Epidemiology of angular leaf spot of bean in monocultures and in bean-maize intercrops

The effect of increased species diversity on disease spread was investigated. Angular leaf spot (ALS) of bean (Phaseolus vulgaris L.) was studied in two cropping systems (bean monoculture, bean-maize intercrop) during three trials (two rainy seasons, one dry season) conducted at the International Center for Tropical Agriculture near Cali, Colombia during 1986-87. The two bean lines (one susceptible, one partially resistant) chosen for this study were found to react differently to cropping system and the reaction varied with the season. The monoculture of the susceptible line experienced significantly greater ALS intensity than the intercrop during both rainy season trials, and during the dry season, the reverse was true. For the partially resistant line, the intercrop had significantly higher ALS intensity than the monoculture during the first rainy season; thereafter, interference from another disease, common bacterial blight, was severe. Interestingly, the effect of cropping system on common bacterial blight was consistent: its incidence was significantly higher in monocultures of both cultivars during dry and rainy seasons.;A significant negative correlation between yield and ALS intensity was found. Seed weight was the yield component most affected by ALS and it was negatively correlated with ALS intensity.;There are several theories on how increased diversity could result in lower levels of disease, but few experimental studies have addressed this question. Microclimate may be affected by changes in species composition, and intercrops may experience microclimatic conditions quite different from those existing in monocultures of the components. Several microclimatic variables were continuously monitored and recorded for the ALS-susceptible line in both cropping systems. Temperatures in- and outside the canopy, vapor pressure, saturation deficit, and duration of leaf wetness were compared by discriminant analysis, and significant canonical correlations were found for the three trials. Duration of leaf wetness was the only variable that followed the same pattern as ALS intensity in rainy and dry seasons. In each trial, the treatment with the longer duration of leaf wetness had the greater ALS intensity.