about ozone impacts here:

Ozone injury on watermelon leaf. Photo by David B. Langston, University of Georgia,

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