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Weekly vegetable update 6/28/2023

Authors: Natalie Hoidal, Shane Bugeja, Marissa Schuh

Most growers got some much needed rain this week, but continued air quality issues has made field work challenging. Due a variety of challenges growers have faced this spring, this update includes some notes about pivoting late in the season, including crops that can still be planted if succession plans have gone awry, and marketing opportunities with schools if buyers have backed out or changed their terms. There are also a couple of new (for this season) pests to add to your radar including aster leafhoppers, Japanese beetles, and birds.

Crop updates

Cucurbits: Melon and early planted winter squash are starting to vine out, and early zucchini and summer squash plants are starting to produce. Squash vine borer reports continue to roll in. We’ve heard some reports of poor pollination, and some confusion about flowering and pollination in these crops. Row covers are an excellent strategy for keeping insects out of cucurbit plantings, but they should be removed as soon as plants begin to flower since plants in this family need to be pollinated by insects (with the exception of parthenocarpic cucumbers). Since pollinators are so important for these crops, take extra care when applying any pesticides during flowering.

Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo,

We’re seeing a lot of curling leaves lately. Is it heat or herbicide drift? Leaf curl from heat stress is common, and tends to look sort of like a hot-dog bun: a clean folding-in of the leaves. Leaves curling from herbicide drift tend to make more of a cupped shape, or they can be puckered. The photos below show examples of both. In general, most growers are seeing some leaf curl right now associated with heat stress, so that’s the most likely culprit of curling leaves. If you suspect drift, look for signs on neighboring plants, as your other vegetables and nearby vegetation should be showing signs too.

Tomatoes just starting to curl from 2,4-D exposure in Waseca. Photo: Charlie Rohwer 

Tomatoes with leaf curl from heat stress. Photo: Natalie Hoidal

Many potatoes are flowering this week, and potato beetle larvae are developing. This is a great time to mulch your potato fields. It’s late enough in the season that you’ve been able to cultivate a few generations of spring weeds, and so straw mulch should be fairly effective at keeping weeds down for the rest of the season. This period of development is when the tubers start to bulk, and it’s critical to maintain consistent soil moisture. Mulch will also help with this. Soil moisture can also make it harder for the pathogen that causes scab to infect potatoes. Finally, mulch is helpful at this stage in the season because Colorado Potato Beetle larvae will soon be pupating in the soil, and the mulch provides a physical barrier between plants and the soil, making it harder for beetles to reach the soil and complete their lifecycles.

Carrots and beets: Spring carrots are being harvested while fall carrots are being planted. We’ve been hearing from multiple farmers that they are losing access to wholesale markets for their carrots or are being priced out of these markets. During the winter, some of the folks on our team conducted interviews with school food service directors, and many of them were excited about buying root vegetables. While many schools already have contracts in place for the coming school year, many of the food service directors we talked to were excited to connect with new growers and said it’s always worth reaching out if you have extra produce, especially if you’re planning for a fall crop. If you’ve lost your primary carrot and beet buyers, it may be worth reaching out to local school districts to gauge interest.

Aster leafhoppers (which transmit aster yellows) are now present in Minnesota, and it’s important to scout for them if carrots are an important crop for you. Leafhoppers are quick moving, so sweep nets are the best way to estimate the size of aster leafhopper populations. Use a 15-inch diameter sweep net in an 180 arc through the top six to eight inches of the crop 20 times in five different locations in the field (100 sweeps) and counting the aster leafhoppers captured. Aster yellows susceptible carrot varieties have a threshold of 20 aster leafhoppers per 100 sweeps. There are no thresholds available for garlic or other impacted crops. Read more about aster leafhoppers here.

Sweeping carrot foliage. Diagram by Marissa Schuh using BioRender.

Garlic and onions: We’ve seen some crispy / yellow onions and garlic lately. This is likely a response to heat and drought. For organic growers who rely on nitrogen sources that are tied up in organic matter and need to be broken down by microbes, prolonged heat and drought can also create nitrogen deficiencies since plants can’t access the nitrogen that’s present in manure, compost, etc. until the microbes make it available, and they tend to be less active in dry soils. See the above note on aster leafhopper and aster yellows.


Filling mid-season production gaps

Drought conditions have caused extremely delayed emergence of some crops, and crop failures for others. Crop failure and delayed germination can alter succession plans, and some growers are asking what they can plant at this point to fill in the gaps. Here are a few things that can be planted now for late summer or fall harvest:

  • Brassicas that form heads like cabbage or broccoli can be planted now for early fall harvest, and as late as July 25 or so in the southern part of the state for a late fall harvest. Brassicas that don’t form heads will produce a crop more quickly (e.g. mustard greens, arugula), and need only 30-40 days to mature. Make sure to plant heat tolerant varieties, or those advertised for summer planting.
  • Carrots and beets mature in 50-70 days. These are great crops for midsummer planting and fall harvest. Carrots and beets are hard to germinate, and can be even trickier to germinate in hot, dry conditions, so make sure you’re irrigating very regularly until they are established.
  • Green beans mature in 45-65 days, depending on the variety. Make sure to choose a heat tolerant variety.
  • Swiss chard tends to be relatively heat tolerant, and can produce a crop in 40-60 days.
Spotty pumpkin germination in Waseca. Photo: Charlie Rohwer

At the U of M research and outreach station in Waseca, pumpkin germination was very uneven, leaving significant gaps in the field. Following the rains this weekend, many of the seeds still in the soil germinated. Seeds that germinate at the end of June will likely not be able to produce full pumpkins before the first hard frost this fall. If you’re in a similar situation, check your seed packets to figure out the days to maturity, as well as the DNR’s first / last frost date map to consider whether it’s worth letting them grow or whether it would be best to hoe / till them into the soil and plant something else in their place. 


Japanese Beetle in Vegetables

Japanese beetle has started emerging in the southern half of the state. Japanese beetles are leaf- and flower-feeders, but the damage is most often cosmetic (unless, of course, they are eating the leaves we harvest). Crops they are most likely to end up causing problems in are asparagus, basil, sweet corn, and okra.

When thinking about managing them...

  • Depending on the size of planting, what the plant is, and how valuable it is, hand-removing beetles when you first start to see them can help delay damage. Japanese beetles release pheromones that cause other beetles to come to them, so removing the first few can push back when they aggregate.
  • Young plants/plants that don't need pollination can be covered with row covers to keep beetles off.
  • As for sprays, there is some research on Bt galleria saying that the foliar formulation can reduce adult feeding. Look for the liquid formulation. The granular formulation is for the immature grubs (which live underground) and hasn’t worked in research trials. Like all Bts, it needs to be consumed to be effective, breaks down in the sun, and washes off with water, so re-application may be needed.
  • Pyganic has Japanese Beetle on the label.
  • Neem oil can deter Japanese beetles, but won’t kill them outright. Will work better before a huge number of beetles aggregate.
  • For specific crop pesticide recommendations, check out the Midwest Fruit Guide and Vegetable Guides.

Deterring birds in sweet corn

While sweet corn harvest is still a month away, it’s important to start thinking about how you’ll keep birds away from developing ears of corn before they start to develop. The most at risk fields will be those nears marshy areas, where red wing blackbirds nest. If you’ve been dive bombed by birds on your farm, you probably have some red-winged blackbirds around.

A red winged blackbird. Image: Joy Viola, Northeastern University,

There are a lot of deterrents available – from owl eyes balloons to reflective tapes to air dancers and spray-applied repellents. Key in getting all of them to work is getting the deterrent in place before the birds discover the corn ears. Once birds know how good your maturing sweet corn is, they aren’t going to leave. Birds most often seem to find corn approximately 3 days before harvest, so have a deterrent (or better yet, multiple deterrents that use different senses) in place before this point.

What deterrents work best? Research in New York found that Avian Control (spray product that smells like artificial grapes), owl eye balloons, air dancers, and detasseling to reduce bird damage (with detasseling being the most effective of the bunch). There is some interesting work in Rhode Island using homemade lasers to deter bird feeding. If you don’t have neighbors nearby, there are auditory scares like propane cannons (for those in truly rural spots) or bird distress calls (if neighbors are a little closer). Use a couple of scares, and rotate them around so that birds don’t become accustomed to their presence.

No silver bullet here, but now is a good time to make sure you have some scare tactics either in the barn or on order so that they can be deployed as sweet corn matures.

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