Weekly vegetable update - July 9, 2025

Natalie Hoidal & Marissa Schuh

After a slow spring, daily highs in the 80’s are pushing warm season crops ahead and growers are catching up.

Mid-season nutritional deficiencies & what to do

With warm, long days and rapidly maturing crops, this is the time of year we start to notice nutritional deficiencies. There are two common issues we’re seeing right now, especially in high tunnels.

• Nitrogen: A nitrogen deficiency looks like yellow leaves on the oldest part of the plant. Plants can efficiently move nitrogen around, and so they tend to push all of their available nitrogen into the youngest leaves / the active growth. If your plants are nitrogen deficient, doing some foliar applications of products like fish emulsion or blood meal can give them a boost. 
  

• Magnesium: We often see high tunnel tomatoes showing magnesium deficiencies around early to mid July. Symptoms are: the leaf veins remain darker green, while the leaf tissue in between the veins turns lighter green to yellow, creating a blotchy appearance in the lower leaves. Just like nitrogen, magnesium is fairly mobile, and so the plant is able to move most of the magnesium to the new growth. We often see magnesium deficiency symptoms even when soil levels seem ok - if soil potassium levels are very high (as they often are on vegetable farms), the high potassium can actually interfere with magnesium uptake. If you’re seeing these symptoms, it’s worth taking a foliar test to confirm. In the meantime, you can begin to apply magnesium sulfate (epsom salts) as a drench through your drip irrigation system, or as a side dressing. 

 

Early Mg deficiency symptoms in tomato

 
As harvest ramps up, review storage guidelines to prevent spoilage

We saw some very rough looking local summer squash at a food co-op in Southern MN this week. It looked partly just a bit old, but also like it had some chilling injury. Cucurbits in particular are very sensitive to cold stress, but every year we also get reports of overly-chilled potatoes and a handful of other vegetables. After you’ve done all of the hard work of growing, tending, and harvesting your crops, storing them properly until you can sell them is the final hurdle. Take a few minutes to review our postharvest handling guide for an overview of ideal temperature and humidity ranges for vegetable crops. We also have an article all about designing a DIY walk-in cooler using a coolbot. 

 

Summer squash with dents and knicks on the surface from cold injury

Crop updates

Cucurbits

We’ve heard a lot of reports about summer squash and zucchini that aren’t producing well. They either have limited fruit set, or they are producing small, misshapen fruit that fall off. This can happen for a few reasons: zucchini and squash plant flowering is extremely delicate. Hot weather can cause an imbalance in the ratio of male:female flowers, rainy weather can cause pollinators to be less active, and early in the season we tend to see fewer male flowers, which can result in incomplete pollination. Whether or not to remove small, poorly pollinated fruit is really dependent on your labor availability. It is time consuming to remove misshapen fruit, but doing so allows the plant to focus its energy on producing viable fruit, and helps to prevent diseases. These small fruit often fall off and can rot, inviting soft rots into the field.

On the plus side, we spoke with a few growers this week who were grateful to be seeing very few striped cucumber beetles this year. This pest is observed to have population spikes every 7-ish years, so hopefully we have a few more years of low cucumber beetle pressure in store after a miserable 2023 with this pest. With more vine crops flowering, the few cucumber beetles that are around are hanging out in flowers and aren’t worth managing.

Broccoli & cauliflower

Growers have been harvesting field grown broccoli and cauliflower for a couple of weeks now. For the most part, broccoli has done quite well this season, but we’ve seen a bit of hollow stem here and there. Plants are more susceptible to hollow stem during periods of rapid growth, so this could simply be due to bouts of hot weather and rapid growth. However, it can also be a sign of too much nitrogen, or a boron deficiency. While hollow stem doesn’t necessarily impact marketability, it can reduce the shelf life of broccoli and cauliflower.

Hollow stem in broccoli

If you’re still looking for a good broccoli variety to plant in July and August, you can check out our variety trial results from a broccoli trial we did a couple of years ago with growers all over Minnesota. Here’s the quick summary for folks who don't want to read the full report:

• Belstar was the most disease resistant variety we trialed, and would be a good choice during growing seasons when high disease pressure is expected. Note that Belstar did not perform as well in the early trial due to brown bead, which is often associated with heat stress.
• Covina may also be a good choice for years when disease pressure is expected to be high.
• Asteroid, Imperial, Luna, Monty, Diplomat, Abrams and Eastern Crown are all varieties that growers liked. Of these, Eastern Crown, Diplomat, Imperial, and Abrams had the best disease tolerance in 2022. During seasons when disease pressure is expected to remain low, these varieties could all be good choices.
• How can you predict disease pressure? If your fields had a lot of broccoli disease last year, your risk of seeing more diseases this fall is fairly high.

Potato and eggplant 

Growers are seeing the hatching of another generation of Colorado potato beetle. The freshly hatched larvae are the easiest to treat with pesticides, and by getting rid of them early, you avoid them becoming the larger larvae that defoliate plants quickly.. The Midwest Vegetable Production Guide highlights management options, remember to use more than one product in your program to prevent resistance. For the squeamish, small larvae and eggs are easier to squish.

By catching Colorado potato beetle when they first hatch, you have a better chance of managing them successfully. Photo: Marissa Schuh, UMN Extension

Sweet corn

Sweet corn is starting to show up in markets, with much more slated for harvest in the next week. If your corn isn’t quite ready yet, don’t feel too behind. Believe it or not, a few Minnesota growers raise sweet corn in high tunnels in order to have the very first sweet corn at the market.

Tomatoes 

 

Tomatoes continue to flower and set fruit. Most high tunnel growers are now harvesting a few tomatoes, and field grown tomatoes are a couple weeks behind. Things to keep in mind at this point in the season…

• Most growers have heard the phrase "an inch of water per week" as a guideline for watering vegetables. However, an inch may not be enough for some vegetables, particularly during hot weather and fruiting. At this point in the season, every single tomato plant in a high tunnel should be getting around 70-75 oz of water each day. This helpful chart and webpage from University of Missouri has some helpful charts for figuring out how much water you're giving each plant based on your water pressure and plant spacing.
  

• Tomato foliar diseases are showing up – we are seeing early blight and septoria on many farms. Both of these diseases are facts of life when growing tomatoes. Mulches, staking, and pruning help reduce the leaf wetness that both diseases need to get started. If considering spraying, start at the first sign of disease. Fungicides cant and won’t do much for a plant that is already covered in spots and specks. The Midwest Vegetable Production Guide outlines options for conventional growers. Organic growers, things are a little less clear, as products perform very differently in university trials. Products like Cease, Stragus, Actinovate, LifeGard, Sonata, Serenade Opti, and copper can work. If using in a high tunnel, make sure the product is allowed for greenhouse use. Please reach out if you have questions about what products might work well in your situation.