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Weekly vegetable update 8/24/23

Authors: Natalie Hoidal and Marissa Schuh

We hope you’re all staying cool and taking plenty of popsicle breaks. This week, humans, pathogens, raccoons, and birds alike are enjoying all of the fruits and vegetables that August has to offer.

Crop updates

Watermelons: During a farm visit this morning we saw a field of healthy watermelon plants with no signs of disease on the leaves and a good crop of nearly ripe melons. However, there were some small melons rotting throughout the patch. We determined that we were seeing small fruit that had aborted due to issues with pollination - when it’s hot, pollinators are less active and pollen can become sticky and deformed, leading to fruit that don’t set correctly. These fruit are then susceptible to secondary / opportunistic fruit rot pathogens as they die and decompose. However, there are plenty of watermelon diseases that you might also be seeing right now - you’re always welcome to reach out with photos to get a second opinion.

We are also seeing very sporadic watermelons with round concentric lesions. Spots like this are either a mosaic virus or target spot. Target spot tends to pop up here and there at very low levels. Plant pathologists aren’t sure what causes this, probably some virus we don’t understand yet, but melons seem to harvest, store, and eat alright. Mosaic viruses will likely be more widespread and you'll likely see mottling on the leaves. Sending a sample to the plant disease clinic can help you verify what you're seeing.

Target spot appears as concentric circles on the rind. Photo: Marissa Schuh, UMN Extension

Asparagus: We’re not sure how widespread this is, but we’ve seen some patches of female asparagus plants in recently planted fields among varieties that are supposed to be all male hybrids. Female plants tend to be less productive, and since an asparagus planting can last 10-15 years, it may be worth taking some time to remove female plants and re-plant *if* it’s a fairly recent planting. For many growers, taking the time to do this will be impractical.

Female asparagus plants produce berries, which are easy to see this time of year. Photo: Natalie Hoidal

Sweet Corn: More and more plantings are being harvested. The heat dome we are under has ushered in some some corn earworm flights. Traps in Blue Earth (Faribault County) caught hundreds of moths in the last week, though traps in other parts of the state caught only a few moths.

Tomatoes: Harvest continues. As we see more morning dew with fall’s approach, we are starting to see an uptick in foliar diseases. Most common among these is early blight, a fact of life when growing tomatoes in Minnesota. Look for brown spots, look closely at the for concentric rings.

Zooming in on an early blight lesion to see the concentric rings. Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo,

Early blight can get into fruit, but is often primarily foliar. More common in fruit is anthracnose. This disease attacks primarily tomato fruit that are ripe, and especially the overripe ones. If you are seeing tomatoes with small, circular depressions, that eventually turn black, this is anthracnose. Early blight fruit rot typically starts from the stem and creeps down, while anthracnose is a distinctly round isolated spot. If you are seeing anthracnose, harvest your tomatoes earlier – remember that the ripening hormone in tomatoes (ethylene) will turn tomatoes picked at the blush stage to a true red. If what you are seeing is more in line with early blight, make a plan to better manage this disease next year. The UMN early blight page includes information on resistant cultivars, cultural controls, and fungicides.

Anthracnose on tomato. Clemson University - USDA Cooperative Extension Slide Series ,


We have gotten some heebie-jeebie inducing reports of high numbers of earwigs hiding out in the wrapper leaves of cabbage and at the base of sweet corn. The earwigs don’t always appear to be feeding on the crop – oftentimes the vegetable looks fine. Our working hypothesis is that the earwigs are seeking the dark, cool, damp places they like to hang out during the day. Earwigs are nocturnal, so the vegetable might just be their daytime hiding place. If you want to make your growing space less attractive to earwigs, removing weeds can be a big help – they don’t like wide open spaces, so even the removal of weeds might draw them out of your main production area. Variety selection may also play a role. Consider napa cabbage vs. a more standard round cabbage head with tightly spaced wrapper leaves: the loose head of the napa cabbage makes it far easier for insects like earwigs to crawl inside. If you’re seeing high earwig pressure (or insect pressure in general, Napa is always the first to get taken out by bugs), consider switching to another variety next summer that may provide less ideal habitat to these insects.

Vertebrate pest damage

Our customers aren’t the only ones enjoying the fruits of a season of labor – deer, rodents, raccoons, and birds are happy to join in. Familiarizing yourself with the damage can help you figure out what type of deterrents or exclusion could be put in place. Also note that there are food safety implications to animal damage – it is best practice to leave a harvest buffer around areas with signs of animal activity.

Raccoon damage in watermelon. Photo: Whitney Cranshaw, Colorado State University,

Racoons are most likely to damage watermelon and sweet corn. In watermelon, they make one hole and use their hands to dig out the flesh. In sweet corn, they yank the ear down towards the ground, strip open the husk, and eat kernels.

Rodents can also feed on fruit. Chipmunks are a common tomato feeder in urban and suburban areas. Groundhogs also feed on a variety of crops.

Crows peck one hole and dig shallowly in the melon. Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo,

Crows are most likely to feed on watermelons and other fruit like berries and cherries. Crows tend to make one hole and feed, then hop over to another melon. Maybe the rudest animal because of how many melons one crow will damage.

Deer are maybe the most devastating vertebrate, as they will feed on lots of crops across many life stages. This time of year, deer will feed on all kinds of stuff. You can tell their feeding apart from other animals because they are kind of messy eaters – because of the way their teeth are laid out, their feeding lacks the distinct, sharp edges around spots where they’ve fed. Their feeding usually involves some tearing because they can’t clip stems and leaves the way rodents can. In sweet corn, deer scrape kernels of ears with their lower teeth. They will eat cucurbits, sometimes making a big mess as they dig in to feed on the seeds.

Note that just because you have seen deer in the field doesn’t mean they are the primary damage doer, confirm by taking a look at the damage you are seeing.

Deer feeding on an immature butternut squash. Photo: Marissa Schuh, UMN Extension.

Algae in soil

One of the themes of the summer has been working with farmers who are struggling with soil management after adding a lot of compost to fields and high tunnels. Some of the issues we’ve discussed already include high salt concentrations in compost and the hydrophobic (water-repelling) nature of compost when it gets dry. It may seem counterintuitive that compost can both be too dry and too wet, but another challenge we’re hearing about is algae growing on the surface of beds.

Essentially, compost is really really good at holding onto water until it dries out. Once it dries, it can be difficult to re-wet since water tends to run right through it. Once it does become wet again, it does a great job of absorbing water.

What we often see in deep compost mulch systems is that a thick layer of compost is added on top of the soil without being incorporated. During hot, dry weather, the compost on the surface can dry out, even if the soil below it is holding on to water, and so farmers have to water really really thoroughly to re-wet the compost at the surface, especially early in the lifecycle of a crop when the roots are small and close to the surface. It’s hard to find balance in this system, and we sometimes see compost layers swinging between extremes of too dry and too wet. If the soil surface is too wet and there are a lot of nutrients present, particularly phosphorus, we sometimes see algae building up on the surface. What can you do in this scenario?
  • Test your soil and make sure your nutrient levels are in the ideal range. If your phosphorus levels are too high, take a break from inputs that contain phosphorus.
  • Try to work your compost into the soil. While compost is often added to no-till farms for soil health purposes, algal growth and the cycle of too wet / too dry compost are signs that it may be worth doing a few tillage passes to work the compost into the soil below it

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