Alternaria longipes (brown spot)

The first symptoms of brown spot disease are small, circular, dark-brown lesions on the lower leaves of field plants and sometimes on old seedlings or the senescing leaves of transplants. The lesions enlarge as the leaves mature and are surrounded by an irregular, yellow halo. The halo is caused by the secretion of toxins. Often there are numerous lesions, about 25 mm in diameter, which may coalesce and affect more than 80% of the leaf area. Large lesions may have numerous small, satellite circular lesions in their vicinity, arising from conidia washed off the main lesion.Infected leaves senesce prematurely and the affected part of a leaf may be yellow when the rest is dark green. A uniform infection, covering up to about 10% of leaf area, in slow ripening conditions, can accelerate ripening and increase the value of the harvested leaves because buyers assess them as being more mature than clean leaves that were reaped at the same time.Lesions on midribs and stems are elliptical, sunken, very dark brown and do not have yellow haloes. They can be numerous in intense inoculum pressure.Seeds taken from infected capsules can have infected debris and spores adhering to them.ToxinsTenuazonic acid (TA), a host-specific toxin, can be isolated from culture filtrates of A. longipes and from infected leaves. Researchers in Japan found that TA produced haloes identical to those on infected leaves and suggested that TA might be used to screen for resistance (Mikami, 1972; Mikami et al., 1971a, b). Disease susceptibility and toxin sensitivity were closely correlated in tests with seven cultivars.

Main et al. (1972) described a comprehensive brown spot assessment programme to evaluate the economic effects of the disease. Cole and Zvenyika (1982) reported that yield and price per kg of saleable tobacco in brown spot epidemics decreased linearly with infection index, based on leaf area damaged. Yield loss varied with the degree of infection; 809 kg/ha of saleable tobacco was lost in one trial in Zimbabwe (Cole and Zvenyika, 1982) and 258 kg/ha in the USA (Lucas, 1961).Lucas (1975) reviewed the economic importance of brown spot in the tobacco-producing regions of the USA. In 1956, the estimated crop loss in North Carolina was US$10 million and in subsequent years it ranged between $2.4 million and $21 million. The estimates were made by summarizing the assessments of county extension agents.Brown spot disease also changes the chemical composition of cured leaves (Main et al., 1973). The sugar concentration decreased linearly with the natural log of leaf infection. The physical characteristics of cured tobacco are also affected. Main and Chaplin (1972) found that filling value, which determines the number of cigarettes that can be made per kg of tobacco, and shatter, which affects the amount of waste, were adversely affected by increasing amounts of brown spot.