Fungal diseases of pigeonpea in South Africa

English: In the literature review, an overview of host, pathogen and environment related factors, showed that by studying sub-components of the disease triangle individually, and then integrating them, a holistic approach to control of pigeonpea (Cajanus cajan) diseases is possible. Sub-components that were identified as being most important in the control of legume diseases, were host resistance and cultural practices. Two fungal species, consistently isolated from pigeonpea leaf spots, were identified as Passalora cajani and Cercospora apii. Pathogenicity could however not be proven for these species, suggesting that they are probably latent-infecting fungi. The C. apii and P. cajani isolates varied significantly regarding nutritional and temperature requirements. Inhibition by fungicides varied, with flusilazole/ carbendazim inhibiting growth of both P. cajani and C. apii most effectively, but mancozeb was found to be least effective. The general trend was that all five fungicides inhibited fungal growth effectively at concentrations higher than 5 :g a.i./ ml. Cluster analysis using physiological data revealed two distinct groups, one group containing the three C. apii isolates, and the other group the eight P. cajani isolates. Results of a molecular study were consistent with those of physiological studies. Development of infection structures were studied for the pathogen Uredo cajani, cause of pigeonpea rust. Uredo cajani was shown to be a well adapted parasite, with leaf topography influencing the growth habit of germ tubes and infection. A comparison of infection structures, at different sampling times and on six pigeonpea varieties, showed some variation in susceptibility which was of a non-hypersensitive nature. This mechanism seems to be sufficient to restrict U. cajani infection, and probably explains why pigeonpea rust is considered of lesser importance. Variety ICP 6927 was least and ICPL 87119 most susceptible to rust. Susceptibility levels in medium-duration (MD) varieties were variable, but more consistent in long-duration (LD) varieties. A rating system was developed which provided a simple and effective aid in determining varietal and treatment differences based on the severity of rust on pigeonpea. Higher disease incidence was observed in MD varieties, due to the shorter maturation period when compared to LD varieties. All varieties used in this trial were susceptible to rust, with variety ICP 6927 most susceptible and ICEAP 0020, least susceptible. Flusilazole/carbendazim and azoxystrobin, sprayed alternately, effectively controlled U. cajani, but fungicide treatments did not significantly increase seed yield nor quality. The highest yielding variety was ICP 6927 and the lowest ICPL 87119. A negative correlation for LD varieties between yield and disease suggests that high yield losses are associated with increasing disease severity over time and the control of rust on pigeonpea with fungicides is an effective, but not economically viable, control measure.