Authors: Natalie Hoidal, Marissa Schuh and Annalisa Hultberg
More oppressive heat is on the way for much of the state in the next week. Some farms are in severe drought, while others are under inches of water (Austin MN received 11 inches of rain in one week!). The heat, humidity, storms, and rainfall have created perfect conditions for plant diseases (and yet another flush of weeds). The conditions next week will also be dangerous for farmers: take heat safety and hydration seriously for both your short-term and long-term health.
A few basic tips (we discussed these at our postharvest field day yesterday with Ariel Pressman, former vegetable farmer and Extension team member):
Most vegetable crops will not continue to ripen after harvest, and need to be harvested when they reach ideal conditions for eating, then quickly cooled. Examples include: blackberry, raspberry, cherry, cucumber, eggplant, grape, pepper (most varieties), pumpkin, squash, strawberry, watermelon, and zucchini.
A few vegetable crops will continue to ripen after harvest. This means you can harvest them before they are completely ripe, then store them to let them ripen quickly. Examples include: tomato, apples, cantaloupe*, honeydew*, and plum. (Cantaloupe & honeydew generally will ripen after harvest, but some varieties do not).
More oppressive heat is on the way for much of the state in the next week. Some farms are in severe drought, while others are under inches of water (Austin MN received 11 inches of rain in one week!). The heat, humidity, storms, and rainfall have created perfect conditions for plant diseases (and yet another flush of weeds). The conditions next week will also be dangerous for farmers: take heat safety and hydration seriously for both your short-term and long-term health.
Extending produce shelf-life
After working hard to grow crops all summer, going the extra mile to handle and store them properly at harvest ensures that your hard work pays off. We have a webpage with most major vegetable crops in Minnesota, whether they are typically washed, how they are typically cooled, their ideal storage conditions, and how long they are expected to last with proper storage.A few basic tips (we discussed these at our postharvest field day yesterday with Ariel Pressman, former vegetable farmer and Extension team member):
- Harvest as early as you can in the morning. Hot plants do not store well, and as the day progresses, plants accumulate field heat.
- Get harvested crops into the pack shed as quickly as possible. If you’re harvesting lettuce, don’t wait until it’s all harvested to bring it back to a cool space. Try to make trips back to the pack shed every 30 minutes at least to get your harvested produce washed or dunked (if needed - some crops don’t need water cooling), and into a cooler.
- Any amount of cooling is better than nothing - don’t let perfect be the enemy of better.
What can you harvest early, and what should you not?
There are two important categories of crops when it comes to deciding when to harvest: those that ripen after harvesting, and those that don’t.Most vegetable crops will not continue to ripen after harvest, and need to be harvested when they reach ideal conditions for eating, then quickly cooled. Examples include: blackberry, raspberry, cherry, cucumber, eggplant, grape, pepper (most varieties), pumpkin, squash, strawberry, watermelon, and zucchini.
A few vegetable crops will continue to ripen after harvest. This means you can harvest them before they are completely ripe, then store them to let them ripen quickly. Examples include: tomato, apples, cantaloupe*, honeydew*, and plum. (Cantaloupe & honeydew generally will ripen after harvest, but some varieties do not).
Why would you want to harvest before produce is fully ripe?
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Cucumbers: Two-spotted spider mite numbers are booming in some high tunnels. While there are predatory mites and other predatory insects who can make a dent in their numbers, if you are seeing leaves with lots of yellow and white flecks on plants, check the underside of the leaves. A hand lens is majorly helpful, as spider mites are a fraction of a millimeter large. High populations will often produce the webbing on the underside of the leaf. There are OMRI listed products that are relatively selective (which will help preserve natural enemies). These include insecticidal soaps such as M-Pede, and horticultural oils (such as SuffoOil-X). These same products also work on aphids, who can often be found alongside spider mites.
Peas have had excellent yields and growth in Southern, MN. The pea stands we’ve seen withstood the last heat wave despite our warnings. We’ll be surprised if they fare well through this next year wave, which is set to be hotter, and longer.
Beans: are ripening just in time to replace peas for the category of crunch, fresh green vegetables in your CSA boxes / farmers market stand, etc. Beans are already labor intensive, with daily harvest required. With the heat wave, your beans will likely be producing even faster than usual.
Sweet corn: Very early sweet corn in the southern portion of the state is almost ready for harvest, and it looks like parts of the state got a flush of corn earworm in late June. While corn earworm are a regular pest in late summer sweet corn, it isn’t every June that we get conducive weather for corn earworm flights from the Southern United States. Trapping on your own farm is the best way to pick up these flights and time protective insecticide applications for fields that are silking.
Corn earworm damage also attracts sap beetles, who are attracted to the smell of rotting fruits and vegetables. You will also see them show up when raccoons or birds get into ears.
Another pest management task this time of year for sweet corn is to prep your bird scares. If you have a history of bird damage or are growing near marshy areas, it is important to get bird scares out before birds start feeding in the field. Research in New York has found that detassling, air dancers, scare eye balloons, and taste-basted deterrent sprays can all work. If using air dancers or balloons, make sure to move the scare around every week or two, as birds learn. Research in New York and Rhode Island has found that laser scarecrows can work if set up correctly, and highlights a grower survey where farmers report getting the best bird control when using multiple tactics.
Tomatoes: The hot overnight temperatures that are on deck in the next week will likely lead to flower abortions, as well as slow the ripening of fruit. We have also gotten questions about tomato leaf roll. In some varieties, exposure to hot weather causes the older leaves to curl in on themselves and become leathery and dull. This can happen relatively quickly, and can be alarming, but doesn’t impact yield.
Diseases continue to progress in tomatoes.
This is also a time when we start to really see fusarium crown/root rot and Verticillium wilts show up. Plants are growing really quickly and producing fruit, and as they use more water, we really start to see wilt symptoms show up when these pathogens are present.
We are always available as a sounding board for any suspected disease issues; the Plant Disease Clinic is also a place to go for a solid diagnosis. This is especially important for plants with symptoms like wilting, rotting roots, and discolored vascular tissue, as diseases with these types of symptoms often need intensive management, so getting an accurate diagnosis is critical.
In the high tunnel, the main issue of discolored lower leaves we see this time of year is magnesium deficiency. This doesn’t necessarily mean there’s not enough magnesium in the soil. Sometimes it means that your plants are struggling to access that magnesium due to excess calcium or potassium. If you’re seeing magnesium deficiency, you can correct for this mid-season with 2-3 foliar sprays equaling 2-4 lbs Mg per acre (this is equivalent to around 20-40 lbs of Epsom salts per acre). This advice comes from the Nutrient Management Guide for Commercial Fruit and Vegetable Crops in Minnesota.
Vine crops: Vine crops have big leaves, it is pretty common for these to get droopy in the heat of the day. Healthy plants should recover overnight, but if you have wilted plants with drooping leaves (a grower recently described this to me as a “melting umbrella” look) in the morning, check in on these plants. For cucumber and melons, bacterial wilt is the most likely culprit; for squash and melons, squash vine borer is frequently to blame.
- If you have more tomatoes than you can sell, harvesting tomatoes before they are fully ripe and storing them in cool temperatures allows you to hold onto your tomato crop and sell more of your tomatoes later.
- For some buyers, it may be a week or more between when you harvest and when the vegetable is eaten. Wholesale buyers will buy produce, store it for a few days, ship to a customer, who may store it for even more days, and then it may be served or sold. It must remain sellable throughout this process and must not be overripe. Harvesting early allows it to have a longer shelf life and become ripe when your buyer is ready to use or re-sell it.
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Hydrocooling Bunched Greens
It is critical to cool down and hydrate bunched greens like kale, collards and Swiss chard when it is this hot, or really any time. Try to harvest early in the day, and then hydrocool them.
If you just harvest greens like these and put them directly into a cooler or straight to the customer, they likely will be wilted within a few days. You may not see the wilt, but your customer will, and you may not get further orders from them.
- After harvesting (and usually bunching in the field, unless you are selling them bulk), bring the bunched greens into your packing area.
- Fill a tank or large tote with cold water
- If possible, add a food-grade sanitizer that is intended for produce wash water like SaniDate 5.0. This is an extra step for food safety, and will reduce the chances that the water will spread germs if there are any amounts of pathogens from animal feces or other contamination on the greens. (For more info see this post).
- Submerge the greens for a few minutes. You do not need to soak them for longer than that.
- Take them out, shaking off most of the water from the bunch, and pack into your box and put them in the cooler. Leave some of the water on, as they will be dehydrated in the cooler, so the extra water helps them stay crisp.
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This step is critical for not only removing any dirt/soil/insects that might be present, but with cooling and hydrating the product so it will last for your customer.
Technologies for labor savings?
We have a chronic labor problem in the Midwest vegetable industry. You’ve likely heard about robotics for bridging the labor gap in places like California and Europe, but we haven’t seen any of these technologies meaningfully proliferate farms in our region. The Great Lakes Tek Flex is coming up this September, where ~18 different robotics vendors will be showcasing their technologies, which are designed for small to medium scale vegetable farms: each vendor will have 1 acre to showcase their products, and everything they’ll be displaying is already on the market. If you’ve considered robotic weeding, spraying, or harvest assist tools, it might be worth the trip to check it out! Click here for the link to the event.Crop Updates
Brassicas: We’re seeing some brown bead and loose heads in broccoli and cauliflower. These are normal responses to heat. Choosing super heat tolerant varieties helps with this - in cauliflower, using varieties that self-wrap (e.g. grow leaves over their heads to shade them), or going out and manually bunching leaves to wrap over the heads also helps. If you’re seeing a lot of brown bead this year in your current plantings, one strategy is to harvest a little early. Early harvest reduces yields since the heads are smaller, but it’s worth it if the alternative is losing your crop.Cucumbers: Two-spotted spider mite numbers are booming in some high tunnels. While there are predatory mites and other predatory insects who can make a dent in their numbers, if you are seeing leaves with lots of yellow and white flecks on plants, check the underside of the leaves. A hand lens is majorly helpful, as spider mites are a fraction of a millimeter large. High populations will often produce the webbing on the underside of the leaf. There are OMRI listed products that are relatively selective (which will help preserve natural enemies). These include insecticidal soaps such as M-Pede, and horticultural oils (such as SuffoOil-X). These same products also work on aphids, who can often be found alongside spider mites.
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| The underside of the leaf is where insects can be found. Here we can see a lacewing larvae (elongated brown-red insect), aphids (small, light green), though much of the flecking damage is done by spider mites (who are too small to photograph). Photo: Marissa Schuh, UMN Extension . |
Peas have had excellent yields and growth in Southern, MN. The pea stands we’ve seen withstood the last heat wave despite our warnings. We’ll be surprised if they fare well through this next year wave, which is set to be hotter, and longer.
Beans: are ripening just in time to replace peas for the category of crunch, fresh green vegetables in your CSA boxes / farmers market stand, etc. Beans are already labor intensive, with daily harvest required. With the heat wave, your beans will likely be producing even faster than usual.
Sweet corn: Very early sweet corn in the southern portion of the state is almost ready for harvest, and it looks like parts of the state got a flush of corn earworm in late June. While corn earworm are a regular pest in late summer sweet corn, it isn’t every June that we get conducive weather for corn earworm flights from the Southern United States. Trapping on your own farm is the best way to pick up these flights and time protective insecticide applications for fields that are silking.
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| Corn earworm caterpillars feed in the tip of ears, sometimes shearing silks. Photo: Charlie Rohwer, UMN Extension. |
Corn earworm damage also attracts sap beetles, who are attracted to the smell of rotting fruits and vegetables. You will also see them show up when raccoons or birds get into ears.
Another pest management task this time of year for sweet corn is to prep your bird scares. If you have a history of bird damage or are growing near marshy areas, it is important to get bird scares out before birds start feeding in the field. Research in New York has found that detassling, air dancers, scare eye balloons, and taste-basted deterrent sprays can all work. If using air dancers or balloons, make sure to move the scare around every week or two, as birds learn. Research in New York and Rhode Island has found that laser scarecrows can work if set up correctly, and highlights a grower survey where farmers report getting the best bird control when using multiple tactics.
Tomatoes: The hot overnight temperatures that are on deck in the next week will likely lead to flower abortions, as well as slow the ripening of fruit. We have also gotten questions about tomato leaf roll. In some varieties, exposure to hot weather causes the older leaves to curl in on themselves and become leathery and dull. This can happen relatively quickly, and can be alarming, but doesn’t impact yield.
Diseases continue to progress in tomatoes.
- One of the most common diseases we see in tomato, especially those grown outdoors, is early blight. This disease primarily affects tomato leaves, and starts on the oldest, lowest leaves. Water splashing facilitates the movement of the disease-causing fungus moving into the leaf canopy, affected leaves will have brown spots with concentric rings inside of them. As time progresses, these spots come together and leaves will die. If you are seeing this, work on airflow – stake and tie tomatoes, as well as keep up on weeding within in the field and mowing field edges.
- For conventional growers, early blight is managed relatively easily with a wide range of fungicides. For organic growers, lean more heavily on the cultural controls that help you manage airflow. OMRI listed products with some degree of early blight efficacy include Double Nickel and LifeGard (suppression only), as well as Regalia used with copper products.
- The other leaf spot disease that is showing up is bacterial spot. This one can be hard to tell apart from the fungal tomato diseases, as they all cause spotting.
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| Bacterial spot is caused by bacteria, and leads to many small brown spots on the leaf. These can cause leaf yellowing when many leaf spots develop close together. Photo: Dan Egel, Purdue University. |
This is also a time when we start to really see fusarium crown/root rot and Verticillium wilts show up. Plants are growing really quickly and producing fruit, and as they use more water, we really start to see wilt symptoms show up when these pathogens are present.
We are always available as a sounding board for any suspected disease issues; the Plant Disease Clinic is also a place to go for a solid diagnosis. This is especially important for plants with symptoms like wilting, rotting roots, and discolored vascular tissue, as diseases with these types of symptoms often need intensive management, so getting an accurate diagnosis is critical.
In the high tunnel, the main issue of discolored lower leaves we see this time of year is magnesium deficiency. This doesn’t necessarily mean there’s not enough magnesium in the soil. Sometimes it means that your plants are struggling to access that magnesium due to excess calcium or potassium. If you’re seeing magnesium deficiency, you can correct for this mid-season with 2-3 foliar sprays equaling 2-4 lbs Mg per acre (this is equivalent to around 20-40 lbs of Epsom salts per acre). This advice comes from the Nutrient Management Guide for Commercial Fruit and Vegetable Crops in Minnesota.
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| Magnesium deficiency appears as uniform yellowing between the veins of the oldest leaves. Photo: Bruce Watt, University of Maine, Bugwood.org |
Vine crops: Vine crops have big leaves, it is pretty common for these to get droopy in the heat of the day. Healthy plants should recover overnight, but if you have wilted plants with drooping leaves (a grower recently described this to me as a “melting umbrella” look) in the morning, check in on these plants. For cucumber and melons, bacterial wilt is the most likely culprit; for squash and melons, squash vine borer is frequently to blame.
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