Etiology of septoria leaf spot on cowpea (vigna unguiculata (l.) walp.). Its interaction with some genotypes of cowpea and other food legumes, and its chemical control

Abstract

The study was initiated to determine the etiology of Septaria leaf spot disease of cowpea that caused severe leaf defoliation in cowpea plots at National Dryland Farming Research Centre (N.D.F.R.C. )-Katumani (Machakos District. Kenya) , the reaction of some cowpea genotypes of Kenyan origin to the disease, the host range of the pathogen, and the efficacy of three foliar fungicides for its control. Laboratory and glasshouse experiments were carried out at Kabete Campus, University of Nairobi. Field trials were conducted at two sites: Kabete Field Station (Faculty of Agriculture, Kabete Campus) and N.D.F.R.C.-Katumani (Kenya Agrcultural Research Institute <K.A.R.I.» during the long rains (April-August) of 1992. The weather conditions at Kabete during the planting season were: maximum and minimum temperatures 21.50C and respectively, total rainfall 671. 3 mm, and mean relative humidity 73%. A t Kat uma n.i , the con d it ion s were: a maximum and minimum temperatures of 23.5 C\:.:; and respectively, total rainfall 233.1 mm , and mean relative humidity 67%. Studies on symptoms of Septoria leaf spot and morphological characters of the fungus causing the disease indicated that Septaria vignicoia V.G. Rao was the causal organism. Growth of the fungu:s on artificial media was restricted, the colony diameter of an eight-day-old culture being 4.9 mm and 4.5 mm on cowpea dextrose agar and potato dextrose agar, respectively. The hyphae on the colonies were also short. S. vignicola could be grown best on cowpea dextrose agar, potato dextrose agar, yeast malt agar, malt extract agar and potato carrot agar. The fungus sporulated better on cowpea dextrose agar, potato dextrose agar and yeast malt agar than on the other media used in the study. The optimum temperature for its growth in culture was 22-24oC. and the maximum and minimum temperatures were above 30 ~ and below l8oC, respectively. Koch's postulates were verified to the effect that S. vignicoia was the causal agent of Septoria leaf spot of cowpea. Conidium of S. vignicoia germinated by eel I elongation and budding. The fungus produced one or two germ tubes that emerged from one conidium, and the budded eel Is germinated independently. It penetrated the host tissue passively through the stomata and directly through the cell wall of the epidermis. The fungus attacked all parts of the plant above the ground except the flowers and pods. Disease severity on cowpea increased with increase in inoculum concentration of the fungus. The conidial concentrations of the fungus disease severity (in terms of were 104 to la conidia/mi. S. which caused above average number of leafspots per leaf) vignicola was found to be host specific to Vigna unguiculata (cowpea). The fungus caused severe leaf spotting and defoliation of most of the cowpea genotypes in the field. None of the genotypes was found to be immune or resistant to the disease. Genotypes of intermediate resistance were Kvu-419, Kvu-HB48E- 10, ICV2 and ICV5, and the remaining, including the high yielding and widely grown varieties like Machakos 66 and Katumani 80, were susceptfble to the fungus. Septoria leaf spot was more severe at Kabete than at Katumani. The disease caused up to 32.82% and 20.85% seed yield loss at Kabete and Katumani, respectively. Kocide 101 and Antracol (non-systemics), and Folicur (systemic), all foliar fungicides, controlled Septoria vignicoia of cowpea at both locations, but only Kocide 101 and Antracol fungicides gave economic grain yields at Katumani. though not at Kabete. This was explained by the fact that high seed yield was realised at Katumani due to favourable weather and soil conditions for cowpea growth, and that Folicur fungicide was phytotoxic to cowpeas leading to considerable yield loss at both sites.