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Weekly vegetable update – July 3, 2024

Authors: Marissa Schuh and Natalie Hoidal

Just as things get dried out, we get another flush of rain.  There are a lot of plant diseases and insects around, but farms are still sending all kinds of vegetables to market. Read on for updates on fertility, protecting pollinators as crops flower, and organic fungicide recommendations.

NOAA has released its outlook for July, and is calling for normal temperatures and above average precipitation.
Source: NOAA

This likely means disease pressure will continue – many diseases thrive not only in wet conditions, but temperatures in the 70s and 80s are in the ideal range for many pathogens.

Plant Disease
As disease pressure will continue to be high, it might be time to lean on chemical tools. Fungicides and bactericides are best used preventatively – for pathogens where thresholds are available, the thresholds are often “trace,” meaning that you start treatment as soon as you see any sign of disease. When you see your first leaf spots, or hear a farmer neighbor talk about a sick vegetable, is the best time to start using products.

For organic growers, the Midwest Vegetable Production Guide has been updated to include more organic fungicides based on what has performed well in university trials. If you haven’t checked this tool out recently, this is a good year to see which products have performed well enough to get into the guide.

Check in on Fertility
Early July is a great time to do some foliar testing on your farm to refine your fertility plan for the rest of the season. For most crops, this should be done when plants are entering the reproductive phase (flowering and starting to fruit). Foliar testing can be especially valuable for high value crops in high tunnels. Read more about how to take a foliar test and how to interpret results here:

Crop Updates
Cole crops: Spring planted kale, radish, turnip, and bok choy harvest is ongoing, and many farms started cabbage harvest over the last week.  All the usual insect suspects are around.

Potatoes: The rainy weather is a blessing and a curse this time of year for potatoes.
Pros: July is the most important time for water in potatoes, as tubers are sizing up. A wet July will hopefully lead to evenly sized, larger potatoes compared to the last few drought-stricken years. The wet soils will also help promote overall soil biology, which can help other microorganisms outcompete the bacteria that causes potato scab.
Cons: It will continue to be wet, promoting diseases like early blight. This pathogen is easily managed with conventional fungicides (though some parts of the Midwest are seeing Group 11 fungicide resistance). No organic products do a great job of managing this disease, instead we have to lean on cultural controls like airflow, watering timing, and variety selection (early maturing varieties tend to get this disease the worst).

Onions are starting to size up their bulbs. Thrips are starting to show up on some farms – scout your farm to see if they are at a level worth treating on your farm. Thresholds are between 1 and 3 thrips per leaf. If the onions on your farm are in the period of intense bulb development/sizing up, use the lower threshold. Products for onion thrips can be found in the Midwest Vegetable Production Guide.

Root rot stressed pea plants make the lower spots in the field more obvious. Photo: Charlie Rohwer, UMN.
Peas are struggling in some spots – root rots are common with all the wet weather, and it is stressful for peas with root rot to fill their pods.

Tomatoes: Hoop harvest tomatoes continue to be harvested, some early maturing tomato varieties are starting to be harvested in the field.

Still seeing blossom end rot despite the moisture? It isn’t likely that you are low on calcium. In a recent study of soils on 100 farms, not a single farm had a calcium deficiency; most had far more calcium than is needed. Things to do if you are seeing blossom end rot? Note that varieties that have this problem, as genetics play a huge issue in the disorder. Remove any affected fruits so plants don’t waste their energy on them.

Leaf spots continue to show up, refresh yourself with this article from last week.

Vine crops: As more and more vine crop plantings start to flower, remember pollinator protection going forward. Cucurbits rely on bumblebees and squash bees for pollination and fruit set, and some growers may also need to think about local honeybee hives.  Research has shown insecticides and fungicides can have a wide range of negative impacts on bees.  You may be able to make fewer applications as cucumber beetles move to hang out in flowers where they aren't worth treating, and it is still early for powdery mildew applications.  If  you do need to spray, do it later in the day, when flowers close and bees are less active.
Squash bees and cucumber beetle together in a pumpkin flower. Photo: Jim Jasinski, Ohio State University Extension,
Cucumber bacterial wilt is popping up in some plantings. If you have cucumbers that look fine one day and are totally wilted a few days later, this disease is the likely culprit. If you are unlucky, even low levels of cucumber beetle feeding can move the disease into your plants. To test to see if what you have is bacterial wilt, use a knife to cut straight across the stem of an infected plant, then rub the two halves of the cut stem together and pull them apart slowly. Plants with bacterial wilt will have white, stringy goo stretching between the two halves (similar to a cheese pull, if you've ever fallen into food TikTok). What you are seeing here are seeing the bacteria that were circulating in your plant's tissue. Remove affected plants promptly to reduce the chance that cucumber beetles pick up and move the disease to more plants.

Plants affected by bacterial wilt can be somewhat randomly placed in a field, reflecting the spots where infected cucumber beetles have fed. Photo: Edward Sikora, Auburn University,
Summer squash and zucchini tend to develop female flowers before they develop male flowers, and so early in the season we often see some small fruit that never got fertilized. These zucchini and squash tend to stay quite small and often have a funny curved shape. This is fine for growers with CSAs and farmers markets who want to get early veggies to their customers, and whose customers tend to be more forgiving than wholesale markets. Harvest these fruit early; since they remain small, we often leave them on the vine too long thinking they'll grow, and they become susceptible to soft rots.

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