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Managing Black Rot of Cabbage with Resistant Varieties and Leaf Removal

Black rot on cabbage. M.Grabowski, UMN Extension

Project Overview

In the summer of 2018, Meg Gable, a UMN Agriculture/Food Business Management student with guidance from Michelle Grabowski, UMN Extension Professor, set out to learn more about how to best manage black rot of cabbage. The goal of the project was to compare susceptible and resistant varieties of cabbage and to determine if removing infected leaves would stop the spread of a black rot infection.
Veins turn black as the black rot bacteria moves through the leaf.  M. Grabowski, UMN Extension

Overview of Black Rot 

Black rot is a common disease of crops such as cabbage, cauliflower, broccoli and other brassicas. It is a bacterial infection that moves through the veins of plants, travels to the stem and affects brassica plants in many ways depending on variety and time of infection.

Cabbage is a labor-intensive crop and if infected, black rot can spread easily and destroy an entire crop. However, there is no simple fix for black rot. Seed treatments are effective in preventing disease but there are few options for control once a plant has been infected in the field.

Do resistant varieties prevent black rot?

Two varieties of cabbage were tested; Capture and Drago. Capture is marketed as a black rot resistant variety; it has a round head and broader leaves and is a milder type of cabbage, best for cooking. Drago is susceptible to a black rot infection. It is taller and denser than the Capture variety and would be better suited for coleslaw/salad recipes.

The varieties were seeded in the greenhouse mid-April and transplanted in mid-May. During the last week of July, black rot bacteria was rubbed onto one leaf per plant. The leaves were then punctured with a needle to allow for infection. The crop was harvested 4 weeks later. At that time, symptoms of black rot were present but there was no progression into the heads. At this time information about lesion length, head weight and stem discoloration were recorded.
Black discoloration in the veins of the cut stem of a cabbage leaf indicate black rot has moved into the stem. M. Grabowski, UMN Extension
The resistant variety, Capture, showed symptoms of black rot but little or no spread from the initial puncture wounds created to allow for infection. The susceptible variety, Drago, showed spreading of the black rot from the initially punctured leaf to other leaves on the plant. This was very noticeable in the susceptible variety that was closest to a patch of tall weeds. Tall weeds shade the crop and create a humid environment where the black rot bacteria thrive and spread.

Side Experiment in Coleslaw

A small, unofficial, side experiment was completed where each variety of cabbage was used to make coleslaw. One recipe was vinegar based and another was mayonnaise based. Coleslaw recipes vary greatly based on personal preferences, but each slaw with the Drago variety was preferred over the Capture in the coleslaw.

Will removal of infected leaves increase or decrease black rot?

Breaking off a black rot infected leaf from a kale plant. M. Grabowski, UMN Extension
A second crop was seeded in the greenhouse the first week of May and transplanted at the end of July. Just as in the first field, black rot bacteria was rubbed onto one leaf per plant and the leaves were punctured to allow for infection. After initial symptoms of black rot became noticeable, the infected leaves were broken off and removed from the field for half of the plants. Disease was allowed to progress naturally on the other half of the plants. Data was collected regarding number of infected leaves and lesion length.

Harvest was completed at the end of October. The heads were not large but data about lesion length, head weight and stem discoloration was collected.

The data showed the Drago variety that was infected without removing any leaves, had more infected leaves per plant, when compared to other plants. The Capture variety that was infected and did not have any leaves removed had the second highest amount of infected leaves per plant. The other plants showed no significant difference in number of infected leaves. This supports the idea that removal of infected leaves will stop the further spread of black rot.

It also needs to be pointed out that, breaking the leaves off does not appear to create a new wound for the black rot to infect. The recommendation is to remove infected leaves in dry conditions when disease spread is less likely.

Other observations about managing black rot

When black rot is present in your field, removal of plants and debris after harvest is essential. We observed black rot on wrapper leaves left in the field, surviving and spreading long after harvest. After harvest plants should be buried below ground or removed from the field. If this is not done and other brassicas are in surrounding fields, black rot will spread to those plants as well. This is not dependent on type or variety.

Also, if black rot infects late in the season, after heads are already well developed, removing infected leaves is not the best use of time due to the maturity of the plant. In our trial infection 4 weeks before harvest did not affect the growth of the cabbage or cause any stem discoloration.

About the Author

My name is Meg Gable; I am a senior at CFANS with a major in Agriculture/Food Business Management and a minor in Environmental Science, Policy Management. Participating in research has been a great privilege for me and allows me to understand the practices that can lead to better policies and decisions for the ag industry. I am grateful to Theodora and Arnold Johnson Research Grant for this opportunity. Thanks Cornucopia, the student organic farm on the St. Paul campus for providing land, seed and use of tools during the project.
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