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Brown to black, round to irregular and often zonate lesions measuring up to 1 cm diam. are produced on the leaves and in severe attacks the leaves dry out and fall off (N'Gabala and Zambettakis, 1970). Stem lesions are either in the form of dark brown spots or streaks. Dark brown, circular lesions are produced on the capsules (Yu et al., 1982) which can cause the capsule to drop (N'Gabala and Zambettakis, 1970). The most visible symptoms are the leaf spots which are dark, irregular patches mostly on the edges and tips of the leaves, but the stem rots can be more significant. Kushi and Khare (1979) note that A. sesami can cause seed rot, pre- and post-emergence losses as well as stem rot and leaf spots.
Cultural Control and Sanitary Methods
As a predominantly seedborne disease the most important sanitary methods are to ensure that the seed is free of infection. See section on Seed Treatments in Seedborne Aspects of Disease.
All investigations into resistance to infection by A. sesami in sesame have found that no variety or cultivar is totally resistant, although some were more resistant than others (Natarajan and Shanmugam, 1983; Dolle, 1984). Dolle and Hegde (1984a) tested 22 cultivars and found that a number were moderately resistant and that X-7732/10-2 developed the least disease in field conditions. Siddaramaiah et al. (1981b) list seven cultivars found to be resistant and Jayaramaiah et al. (1981) note four cultivars that are highly resistant and suitable for breeding programmes, or for direct cultivation.
The mechanism of resistance has also been examined. Gupta et al. (1987) compared susceptible and resistant cultivars and found that wax, phenol and chlorophyll contents were higher, and reducing sugars and soluble nitrogen were lower, in resistant plants than in susceptible ones. Phytoalexins also appear to have a role. Kulshrestha and Chauhan (1988) showed that phytoalexin produced in sesame fruits infected by A. sesami was fungicidal to conidia of A. sesami. The phytoalexin was inactivated by exposure to UV radiation but not by high temperatures. Its production could be induced by the addition of a range of chemicals. Kulshrestha and Chauhan (1987) carried out a chromatogram bioassay which revealed the presence of an inhibitory compound at Rf 0.14 which they suggest is the phytoalexin. Aqueous leaf extract of neem (Azadirachta indica Juss.) provided the control of Alternaria leaf spot pathogen (Alternaria sesami) of sesame (Sesamum indicum L: Syn. S. orientale L). Treatment with this extract led to the changes in plant metabolism as leaves of the treated plants exhibited significantly high level of enzymes phenylalanine ammonialyase (PAL), peroxidase (PO) and content of phenolic compounds (Sanjay and Ashok , 2006).
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:
Although considered an important fungal disease of sesame, there is little information about actual economic impact of A. sesami. Kumar and Mishra (1992) state that most of the common diseases of sesame cause yield losses of 20-40%. This yield loss is caused by the premature defoliation of the plants leading to smaller capsules and loss of capsules due to infection. However, in the USA up until 1961, infection by A. sesami was not considered a problem as yield loss was minimal and the presence of the fungus defoliated and dried the sesame prior to harvest (Culp and Thomas, 1964). Yield losses are greatest in dry years as plants under moisture stress are more susceptible. Siddaramaiah et al. (1981a) found that yield losses in Karnataka, India, due to Alternaria blight were 0.1-5.7g/100 fruits.