After a cool, rainy week with some impressive storms, we finally have a few warm days ahead of us (and some really really hot weather in the forecast). This should give recently planted crops a chance to perk up and catch up.
Potassium deficiency or salt stress?
We received some photos of tomatoes growing in a high tunnel this week with yellow leaf edges and necrotic tips. Sounds like a potassium deficiency, right? But, the photos didn’t quite match a potassium deficiency. The edges of the yellow ring were too neat, and the tips more crispy than what you might expect from a potassium deficiency. We reviewed the soil tests and saw off the charts potassium (literally, the UMN soil lab caps reporting at 300 ppm). So what was the culprit? Soluble salts!
Salt stress in high tunnel tomatoes. Anonymous grower submitted photo.
We know from our study of 100 MN high tunnels that most high tunnels have more fertilizer than they need (fertilizer is loosely defined here - this could include compost, manure, or any inputs that contain nutrients). Just under half of the high tunnels in our study had challenges with soluble salts, and these salt levels were correlated with soil nutrients, especially nitrate.
So, this is a good reminder that too much fertilizer and not enough can sometimes cause symptoms that look kind of similar. If you’re seeing plant stress symptoms that seem to be nutrient related, it’s so important to take a soil test, or even a foliar test to help get to the bottom of it so that we don’t misdiagnose issues and accidentally make things worse (in this case, “make things worse” would have meant adding more potassium, which is itself a salt).
Another fun mystery: Bright yellow zucchini leaves
We received a photo this morning of some zucchini plants with bright yellow leaves that almost looked like a virus (but similar to the potassium / salt photos, it didn’t quite have the right characteristics to be a virus like curly distorted leaves, rings, or a mottled texture). The grower had other varieties of zucchini and squash that were grown in the same area, and started in the same potting soil without any issues, so that helped us rule out some potential stressors. Thankfully our colleagues around the country have been seeing this too, so we were able to get to the bottom of it quickly.
B gene in zucchini causing the leaves to turn yellow with the cold. Anonymous grower submitted photo.
It turns out many zucchini, summer squash, and squash varieties have a gene referred to as the “B gene”, which may increase the number of female flowers (which increases yield) and contributes to early ripening. Plants with this gene typically have yellow stems rather than green ones, and the leaves have a tendency to turn bright yellow under stress, especially cold stress. Our spring has been relatively cool, and so farmers around the region are seeing these bright yellow leaves more often this spring. The good news is: your plants should grow out of it with no impacts to yield.
Disease scouting refresher
We’ve had pretty regular rains, and looking at the short term forecast, those look to continue over the next week. This is nice because it means we don’t have to spend time irrigating, but we will need to spend time scouting for disease. Things to keep in mind when scouting/moving around the farm during periods of potential disease development…- Stay out of fields when leaves are wet
- Start the day’s work, as well as scouting route, in the youngest, healthiest plants. End in the oldest plants or in places where you suspect disease.
- Start each day with clean clothes
- Regularly clean tools and boots. If the tools isn’t wounding plants, the key thing to do is remove clumps of dirt. Pruners should be cleaned regularly during their use.
It’s caterpillar season
In cole crops, imported cabbage worms are starting to be found (in addition to diamondback moths and flea beetles). Imported cabbageworm is a bigger, hungrier caterpillar, so it only take a few of these to do a fair amount of damage. The white butterflies, the adult form of this pest, can be seen flying around brassica fields..jpg)
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Frass left behind after an imported cabbageworm fed. Photo: Marissa Schuh, UMN Extension.
- Transplant to cupping stage
- 50% infested with 5 or more diamondback moth larvae.
- 30% plants infested with 1 or more medium to large imported cabbageworm larvae.
- Cupping to harvest stage
- 10% of plants infested with 1 or more diamondback moth larvae.
- 10% plants infested with 1 or more medium-large imported cabbage worm larvae and cabbage looper eggs or larvae.
- For cabbage and broccoli, treat after the head starts to form and if 20% of the plants are infested.
- For cauliflower and leafy cole crops, treat if 10% of the plants are infested
Crop updates
- Cucurbits will love the warm weather ahead this week and should start to rebound from early season cold distress.
- Peppers continue to perk up with warmer weather. We saw some peppers growing out of an aphid infestation. The leaves were cupped, when looking into the curled underside, I found what are called aphid mummies. These grey aphid husks are left behind when a parasitoid wasp kills an aphid. When and how to best manage aphids varies by crop, and in the case of peppers, if the peppers are outside or indoors. We don’t have any thresholds developed in the Midwest, but on the east coast thresholds vary anywhere from 2-10 aphids before fruit set to 4-5 aphids after fruit set. Management options range from spraying plants with strong stream of water (aphids are good at many things, but not at holding on tight) to a variety of pesticides, including some that are targeted at small, soft bodied insects. The more targeted the product, the better chance you have of preserving the predatory insects that can tamp down future aphid populations.
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Live aphid (very small, green) and aphid mummies (grey). Photo: Marissa Schuh
- Tomatoes are being staked and strung up as they get taller. The first Minnesota tomatoes from *heated* high tunnels are hitting grocery stores shelves this week, and non-heated high tunnel tomatoes are still a few weeks out. We have two upcoming field days where we’ll highlight pruning and trellising strategies for tomatoes, peppers, and cucumbers in high tunnels. We hope you can join us! More information here.
- Cole crops: We discussed cole crop caterpillars at length above, but here’s a quick reminder that the cole crops (broccoli, cabbage, collards, bok choy, etc.) that you are seeding right now should be different varieties than the ones you planted a few weeks ago. Summer plantings should be varieties that are specifically bred for heat tolerance and less bolting. Another fun update: the broccolini we highlighted a couple weeks ago that was experiencing brown bead after hot, humid weather grew out of the brown bead and the new shoots are looking great.
- Garlic: Scape harvest continues this week - check out our update from last week for a more thorough discussion of scape harvesting practices.
- Late spring crops: We’re seeing some beautiful head lettuce, green onions (and green garlic), beets, parsley + cilantro, turnips, and strawberries being harvested from open field plantings this week, with no major (reported to us at least) problems yet!
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