Alternaria brassicicola (dark leaf spot of cabbage)

The IPM approach recommended for A. brassicae (Eastburn, 1989) can also be applied to A. brassicicola. Fontem et al. (1991) found that intensification of dark leaf spot over time is slower than many other common leaf spot and rust diseases, and suggested that this disease could be quite easily controlled by planting disease-free seed and avoiding inoculum from debris. Planting disease-free seed should be relatively easily accomplished. However, it would be difficult, if not impossible, to avoid inoculum from infested debris. Spores are mostly contained within the crop during the growing season, but large numbers of spores are released during harvesting and deposited for some distance downwind (Humpherson-Jones, 1992). Kolte (1985), Humpherson-Jones (1992), Saharan (1992), and Verma and Saharan (1994) have all given many control measures for this disease.

A. brassicicola is distributed around the world, is widely dispersed and has rather low strain variation. Perhaps for these reasons there are no known quarantine restrictions.

A. brassicicola survives on crop debris, seed and in association with weed hosts (Humpherson-Jones and Maude, 1982; Humpherson-Jones, 1989, 1992; Chung and Huang, 1993a; Cobb and Dillard, 1998). Therefore, crop debris management, for example through crop rotation and deep tillage, the use of clean seed and proper weed control should alleviate dark leaf spot disease. Seed infection by A. brassicicola declines very slowly with time unlike that caused by A. brassicae (Humpherson-Jones, 1992; Sivapalan and Browning, 1992). Therefore, storage of seed is not a good way to reduce the seedborne inoculum of A. brassicicola. Adjusting the sowing date affects disease severity (Mian and Akanda, 1989; DasGupta et al., 1991). The disease is also affected by various fertilizer regimes (Mian and Akanda, 1989; DasGupta et al., 1991; Walker and Booth, 1994).

The biochemistry and molecular biology of resistance to A. brassicicola is being investigated as a model system by many workers. Deficiency of camalexin, a phytoalexin, production increases the susceptibility of Arabidopsis thaliana to A. brassicicola (Browne et al., 1991; Thomma et al., 1999; Zhou et al., 1999). Jasmonic acid (JA) and ethylene (ET) are involved in providing basal resistance responses of the host to A. brassicicola, and JA and ET also mediate the resistance response in induced systemic resistance (Ton et al., 2002). However, microarray analysis has shown regulatory interactions and coordination among different signalling pathways, especially between salicylate and jasmonate pathways (Schenk et al., 2000).Resistance to A. brassicicola has been identified in some selections of vegetable, oleiferous, and weedy crucifers (Galvez-Ramirez et al., 1988b; Ahmad et al., 1991; Sharma et al., 1991; Humpherson-Jones, 1992; King and Dickson, 1994; Scholze and Hammer, 1998; Dzhalilov et al., 1999; Westman et al., 1999); this resistance, however, may not be high. Multiple disease resistance, including that for A. brassicicola, has been identified in some crucifers (Scholze and Hammer, 1999).Mora and Earle (2001) described significantly increased resistance to A. brassicicola in broccoli transformed with Trichoderma harzianum endochitinase gene. Sinapis alba (white mustard), Capsella bursa-pastoris (shepherd's purse) and Camelina sativa (false flax) are resistant to A. brassicicola and efforts have been made to transfer this resistance to B. oleracea through somatic hybridization (Hansen, 1998; Sigareva and Earle, 1999; Sigareva et al., 1999).

Several bacteria have been patented as they are inhibitory to postharvest development of A. brassicicola in cabbage (Leifert et al., 1999). Some naturally occurring compounds have potential for being used in the management of A. brassicicola. Fistupyrone, a metabolite from Streptomyces sp., inhibits infection by the pathogen (Igarashi et al., 2000) and potato alkaloids were shown to be inhibitory to A. brassicicola (Fewell and Roddick, 1997).

Many fungicides have been tested as foliar sprays or as seed-treatment agents (Maude and Humpherson-Jones, 1980b; Humpherson-Jones and Maude, 1982; Humpherson-Jones, 1992; Babadoost et al., 1993; Panja et al., 2000). Tolerance of isolates of A. brassicicola to iprodione has been reported (Huang and Levy, 1995).

Various physical, chemical and biological methods of seed treatment have been reviewed by Humpherson-Jones (1992), Verma and Saharan (1994) and Maude (1996). Hot-water soak treatment is not especially useful because only small lots can be treated and the seeds have to be dried afterwards. Treatment of seed with fungicides such as iprodione is useful (Maude and Humpherson-Jones, 1980b; White, 1984; Humpherson-Jones, 1992). None of the biological seed treatment methods has yet been used commercially.

A sporulation prediction model for A. brassicicola has been developed (Kennedy et al., 1999). However, disease forecasting systems are not well developed for dark leaf spot disease. There is a need to develop them further, for example using the work of Humpherson-Jones (1991).

The disease assessment keys used for grey leaf spot (Alternaria brassicae) can also be used for dark leaf spot (Kolte, 1985; Conn et al., 1990; Verma and Saharan, 1994).

Although A. brassicicola occurs both on oleiferous and vegetable (oleraceous) Brassicas, the latter are the primary hosts of this pathogen, suffering considerable yield reduction (Humpherson-Jones, 1992). However, the leaf spot of oilseed rape in Germany is mainly caused by A. brassicicola (Daebeler et al., 1986). A. brassicicola causes economic losses in several different ways (Humpherson-Jones, 1992; Strandberg, 1992; Rotem, 1994; Verma and Saharan, 1994). Seed infection causes reduced germination and seedling vigour, in addition to pre- and post-emergence damping-off, and affects the sale and use of infected/infested seed. Lesions on leaves, stems and siliques reduce the photosynthetic area and accelerate sensescence in the plant. Heavily infected seeds add to dockage. This pathogen is responsible for major seed yield losses in the oleraceous Brassicas and this is the most important component of its economic impact. The unsightly cosmetic blemishing or rotting of the head or wrapper leaves in vegetable Brassicas as a consequence of disease causes downgrading and crop losses in both fresh and stored produce. A. brassicicola often occurs in conjunction with A. brassicae and some other pathogens of the Brassicaceae. This confounds precise estimates of losses caused individually by this pathogen in the field. A. brassicicola is distributed worldwide, and is established and widespread in many countries (Anon., 1988; Verma and Saharan, 1994).

Brussels sprouts were heavily infected with A. brassicicola in the Irish Republic (Ryan et al., 1984). Major yield losses have been reported from the UK. Maude and Humpherson-Jones (1980) noted seed yield losses of 80% in B. oleracea in some years. Maude et al. (1986) reported 86% infection in cabbage seed and Humpherson-Jones (1985) reported that 88% of seed samples and up to 55% of seed of B. oleracea-types were affected by A. brassicicola in the UK. In another study from the UK, 48 and 42% of oilseed samples were infested with A. brassicae and A. brassicicola, respectively. Seed samples from Scotland showed higher infestation (80 and 45%, respectively) than those from England (40 and 28%, respectively) (Anon., 1983). Further studies reported damage to Brassicas by species of Alternaria as being associated particularly with seed crops, especially by A. brassicicola, but A. brassicae was found to be the major pathogen of oilseed rape in Scotland (Anon., 1985). Levels of infection caused by A. brassicicola, however, vary in the UK (Sansford and Hardwick, 1992), and may be based on the location and year. This pathogen also causes storage rot of white cabbage in the UK (Geeson, 1978). Extremely high economic losses caused by A. brassicicola have been reported from the Netherlands. Neergaard (1945) described about 90% of the Brassicaceae seed being infested with A. brassicae and A. brassicicola, and a 70% yield reduction in B. oleracea seed crops as a result of the latter pathogen. Belgium is also a hot spot of the disease caused by A. brassicicola on purple broccoli (Vanparys, 1999), winter cauliflower (Vanparys, 1998a), white cabbage (Vanparys, 1998b) and Brussels sprouts (Vanparys, 1998c). The disease is generally severe and occurs in combination with A. brassicae. A. brassicicola is economically important in several other European countries too. This fungus and Mycosphaerella brassicicola are the most important pathogens of winter cauliflower seed plants in France (Jouan et al., 1972). A. brassicicola is the main causal agent of leaf spot of winter rape in northern Germany, the losses ranging from 20% in a medium infestation to >50% in a severe attack (Daebeler et al., 1986). This pathogen is also the main pathogen of stored Chinese cabbages in Schleswig-Holstein in Germany along with Erwinia sp., A. brassicae and Leptosphaeria maculans (Todt and Schulz, 1987). In Austria, A. brassicicola and Pseudocercosporella capsellae, L. maculans and A. brassicae are important pathogens of Chinese cabbage and yield is improved by about 35% following fungicide application (Bedlan, 1987; 1991). A. brassicicola was most widely found, particularly on cabbage seed, in a study on the seedborne fungi of cultivated cruciferous plants in Finland and 20% seed infestation was found to cause 10% damping-off in a sand substrate (Tahvonen, 1979). Fungicide and insecticide applications resulted in significantly higher (2-22 times) seed yield of cauliflower in the Czech Republic in crops infected with A. brassicicola, A. brassicae and L. maculans (Kudela et al., 1978). A. brassicicola rot causes serious postharvest damage in cauliflower in Italy (Menniti and Casalini, 2000). In Poland, A. brassicicola was present in 0.5% kohlrabi to 100% white cabbage seed but did not appear to affect emergence (Tylkowska and Beresniewicz Dudala, 1988). Blight caused by A. alternata, A. brassicicola and A. brassicae is also the most damaging disease of Crambe abyssinica in Poland (Kwasna, 1992).

Knox-Davies (1979) recovered A. brassicicola from 41-45%of naturally infected seed of Brussels sprouts from South Africa. It was also seedborne in Japanese radish in the Transvaal region of the country (Holtzhausen, 1978).

In Washington State, USA, Brussels sprouts and cabbage seed fields were severely affected by A. brassicicola and A. brassicae, of which the former was more virulent on seed plants (Babadoost and Gabrielson, 1979). These crops suffered serious seed losses and reduced seed germination, and A. brassicicola was found in 79% of the seed production fields (Babadoost and Gabrielson, 1979; Strandberg, 1992). A. brassicicola is not common in Canada.

Alternaria leaf spot caused by A. brassicicola was present in 95% of the areas surveyed in Pernambuco, Brazil, during 1997 and 1998, and caused impairment of production (Azevedo et al., 2000).

A. brassicicola is a major pathogen of cabbage in the Mie prefecture in Japan causing spotting in adult plants and often hypocotyl rot and damping-off in plug seedlings (Kubota and Abiko, 2000). Field infection of 36.4% with A. brassicicola in kale has been reported from the Korean Republic (Choi et al., 1998). A high proportion of commercially available B. oleracea seed from Victoria, Australia was contaminated with this pathogen (Sivapalan and Browning, 1992). A. brassicicola was detected in 26 out of 44 seed samples and in 24-37% seed (with 4-8% embryo infection) from this region. Inoculation of seed with this pathogen resulted in loss of vigour and death of the seedlings. Only A. brassicicola is widespread in this region; A. brassicae was not detected in any of the seed samples. In another study from the same general area, A. brassicicola was present in 29 out of 38 seed lots of broccoli, infecting 68% of seed overall (Sivapalan, 1993). A. brassicicola and A. brassicae are also reported to cause blight of B. juncea in Bangladesh (Mian and Akanda, 1989; Ayub et al., 1996). Both species are also widespread on Brassicas in India. They are reported to cause disease on many crop plants in the Darjeeling Hills of NE India (Das et al., 1998). On B. juncea in West Bengal, India, 38 and 14% reduction in seed yield and in oil content, respectively, resulted from infection (Dasgupta et al., 1991). On the cauliflower seed crop in Bihar, India (Sinha and Prasad, 1989), and on B. rapa and B. juncea in Uttar Pradesh, India, losses of 46.57 and 35.38%, were recorded, respectively (Kolte et al., 1987). Eruca sativa in Haryana, India has also been infected by this pathogen (Sharma et al., 1993).