The effect of cropping history and the role of cowpea debris in the epidemiology of cowpea scab

The role of cowpea ( Vigna unguiculata) debris as a primary source of inoculum for Sphaceloma sp., the pathogen of cowpea scab, was studied in field experiments. Three fields were selected in 1993 and three in 1994, in which cowpea had been grown 1, 2 or 3 years previously as part of a crop rotation. Polyethylene mulch was spread over the soil to prevent soil/debris splash in half of the plots. No scab symptoms were observed on the primary leaves. It took an average of 25 days for primary symptoms to be observed in each field, irrespective of mulching. Mulching had a significant effect on disease severity only in the two sites where cowpeas were last grown a year before the trial (sites 1a and 1b). In both years, interaction between time (days after sowing) and site (fields in which cowpea had been grown 1–3 years earlier) had a significant effect on disease incidence while in 1994, interaction between time, site and mulching was also significant ( P  < 0.05). Higher disease incidence was observed in 1994 than in 1993. In all fields, there were increases in disease incidence over time. Rain splashing may have contributed to higher disease incidence in plots without mulch. The presence of scab in the mulched plots in fields last sown to cowpea 3 years before the trial suggests that the pathogen may survive on infected cowpea debris, which acts as one of the sources of primary inoculum. Hence longer periods of crop rotation with nonhosts may be required to control the disease.     

Assessment of loss in yield due to sorghum downy mildew (Peronosclerospora sorghi) of maize in Nigerian guinea savanna

Abstract

The losses in yield due to sorghum downy mildew of maize induced by Peronoscleospora sorghi were assessed from 1982 to 1984 in the epidemic‐prone area of Nigeria. Plots of maize with different downy mildew incidences were obtained by planting seedlots containing different percentages of metalaxyl‐treated seeds. The losses in yield were directly proportional to the incidence of the disease. There was highly significant negative correlation between the incidence of downy mildew and grain yield.