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Weekly update 7/27/2023

 Authors: Natalie Hoidal, and Marissa Schuh

At this point in the season most growers are juggling a bit of everything: planting, weeding, harvesting, and dealing with mid-season issues like increasing insect and disease pressure. It’s also a key time where your management practices still have a big impact on the health of your crops later in the season, so it’s worth taking time to scout for insect and disease issues. 

Crop updates

In high tunnel cucumbers, spider mite and aphid numbers have really jumped on a lot of farms.


For aphids, a threshold is that if more than 20% of the runner vines have aphids, you should consider treatment. There are lots of organic products with aphids on the label (microbials, oils, soaps, etc.). In university trials, the efficacy of each product has varied widely.  M-Pede and other horticultural soaps can work well on aphids, be sure to get good spray coverage in new growth, young leaves, and the underside of infested leaves. These types of produces need to touch the pest to work, as they land on the aphid and cause them to dry out.

Be mindful of the weather when applying – phytotoxicity can happen for some products if it is hot when the application is made. Check the label of what you’re using for specifics. If using in a high tunnel, make sure the product is labeled for greenhouse use.

Two-spotted Spider Mites 

Spider mites are on the tipping point on many farms. Populations of this pest can flare to super high numbers, especially during hot dry weather. A female spider mite can lay 100 eggs, and the lifecycle for these pests is one to two weeks (the hotter the weather, generally the faster this will be).  If you noticed spider mites earlier in the month, check again on populations and consider treating, the horse may be about to get out of the barn.

These guys have hot spots, so scouting and spot treating can be a good option. Horticultural oils can provide good spider mites control (paraffinic oil, petroleum oils (check label), and some plant-based oils).  Again, these products need to touch the mites (the oils suffocate them), so good coverage in problem areas and re-application is important. Check label for weather-related restrictions to avoid harming plants, as well as for greenhouse use notes.

Don’t forget your allies. 

Many of the aphid pics we’ve gotten from growers also contain a glimpse of hope - syrphid fly larvae. While these look like green, brown, or yellow caterpillars, they are actually there to eat the aphids.

A syrphid fly larvae getting ready to snack on some aphids. Photo: Whitney Cranshaw, Colorado State University,

Garlic: Garlic harvest is underway across the state, and there are a few important diseases and mites to look for as you begin to cure your garlic. Since we practice asexual reproduction with garlic (planting the cloves rather than from true seed), it’s easy for problems to carry over from year to year, and so carefully examining your garlic crop this year can help you to avoid problems in the future. 

  • While aster leafhopper typically prefers carrots and other ornamental plants to garlic, it is an occasional pest of garlic, and leafhopper numbers have been high this year. Bulbs with aster yellows tend to be small and soft with dark streaks or discoloration of the wrapper. They can also smell bad. 

  • Fusarium is a common soilborne pathogen that can infect garlic, especially when the garlic has already been weakened from mechanical damage, insect damage, or other diseases. Garlic bulbs with this disease have rotting on the “basal plate” - the base of the bulb.

  • Garlic bloat nematode: Garlic with garlic bloat nematode tends to not form a proper bulb, or if they do, the bulb will rot.

  • Mites: Garlic bulb mites are very common in Minnesota. While the mites are too small to see with your eyes, you can see their damage when you pull back the wrappers. Damage from bulb mites looks like sunken tan or brown spots on the cloves, and they carry over from year to year if infected garlic is replanted. 

  • These are only the most common problems, there are many other diseases and insects that can infect garlic. If your garlic looks or smells not quite right, consult the new Growing Garlic in Minnesota Guide and consider sending a sample to the UMN Plant Disease Clinic. 

Tomatoes: As tomatoes push through their first few flushes of fruit, the plants tend to get better at transporting calcium to their fruit, and blossom end rot starts to become less pronounced. However, at this point in the season we start to see a lot more issues with potassium. Potassium deficiency symptoms first show up in leaves that are about halfway up the plant stem (not the youngest, not the oldest), and deficiencies can lead to a myriad of issues with fruit including yellow shoulder, uneven ripening, and hard, tasteless fruit. If you’re seeing deficiency symptoms, it’s not too late to correct them! Fertigating with water soluble potassium can help to mitigate deficiencies. Potassium sulfate works well for fertigation because it is water soluble and in a form that plants can readily use. 

Peppers: Peppers are uniquely susceptible to sunscald. We’re starting to get questions from growers who have sporadically rotting peppers (as in, a few here and there but it’s not widespread), and in most cases it looks like the peppers got sunscald, and were then infected with opportunistic pathogens that feed on rotting plant material. It’s almost inevitable that some of your peppers (particularly bell peppers) will get sunscald, and once they do, they cannot bounce back. The best thing to do is to walk through your fields at least weekly for the next few weeks and “weed” out peppers with sunburn so that your plants can focus their energy on producing new healthy fruits and so that opportunistic fungi do not become a problem.

For pictures of sunburn and other causes of mushy peppers, see this article from last year about pepper meltdowns.

In all cucurbit crops, squash bugs continue to lay eggs, and those eggs are now hatching. The easiest time to kill squash bugs with insecticides is when they are small - so if you’re seeing lots of squash bug nymphs and have had losses at the hands of squash bugs before, make a plan to manage them. The threshold for treatment is on egg mass per plant (though you only make treatments when eggs hatch).  For organic growers managing nymphs, pyrethrin and azadirachtin (or even better, both in combination) can do a number on nymph populations. For conventional growers, see the Midwest Vegetable Production Guide for options. Regardless of what product you choose, time applications for later in the day when cucurbit flowers have closed and many bees are done foraging for the day.

Freshly hatched squash bug are small with black fronts halves and green rear ends. Photo: UMN Extension. 

A reminder for carrot growers that we have been seeing high numbers of aster leafhopper across the state this year. This insect vectors aster yellows.  If you have been managing leafhoppers, a reminder that because of the lag between infection and symptoms, you can stop spraying for leafhoppers 2-3 weeks before harvest.

Sweet corn is being harvested. Corn earworm numbers have been low up to this point. This insect rides south to north airflow up to Minnesota, so as long as we keep getting air (and wildfire smoke) out of Canada, we will continue to have low corn earworm pressure. If you are still early in harvest and have had bird issues in the past, remember to get your bird deterrents out early. For a rundown of bird deterrents, see the end of this earlier vegetable update.

Crops that perform well at the edges of tunnels

We’ve been noticing a lot of high tunnels where the plants at the ends look great, while the crops in the middle are struggling. While the causes may be different at each farm, here are a few questions to ask if you’re seeing this: 

  • Is your drip line leaking at the connection point between your drip tape and the main line? If so, plants nearby this point may be getting extra fertilizer and water. 

  • This phenomenon may also be temperature related since plants near open sides have more airflow. How is the airflow in the center of the tunnel? Adding fans may help to improve airflow, which helps with both heat stress and disease management. 

  • Is there water flow from outside the tunnel into the outer edges? This extra water could help to leach salts from the soil and keep the pH under control. 

  • Are there distinct pockets where things look worse? If so, it may be worth investigating the roots at the end of the season to check for root rot pathogens. 

Vegetables struggle with heat too!

This has been a tough week for field work, with temperatures over 90 degrees fahrenheit and high humidity. Plants struggle with this weather as much as we do, and so we can expect some issues with flowering and fruit set over the next couple of weeks based on the heat we’re experiencing more. This is due to a few factors: 

  • Hot day time and night time temperatures cause flowers to drop

  • Cucurbits can develop more male flowers than female flowers when temperatures are high

  • Pollinators are less active in high temperatures, so flowers are less likely to get pollinated. 

  • Tomatoes struggle to produce the pigments that cause them to ripen and turn red when temperatures are too high

For a more in-depth look at these issues, check out this article from last year. 

A cooler month ahead?

Every month the National Oceanic and Atmospheric Administration (NOAA) releases 30 day forecast maps for temperature and precipitation across the US. The map for next month is showing cooler than average temperatures for most of Minnesota, while most of the country is likely to experience warmer than average temperatures

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