Black rot is a serious bacterial disease of cole crops worldwide. With the hot, humid weather experienced in Ontario recently, cole crop growers should be vigilant at monitoring for black rot in their crops this summer. Hot (25-30°C) and humid weather favour the multiplication of the pathogenic bacteria, however; water splashing is required for disease spread, infection and development. The disease is more severe and widespread in fields that either receive a frequent early morning shower, frequent heavy rains or are overhead irrigated during warm weather. In fact, black rot bacteria counts in the atmosphere around infected crops are highest during periods of rain. Early prevention should help protect the crop from later infections. Black rot also provides an entry point for other pathogens, resulting in storage rot problems later. The pathogenic bacteria that cause black rot (Xanthomonas campestris) are often introduced into a field of cole crops on infected transplants but can also survive on cruciferous weeds during the summer and in contaminated crop residue left in or on the soil from previous years. At transplanting, the plants may not exhibit black rot symptoms due to unsuitable environmental conditions.

The pathogenic bacteria are easily spread by rain splashing. The bacteria enter leaves through wounds caused by damaging winds, hail or insects as well as special pores on the edge of leaves called "hydathodes". The disease spreads very quickly when bacteria-contaminated water droplets exude from the hydathodes of infected plants and are rain-splashed to neighbouring healthy plants. Equipment, people and animals can also spread the disease and result in significant losses.

On older plants, black rot symptoms often appear as yellow or dead tissue at the edges of leaves, similar to tip burn, except the lesion frequently progress into a U- or V-shape with the base of the U or V usually directed along a vein (Figure 1 and 2 ). Close inspection of infected leaves and stems may reveal black veins running through the infected tissue from which the disease gets its name (Figure 3). Lesions on leaves can expand down toward the base of the leaf causing the leaf to wilt and die.

Figure 1. U-shaped lesion with black veins on edge of a leaf is a typical symptom of Black rot in cauliflower, Brussels sprouts and broccoli.

Figure 2. V-shaped lesions containing black veins extending from the edge of a cabbage leaf along a major vein are typical.

Figure 3. Close-up of black rot lesion with blackened veins on Brussels sprout leaf.

During hot humid conditions, the bacteria can move from the leaf into the stem. Once inside the stem, the bacteria can move up or down to other parts of the plant including the roots. Severely infected cole crops such as kale and cauliflower tend to shed their leaves from the bottom up leaving only a tuft of distorted leaves separated from the root system by a scarred barren stem. Symptoms on cauliflower often appear as black flecks or scorched leaf margins. The curds of infected cauliflower heads often become blackened.

There are no magic bullets available to control black rot in cole crops and disease management relies mostly on sanitation. The following are a few tips to help reduce the risk, spread and development of black rot.

  • plant disease-free seed and transplants (3 infected seeds per 10,000 or 0.03% infected seeds can result in a black rot epidemic)
  • 3 year rotation with non-crucifer crops (cole crop residue takes about 2 to 3 year to completely break down)
  • reduce plant densities to allow good air circulation and facilitate the quick drying of plants, if possible
  • restrict activities in fields until later in the day when fields are completely dry
  • work in diseased fields at the end of the day (reduces the risk of spreading the disease to non infected fields on contaminated equipment )
  • control cruciferous weed hosts within and around the field (the pathogen can be water splashed up to 30 meters (~100 ft) from infected weeds to cole crop plants)
  • insect management will help to reduce disease (wounds caused by insects provide an entry point for the pathogen)
  • prepare crucifer crops for market away from fields and immediately chop and bury diseased tissue cut from plants
  • grow black rot tolerant cole crop varieties whenever possible particularly in fields neighbouring those that were planted to infected cole crops the previous year
  • do not apply excess nitrogen which encourages lush vegetative growth and can make the plants more susceptible to this disease
  • scout fields regularly and remove all infected plants if possible from the field
  • greenhouse sanitation is essential to produce seedlings that are free of black rot. Thoroughly disinfect all surfaces. Where black rot has been present in the greenhouse, discard or thoroughly disinfect used trays.