Black Rot of Cole Crops in Hot, Humid Weather

Michael Celetti, Plant Pathologist – Horticulture Crops Program Lead, OMAFRA, Guelph

With the hot, humid weather experienced in Ontario recently, cole crop growers should be vigilant at monitoring for black rot in cole crops this summer. Black rot is considered a serious bacterial disease of cole crops worldwide. Hot (25-30^oC), wet and humid weather favour the spread, infection and disease development in cole crops. Black rot is more severe and widespread in fields that either receive a frequent early morning shower or are overhead irrigated during this heat. Early prevention will protect the crop from late infections that provide wounds for other pathogens to infect, resulting in storage rot problems in late fall and winter.

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 environmental conditions.

These pathogenic bacteria are spread by rain splashing. In fact, black rot bacteria are highest during periods of rain. The bacteria enter 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 theses hydathodes of infected plants and are rain-splashed to neighbouring healthy plants. Equipment, people, animals as well as overhead sprinkler irrigation can also spread the disease and result in significant losses.

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
• work in diseased fields at the end of the day (reduces the risk of spreading the disease to non infected fields on contaminated equipment)
• restrict activities in fields until later in the day when fields are completely dry
• 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 entrance 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 no 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 and disinfect used trays.