Studies on ascochyta blight on cowpea (vigna unguiculate l. (walp) and characterization of isolates of its causal organism, Ascochyta phaseolorum Sacc.
Wamari, Phoebe A.
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Cowpea leaf blight cause by Ascochyta phaseolorum Sacc. was found to be widely distributed in Kenya. Blight symptoms consisted of light to dark brown circular lesions with darker, regularly spaced, wavy concentric rings with occasional shot holes or tatterd leaf laminae. In a survey conducted from October to December 1980, the disease was found to occur in Kirinyaga, Kiambu, Meru, Siaya, Kisii, South Nyanza and Kakamega districts. The highest incidence of 46% and severity of 42% were recorded in Kakamega District. Although it was observed under experimental conditions that Vita 3, Vita 4, MM4, Mak 1/39/B and KCIP/AG/77/99 were mildly infected, some of the land races grown by farmers were severely infected. Seedlings grown from seed samples collected from infected plants did not develop the blight. Isolates of A. phaseolorum obtained from diseased samples varied in growth and sporulation characteristics when cultured under different temperatures, lighting conditions and nutrient media. Most isolates grew optimally either at 20oC or at 25oC. The maximum and minimum temperature for mycelial growth were above 30oC and below 10oC respectively for most isolates. Optimal sporulation temperature was either 20oC or 25oC for most isolates. Both continuous light and continuous darkness supported the greatest sporulation in different isolates. For most isolates the temperature range for sporulation was 15 to 25oC. Some isolates formed chlamydospores under the temperature and light treatments. A. phaseolorum showed versatility by growing on the six nutrient media used. Chlamydospore production was notable on potato dextrose agar. Spore germination occurred only in host sap. Host penetration was preceded by germination by 1 to 4 germ tubes. Bipolar germination was most prevalent. Both stomatal and cuticular penetration were observed. Penetration was followed by proliferation of hyhae below the host epidermis. The incubation period for A.. phaseolorum was 4 to 5 days in the greenhouse. Virulence levels of the isolates varied with regard to the eleven-cowpea lines tested for resistance. None of the isolates infected all the lines and the lines showed different degrees of susceptibility. No line was resistant to all the isolates of A. phaseolorum. Vita 4 and Machakos 74 were less susceptible than Mak 1/39/B, KCIP/AG/77/10 and KCIP/ag/77/11 which were the most susceptible. The other six lines were of intermediate susceptibility. Isolates Ka and K3 were the most virulent and least virulent respectively.