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Weekly vegetable update 9/7/2022

Author: Natalie Hoidal, UMN Extension educator, local foods and vegetable crops 

This is the second to last vegetable update of the season. This update includes an overview of some disease issues I've been seeing in the field, a discussion about what the hot fall nights mean for cool season crops, and reminders about fall tasks like thistle management and manure applications. 

Crop updates

  • Tomatoes and peppers: I've been seeing tomatoes here and there with fruit spots that look to be caused by a pathogen, but the leaves are still disease-free. Seeing these tomatoes reminded me of a case last year where a farmer was harvesting fruit that looked great at harvest, but was developing lesions in storage which were caused by Alternaria. The best things growers can do to prevent these problems are: 1. Harvest as often as possible so that fruit do not become overripe. Soft spots in overripe fruit are very prone to secondary infection. 2. Store fruit at the right temperature and humidity. For tomatoes, this is 50 degrees F with 95% humidity. 
Tomato fruit with lesions despite healthy leaves. Photo: Natalie Hoidal

  • Asparagus: Asparagus plants still have another month or so of growth before thy go totally dormant for the winter. Towards the end of the season we often see foliar diseases starting to build up. This is important, because disease pressure in the fall can cause early dieback, which translates to less energy stored in the roots for spring. These diseases include rust,  purple spot, cercospora leaf spot, and Fusarium. Scout for these diseases this week, and consider a fungicide if you're seeing disease pressure. Towards the end of the fall as the ferns start to senesce, consider removing infected ferns from the field if it is practical to do so at your scale. Read more about these diseases in the MN asparagus guide, and review fungicide options at
  • Potatoes: I've been seeing some potatoes with scab. These potatoes are fine to eat, but they are more susceptible to soft rots in storage. As you make the shift from fresh market to storage potatoes, make sure you're sorting out any potatoes with scab and eating / selling them right away.
  • Cole crops: I heard a statistic on the radio yesterday from meteorologist Sven Sungaard. I haven't been able to track down the dataset yet, but he said that historically around the Twin Cities, 50 degree nighttime lows have historically started at the end of August, but in recent years temperatures haven't begun to drop into the 50s until around September 16th. I did spend some time clicking through the climate central models, which he referenced, and our nighttime temperatures this month have indeed been very high. This has major implications for fall Brassica crops, especially those that form heads. These crops need to experience a cold period ("vernalization") in order to trigger head formation. Some varieties need to experience more cold weather than others before forming a head. If our fall nights are not getting cool enough, head formation can become delayed, increasing the risk that crops will not form heads in time for winter. Once they do begin to form a head, they can bolt at any time if they experience too much heat. We usually worry about this in the spring, but not the fall. However, I saw plenty of bolted broccoli heads last night at a Twin Cities farmers market. 

Problems in the field / things to note this week

The fall window for perennial weed management

Most farms have at least a few difficult perennial weeds like thistles. For many perennial weeds, late summer / early fall is the best time to manage them. Perennial weeds with extensive root systems are generally difficult to manage because you can mow or pull them, but because they have so much stored energy in the roots, they easily come back.
If we look at the movement of sugars and other carbohydrates in plants, we can better understand how to manage them. Plants store energy as sugars and carbohydrates. Early in the summer, perennial plants tend to use some of their energy to grow, while saving some of their energy below ground. Towards the end of the summer, plants like thistles tend to push a lot more energy aboveground so they can do energy-intensive things like flower and set seeds. As the cool weather begins, thistles begin to shift their stored sugars and carbohydrates back to the roots.

Managing perennial weeds right now is a great time because they've moved a lot of their stored sugars / carbs aboveground. By mowing or removing the aboveground parts of the plant, you're removing a lot of the plant's energy before it's able to send it back underground for the winter. If there's any time of year to invest in hand pulling weeds, now is the time. Mowing can also work, but be mindful of spreading seeds if the weeds in question have already begun to set seeds. 
Image by Annie Klodd

Annual fall manure application reminders

For growers applying manure to fields this fall, remember a few basic best practices: 
  • Wait until soil temperatures drop to 50 degrees F to avoid losses due to nitrification
  • Incorporate your manure into the soil to prevent volatilization.  
  • Test your manure if possible. At the very least, make applications based on a soil test, and consider testing your soil again in the spring. 
  • Pay attention to food safety. Applying manure in the fall is a better practice from a food safety perspective than applying it in the spring, because you're allowing more time for pathogens to break down.

Read more about fall manure applications in this article by Chryseis Modderman, Melissa Wilson, and Annalisa Hultberg. 

Manure spreader, MPCA photos

Connect with the fruit & vegetable team

If you're seeing interesting things in your fields, need help identifying problems, or just want to share photos, we'd love to hear from you! Growers can reach out directly to me any time at, and you can submit questions and diagnostic help below.  

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