Epidemiology of Bean Common Bacterial Blight and Maize Rust in Intercropping

Abstract

Varied cropping systems and production situations can influence disease occurrence, epidemic development and damage to crops. Rust and common bacterial blight on common bean (Phaseolus vulgaris), and common rust and leaf blight on maize (Zea mays) are economically important diseases. This thesis, based on surveys and field experiments, deals with the association of these diseases with cropping systems, and common bacterial blight and maize rust epidemics. Using logistic regression analyses, intercropping systems were associated with lower levels of bean common bacterial blight and rust incidence and severity. An intermediate altitude area was more associated with higher levels of the diseases. Common maize rust incidence was low at low crop densities, at early and optimum sowing time and in maize-bean. Sowing at an optimum time also had a high probability of association with lower levels of maize leaf blight incidence. A slower common bacterial blight progress rate and reduced incidence and severity occurred on beans planted with maize or sorghum in row, mixed, and broadcast intercropping than on beans planted alone. Under conditions less favorable, maize common rust incidence and area under the disease progress curve were significantly lower by 32% in broadcast and 20% in row intercropped maize compared to sole cropped. The disease incidence gradient curves and the area under the disease gradient curve generally were consistent with less disease in intercropping systems compared to sole cropping. Disease gradients in intercropping were steeper than in sole cropping. Sole bean cropping had higher temperatures and lower relative humidity during daytime compared to the intercropping systems. Overall, sole cropping was 1.4 °C hotter and 1.9-3.8% less humid. Common bacterial blight incidence changes in the systems were positively correlated with weekly mean microclimatic temperature and negatively correlated with microclimatic relative humidity. Weekly mean relative humidity from a nearby meteorological station also had a negative correlation with the bacterial blight incidence changes while sunshine duration had a positive correlation.