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Weekly vegetable update 8/31/23

Our food systems team has been on the road this week touring farms across north central Minnesota to make new connections. In general, crops are ahead of schedule this year. We had our first sweet tango apples (about two weeks early near Little Falls) and saw plenty of winter squash ripening early. For the most part, peoples' crops are doing quite well despite the dry summer. It seems like in our third year of drought most growers have caught up, either by investing in more irrigation infrastructure or by adapting farm plans to accommodate less water availability. This update includes discussion of how heat and drought impact curing and potassium availability, as well as two types of Alternaria causing problems for fall crops.

A hot labor day

Forecasts are showing temperatures potentially reaching 100 degrees this weekend in Southern Minnesota, and low to mid 90s in the north. Thankfully, forecasts are projecting lower humidity than the last heat wave.

We’ve seen a lot of onions and garlic curing in high tunnels this week, and while warm, dry weather is ideal for curing, really hot sunny weather can be detrimental. If you’re curing in the high tunnel, use shade cloth to prevent sunscald, and leave the side walls, vents, and sides wide open. Use fans to provide good airflow. If curing in the soil, keep in mind that sandy soils tend to heat up faster. See crop specific curing instructions here.

Potassium Issues and Drought

Often it seems like potassium is the third wheel of macronutrients. Nitrogen and phosphorus tend to get all the headlines nowadays, and perhaps that is understandable due to their impacts on the environment. However, to the plant, all three nutrients are equally important to their life cycles. Potassium deficiency symptoms differ between vegetables, but most include yellowing (chlorosis) of some sort on older leaves as opposed to the newest ones. This is because potassium is pretty mobile in plant tissues, similar to nitrogen. When the problem is severe, leaves can prematurely drop or develop necrosis (dead areas).

Crops absorb potassium mostly from soil water. This means droughts and K deficiencies frequently appear together. Clay layers can hold onto potassium quite tightly during dry periods, and often need water to release K into solution. You can think of it kind of like a zipper unzipping—with water moving in and peeling the layers apart. This peeling action allows trapped K ions to escape and (perhaps) get absorbed by the plant as it takes in water.

Shrink-swell type clays (also called smectites) tend to release more K compared to other types of minerals when the rain finally falls. In these types of minerals, the “zipper” works really well when water moves in. In other kinds of clays—like illites—the “zipper” tends to get stuck and not fully open, releasing less K. Regardless of the clay type you may have, proper irrigation and responsibly increasing soil organic matter (to store more water) helps to avoid potassium deficiencies. If you are interested in where and what types of clay minerals are found in Minnesota, visit to learn more.

Severe K deficiency in pepper. Photo: Natalie Hoidal

Finally, when doing your regular soil testing, make sure to keep an eye out for “true” deficiency of potassium. This photo of a pepper with severe potassium deficiency symptoms was taken in a high tunnel with low soil K levels.

Post-harvest Alternaria in tomatoes

While any disease on your tomatoes is frustrating, it’s particularly frustrating when you’ve harvested a perfectly healthy looking tomato, only to find lesions developing a day or two after harvest. This can be especially confusing for growers and customers when the lesions show up after the CSA box or wholesale order has left the farm, or after the produce has left the farmers’ market stand.

Alternaria alternata is a pathogen that readily infects ripe tomato fruit, often with symptoms showing up after harvest. It’s one pathogen in a group of Alternaria species that are common in gardens and feed on decaying plant tissue. It’s most prevalent towards the end of the summer / beginning of fall when humidity is high and dew forms on plants and fruit in the mornings. 
Alternaria fruit rot on tomatoes. These tomatoes looked unblemished at harvest, but developed spots about two days later. Photo: Natalie Hoidal

These Alternaria species that feed on decaying plant tissue are in the same family as early blight, but they are quite different. These pathogens tend to be a lot weaker, predominantly feeding on already damaged or decaying material, and they have a wider host range. Alternaria alternata needs exposure to 3-5 hours of free standing water to sporulate, and can only infect very ripe tissues (e.g. the thin skin of very ripe tomatoes).

There are a few things you can do to prevent these pathogens from causing damage to tomatoes:
  • Pick your tomatoes as soon as they are ripe. The more likely the fruit are to become infected and the more likely the fungus is to sporulate and spread.
  • Lesions are far more likely to occur on fruit that already have damage from chilling, sunscald, calcium deficiency, or insect injury since these pathogens are fairly weak and mostly infect already damaged tissue. As you harvest, sort tomatoes that show signs of damage into separate bins.
  • Over-pruning may actually cause more damage since the fruit are more susceptible to sunscald or sun damage when they do not have a canopy of leaves to shelter them. (This is especially true in peppers, which are also susceptible to these pathogens).
  • Avoid overhead irrigation
  • Remove dead tissue to the extent that it is practical
  • Handle tomatoes as gently as possible during harvest.
  • Cool fruit to 35-50 degrees fahrenheit promptly after harvesting.

Brassica diseases

Fall weather tends to bring Brassica diseases, and this year is no exception. During farm visits this week we saw quite a bit of Alternaria. Thankfully we haven’t noticed much black rot yet, which makes sense given how dry the summer has been. Alternaria has been a significant challenge over the last three years or so for Brassica growers in MInnesota, as a single lesion can impact the marketability and shelf life of broccoli, cauliflower, and leafy Brassica crops.

Our 2022 broccoli trial helped to identify some varieties of fall broccoli that could better withstand Alternaria and black rot pressure. If you’re seeing Alternaria take off this year, consider trying the following broccoli varieties next year:
  • Eastern Crown, Green Magic, or Wolfman for spring planting & summer harvest
  • Belstar or Covina for summer planting & fall harvest
Unfortunately there are no OMRI approved fungicides that are currently recommended for black rot. If you’re not an organic grower, check out this list of recommended products from the Midwest Vegetable Production Guide. Alternaria japonica has shown resistance to group 11 fungicides, but so far we have only seen Alternaria brassicicola in Minnesota (last year we collected samples from farms around the state and sent them to our colleagues at University of Connecticut for sequencing).

An important and related question a grower asked our team this week had to do with rotations. This grower has a great rotation where all of the Brassica crops are planted together in one field, cucurbits are planted together in another, and the final fourth field has a mix of “small stuff” like lettuce, beets, and other crops. They have a thoughtful rotation plan that was based on spacing in landscape fabric that allows them to use the same landscape fabric for 3 years, adding closer and closer holes each year as they move from wider to more narrowly spaced crops. The caveat was that some of the smaller Brassicas fit better in the fourth group based on their spacing, and so they were planing Brassicas in the same fields for 2 out of their 3 rotations. Since Alternaria was beginning to build up in their fields, we discussed that it really is important to have a 3-4 year rotation for Brassicas to avoid this disease. The solution we discussed was to re-assess the importance of the smaller Brassicas and whether they could swap them with other crops that would fit their spacing better, and to potentially add a fourth rotation with sweet corn. Making sure your cover crop mixes are also Brassica-free (e.g. do not contain tillage radish) is also key to getting a proper rotation.

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