Mycosphaerella cruenta (leaf spot of cowpea)

Common Bean (Phaseolus vulgaris)On infected leaves (especially those more mature) look for brown or rust-coloured lesions that vary from circular to angular, are 2-10 mm, and may coalesce. Lesions may have a grey centre with a slightly reddish border. Conidia develop at the centre on short conidiophores. Severely affected leaves become chlorotic. Lesions may dry and portions may fall out, giving the leaf a shot-hole appearance. Lesions and blemishes may occur on branches, stems and pods.

Cercospora leaf spot, caused by M. cruenta, has caused considerable yield losses in cowpeas in Nigeria where considerable efforts have been made on control of the disease (Williams, 1977). Under natural conditions, M. cruenta reduced the number of pods per plant, average number of seed per pod and average 100-seed weight of a susceptible cowpea variety by 25.4, 9.6 and 3.1%, respectively (Fery and Dukes, 1977a). Seed yield loss of cowpea was correlated with Cercospora leaf spot disease (caused by M. cruenta) severity from artificially simulated, chemically regulated and naturally occurring epidemics at Ibadan, Nigeria. Areas under disease progress curves were correlated with yield loss regardless of the shapes of the curves. The yield-loss estimation model was not accurate for low disease levels. A critical-point model was constructed and tested that related date of occurrence of specific disease severities to percentage yield loss, though this could be used only for uninterrupted disease progressions (Schneider et al., 1976).In a study in USA, M. cruenta leaf spot of cowpea reduced the seed yield of the susceptible cv. Colossus by 35.6%, 100-seed weight by 3.1%, number of seeds/pod by 9.6% and number of pods per plant by 25.4% compared with crops protected with benomyl and chlorothalonil sprays. The resistant line CR 17-1-34 was unaffected by leaf spot and applications of benomyl or chlorothalonil. Although M. cruenta is present to some degree in the US in most years, losses are slight from this pathogen (Schwartz and Pastor-Corrales, 1989).In Varanasi, India, leaf spot caused by M. cruenta was found to cause serious disease in lobiya (Vigna unguiculata) (Pant, 1989).