2021 Considerations: Brassica Bonanza

Authors: Natalie Hoidal and Marissa Schuh, Extension Educators, University of Minnesota Extension


Our biweekly crop by crop growing season prep series continues. This week we are focusing on brassica crops. The more you know about these likely issues, the better prepared you will be to deal with them.

Bolting

Brassicas originated in the Mediterranean, where they were originally biennial plants that required a 2-3 month cool period (winter vernalization) to induce flowering. Since winter in Greece is a bit like spring in Minnesota, growers in cool climates have been able to grow Brassicas in the spring. Because our spring is followed by a very warm summer, growing spring Brassicas in Minnesota can be a bit of a dance with the weather. While each plant is a bit different in its requirements, the basic idea is that they prefer conditions in the 60s early in development where they can put on leafy biomass, and eventually with enough cool nighttime temperatures, they will experience enough cold weather to trigger vernalization, which allows for head formation (or enlarged storage roots in the case of kohlrabi, turnips, etc.). If plants experience too much cold early on, they can experience “buttoning up”, which happens when the plant produces a head before it’s really ready. Once plants enter their head formation stage, they are far more susceptible to bolting with high temperatures. For a very in-depth look at the conditions that cause bolting in a range of crops, see this article from our colleagues in Michigan.

Bolting broccoli, photo NH

Based on this information, some tips for preventing bolting include:

• Choose varieties based on season: plant varieties recommended for spring in the spring, fall varieties in the fall, and heat tolerant varieties in summer. Certain varieties have been specifically bred to better withstand the temperature fluctuations of each season.
• Consider using row cover early in the spring if temperatures are consistently dipping below 50 degrees F to hold off vernalization.
• Transplanting can help, especially for spring broccoli; plants will already have a few leaves, and may be able to more quickly reach maturity before hot summer weather arrives.
• Hedge your bets with succession planting. Since spring weather is so unpredictable, planting a new succession every couple of weeks will help to provide some resilience in the case of very high or very low temperatures.
• For broccoli and cauliflower, harvest based on bud size rather than head size.
• You can always stick to growing your head-forming Brassicas in the fall if your markets allow for it - fall grown Brassicas get to experience conditions much more similar to the Mediterranean conditions they were bred for: a warm summer to put on leafy green biomass, followed by a cool fall with ideal head formation temperatures.

The Very Hungry Caterpillar Complex

Size alone isn’t enough for caterpillar ID. Diamondback moth is smooth, and tapered at each end. Imported cabbageworm is velvety. Cabbagelooper and smooth and moves in an inchworm fashion. Photo: Marissa Schuh, University of Minnesota Extension.

The three pests in the cole crop caterpillar complex make sure there is some level of pest pressure no matter where we are at in the season. Diamondback moth is the first arrival most years, with larvae chewing many small holes in leaves. In the spring the adult imported cabbageworm is one the first butterflies flying, though it sometimes takes a while to start seeing the velvety larvae feeding on brassicas. Cabbage looper is the last arrival. Unlike the other two, cabbage loopers cannot survive the Minnesota winters, so when they show up in the season is dependent on weather fronts bringing them up from the southern US.

While these three can sometimes be found feeding on one unfortunate plant, they vary in their timing and capacity for damage, so being able to tell them apart aids in scouting and spray decisions.

Scouting Thresholds for Caterpillar Pest in Broccoli and Cauliflower 

From Michigan State University’s Caterpillar Pests in Cole Crops.

• Plants in Seedbed Stage
• Imported Cabbageworm, Cabbage Looper - 10% of plants with larva
• Diamondback moth- 10% of plants with larvae
• Plants in Transplant through First Curd Stage
• Imported Cabbageworm, Cabbage Looper - 40% of plants with larvae
• Diamondback moth- 20% of plants with larvae
• Plant in Fist Curd to Final Harvest
• Imported Cabbageworm, Cabbage Looper - 10% of plants with larvae
• Diamondback moth- 10% of plants with larvae

Scouting Thresholds for Caterpillar Pests in Cabbage

From University of Minnesota

• Plants in Transplant through First Cupping Stage
• Imported Cabbageworm, Cabbage Looper - 10% of plants with larvae
• Diamondback moth- 10% of plants with larvae
• Plants in Transplant through First Curd Stage
• Imported Cabbageworm, Cabbage Looper - 30% plants infested with 1 or more medium-large imported cabbageworm larvae and/or 1 or more cabbage looper eggs or larvae.
• 50% plants infested with 5 or more larvae each.
• Plant in Cupping to Harvest Stage
• Imported Cabbageworm, Cabbage Looper - 10% plants infested with 1 or more medium-large imported cabbage worm
• larvae and cabbage looper eggs or larvae.Diamondback moth- 10% plants infested with 1 or more larvae each.

Bt is effective against all caterpillars in the complex. Understanding the nuance of using Bt is key in its successful control. Caterpillars must ingest the product while it is active. This means it needs to reach the caterpillars where they are feeding. Bt also breaks down in direct sunlight and washes away with water, so reapplication will be needed. Finally, Bt works best against these caterpillars when they are young and small (under a quarter inch in size). This highlights how scouting and early detection will help catch these pests when they are most easily controlled. 

 

Cabbage Maggot

The biology of cabbage maggot makes it an incredibly hard pest to control. The adult flies look similar to a common house fly, making scout or trapping untenable. The eggs are too small to scout for, and once they hatch, the maggots enter the plant, where they are protected from sprays. This means the place in the pest’s biology where we can intervene is at egg laying and egg hatching. 

Cabbage maggot larvae and pupa in the root zone of a wilted cabbage plant. Photo: Marissa Schuh, University of Minnesota Extension.

Predicting this is difficult, though we do have some tools. Models based on how temperatures impact cabbage maggot development can be found on the University of Minnesota VegEdge page. As temperatures are warm enough for cabbage maggot to develop (over 43° F), the map will start to change colors. In the spring, green is the color to look for. If the area you are in is green, it means we’ve had enough warm weather to allow cabbage maggot adults to start flying and laying eggs. Our window for cabbage maggot control is at egg-laying. This means that when the model says cabbage maggot adults are flying in our area, we should do what we can to prevent them from laying eggs. This could be by deploying a row cover, transplanting large plants with more robust root systems, or by holding off on planting our most susceptible crops.

As temperatures warm, cabbage maggot is able to develop. The models that make maps like this help us predict when we need to protect plants. Photo: University of Minnesota Extension

Flea Beetles

Flea beetles are typically an early spring problem in Minnesota, with adult beetles peppering seedlings and leaves with many small holes. In recent years, we have observed flea beetles causing problems as late as August. 

Caption: Whitney Cranshaw, Colorado State University, Bugwood.org.

Options for control include

• Waiting to plant until June when the first wave of flea beetles has passed
• Plant larger transplants that will be able to power through flea beetle feeding
• Using a row cover when plants are small (though this makes weeding hard)
• Use a trap crop like mustard, and plant the trap crop early so it will 7 to 14 days ahead of your main crop
• Kaolin clay can provide a physical barrier to feeding (but doesn’t always work)
• If 10-20% of the leaf area of plants are damaged, consider spraying

If you want to learn more about flea beetles, visit VegEdge or eOrganic.
For more information on chemical controls for the pests mentioned above, visit the Midwest Vegetable Production Guide.

Brassica Diseases

While bugs are often more noticeable, diseases like black rot and alternaria are increasingly worrisome. For more information on brassica diseases and their management, see this post from last year.

Variety selection

In preparation for our 2021 broccoli trials, we asked Minnesota growers to share their favorite current varieties with us. These are the varieties they shared:

• Favorite spring varieties: Belstar, Emerald Crown, Green Magic, Imperial, Gypsy, various sprouting broccolis, Amadeus, Covina
  
• Favorite fall varieties: Diplomat, Gypsy, Imperial, Emerald Crown, Marathon, Happy Rich, Arcadia
  

At the end of 2021 we should have some updated recommendations based on our trials! We’ll be trialing 17 early season and 17 late season varieties. While we’ll be primarily focusing on black rot and Alternaria tolerance, we have 60 growers participating in our trial, who will be able to provide valuable insight about additional characteristics.