Septoria apiicola (black blight)

Leaves Under optimum conditions for disease development, the initial visible symptoms are small chlorotic flecks which enlarge into brownish-black to grey spots, reddish-brown in the centre, variable in size, 1-10 mm wide, with or without a well defined dark reddish-brown margin. Numerous pycnidia appear within the leaf spots (Cochran, 1932; Gabrielson and Grogan, 1964; Sheridan, 1968a).Leaf stalksInfection of stalks depends on inoculum density. Lesions on stalks are elongated and brown without a well defined margin (Gabrielson and Grogan, 1964; Snowdon, 1991).SeedsInfected seeds usually bear pycnidia which can be detected by visual or microscopic examination of the seed coat. This would reveal mycelium in the pericarp and testa of infected seeds. When an infected seed is soaked in water, pycnidia in the seed coat swell and become easy to distinguish.

Introduction

Celery is an important salad crop and ranks as the third most important in the USA (Lacy et al., 1996). The unprotected crop is liable to heavy losses by Septoria infection during crop production, storage and transport. Control of Septoria blight is essential to prevent heavy crop damage and susceptible cultivars being wiped out, resulting in a serious economic impact on crop production in countries where the crop is grown in conditions favourable for disease development.

Host-Plant Resistance

To date, screening of celery cultivars for late blight resistance has shown no pronounced resistance but only differences in susceptibility to S. apiicola (Thomas, 1921; Cochran, 1932; Marshall, 1960; Dixon, 1988; Ochoa and Quiros, 1989). The notable results of selected trials are in Italy, where the American celery cultivars Utah 52-70 and Florida 683 were shown to be much more susceptible to S. apiicola than two of the Italian cultivars (Bazzi and Dalli, 1973). In Czechoslovakia, in a trial of 42 celeriac cultivars by the F1 hybrid Vatters Globus x Prazsky Obrovsky (Prague Giant) was the most resistant, followed by Wiener Reisen (Viennese Giant) and Wiener Markt (Viennese Market) (Janyska et al., 1979). In another trial, six varieties with superior resistance were recognized (Moravec, 1980). In Poland, as a result of self and sib pollination in celery cultivars, celery lines more resistant than currently available resistant varieties to infection by S. apiicola have been selected for breeding (Wasilewska and Masternak, 1987).

During the period 1959-1986, breeding by hybridization, double crossing and selection for performance to resistance to artificial infection, a new celeriac cultivar named Kompakt with improved quality and increased resistance to S apiicola, was developed in Czechoslovakia (Moravec et al., 1988). In Canada, no celery cultivars are known to show a high level resistance to S. apiicola but cultivars Emerson Pascal, Florida Green Pascal, Earligreen, Green Giant and June-Belle exhibited tolerance (Cerkauskas, 1994).

Attempts have also been made to transfer resistance from novel sources of wild Apium species to the cultivated celery A. graveolens. As a result of crossing A. graveolens with the wild Apium species, A. chilense and A. panul, the resulting F1 hybrids had intermediate resistance, useful for breeding celery lines with improved resistance (Ochoa and Quiros, 1989).

In Bulgaria, breeding by crossing celery cultivars Pioneer and Prolet with parsley cultivars Listen and Berlinski and with Festival 68 (parsley X celery), new leaf forms were produced having tender leaves rich in nutrients with celery aroma and certain lines were relatively resistant to S. apiicola (Madzharova, 1973). In the USA, Apium graveolens var. dulce cv. Golden Spartan was successfully hybridized with Petroselinum hortense (which is immune to S. apiicola blight) and from over 1000 F1 seedlings, three F1 hybrids which were intermediate for most characters, were selected. In the F2 generation, about one-third of the plants showed some resistance to S. apiicola (Honma and Lacy, 1980).

Crop improvement by somaclonal variants of celery by selection for resistance to S. apiicola has been investigated by culturing in the fungal culture filtrate or co-culturing callus cells with isolates of S. apiicola and regenerating whole plants from callus cultures started from axillary buds of celery cultivars. The results have been claimed as promising for breeding celery lines with improved resistance (Wright and Lacy, 1988; Donovan et al., 1994; Evenor et al., 1994).

Cultural Control

Since conidia on seeds remain viable for two years under normal conditions it has been suggested that celery should not be grown within two years in fields which previously produced infected celery or to practise crop rotation with a non-susceptible crop such as onions (Lacy et al., 1996). Field sanitation, crop rotation involving at least a 2 year break between planting susceptible varieties on the same land, removal of plant debris, deep ploughing at the end of each season, destruction of infected plants after harvest and the avoidance of excess nitrogenous fertilizer have been recommended (Maude and Shuring, 1970; Dullahide, 1979; Bedlan, 1985; Snowdon, 1991). Trials in the use of pig manure as fertilizer, either alone or mixed with industrial flax waste, have been carried out (Vulsteke et al., 1994b; Vulsteke et al., 1995a).

Biological Control

Attempts to control Septoria leaf spot of celery in the field and greenhouse with an antagonist, Trichoderma harzianum, produced only partial control when the antagonist was applied 5 days before inoculation and produced no control when applied after inoculation of celery with S. apiicola (Ciccarese et al., 1995).

Septoria disease of celery is often destructive, affects yield, reduces market value, causes heavy crop losses when the crop is not protected in the seedbed or field, during transport (Middleton, 1952; Marshall, 1960; Conners, 1967) and in storage (Waterston, 1947; Snowdon, 1991). During the first half of the 20th century, severe losses were recorded from all celery growing regions in Canada (Conners and Savile, 1952) and the USA (Coons, 1923; Cochran, 1932). During 1964, losses of 50% or more in a Hawaiian celery crop were reported when late blight due to S. apiicola was not controlled by fungicide sprays (Raabe and Matsuura, 1964). In Bermuda, during 1932, one-third of the celery crop was destroyed (Russell, 1933) and during 1933 most of the celery crop was wiped out in all parts except a small area in Southampton (Russell, 1934). The cause was attributed to a virulent source of infection provided by the early crop, started in Canada and transplanted to Bermuda in the autumn of 1932. In Chile, losses estimated at 10-15% of the value of crop covering 150 to 200 ha amounting to a minimum of $150K per annum were reported (Mujica, 1943). In Europe, celery crops sustained different degrees of heavy damage in Denmark (Weber, 1950), Belgium during 1982-1984 (Vulsteke and Meeus, 1986), East Germany where yield losses up to 90% was recorded during June-July (Daebeler and Giessmann, 1971), Switzerland which reported maximum damage in the autumn (Baehni, 1933) and the UK where celery blight was serious everywhere in 1910 (Chittenden, 1914) and 1968 (Baker, 1972). In the Philippines, Septoria blight of celery is a destructive disease of economic importance causing losses estimated in the region of 10-20% of the crop (Palo and Fajardo, 1934). In the coastal plain of Israel, frequent losses of celery due to Septoria blight were also recorded (Chorin et al., 1954). Severe annual crop losses of celery crops due to Septoria blight were reported from the Natal district of South Africa (Wager, 1952) and Mauritius (Felix, 1961). In Australia, Septoria blight of celery was reported to be a serious problem in South Australia (Rogers, 1969) and in the Melbourne market-garden districts of Victoria, Septoria leaf spot has been a limiting factor to celery production since infection was prevalent in early and late plantings and the popular, locally grown variety Golden Self Blanching was found to be highly susceptible to the disease (Stubbs, 1945).The current practice of using treated seeds, soil fumigants, application of fungicides at regular intervals in the field, crop rotation and the avoidance of excess nitrogenous fertilizers and overhead irrigation have minimized or eliminated serious crop losses.