How to Identify Black Rot on Brassicas

Black rot is one of the most common diseases of broccoli, kale and other such crops brassica. It affects the leaves and often causes head rot.

Under humid conditions and without preventative practices, black rot can severely limit yield.

Some cabbage varieties have some degree of tolerance, but in most cases resistant cultivars are not currently available. Black rot is a bacterial disease that spreads in water and in conditions of high atmospheric humidity.

Therefore, irrigation management is essential to manage this disease caused by the bacteria Xanthomonas campestris pvcampestris.

read more: Bacillus subtilisa bacteria that fights disease.

Characteristics of black rot (Xanthomonas campestris p.v. campestris)

Some key features of disease caused by bacteria are:

• Black rot spreads more easily in warm conditions (above 25°C) and in conditions of high humidity (>80%).
• Bacteria Xanthomonas can infect any plant of the genus brassicabut tends to affect broccoli, cabbage, cauliflower and kale more severely.
• Black rot is often spread through previously infected seeds.
• The bacteria can stay on the plant debris and cut the leaves to the ground.
• Bacteria enter plants through pores in leaf edges called hydrothodes, where morning dew usually collects. It can also get inside the plant in case of hail damage, mechanical injury or insect bites.

Leaf symptoms of black cabbage leaf rot. Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

How to Differentiate Black Rot From Other Problems

The spots and visual symptoms created by black rot can be mistaken for other diseases. In general, the attack of bacteria such as Xanthamones they are easily identifiable and distinguishable from nutrient deficiencies, herbicide toxicity and other fungal diseases.

• Symptoms initially begin as yellowing of the leaf margins, which develops into characteristic V-shaped lesions.
• As black rot symptoms progress in crucifersplants may develop blackened vascular tissue and head rot.
• Plants can be affected at any stage of maturity.

Symptoms of cabbage black rot. Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

Tips to prevent illness

Against bacteria, there is no phytosanitary treatment, except the application of different forms of copper. Either way, if conditions are very humid, copper is unlikely to have much effect.

• Buy seedlings that are certified and in favorable sanitary condition at the time of delivery by the nursery or nursery.
• It is better to use drip irrigation instead of sprinkler or any other irrigation method that mists the environment or creates drips on the leaves of the genus brassica.
• Crop rotation is beneficial to prevent bacteria that may remain in the soil through plant debris from affecting the new crop.
• Increase airflow by spacing plants appropriately. A very dense planting frame increases production, but also the risk of losing the entire crop to this bacterium in high humidity conditions.
• After harvest, bury crop residues to facilitate composting.
• Disinfect field tools each time they are used.

prevent the spread

If black rot symptoms are limited to a few plants, remove infected plants as soon as symptoms appear. Be sure to remove the whole plant, not just the leaves. If all plants are affected, remove infected leaves.

If you touch infected leaves, wash your hands thoroughly before working on healthy plants of any plant brassica. If you use pruning tools, wash and disinfect them after touching infected plants.

Treatment of black rot

To date, no antibiotics are authorized against bacteria such as Xanthomonas campestris. The only phytosanitary products authorized are those formulated with copper, which have a preventive rather than a curative action.

If applied before high humidity conditions or just after mechanical damage such as hail, it can help heal wounds and prevent the entry of any type of bacteria or fungi. Hydrogen peroxide can also reduce the proliferation of bacteria present in the soil or under conditions of lack of oxygen in the roots.

Some formulations of copper permitted to control black rot in crops such as broccoli, cauliflower, cabbage or collards are as follows: