Alternaria brassicae (dark spot of crucifers)

An IPM programme for the control of A. brassicae has been developed (Eastburn, 1989). Some other studies have also presented an integrated multi-pronged approach to disease control (Kolte, 1985; Tewari, 1985; Humpherson-Jones, 1992; Saharan, 1992; Verma and Saharan, 1992; Seidle et al., 1995; Kharbanda and Tewari, 1996).

There are no known quarantine restrictions on A. brassicae, perhaps because the pathogen is dispersed effectively by wind, is widely distributed around the world, and does not possess an extremely high pathogenic variability.

A. brassicae survives on the stubble of crop and weed host plants (Tewari, 1985; Madej, 1986; Humpherson-Jones, 1989; Ansari et al., 1989; Kharbanda and Tewari, 1996). Therefore, crop rotation will reduce the severity of blackspot disease.Proper fertilization and adjustment of sowing and harvesting time based on local weather conditions will also help reduce losses caused by A. brassicae (Mian and Akanda, 1989; Dasgupta et al., 1991; Humpherson-Jones, 1992; Chung and Huang, 1993c; Sharma and Kolte, 1994; Verma and Saharan, 1994; Seidle et al., 1995; Kharbanda and Tewari, 1996). Weed control is important in reducing the severity of blackspot because A. brassicae has a rather wide host range (Saharan et al., 1982; Tripathi and Kaushik, 1984; Tewari, 1985; Ansari et al., 1990). Seed infection by A. brassicae declines significantly with time, especially in the tropics where the summer temperatures are high (Chahal, 1981; Humpherson-Jones, 1992; Kumar and Gupta, 1994; Verma and Saharan, 1994; Seidle et al., 1995; Tewari and Kharbanda, 1996; Rimmer et al., 1999). A. brassicae levels are lower in swathed than in straight-combined seed of B. rapa (Duczek et al., 1999). These may be a good ways of reducing seedborne inoculum of A. brassicae.

Oleiferous brassica species do not differ much in their susceptibility to A. brassicae, although some differences have been noted: B. carinata and B. napus are less susceptible than B. rapa and B. juncea (Skoropad and Tewari, 1977; Bhowmik and Munde, 1987; Conn and Tewari, 1989; Katiyar et al., 1994). Lately, there has been interest in B. carinata as a source of multiple disease resistance, including A. brassicae (Bansal et al., 2000; Dang et al., 2000; Zheng et al., 2001). Some genotypes of Chinese cabbage are highly resistant to A. brassicae (Cao et al., 1998; Fuyou and Tewari, 1999; Deng and Tewari, 2000; Yang et al., 2000; Wang and Tewari, 2001). Sinapis alba is fairly resistant to A. brassicae (Dueck and Degenhardt, 1975; Brun et al., 1987) and efforts are underway to transfer this resistance in Brassica background (Hansen and Earle, 1997). High degrees of resistance to A. brassicae occur in some wild crucifers (Tewari, 1991a; Tewari and Conn, 1993; Tewari, 1993) and research programmes are currently in progress in some laboratories around the world to try to transfer this resistance to crop brassicas (Hansen, 1998).Induction of resistance in B. juncea through the application of an avirulent isolate of A. brassicae has been reported (Vishwanath et al., 1999) and has the potential for being developed into a management strategy.

Some microbes which are competitive, antagonistic or parasitic on A. brassicae have been described (Tsuneda et al., 1976; Sharma et al., 1989; Sharma, 1989; Mercer et al., 1992; Boyko and Tewari, 1994; Garbe, 2000) but none has yet been developed for commercial application.

Many fungicides are active against A. brassicae (Tewari and Skoropad, 1979; Humpherson-Jones, 1992; Verma and Saharan, 1994; Seidle et al., 1995). Of these iprodione is perhaps the most widely used.

Various methods of seed treatments have been reviewed: see Humpherson-Jones (1992), Verma and Saharan (1994) and Seidle et al. (1995). Hot-water soak treatments recommend exposure to temperatures of 40-50°C for 20-30 min (Verma and Saharan, 1994). Rimmer et al. (1999) reported that hot-water treatment of seed at 45°C for 15 min was optimal both for reducing the fungal infection and for maintaining seed germination. However, soak treatments have a limited value because only small quantities of seeds can be treated at one time and the seeds must be dried after soaking (Humpherson-Jones, 1992).Many fungicide seed treatments give effective control of A. brassicae. Iprodione and fenpropimorph are used commercially to treat Brassica seed (Maude et al., 1984; Humpherson-Jones, 1992). Extracts of garlic and celery are also effective as seed treatments (Kuprashvili, 1996). Leaf extracts of Ocimum sanctum and Adhatoda vasica are also inhibitory to spore germination in A. brassicae and may potentially have a role in blackspot disease management (Ram, 1997).

Comprehensive disease-forecasting systems have not yet been developed. Humpherson-Jones (1991) indicated the possibility of timing fungicide sprays on the basis of predicted sporulation and infection periods. Seidle et al. (1995) observed that during late flowering, a symptom of Alternaria blackspot is the formation of pinhead lesions on fruits of rape and indicated that it may be possible to develop a disease-forecasting system based on the frequency of these lesions and weather forecasts. To these parameters one may also add cultivation and disease history of the field and neighbourhood, and some measure of inoculum build-up during the season, for example on leaves. A model for predicting sporulation of A. brassicae has been developed (Kennedy et al., 1999) and may help in developing blackspot forecasting strategies. Sangwan et al. (2000) showed that the Gompertz model can be helpful for planning the timing of protection measures for A. brassicae management.

Keys and standard-area diagrams have been developed for assessing blackspot infection in the field (Kolte, 1985; Conn et al., 1990; Verma and Saharan, 1994).

A. brassicae affects yield and quality of Brassicas in several different direct and indirect ways. These include pre- and post-emergence damping-off of seedlings, reduction of foliar and stem photosynthetic areas, accelerated senescence causing premature ripening, defoliation, floret abortion, spotting of upper leaves and curds of vegetable produce, infection of seeds through the fruit wall, reduction of the photosynthetic areas of inflorescent stems and fruit walls, and premature shattering of fruits (Tewari, 1991a, b; Humpherson-Jones, 1992; Verma and Saharan, 1994; Seidle et al., 1995). These effects result in the downgrading of vegetable produce, reduced seed yield, loss of seed during harvesting, increased green seeds, shrivelled seeds, reduced 1000-seed weight, increased dockage, poor seed quality in terms of reduced oil and protein content, and poor seed germination. The disease caused by A. brassicae inflicts high economic losses in some parts of the world. It has been economically important in the Indian subcontinent for a long time; B. rapa was the original economically important host and cultivation of the highly susceptible B. rapa var. yellow sarson has been greatly curtailed as a result of this disease. Tremendous proliferation of the susceptible B. juncea has further increased disease intensity in the area. Similarly, over several decades, intensification of oleraceous Brassicas (B. napus and B. rapa) cultivation, prolongation of the cropping season in some areas for some crops, and dispersal of A. brassicae-inoculum through seed may have contributed to the increase of this disease in Europe and Canada (Humpherson-Jones, 1991, 1992).

Blight caused by A. brassicae occurs with utmost regularity in northern, north-western and north-eastern India, and of all the countries where this disease occurs, maximum yield losses are reported from this country. The economically important hosts of A. brassicae in India include Brassica rapa var. yellow sarson, B. rapa var. brown sarson, B. rapa var. toria, B. juncea (Singh and Bhowmik, 1985; Chahal, 1986; Singh et al., 1990; Saharan, 1991; Sinha et al., 1992; Kumar, 1997; Ram and Chauhan, 1998), B. oleracea var. botrytis (Singh et al., 1990; Shyam et al., 1994), Eruca sativa (Jain, 1992), and Raphanus sativus (Sandhu et al., 1985). Brassica rapa suffers more yield loss compared to B. juncea (Chahal, 1986; Kumar, 1997). In a field experiment in the province of Himachal Pradesh in India, B. rapa var. yellow sarson suffered maximum yield loss (27.53%), followed by B. rapa var. brown sarson (25.01%) and B. juncea (20.28%) due to blight caused by A. brassicae (Kumar, 1997). In the same experiment, B. napus and B. carinata suffered yield losses of 17.16 and 10.72%, respectively. Both these are relatively uncommon crops in India. However, yield losses of up to 71.5% are not uncommon in India in B. rapa and B. juncea (Kolte, 1985; Singh and Bhowmik, 1985; Kolte et al., 1987; Verma and Saharan, 1994; Ram and Chauhan, 1998). Brassica oleracea var. botrytis is reported to suffer severe damage from germination until harvest, in storage, and in transit, in the province of Bihar in India (Singh et al., 1990). Losses in seed weight in E. sativa were estimated to be from 6.67-55.85% (Jain, 1992). Alternaria blight caused by A. brassicae var. phaseoli is also reported to cause yield losses of 25-40% in faba bean at elevations of <1600 m in the Garhwal Mountains in India (Bisht et al., 1997).A. brassicae is a major pathogen of rapeseed and mustard in some other parts of the Indian subcontinent too, including Nepal (Shrestha et al., 2000), Bangladesh (Ashrafuzzaman et al., 1996) and Pakistan (Shah et al., 2000). The disease is a major problem in the Inner Terai and Terai (plains) areas of Nepal, and yield loss estimates in rapeseed and mustard in the Chitwan district due to A. brassicae range from 20-50% (Shrestha et al., 2000). In a recent study from Pakistan, yield losses in different genotypes of rape and mustard were found to range from 8.62-17.71 and 13.58-38.50, respectively (Shah et al., 2000).

Besides the Indian subcontinent, diseases caused by A. brassicae also have an economically high profile in parts of North America and Europe. In Canada, detailed studies have been carried out in the provinces of Alberta and Saskatchewan. Disease epidemics in this region vary from place to place and from season to season, primarily depending on rainfall during the silique maturation phases of B. rapa and B. napus, severely damaging up to 75% of the crop (Conn and Tewari, 1990; Stonehouse, 2000). In the Canadian prairie provinces, yield reductions due to Alternaria blight have been estimated to be up to 36% (Conn and Tewari, 1990; Tewari, 1991a; Seidle et al., 1995; Duczek et al., 1999; Stonehouse, 2000). In 1990, the weather was conducive for A. brassicae development in Alberta and monetary losses in this province alone were estimated to be more than Canadian $23 million calculated using a Canadian $6.00 per bushel price of rapeseed at that time (Tewari, 1991a). A. brassicae is also a major pathogen of oriental cruciferous vegetables in southern Ontario, Canada (Cerkauskas et al., 1998). Unsightly spots caused by A. brassicae, affecting consumer acceptance, are common on the produce of oriental cruciferous vegetables being sold in the grocery stores in Alberta, Canada. Alternaria blight caused by A. brassicae is a minor disease on seed crops of brussels sprouts and cabbage in the USA (Babadoost and Gabrielson, 1979) but it is the most serious disease of B. napus and B. rapa in the high valleys of Mexico (Ponce and Mendoza, 1983). With the current increase in A. brassicae-susceptible oleiferous B. napus cultivation in some parts of the USA, the intensity of this disease is likely to increase in future.

A. brassicae is reported to cause head rot of cauliflower in Colombia resulting in 30% losses (Tamayo et al., 2001).

A. brassicae blight is a serious yield limiting disease on oleiferous and vegetable Brassicas in some parts of Europe (Humpherson-Jones, 1992; Paul and Rawlinson, 1992; Verma and Saharan, 1994). The disease is widespread in Germany and yield losses may be more than 50% in oilseed rape (Daebler et al., 1986; Daebler and Amelung, 1988; Humpherson-Jones, 1992). A. brassicae is also one of the main pathogens of Chinese cabbage under storage in Germany in the Schleswig-Holstein region (Todt and Schulz, 1987). In Lithuania, the spring oilseed crop is infected with A. brassicae every year resulting in it being the most common and important fungal disease of this crop; seed yield increased by 13.4-17.5% by the application of fungicide (Brazauskiene et al., 2000). In Romania, 28.4% losses were reported in stored cabbage due to A. brassicae, Botrytis cinerea and Rhizopus stolonifer (Tasca and Trandaf, 1984). Severe damage on purple broccoli, winter cauliflower and white cabbage have recently been reported from Belgium (Vanparys, 1998 a, b, 1999). Penetrating infections develop from the outer leaves in stored vegetable Brassicas requiring costly trimming and yield loss (Humpherson-Jones, 1992). Alternaria blight is also regarded as a major disease in France, Poland and the UK (Paul and Rawlinson, 1992). In 1978, Maude and Humpherson-Jones (1980) found A. brassicae to cause damage in some seed crops of cabbage and kale in England for the first time. Since then there have been many reports of infection and yield reduction by this pathogen on cruciferous crops in the UK (Anon., 1983; Ogilvy, 1984; Gladders, 1987; Sansford and Hardwick, 1992).

Brassica seed can be infected heavily with A. brassicae which can result in seed rotting, seedling mortality, increase in dockage and production of primary inoculum initiating early epidemics. A relationship of 1:0.88 between seed infection, and seed rotting and seedling mortality in rapeseed and mustard, using a rolled paper method in the laboratory, has been reported from India (Chahal and Kang, 1979; Shrestha et al., 2000). A. brassicae is common in oilseed Brassica seed in the Canadian prairies (Clear, 1992) and was shown to reduce seed germination significantly in B. rapa in a study from Canada (Rude et al., 1999). Many years ago samples of Brassicaceae seed with approximately 90% seed infection by A. brassicae and A. brassicicola were described from Denmark (Neergaard, 1945; Rotem, 1994). Seed samples of oilseed rape from Scotland and England, UK, have shown higher infestation with A. brassicae than with A. brassicicola (Anon., 1983). Seed from Scotland showed 80 and 45% infection, respectively, whereas those from England showed 40 and 28% infection, respectively. Verma and Saharan (1994) have discussed the extent of seed infection by A. brassicae in many countries.In Canada, some seeds of Brassica napus do not degreen completely upon maturation and retain chlorophyll and other pigments. Even 2% admixtures with green seed result in downgrading of the crop, as these pigments impart off-flavours and odours, reducing the shelf life of oil (Politeski Morissette et al., 1998). Thus, additional costly steps are required to remove these pigments from the oil (Green et al., 1998). The green seed problem is estimated to cause losses of nearly $100 million per year to producers in Canada. Mild freezing stress and switching in embryonic development from pre-desiccation to desiccation phase can cause the green seed problem in Brassica seed leading to retention of chlorophyll and other pigments at maturity (Green et al., 1998). Alternaria black spot has been shown to increase green seed counts in B. rapa (Seidle et al., 1995). Higher levels of A. brassicae are present in green seed (39%) than in normal seed (4%) of B. rapa grown under field conditions (Seidle et al., 1995). In growth chamber experiments, A. brassicae-inoculated B. rapa plants produced 9% green seed whereas there were no green seed in uninoculated plants (Seidle et al., 1995). There are many reports on changes in chemical composition of oilseed Brassica seed from crops infected with A. brassicae (Humpherson-Jones, 1992; Rotem, 1994; Verma and Saharan, 1994). Degenhardt et al. (1974) from Canada showed reductions in oil and protein contents of rapeseed upon field inoculation with A. brassicae and A. raphani. Reduction in oil content of rape and mustard of up to 35% due to infection with A. brassicae was reported from India (Ansari et al., 1988; Humpherson-Jones, 1992). In a recent study from Pakistan, oil content reductions in rape and mustard were reported to be from 5.26-17.94% and 8.57-17.50%, respectively, based on genotype of the crop infected with A. brassicae. However, there have been some conflicting reports as well. Reports from Australia (Stovold et al., 1987) and Canada (Seidle et al., 1995) did not find any significant relationship between A. brassicae and seed oil content.