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Organic management of black rot in cole crops: an overview

Photo: Flickr, Scot Nelson
Author: Natalie Hoidal, University of Minnesota Extension - Horticulture

In certain regions of the state, black rot (Xanthamonas campestris pv campestris) is becoming a major disease of cole crops. Once black rot is present, it is exceedingly difficult to get rid of.

Black rot bacteria enter the plant through hydrathodes, or pores at the leaf margins. The bacteria can also enter the plant through wounds such as hail damage, mechanical injury, or in some cases insect feeding. Symptoms begin as yellowing at leaf edges, which turns into characteristic v-shaped lesions (see photo). As symptoms progress, plants can develop blackened vascular tissue in severe cases.

If you’re seeing black rot symptoms on your farm, there are a few strategies you can take to minimize the spread:

  • Remove infected plants as soon as symptoms appear (not just leaves, but the entire plant) if symptoms are isolated to a few plants.
  • If one field is impacted but not another, make sure to practice excellent sanitation. Clean shoes and tools before entering another field after working in a field infected with black rot. 
  • Various sprays can be used to prevent the spread. These products are not curative, but rather preventative - they do not get rid of the bacteria, they just help to prevent it from spreading. Copper products can be effective if applied at the right time, and some are approved for organic systems. Some growers have also had success with various sprays labeled “probiotic”, “plant defence-enhancing”, or sprays containing strains of antagonistic bacteria such as Bacillus spp. However, experimental results are mixed regarding the efficacy of such products, and by activating a plant's defense system, the plant may put less energy into growth and yield. Black rot spreads in warm (75° to 95°F), humid conditions, so keep an eye on the weather, and make preventative applications when conditions are right rather than taking a calendar spraying approach. If you are using chemicals, make sure to consult your organic certifier, only apply to crops listed on the label, and take note of personal protective equipment, re-entry interval, and pre-harvest interval requirements on the label. 

If you do not currently have black rot on your farm, it’s important to take steps to prevent it. Overall, there are four steps to prevent and mitigate black rot:

Start with clean seed

Under favorable conditions (warm, humid weather), a single infected seed in as many as 10,000 could lead to a black rot outbreak. In an informal survey of seed distributors, a former Extension plant pathologist found that different companies have very different approaches to screening for and treating black rot, and this information is not always disclosed on their websites. Before purchasing cole crop seeds (broccoli, cauliflower, Brussels sprouts, mustard greens, etc. and even brassicas in cover crop mixtures such as tillage radish), call your seed company and make sure they can certify that their seed is disease free.

Another approach is to treat your own seed; the most common approach is hot water treatment. See the University of Massachusetts Extension’s overview chart of water temperatures and length of treatment for various plant species and diseases. If you’re purchasing transplants, make sure that your supplier purchased disease free seed or treated their seed.

In addition to clean seed, use resistant varieties when possible. There are a number of resistant cabbage varieties on the market.

Maintain a four year rotation

If you’ve had black rot in your field, a full four year rotation is critical for prevention of the disease in the future. This includes cover crops - while tillage radish is great for aerating soil, make sure you’re considering it as part of your brassica rotation. While some sources say that only a 2 year rotation is necessary, waiting three to four years is recommended in Minnesota. In addition to rotation, make sure to completely till under any diseased tissue, and remove any severely diseased tissue throughout the season.

Weed management

Weeds can harbor disease, and in Minnesota there are plenty of weeds in the Brassicaceae family that serve as hosts for black rot. If you have shepherd's purse, black mustard, field mustard, or other weeds in the brassica family near your fields, the pathogen can travel from field margins into your crop.

Practice good sanitation

Even if you’re not seeing black rot symptoms, good sanitation is critical for disease prevention. Use new or sterilized trays and soil for seeding transplants, and clean your boots and machinery as often as possible.

Black rot (Xcc) bacteria can disperse short distances via irrigation water and rain. Water your plants in the morning, but after the dew has evaporated from plant leaves. Consider wider spacing between plants if you’ve had problems with black rot in the past, and maintain your irrigation equipment to prevent pooling. Consider water movement in site selection; avoid fields with poor drainage and fields that may receive runoff from fields where cole crops were recently grown.

Author: Natalie Hoidal, Extension Educator, Fruit and Vegetable Production Systems,
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