Weekly vegetable update 7/5/2023

Authors: Natalie Hoidal, Shane Bugeja, Marissa Schuh, Anthony Adams, Claire LaCanne

With clearer skies and cooler nights, this week is providing some respite from a hot and smoky summer in Minnesota. However, temperatures across the globe are hitting record highs. This week's update includes an overview of fruiting problems in cucumbers, nutrient deficiency symptoms in tomatoes, causes for funky looking peppers, reasons your sweet corn might be lodging (falling over), a discussion of June weather, and some tips for dealing with blister beetles.

Crop updates

Cucurbits

There are two main issues to look for this week in cucurbits: blossom end rot (mostly on zucchini and summer squash), and fruit abortion. If your fruit is developing with rot on the blossom end, it could be Choanephora rot (which is easily distinguishable due to its fuzzy appearance) or blossom end rot. Fluctuating water levels are the main culprit for blossom end rot, so trying to provide more consistent irrigation may help. Remove any fruit with blossom end rot symptoms, as they will not grow out of it, and will take resources away from healthy fruit.

We’ve also had multiple reports of fruit abortion in cucumbers (and it seems that growers from across the Great Lakes are having similar issues). We often attribute fruit abortion to problems with pollination. There are a variety of reasons pollination doesn’t go well in cucurbits, but heat tends to be one of the more important factors. Air quality issues may also impact pollinator behavior.

That said, many growers use parthenocarpic varieties, which do not require pollination to produce cucumbers. We’ve been hearing about abnormally high fruit abortion rates in parthenocarpic cucumbers in addition to regular cucumbers. If parthenocarpic cucumbers are planted alongside regular cucumbers, they can still be pollinated normally, which could lead to improperly pollinated fruit. However, if they are planted without any other varieties in a controlled environment, we need to look for other causes of fruit abortion:

• Cucumbers will self-thin if they produce more flowers and fruit than the plant can support. This is common early in the season.
• Very high temperatures can cause fruit to drop. Keeping your high tunnel properly ventilated with gable vents, fans, and open sides will help with this.
• Insufficient irrigation could be a cause: we recommend using soil moisture sensors in high tunnels since soil moisture can be rapidly used up in tunnel environments.

 

Cucumber fruit abortion on a parthenocarpic variety. Photo: Anna Racer

Potatoes

The rain some parts of the state received will benefit growing tubers. Some spots have seen an unusually high number of volunteer potatoes. The number of volunteer potatoes you have will be impacted by the varieties you grow (short season varieties tend to have fewer volunteers), how you harvest them (leaving potatoes buried deeply=volunteer potatoes), and what type of winter you had in your area. If potatoes are left buried deeply in the soil at the end of the season, they will be protected from freezing winter temperatures and be able to come back next year as volunteers. Potatoes on the surface and in the top one or two inches of soil are likely to be killed by winter temperatures, but those buried more deeply, especially in dry soil, have a better chance of surviving. Research suggests that volunteer potatoes are killed when dry soil reaches a temperature of 25°F degrees. The early, prolonged snow cover last winter may have given some left behind, buried potatoes the extra insulation they needed to come back this year as volunteers.

Why are volunteer potatoes a problem? They act as weeds, using resources the crop needs, but more importantly, they can be a reservoir of diseases, including viruses and late blight. There haven’t been any reports of late blight in the US this year so far, but with late blight reportedly being widespread in northeast Minnesota last year, volunteer potatoes should be managed if possible.

Brassicas

Fall plantings of Brassica crops are underway, and we’re getting some questions about the need to rotate away from spring Brassica areas. A lot of this comes down to your farm’s rotation and if your spring crops were healthy. Did you see diseases like black rot and alternaria? Place your fall brassicas somewhere else. Did you have lots of flea beetle pressure? This is a little trickier. Historically, flea beetle pressure has wrapped up by now, but increasingly growers are reporting more intense flea beetle feeding later into the year. If you are planting brassicas in the same spot, row covers might not help if you suspect a second generation of flea beetles on your farm. This second generation may potentially pupating under the soil where your first planting was (the second generation in other states emerges in late July). Regardless of where you plant, make sure you factor that into your future rotation plants.

Tomatoes

This is the time of year when it’s worth taking stock of potential nutrient deficiencies in tomatoes. High tunnel tomatoes are fruiting, and some field tomatoes are beginning to set flowers and fruit. Taking care of deficiencies now will save you a lot of stress and potential yield losses later. A foliar test is one easy and effective way to make sure your plants are taking up the nutrients they need. While fall or spring soil tests are the best option for making sure your plants have enough fertility available to them in the soil, a summer foliar test will tell you whether your plants are actually able to access those nutrients. Read more about foliar testing here.

Potassium and magnesium deficiencies are both common in high tunnel tomatoes, and they look relatively similar. Both are characterized by interveinal chlorosis (yellowing of the leaf tissue between veins). If there is a potassium deficiency, the leaf margins tend to be more tan / scorched looking and they tend to cup downward. The symptoms first appear on “recently matured leaves”, then progress towards the older leaves, and show up last on young leaves. If there is a magnesium deficiency, symptoms first show up in younger leaves, and the leaves tend to curl upward.

Potassium deficiency. Photo taken from the American Phytopathological Society’s Compendium of Tomato Diseases and Pests (2nd edition) , photo courtesy of J. Arboleya.

Magnesium deficiency. Photo taken from the American Phytopathological Society’s Compendium of Tomato Diseases and Pests (2nd edition) , photo courtesy of G.J. Hochmuth.

Sweet Corn

Lodging sweet corn during late June and July can have multiple suspects. Quick, intense summer storms often expose issues as the wind stresses the stalk of the plant. While there may be cultivar differences and/or bad timing in regards to crop stage (aka “greensnap”)--- insects should also be investigated. If your field of sweet corn is larger, make sure to actually venture out into the middle of the plot. In other words, avoid doing a “60 mile per hour scout” where you examine just the borders of the field.

If sweet corn has been planted multiple years in a row—or is in only a two year rotation—corn rootworm larvae could be responsible for lodging. The species most responsible for damage are northern and western corn rootworms. Carefully pull up any downed or “goosenecked” plants and examine the roots for any pruning injury. In their adult form, corn rootworm beetles can also cause silk clipping injury. We have a great resource that can walk you through adult beetle scouting on our website. Right around silking is a good time to scout if you find yourself in a high risk continuous corn or short rotation situation. Adult corn rootworms usually do not stray far from where they emerge, and can give you a decent idea about how much larvae were in your field and help you plan for the future.

European corn borers are another suspect if your sweet corn is looking terrible this year. Larva avoid roots but everything else is on the menu. Leaf damage consists of ragged “shot holes”. As the critters become larger, they bore into stalks and corn ears which can cause them to fall over and fall off, respectively. Unlike corn rootworm, European corn borer has been extremely susceptible to most Bt proteins, and 2023 adult moth flights have still been pretty low by historical standards.

One resource that could be helpful for sweet corn growers is the Sweet Corn Pest

Identification and Management Pocket Guide. This handbook is published by Purdue Extension but has the University of Minnesota and other nearby university extension services as co-authors. Both corn rootworm species and European corn borer are listed in the publication.

Peppers

Pepper fruit continue to form. Cooler weather will help fewer blossoms drop. Most pepper questions have revolved around odd coloration on new growth – is it a virus, herbicide drift, or just peppers being peppers? All three can cause off-color and shrunken new growth.

Pepper plants with discolored, slightly distorted leaves due to infection with tomato spotted wilt virus. Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

 

Pepper leaves infected with cucumber mosaic virus with discoloration and circles on leaves. Photo: Anette Phibbs, WI Department of Agriculture, Trade & Consumer Protection, Bugwood.org

Viruses are moved around by insects. Different viruses cause weird things to happen with different , but things unique to viruses is yellow or green circles on the plant leaves. These will be in the color of the leaf, so lesions or hole needed. I almost think these look like crop circles in the leaves or even on the fruit. The can also cause tye-die like discoloration on leaves and fruit.

Herbicide drift this time of year is most likely to be caused by glyphosate and 2,4-D (the window for dicamba applications has passed). Both are systemic herbicide and will work through the whole plant. Depending on the amount of product that the plants are exposed to, plants may be killed, maturity may be delayed, and yields may be lower. 

Glyphosate will cause yellowing on the bottom half of leaves. Notice the yellow from the base of the leaf outward on the youngest leaves, caused by glyphosate injury:

Photo: Paul Bachi, University of Kentucky Research and Education Center, Bugwood.org

 

2, 4-D affects the ways the plant grows, so look for cupping, twisted stems, and leaves that become misshapen and twisted:

2, 4-D causing distortion of all parts of the plant. Photo: Annie Klodd.

Peppers are just a little dramatic. Every year we deal with these questions, with some growers asking us about plants with these symptoms, don’t end up sending in a sample or contacting the MDA, and in a few weeks have productive, beautiful peppers.

How to sift through this?

• Check the distribution in the field.
• While herbicide can drift somewhat unpredictable, the damage they cause is most likely to be concentrated along a field edge bordering field crops.
• Virus infected plants are more likely to be sporadic throughout the field. If plants were infected as transplant, the pattern may reflect greenhouse trays. Look for sporadically stunted plants.
• Check out other plants. If tomatoes are nearby, take a peak at those, as tomatoes are probably the most susceptible vegetable to herbicide drift. You can also look at trees and weeds in the area, as they can also show signs of herbicide injury.
• Check for aphids. Aphids vector a lot of our plant problems. Even if the aphids themselves have been treated or moved on, you may still see the shiny, sticky honey dew and shed exoskeletons they leave behind.
• Send a sample into the UMN disease clinic if you suspect a virus
• If you suspect herbicide drift, freeze affected plants as soon as possible to preserve them, and contact the MDA.

How does the weather this summer compare to other summers? 

Every month Mark Seeley posts a climate summary for the previous month on the Weather Talk blog. Here's a summary of the last two posts: June 2023 was the fourth warmest June on record for Minnesota; only 2021, 1988, and 1933 had warmer June weather. Last month was also one of the driest Junes on record, and the hot, dry weather is expected to persist. 

While the 4th of July weather was fairly pleasant around Minnesota, many news sources are now reporting it as Earth's hottest day on record across the globe. What does this mean for farmers? It will continue to be critical to provide heat stress relief for yourselves and your employees. Here's a recap of some tips we posted a few weeks ago: The following tips were taken from the NIOSH/CDC "Protect your workers from heat stress" infographic. 

•  Follow best practices for acclimatization for any new employees or volunteers: workers should gradually increase the time spent in hot conditions over a 7-14 day period. New workers should start at 20% of their time in hot weather, with a 20% increase each day.
  
• Check in with each other. This could be a buddy system, or just regular check-ins with employees. Often new employees are unsure about asking to take breaks, or embarrassed to admit that they need one. Do your best to make employees feel comfortable asking for breaks when needed, especially at the beginning of the season.
  
• Encourage workers to wear clothing that is breathable, light-colored, and loose-fitting. Soak cotton bandanas or other clothing in water if needed to stay cool. 
• Make sure everyone is drinking 1 cup of water every 15-20 minutes. 
  

Blister beetles

We are hearing reports of blister beetles in some vegetable crops, and some damage can be caused by adult blister beetles. Blister beetles contain a defensive chemical (cantharidin) that may be harmful to humans and other animals.

Blister beetles. Photo: Doug Danforth

In their immature stage, blister beetles can provide some biological control of several insects, including grasshoppers. Immature blister beetles, called larvae, feed on grasshopper eggs which is why we sometimes see high numbers of blister beetles in dry years, when there are larger populations of grasshoppers. Adult blister beetles have chewing mouthparts and feed on a wide range of plants, including some weeds. Unfortunately, they may also feed on vegetable crops including tomatoes, corn, melon, peas, beans, cabbage, and beets, to name a few.

You can remove adult blister beetles by hand - while wearing gloves - and place them into a container of soapy water. Their defensive substance may cause blisters when in contact with skin, which is why wearing rubber gloves is recommended. Cloth gloves may not prevent the chemical from contacting your skin. If you have areas on the farm where you harvest hay, it is worth keeping an eye out for blister beetles, as the cantharidin is harmful to livestock.