We are transitioning into full-on summer: zucchini, cucumbers, and tomatoes are being harvested, and sweet corn isn't far behind. The good news for next week is that a cool front is expected. The bad news is that the cool front is projected to bring in more wildfire smoke, at least for a few days. In this update we'll discuss some midsummer insects and diseases to watch for, some reminders about harvesting garlic, and updates about our 100 farms soil testing project.
Crop updates
Cucurbits
Cucurbits are vining out. Squash bug eggs are being found on many farms, squish these as you see them. Their eggs are coppery and laid in clumps. The clumps are most often found nestled between leaf veins on the underside of leaves, but squash bugs also don’t seem to be married to laying their eggs there – you can find them on all parts of the leaves, stems, and even sometimes on flowers. You may also be seeing recently hatched immature squash bugs, which are green when they first hatch then turn an ashy gray.
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| Squash bug eggs. Whitney Cranshaw, Colorado State University, Bugwood.org |
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Squash bug nymphs. Whitney Cranshaw, Colorado State University, Bugwood.org |
The most commonly mixed up egg might be lady bug eggs, which are also laid in clumps. Lady bug eggs are more oblong, yellow, and are not metallic. It is worth being able to identify these as a “do not squish”, both immature and adult ladybugs are aggressive predators of pests.
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| Ladybeetle eggs and a recently emerged ladybeetle larvae. Photo: Bradley Higbee, Paramount Farming, Bugwood.org |
We have also been continuing to investigate shrunken new growth and fruit abortion in cucumbers being grown in high tunnels. We’ve had reports from growers across the state that cucumbers are aborting fruit at a higher rate than normal, sometimes accompanied by new growth that is very condensed with very short nodes between new leaves and flowers. We wrote last week that we were hearing reports of this from growers in other states too, and hypothesized that it might be a response to air quality. However, we’ve now seen plenty of high tunnel cucumbers that are not exhibiting these symptoms, so it’s fairly unlikely that this is a universal response to air quality. Instead, after consulting some cucumber colleagues in other states, we think it is simply a mix of stressors related to inconsistent irrigation, heat, and nutrient deficiencies or high soil pH and salinity. Venting your high tunnels as well as possible and using soil moisture sensors to ensure that you’re watering consistently should help. The upcoming cold front should also hopefully help.
Garlic
Some growers have already started harvesting garlic. Others will wait a few weeks. How do you know when your garlic is ready? Here’s some guidance directly from the new Growing Garlic in Minnesota guide: Harvesting too early will result in small bulbs that do not store well. Harvesting too late will force the cloves to pop out of the skins, making them susceptible to disease and resulting in unmarketable bulbs. There are a couple of methods that can be used to determine when to harvest:
- By early July the lower leaves will start to brown, and harvest is usually optimum when half or slightly more than half of the leaves remain green; or when a third of the leaves at the bottom have gone brown or yellow.
- Pull a few bulbs and cut them in half; if the cloves fill the skins, then the bulbs are ready to harvest.
Garlic should be “cured” or allowed to dry in a well-ventilated area until the outer wrappers dry out. After curing, garlic should be stored at 2°-40° F and a relative humidity of 60-70%. If you’re selling un-cured garlic at the market, make sure your customers know it has not been cured. Let them know to keep it in a well ventilated area to prevent it from rotting.
Tomato
Tomatoes, both in hoops and outdoors, continue to develop. As fruit forms, this is when some differences between varieties in terms of disease resistance and blossom end rots become obvious – take notes about what varieties are great and which ones are duds now.Tomato leaf mold is making itself known –surveys of hoop house tomatoes in Minnesota have found that this disease is very widespread. Why only in hoophouses? It is driven by humidity higher than we ever see for prolonged periods outdoors (85% humidity), but is commonplace in hoops. This disease likes temperatures in the 70s, but can develop when temps are anywhere from 50-90 degrees. It appears as yellow spots on the leaf surface, and when these leaves are flipped, each yellow area will correspond with a very diagnostic olive-colored, fuzzy spot.
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| Tomato leaf mold produces distinctive, fuzzy, olive spots on the underside of leaves. Photo: Metin GULESCI, Leaf Tobacco, Bugwood.org |
Research has shown that there are many strains of tomato leaf mold. This makes making management recommendations very difficult – organic products provide variable levels of control from one trial to the next, and even varietal resistance can be different in different spots depending on what strain is present. The most consistently performing products according to research at Cornell have been Champ 30 WG and Zonix.
Making notes about what is performing well in your high tunnel is important if you have this disease. Research in New York has found the varieties Red Mountain, Primo Red, Geronimo, Turst, Boa, Cherokee Purple, Amish Paste, and Prudens purple to demonstrate resistance.
Sweet corn
Sweet corn plantings are beginning to silk. Look out for insects that clip silks, we most often see corn rootworm beetles and Japanese beetles doing this. The threshold for corn rootworm silk cleaning is treat if scouting 20 plants at 5 locations reveals five rootworm beetles per ear/silk. For Japanese beetles, the threshold is two beetles per ear before pollination (silks still green, can still see pollen on the tassel). For conventional growers, there is a lot of overlap between products used on silk clippers and those used on corn earworm. Check the maximum seasonal limits on the products in your corn earworm program to make sure you still have enough left in the seasonal allowance to get you through earworm flights. See the Midwest Vegetable Production Guide for more information.
Corn rootworm looks like cucumber beetle, but they favor corn and flowers instead of squash plants. The easiest way to tell them apart it to look at the underside of their bellies – cucumber beetles have a black abdomen, corn rootworm has a yellow one.
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| Photo: Whitney Cranshaw, Colorado State University, Bugwood.org |
Potatoes
The period between flowering and harvest is the most important period in potato development to focus on consistent irrigation. A few farmers have started to harvest early potatoes, and others will be harvesting soon. During dry summers we often see splitting, scab, and hollow heart. While potatoes with these symptoms are still safe to eat, they are unattractive and have more limited shelf life, and thus many growers struggle to market them. As your potatoes near harvest, make sure you’re prioritizing irrigation in this crop.
Wilting in cucumbers and squash: when to worry
With cucurbit plants getting bigger, there are always questions about when to worry about wilting plants. It is normal for large cucurbit leaves to wilt in the heat of the day, but check on plants that are still wilted the next morning. There are two common causes…- In cucumbers, bacterial wilt is the most common cause of wilting and dying cucumber vines. Other vine crops get this disease occasionally, but cucumbers are very susceptible. This disease is spread by cucumber beetles, who have the bacteria in their gut, then poop on leaves, and cucumber beetle feeding provides the wounds for the bacteria to go from bug poop to plant problem. You can see this disease even if cucumber beetle pressure was low - one infected beetle can cause problems, and we don’t have a good handle on how common these infected beetles are. A rough test for bacterial wilt involves a knife and a cucumber vine that’s on its way out. Cut a straight line through a dying vine, then rub the two halves of the cut together. Pull them apart slowly – if you see a stretching ooze between the two halves, what you are seeing is the gunky ooze bacterial wilt infection causes in the plant.
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| While plant pathologists call this a “bacterial stream,” it also looks like what a food influencer would call a “cheese pull.” Charles Averre, North Carolina State University, Bugwood.org |
- In other cucurbits, check the base of the vine for squash vine borer. Looks for piles of wet sawdust, this is what the borers are pushing out as they burrow into the vine. Cutting around the stems will reveal the borer (actually a caterpillar). Some people are able to surgically remove the borer and their plants end up doing fine. Plants that are very vined out and have started putting down root nodes may also be able to ride out squash vine borer infestation.
Soil testing update
Many of you are participating in our 100 farms project, so we thought we would share an update about what we’re starting to see from the initial data. We’ve now received all of the basic soil series data from the UMN Research Analytical Laboratory. If you’ve not yet received your individual report, you should be receiving it in the next couple of days. Here are the median values for some key soil measures so you can see how your farm’s soil tests compare to 100 other vegetable farms around the state.
| Measurement | Field median | Tunnel median |
|---|---|---|
| Organic matter | 3.95 | 5.4 |
| Soil pH | 6.9 | 7.2 |
| Phosphorus (Bray) | 67 | 138 |
| Potassium | 173 | 231 |
| pH of irrigation water | 7.9 |
We also visited the Khokhani and Grossman labs this week to see how the biological soil health tests are going. These labs are measuring arbuscular mycorrhizal fungi (counting the number of spores in each sample - read more about it here) and active carbon (the fraction of carbon most readily available to microbes). Here are a few photos:
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| Arbuscular mycorrhizal fungal spore. Photo: Nina Charlier, Khokhani lab |
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| Potassium permanganate reacting with soil in the Grossman lab. Photo: Natalie Hoidal |
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| Solution after soil reaction with potassium permanganate being plated to be read on a spectrophotometer. The color of the solution indicates the amount of active carbon that reacted. Photo: Natalie Hoidal |
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