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Black rot symptoms can be observed in plants at any stage of growth. Symptoms induced by the pathogen are variable depending upon the host genus, species, cultivar, age of plant and environment. The disease appears primarily on the above ground parts of the plant, but in hosts such as turnip and radish the fleshy roots may be affected showing internal signs of vascular discoloration (Lambe and Lacy, 1982; Sherf and MacNab, 1986). The pathogen can cause turnip seedlings to collapse and rot (Roberts et al., 1987). Infected seeds usually do not show any sign of infection. However, seed discoloration or seed stain have been reported by Shiomi (1991). Severely affected rape and mustard seeds may appear shrivelled and discoloured (Sharma et al., 1992). The susceptibility of cauliflower to infection was greater when boron was deficient or excessive than when plants were grown at optimum levels of boron (Kumar and Kotur, 1991).
In young plants raised from infected seeds, cotyledons show dark discoloration in the margins which later turn black, shrivel and drop off (Matsumoto, 1975). In older seedlings infection usually takes place from the lower leaves through the hydathodes. Large, yellow to dark-brown lesions form along the margins of the lower leaves. These lesions develop in the shape of a wedge or 'V' around a hydathode. Initial lesions are surrounded by an indistinct pale-green, withered area. As disease advances, veins turn dark brown to black with a narrow yellow halo (Goto, 1992). Wilting of leaves and early defoliation and occasionally mishappen or deformed plants can be observed (Matsumoto, 1975). Leaf spots may appear dry with a brown centre. A network of black or brown veins may appear (depending on cultivar and bacteria strain) in the leaf spots and often in advance of yellow zones. The name black rot originates from the occurrence of a dark vein and black, rotting plants (Schaad and Thaveechai, 1983).The midribs when cut open show internal discoloration of the vascular tissues. Chlorotic spots or 'pale mottle' (Cook et al., 1952; Alvarez et al., 1994) may appear on young leaves before the typical lesions, wilting and blackening of veins without yellowing can also occur. Symptomless plants are common during the vegetative period until flowering. Pods from infected plants may show shrivelling and darkening of irregular areas. Early invasion of the pods usually results in the abortion of all seeds (Cook et al., 1952). During periods of high temperature when the plant is near maturity, soft rot may occur. Soft rot may also be due to secondary invasion of Pseudomonas or Pectobacterium species.
The type strain and several other clones of Xcc are known to cause a distinctive blight symptom that later develops into black rot. Other strains of the pathogen cause typical black rot symptoms but not blight (Alvarez et al., 1994). Mguni et al. (1996b) isolated three Biolog types (Xcc, XccA and XccB) from Brassica spp. cultivated in Zimbabwe. Type XccA strains were normally associated with plants showing initial blight symptoms, Biolog type XccB was mostly isolated from plants showing wilt and stem rot symptoms, while Biolog type Xcc was associated with 'V'-shaped lesions.
In cauliflower, numerous brown specks resembling peppery leaf spots may appear. Entire leaves may turn yellow or wilt and finally drop to the ground. Occasionally, affected plants have a long bare stalk with a tuft of leaves at the top (Sherf and MacNab, 1986). In certain susceptible cultivars, the infection sometimes occurs in the stems, causing internal blackening of the vascular tissue which may result in dwarfing, wilting or uneven or twisted plant growth (Goto, 1992). Curd rot symptoms caused by the black rot organism are characterized by the appearance of yellowish-brown to black areas on the fringes of the bolt surface often accompanied with bacterial ooze in wet and humid weather. The rot spreads downwards towards the base of the branches forming dark-brown to black streaks. The affected branches may lodge down near the forks (Shyam et al., 1994).
Certain weeds exhibit symptoms caused by the black rot organism. Yellow, 'V'-shaped lesions with brown centres on the leaf margin have been observed on infected Brassica campestris. Similar symptoms have also been observed on B. geniculata, B. nigra and Raphanus sativus, but the yellowing was less pronounced (Schaad and Dianese, 1981). Semi-oval necrotic and chlorotic lesions may appear on the margin of the leaves of Arabidopsis thaliana (Tsuji and Somerville, 1992).
Control measures have been recommended to minimize the threat of black rot. Management of the disease in crucifers is mainly focused towards the control of the pathogen in seed and reduction or eradication in the soil. Breeding for the development of varieties which are resistant to the disease has been attempted in many cases. A set of guidelines has been designed for the control of the disease which cover all aspects of crucifer seed production, transplants and crops.
Cultural Control and Sanitary Methods
Recommended measures in transplantation and crop production are: selection of land which has been free of undecayed cruciferous debris for over 12 months, use of pathogen-free seed or transplants of resistant varieties if possible, control of weeds and volunteer plants, removal of and destruction of diseased plants, use of pesticides to control insects that may mask disease symptoms, use of clean seedbed equipment, avoidance of clipping to toughen or reduce transplant size and spraying of transplants with water, and use of disinfected containers for transport of transplants into the fields or for shipment. Growers are also encouraged to follow cultural practices recommended for the production of vigorous and healthy plants, including the application of copper fungicides to prevent the spread of disease (Williams, 1980; Lambe and Lacy, 1982).
In Georgia, USA all crucifer seed lots for transplant must be assayed for the pathogen for which a zero tolerance in 10,000 seeds has been established. For direct seeding, a tolerance of 0.01% infected seeds has been established by the agar plating assay (Schaad et al., 1980c).
In India, the selection of planting dates was found to influence the incidence of black rot in cabbage and cauliflower (Santiranjan Bandyopadhyay et al., 1986b). In Kenya, a management scheme has been adapted to help farmers to keep the disease within tolerance levels. Farmers are recommended to raise nurseries in areas without records of cruciferous weeds and crops; proper timing of physiological maturity of the crop, use of dip irrigation to reduce secondary spread (in medium to low rainfall areas) and the use of grass mulch to reduce spread by water splash by rain or irrigation water are also advocated (Onsando, 1988, 1992).
Soil fumigation with metham-sodium in combination with seed treatment has been recommended for the production of disease-free seeds in Israel (Kritzman and Ben-Yephet, 1990).
Rotational cultivation of crucifers with non-cruciferous crops over a long period of time is often recommended. However, this recommendation could be difficult to practice in certain crucifer production areas of the tropics where the alternatives for crop production are limited (Onsando, 1992). In the humid tropics of Thailand where black rot does not survive in the soil, the use of black rot-free seeds and 1-year crop rotation have been considered (Schaad and Thaveechai, 1983).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Black rot is considered the most important worldwide disease of crucifers. The disease is known to exist in the cool coastal climates of northern Europe and North America but was seldom a problem there until the 1990s. Its potential for crop damage is also considered low in New Zealand, and parts of Australia. In many regions of Central and Eastern Europe, Central Asia (Kazakhstan), China, tropical and subtropical regions of Asia, Africa and South America where brassicas are common and cultivated without crop rotation, black rot is always present. Seed production in those regions is commonly associated with high levels of seedborne Xcc (Williams, 1980). Even minor, visually undetectable development of black rot may considerably increase damage to plants by soft rot caused by Erwinia carotovora, Pseudomonas spp. and other opportunistic pathogens (Djalilov et al., 1989).
For many years the disease was considered of relatively minor importance to crucifer growers in the major northern production areas of the USA and Western Europe. Outbreaks of the disease were sporadic and limited. During the late 1960s, early 1970s and 1990s the frequency and severity of the disease increased. Approximately 70% of several million transplants from one single seedbed were systemically infected in the USA in 1973 (Williams, 1980). In 1976, losses of $US 1 million were estimated (Kennedy and Alcorn, 1980). In Florida, two cabbage crops are commonly grown every year. If temperatures remain cool in the late winter and early spring, black rot does not become a problem, but if temperatures turn warm, serious outbreaks often occur (Schaad, 1988). In Illinois and Virginia, black rot is one of the most important diseases of cabbage and cauliflower (Lambe and Lacy, 1982; Eastburn, 1989). In Canada, rutabaga (swede) producers lost up to 60% of their crop to black rot during the winter of 1979-1980 (McKeen, 1981).
Nemeth and Laszlo (1983) reported black rot as the cause of considerable damage in cabbage and cauliflower in Hungary. Radunovic and Balaz (2012) reported the presence of black rot in cabbage, kale, broccoli and collard crops. In some regions of Russia, black rot caused 23-57% losses on susceptible cabbage cultivars (Ignatov, 1992; Djalilov et al., 1989). Recurrent black rot epidemics have been reported from Italy during 1992-1994 and 1997 (Caponero and Iacobelis, 1994; Scortichini et al., 1994; Catara et al., 1999). In Israel, black rot causes major economic losses in cabbage, cauliflower, radish and kohlrabi, especially during the winter season (Kritzman and Ben-Yephet, 1990).
In Korea, black rot is considered an important disease of cabbage (Kim, 1986). Surveys conducted in 25 crucifer fields in eight provinces in Thailand, where black rot is known to cause severe losses, revealed the presence of the black rot organism from plants showing disease symptoms in 21 fields (Schaad and Thaveechai, 1983). Infected seed lots were reported from commercial seed lots in Japan (Shiomi, 1992) and in 1997-1998 black rot infected from 50 to 90% of plants of susceptible cabbage cultivars grown in three prefectures of Japan (Ignatov et al., 1997b). During 1989-1992, Xcc caused seed yield reductions in cauliflower in India (Shyam et al., 1994). In Himachal Pradesh, curd rot of cauliflower has been a menace to the seed crop and is the cause of huge losses to farmers in India (Shyam et al., 1994). Black rot appears annually in Manipur near the end of February. Its effects are more severe (up to 50% losses) in susceptible cultivars (Gupta, 1991). The widespread occurrence of black rot in Rajasthan, with a high incidence of seed infection, can be the cause of severe losses (Sharma et al., 1992).
In Kenya, black rot is endemic and the cause of much damage (Onsando, 1988, 1992). The disease is considered of intermediate economic importance in Mozambique (Plumb-Dhindsa and Mondjane, 1984). Black rot is widespread in Zimbabwe where it is considered the most important disease of brassicas (Mguni, 1987, 1995). The pathogen was found in crucifer crops from the five agro-ecological regions of the country. Disease incidence was higher during 1994 (10-80%) than during 1995 (10-50%) (Mguni, 1996). In Mozambique, the disease was reported in the southern districts of Boane, Mahotas and Chòkwé. In the Boane district, the highest incidence of black rot was recorded on Copenhagen Market (70%), Starke (67.9%) and Glory F1 (67.3%). In Chòwké, Tronchuda (Portuguese kale) was the least affected Brassica crop (Bila et al., 2009).