Black rot

Black rot

Black rot is a name used for various diseases of cultivated plants caused by fungi or bacteria, producing dark brown discoloration and decay in the leaves of fruit and vegetables:...

, caused by the bacterium Xanthomonas campestris pv. campestris (Xcc), is considered the most important and most destructive disease of crucifers, infecting all cultivated varieties of brassicas worldwide . This disease was first described by botanist and entomologist Harrison Garman in Lexington, Kentucky, USA in 1889 . Since then, it has been found in nearly every country in which cabbage is commercially cultivated .

Host infection by Xcc can occur at any stage of the plant life cycle. Characteristic symptoms of black rot caused by Xcc are V-shaped chlorotic to necrotic lesions extending from the leaf margins and blackening of vascular tissues.

The pathogen thrives in warm and humid climates and is rapidly disseminated in the field. Use of clean seed, crop rotation, and other cultural practices are the primary means of control of black rot. However, in developing countries such as those in South and Eastern Africa, black rot remains the greatest impediment to cabbage cultivation due to unreliable "clean" seed, multiple croppings annually, and high susceptibility of popular local cultivars to the disease.

Hosts and symptoms

Members of the plant family Brassicaceae

Brassicaceae

Brassicaceae, a medium sized and economically important family of flowering plants , are informally known as the mustards, mustard flowers, the crucifers or the cabbage family....

 (Cruciferae), which includes cabbage, broccoli, cauliflower, oilseed rape, radish, and the model organism

Model organism

A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. Model organisms are in vivo models and are widely used to...

 Arabidopsis thaliana

Arabidopsis thaliana

Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...

are affected by black rot .

Host infection by Xcc causes V-shaped chlorotic to necrotic foliar lesions, vascular blackening, wilting, stunted growth, and stem rot symptoms . As the pathogen proceeds from the leaf margins towards the veins, water stress and chlorotic symptoms develop due to occlusion of water-conducting vessels by bacterial exopolysaccharides and components of degraded plant cell walls . The darkening of vascular tissues following bacterial invasion gives the black rot disease its name . Lesions produced by Xcc may serve as portals of entry for other soft-rot pathogens such as Pectobacterium carotovorum (formerly Erwinia

Erwinia

Erwinia is a genus of Enterobacteriaceae bacteria containing mostly plant pathogenic species which was named for the first phytobacteriologist, Erwin Smith. It is a gram negative bacterium related to E. coli, Shigella, Salmonella and Yersinia. It is primarily a rod-shaped bacteria. A well-known...

carotovora) and Pseudomonas marginalis

Pseudomonas marginalis

Pseudomonas marginalis is a soil bacterium that can cause soft rots of plant tissues. Based on 16S rRNA analysis, P. marginalis has been placed in the P. fluorescens group....

.

These symptoms may be confused with fusarium wilt of cabbage (fusarium yellows), caused by the fungus Fusarium oxysporum f. sp. conglutinans. In contrast to black rot, in which the pathogen invades leaf margins and causes chlorotic to necrotic symptoms that progress downwards in the plant, fusarium wilt symptoms first develop in the lower portions of the plant and move upwards. Furthermore, leaf veins invaded by Xcc turn black compared to the dark brown vein discoloration found in fusarium wilt.

Symptoms of black rot may vary widely among different species of crucifers. On cauliflower, Xcc infection via stomates causes black or brown specks, scratched leaf margins, black veins, and discolored curds. Additionally, the severity of symptoms and aggressiveness of the disease varies between different strains of the Xcc pathogen.

Disease cycle

The primary source of inoculum is Xcc infected seed . During germination, the seedling becomes infected through the epicotyl

Epicotyl

In plant physiology, the epicotyl is the embryonic shoot above the cotyledons. In most plants the epicotyl will eventually develop into the leaves of the plant. In dicots, the hypocotyl is what appears to be the base stem under the spent withered cotyledons, and the shoot just above that is the...

  and cotyledons may develop blackened margins, shrivel, and drop . The bacteria progress through the vascular system to the young stems and leaves, where the disease manifests as V-shaped chlorotic to necrotic lesions extending from the leaf margins. Under humid conditions, bacteria present in guttation droplets can be spread by wind, rain, water splashes, and mechanical equipment to neighboring plants .

The natural route of invasion by Xcc is through the hydathodes, though leaf wounds caused by insects and plant roots may also be portals of entry . Occasionally, infections occur through stomata. Hydathodes provide the pathogen a direct path from the leaf margins to the plant vascular system and thus systemic host infection. Invasion of the suture vein leads to production of Xcc infected seed.

Xcc can survive in plant debris in soil for up to 2 years, but not more than 6 weeks in free soil . Bacteria present in plant debris can serve as a source of secondary inoculum.

Environment

Warm and wet conditions favor plant infection by Xcc and the development of disease . Free moisture is required for host invasion, considering that the natural route of infection is through the hydathodes.

The optimum temperature range for bacterial growth and host symptom development is between 25° to 30°C . A slower rate of growth is observed at temperatures as low as 5°C and up to 35°C . However, infected hosts are symptomless below 18°C .

Management

Management of black rot relies heavily on cultural practices:

• Use of certified disease-free seeds and transplants
• Hot water treatment of non-certified seeds; chemical treatments with sodium hypochlorite, hydrogen peroxide, and hot cupuric acetate or zinc sulfate may also be used
• Control of insects
• Crop rotation with non-cruciferous plants (3-4 years)
• Removal of crop debris after harvest
• Control of cruciferous weeds that may serve as reservoir for the pathogen
• Sanitation (e.g. clean equipment, avoiding work in wet fields, etc.)

Resistant cabbage cultivars are available and include :

• Atlantis
• Blueboy
• Bravo
• Bronco
• Cecile
• Defender
• Dynasty
• Gladiator
• Guardian
• Hancock
• Ramada

Economic impact

Cabbage cultivation is a multi-billion dollar industry worldwide, reflecting its value as a vegetable crop, source of vegetable oil, component of fodder crop for livestock feed, and ingredient in condiments and spices. In 2007, the cabbage crop in the US exceed $413M (1.4M+ tons) . Black rot is considered the most important disease of cabbage and other crucifers because Xcc infections may not become apparent until the warm summer months (well after planting), the pathogen spreads rapidly, and losses due to the disease may exceed 50% in warm, wet climates . The importance of using disease-free seed and/or transplants is highlighted by the fact that “as few as three infected seeds in 10,000 (0.03%) can cause black rot epidemics in a field .” In transplant beds, an initial infection level of 0.5% can rise to 65% in just three weeks . In fact more recent work indicates that spread can be much more rapid than this: with overhead gantry irrigation, spread of the pathogen greatly exceeded symptom spread to the extent that in one experiment almost 100% of the transplants were infested in a block of 15 module trays (approx. 4500 plants) six weeks after sowing from a single primary infector. Modelling of the rate of spread in transplants indicates that the widely-used tolerance standard for seed health testing (0·01%) should be revised to 0·004%

Xanthan

Xanthan is an exopolysaccharide produced by Xcc. Commercially produced xanthan is used as a thickening food additive and lubricant, amongst other industrial applications .

Genomics

The genomes of three Xcc strains — ATCC 33913, B100, and 8004 — have been fully sequenced and are publicly available .