Weekly vegetable update 8/10/22

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

This update includes suggestions for late summer cover crops, a reminder about postharvest intervals when using pesticides, notes about diseases and insects we've started to see this week, reminders about postharvest handling, and more.

This weekend brought some desperately needed rain to grower across the state. However, according to the Minnesota WeatherTalk blog, "the NOAA Climate Prediction Center models continue to show a likelihood of warmer than normal temperatures and less than normal rainfall prevailing across Minnesota through the first half of August."

Problems in the field / things to note this week

Make time for cover crops!

You've harvested your garlic and quite a few onions. Your cucumbers may be slowing down. While many growers stick to September planted oats and peas as their primary cover crop, planting a cover crop right now in open beds opens up some exciting opportunities for cover crop diversification, and allows you to get more biomass from your cover crops than if you were to wait until September. Some great options for cover crops to plant right now include:

• Buckwheat - will help control weeds, provide organic matter, and will provide high value flowers for pollinators in 35-40 days
• Crimson clover and berseem clover - will die over the winter, and provide some nitrogen and organic matter
• Oats and peas - planting now will give you quite a bit more biomass (meaning more organic matter and nitrogen fixation) than if you wait until fall

Buckwheat cover crop in bloom. Photo: Adria Fernandez

A reminder about preharvest intervals

When spraying for insects, remember to pay attention to preharvest intervals. The preharvest interval (PHI) refers to the amount of time you need to wait before applying an insecticide and harvesting. Applying too early is against the law, and it puts your customers at risk. You usually have to search for this information on the label, because the PHI can be different for different crops. Check the label for the crop you're growing, and you'll find the PHI and other important information about application rates. Here's an example of what this section of a label looks like (example label is Entrust, Cole Crops section):

Label language. Click to make larger / clearer.

Crop updates

• Cucurbits: Squash bugs have been more active this week, and we're starting to see more powdery mildew. Remember to scout the lower leaves for the first symptoms of powdery mildew. Last year on this same date, Marissa posted an in-depth article about managing both. Review it here.

Check for powdery mildew on older leaves and those deep in the canopy. Video: Marissa Schuh, UMN Extension.

• Tomatoes: At this point in the season, new flowers are unlikely to ripen in time. This is especially true for larger fruited tomatoes like beefsteak varieties. For maximum production, consider topping indeterminate tomato plants to encourage the plants to put their energy into ripening fruit.
  

• Garlic: Just as garlic season is wrapping up, it's time to start purchasing garlic again for fall planting. Review the guide we posted last year about purchasing local garlic if you do not currently have a source.

• Cole crops: Typically we don't see significant disease pressure in cole crops until around this time, at which point we see persistent disease pressure for the rest of the season. The hot, dry weather ahead should help to keep disease pressure low, but we've started to see more diseases impacting head quality in our Waseca broccoli trial. In addition to our old friends Alternaria leaf spot and black rot, we've been seeing bacterial stem rot in some accessions.

Likely Alternaria, Photo: Charlie Rohwer

Bacterial stem rot. Photo: Charlie Rohwer

• Potatoes and onions: Both potatoes and onions are very susceptible to soft rots at harvest time, especially when conditions are hot. Take extra care as you harvest to avoid scrapes and bruises, and if possible, take some extra time to carefully sort out any tubers / bulbs that have obvious damage. Both can be field cured if there is no rain in the forecast; simply use an undercutter bar or harvest forks to loosen the soil. Read more on our postharvest handling page.

• Sweet corn: Corn earworm trap counts have remained fairly low this week. In all of the trap sites around the state, they were below the threshold of 5 adults per trap. Some growers may be noticing stem silk-clipping, which usually happens as a result of beetle feeding. Corn rootworms and Japanese beetles can both feed on silks, which results in uneven pollination.

Silk clipping. Photo: Charlie Rohwer

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 hoida016@umn.edu, and you can submit questions and diagnostic help below.