What Is It? | Facts in Depth | For the Professional Diagnostician
Seedlings are very susceptible to black rot infection. When infected, seedlings become systemically infected, exhibiting chlorosis on the young leaves. As the disease progresses, seedlings prematurely drop leaves and eventually die.
On mature plants, leaf edges are chlorotic or necrotic. Lesions form in a distinct “v-shape,” with the base of the “V” directed along the vein. Closer inspection of these infected areas with a hand lens reveals vein discoloration. These veins usually appear brown to black in color, hence the name black rot. Lesions eventually expand downward toward the base of the leaf and may move down the vascular tissue of petioles and then spread up and down the stems. When infected stems and petioles are cut crosswise or lengthwise, vascular tissue is black to brown in color with yellowish bacterial slime. This leads to wilting and eventual death of the plant.
The disease black rot is caused by the bacterium Xanthomonas campestris pv. campestris. The bacteria can overwinter in plant debris and weeds. The pathogen can also be introduced into the field by infected seeds and transplants. When hosts are present, the bacteria can enter through wounds and natural openings, such as the hydathodes and the stomates, of young seedlings. The pathogen can survive on leaf surfaces for several days and disseminate in the field by rain splash, wind, insects, machinery, and irrigation or drainage waters. Once a host is infected, symptoms can develop 7 to 14 days after infection under optimum conditions, starting with chlorosis. V-shaped lesions form on the leaves, which are characteristic of black rot. As the disease progresses, the bacteria start producing a sticky polysaccharide known as xanthan. Xanthan clogs the vascular tissue of the host, turning the veins in the plant brown and black. The above vascular tissue then wilts and dies. The bacteria can then move via the vascular system and infect the plant systemically until the plant eventually dies. The bacteria can survive in the free soil in infected plant materials for up to two months or for years when buried deeper in the ground.
Favorable Environmental Conditions
The pathogen favors warm, humid environmental conditions with occasional periods of rainy weather. Rainy or damp days help with the spread of the disease and temperatures of 75°F to 95°F are most favorable for disease development.
Often Confused With
- Nutrient deficiencies – When there are nutrient deficiencies in crucifers, the symptoms can resembles those of black rot. For example, a magnesium deficiency in crucifers causes chlorosis on interveinal areas of lower leaves. These chlorotic regions eventually become necrotic.
- Fusarium yellows – Symptoms are very similar to black rot; however, vein discoloration is brown, and bacterial slime is not present.
Young seedlings are the most susceptible to black rot infection, so scouting for symptoms in the early growth stages is very important. It is especially important to scout after periods of rainfall in warm and humid weather. On mature plants, a very distinct chlorotic V-shaped lesion forms on the leaves. Within these chlorotic regions, the veins become brown or black in color.
Currently, there is no threshold information for black rot on crucifers.
- Use host resistant plants – There are many Brassica plants that are highly or partially resistant to black rot. A few black rot-resistant cultivars of cabbage and other crucifers are commercially available. These resistant cultivars should be used in crucifer growing regions where black rot is a common problem. Consult the Midwest Vegetable Production Guide for Commercial Growers for additional information on variety selection (https://ag.purdue.edu/btny/midwest-vegetable-guide/Pages/default.aspx).
- Plant clean seed – Planting certified disease-free seeds is an effective way to prevent introduction of block rot into the field. Hot water treatment can be used to eliminate Xanthomonas both inside and outside the seed coat. Some seed companies offer hot water-treated crucifer seeds to reduce the risk of black rot. Refer to the link “Hot Water and Chlorine Treatment of Vegetable Seeds to Eradicate Bacterial Plant Pathogens” for more information about the hot water treatment method. This treatment may reduce the viability of old or damaged seed. Some other chemical seed treatments, including, sodium hypochlorite (chlorine), hydrogen peroxide, and hot acidified cupric acetate or zinc sulfate can be applied to crucifer seeds, but do not eliminate bacteria under the seed coat.
- Use sound sanitation practices – Removing infected plant material from the field and destroying cull piles will help prevent the spread of the disease; this includes any cruciferous weeds serving as volunteer crops. Rotating hosts with non-hosts in the field is also a great way to prevent black rot. To be safe, a rotation 3 to 4 years away from crucifers will help eliminate any remaining plant material that is infected with black rot.
- Prevent splash from irrigation – Avoid using any sprinkler irrigation systems because this will cause “rain splash,” which is one way the pathogen can move from diseased to healthy plants.