Maize root rot in South Africa.
Numerous fungal species are known to infect maize roots and eventually cause rot. The spectrum of fungi differs over localities as well as their relative frequencies. Fungi isolated from discoloured root tissue and root tissue without visible discolouration were classified as root pathogens and root colonizers, based on their isolation frequency from the respective samples. Exserohilum pedicellatum, Macrophomina phaseolina and Fusarium oxysporum were classified as the major root pathogens and Phoma spp., Curvularia spp. and F. chlamydosporum as root colonizers. Fungi classified as root pathogens tended to occur early in the growing season in juvenile tissue as opposed to root colonizers which occurred later. Treatments in an existing long-term water stress trial included no stress (irrigation), normal rainfall, stress until flowering, stress after flowering until mid grain-fill, stress after flowering and total stress. Root pathogens were isolated at higher frequencies in the no stress and normal rainfall treatments than in the total stress treatment. A positive linear relationship between the water stress index and the isolation frequency of M. phaseolina was obtained. Negative, non-linear relationships were, however, recorded for E. pedicellatum, F. oxysporum and F. moniliforme. The effect of tillage practices on disease severity was carried out in field trials at two localities. Tillage practices applied included rip, plough, chisel plough and no-till treatments. Significant differences between isolation frequencies and tillage treatments were recorded for F. oxysporum at Bloekomspruit and Trichoderma spp., Alternaria spp. and M. phaseolina at Mmabatho. Differences in isolation frequency of fungi involved in maize root rot, were determined in a crop rotation system where maize was rotated with soybeans, sunflowers and groundnuts. Crop rotation had a significant effect on the isolation frequency of Thielavia ferricola, E. pedicellatum, F. moniliforme and F. graminearum. The effect of crop rotation, however, seems to be complex since fungi were affected differently in the various treatments.