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Black rot

V-shaped black rot lesions on cabbage.

Discolored vascular tissue in cabbage stem and petiole.

Black rot symptoms on cabbage transplants.

Figure 1. V-shaped black rot lesions on cabbage.

Figure 2. Discolored vascular tissue in
cabbage stem and petiole.

Figure 3. Black rot symptoms
on cabbage transplants.

Black rot is the most serious disease of crucifer crops world wide when environmental conditions (relatively high temperature and humidity) are favorable. The disease affects primarily aboveground parts of plants at any stage of growth and causes high yield and quality losses, especially in tropical and subtropical regions during the rainy season. All vegetables in the crucifer family, including broccoli, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, kale, mustard, radish, rutabaga, and turnip, are susceptible to black rot. Many cruciferous weeds such as Shepherd's Purse, wild mustard, and yellow rocket may also be hosts of this pathogen.

The characteristic black rot symptom on most cultivated crucifer plants is the appearance of yellow, V-shaped lesions along the margins of leaves. The point of the V-shaped lesion is directed toward a vein (Figure 1). When lesions enlarge, wilted tissue expands toward the base of leaves. Eventually the diseased areas become necrotic and the veins turn black or brown. The infection may move down the vascular tissue of petioles and then spread up and down the stems. When stems and petioles of an infected plant are cut crosswise or lengthwise, the black-brown vascular tissue with yellowish bacterial slime is observed (Figure 2). These symptoms may be confused with Fusarium yellows, except that Fusarium causes brown vein discoloration without bacterial slime. Moreover, symptoms of black rot may vary according to age of host, host genus, species, and cultivar and even environmental conditions. For example, symptoms on cauliflower may appear as numerous black or brown specks, scratched leaf margins, black veins, and discolored curds (Figure 3). Many cruciferous weed species do not exhibit any of these characteristic symptoms even when infected.

Symptoms

Causal Organism

Black rot of crucifer is caused by a bacterium, Xanthomonas campestris pv. campestris (Xcc). The bacteria can overwinter in plant debris, in and on seeds from diseased plants, and in and on weeds. The pathogen may survive in diseased crop residue buried in soil for up to 2 years, but not more than 60 days free in soil. The major source of these bacteria is infected seeds, which enable long-distance spread of the disease. The pathogen is spread within and between fields by splashing water, wind, insects, machinery, and irrigation or drainage waters. The bacteria infect the cotyledons and young leaves through natural plant openings (stomata, hydathodes) or wounds and then migrate between cells until they reach the xylem tissue where they spread throughout the plant. Free moisture is required for infection by the pathogen. After infection, symptoms may appear on plants within 7 to 14 days under optimum conditions (25 to 30 degrees C).

Management

Effective management of black rot of crucifers depends on the application of the following practices in combination:

  1. Use black rot-tested, disease-free seed grown in an arid production area.
  2. a. If source of the seeds is unknown, or infested seedlots must be used, treat seed with hot water to eradicate pathogenic bacteria. Cabbage, broccoli, and Brussels sprouts can be treated at 50 degrees C for 25 minutes, while seeds of cauliflower, kale, turnip, and rutabaga are treated for 15 minutes. However, this treatment may reduce the viability of seed. Therefore, some other chemical seed treatments, including, sodium hypochlorite, hydrogen peroxide, and hot acidified cupric acetate or zinc sulfate can be applied to eliminate the bacteria from crucifer plant seeds.

  3. Use certified disease-free transplants.
  4. Practice crop rotation where crucifers are grown only every 3 to 4 years to eliminate the inoculum sources from diseased crop debris in the soil.
  5. Good sanitation practices should be performed to prevent disease spread.
  6. a. Eliminate all volunteer crucifer plants from previous crops and alternative wild host plants within and around the field.

    b. Do not apply manure that may contain crucifer residues.

    c. Do not use sprinkler irrigation.

    d. Avoid working in the field when plants are wet.

    e. Do not allow machinery and equipment movement from infested areas to non-infested fields.

    f. Deep plow to bury all crucifer residues after harvest.

  7. Application of fixed copper pesticides in the field may help to reduce spread of the disease.
  8. 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.

Grape Black Rot

Black rot is one of the most damaging grape diseases. All cultivated varieties of grapes are susceptible to infection by the black rot fungus. If not controlled, some or all of the grapes within a cluster will be rotted. The disease is favored by warm, humid weather as is found during the summer throughout most of Ohio. Before good control measures were devised, vineyards along the Ohio River often were hard hit. Grape growers commonly lost most of their crop, and the grape industry was literally driven out of the area.

Symptoms

Symptoms of black rot first appear as small yellowish spots on leaves. As the spots (lesions) enlarge, a dark border forms around the margins. The centers of the lesions become reddish brown. By the time the lesions reach 1/8 to 1/4 inch in diameter (approximately two weeks after infection), minute black dots appear. These are fungal fruiting bodies (pycnidia) and contain thousands of summer spores (conidia). Pycnidia are often arranged in a ring pattern, just inside the margin of the lesions. Lesions may also appear on young shoots, cluster stems, and tendrils. The lesions are purple to black, oval in outline, and sunken. Pycnidia also form in these lesions. Fruit symptoms often do not appear until the berries are about half grown. Small, round, light-brownish spots form on the fruit. The rotted tissue in the spot softens, and becomes sunken. The spot enlarges quickly, rotting the entire berry in a few days. The diseased fruit shrivels, becoming small, hard, black and wrinkled (mummies). Tiny black pycnidia are also formed on the fruit mummies. The mummies usually remain attached to the cluster.

Causal Organism

Grape black rot is caused by the fungus, Guignardia bidwellii. Black rot survives the winter in cane and tendril lesions and fruit mummies. In the spring during wet weather, the pycnidia on infected tissues absorb water and conidia are squeezed out. Conidia are splashed about randomly by rain and can infect any young tissue in less than 12 hours at temperatures between 60-90 degrees F. A film of water on the vine surface is necessary for infection (Table 1). A second type of spore, an ascospore, may also be produced in overwintered fruit mummies. Ascospores are forcibly discharged into the air and can travel considerable distances. Research has shown that ascospores are an important source of primary infections in the spring.

Control

  1. Sanitation is important. Destroy mummies, remove diseased tendrils from the wires, and select fruiting canes without lesions. It is very important not to leave mummies attached to the vine. Research has shown that mummies on the ground release most or all of their ascospores before the end of bloom. Mummies left up in the trellis can produce ascospores and conidia throughout the growing season, thus making control of this disease much more difficult. If only a few leaf lesions appear in the spring, remove these infected leaves.
  2. Plant grapes in sunny open areas that allow good air movement. Proper row orientation to prevailing winds and good weed control beneath the vines also enable plants to dry more quickly during wet weather.
  3. A good fungicide spray program is extremely important. Early season control (bud break through bloom) must be emphasized. If controlled early, the need for late season (post bloom) applications of fungicide is greatly reduced.

Table 1. Leaf wetness duration and temperature necessary for infection by the black rot fungus.

Temperature
(degrees F)

Hours of leaf wetness
required for infection

   

45

No infection

50

24

55

12

60

9

65

8

70

7

75

7

80

6

85

9

90

12


Figure 12. Disease cycle of Grape Black Rot

Source: New York State Agricultural Experiment Station, Grape IPM Disease Identification Sheet No. 4.

The causal fungus overwinters in mummified fruit on the vine or on the ground. Spring rains trigger the release of airborne ascospores and/or rain splashed conidia from the mummies. Primary infections occur on green tissues if temperatures and duration of leaf wetness are conducive (Table 4.1). Recent research indicates that the majority of ascospores from mummies on the ground are discharged within a time period from one inch shoot growth to 10-14 days after bloom. If mummies are allowed to hang on the vines, they can discharge ascospores and conidia throughout the growing season.

Lesions on canes from the previous season can also produce conidia for a period of at least one month starting at budbreak. Cane lesions are probably most important in mechanically pruned or hedged vineyards that have an abundance of canes in the canopy. All green tissues of the vine are susceptible to infection. Leaves are susceptible for about one week after they reach full size. Brown circular lesions develop on infected leaves about 9 to 11 days after infection. Within a few days, black spherical fruiting bodies (pycnidia) form within the lesions. Each one of these pycnidia can produce a second type of spore (conidia). These conidia are spread by rain splash and can cause secondary infections of leaves and fruit throughout the growing season. It is important to emphasize that a single ascospore can cause a primary infection (leaf lesion). Within each leaf lesion (primary infection), many pycnidia form. Each pycnidia can produce hundreds of thousands of conidia, each of which can cause another infection (secondary infection) later in the season. Therefore, it is extremely important to control the early season primary infections caused by ascospores. Remember, infection by one ascospore can result in the development of millions of secondary conidia in the vineyard.

The fruit infection phase of the disease can result in serious economic loss. Berries are susceptible to the infection from bloom until they begin to ripen (reach 5 to 8 percent sugar). An infected berry first appears light brown in color. Soon the entire berry turns dark brown, and then black pycnidia develop on its surface. Infected berries eventually turn into shriveled, hard, black mummies. These mummies also serve as a source of secondary inoculum later in the growing season and are the primary means by which the fungus overwinters.

The bottom line for black rot control

In order to effectively control black rot with minimal fungicide use, primary infections that develop from ascospores must be controlled. If ascospores are prevented from infecting fruit and leaves early in the season, no further black rot control measures are needed after the supply of ascospores is depleted. However, if early season infections are not controlled, additional fungicide protection may be needed throughout the summer to protect fruit against secondary infections by conidia. The number of conidia produced in just a few early-season leaf lesions is tremendous.

Important cultural note

Sanitation is critical to successful black rot control. As mentioned above, mummies are an important overwintering source of the black rot fungus. If all mummies and infected canes are removed from the vineyard, there is no source of primary inoculum in the spring and, thus, the disease is controlled. Any practice that removes mummies and other infected material from the vineyard will be beneficial to the disease management program. If all mummies cannot be removed from the vineyard, it is extremely important that they are not left hanging in the trellis. As mentioned previously, mummies on the ground appear to discharge their ascospores early in the season, while those hanging in the trellis may discharge ascospores throughout the growing season.

Black rot affects many of the crucifer family of plants including cabbage, cauliflower, and brussels sprouts. The leaves of affected plants will develop wedge-shaped areas yellow in color. Plants will have black or discolored areas inside the stem.

Plant in a well drained location to discourage rotting. Purchase plants that are certified as being disease free, such as those that are grown in the far west. Also, purchase seed that has been hot water treated. Remove and destroy diseased plants to prevent the spread of the disease. Frequent crop rotation is advisable.

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