Fungal diseases of pigeonpea in South Africa

[摘要] English: In the literature review, an overview of host, pathogen and environmentrelated factors, showed that by studying sub-components of the disease triangleindividually, and then integrating them, a holistic approach to control of pigeonpea(Cajanus cajan) diseases is possible. Sub-components that were identified asbeing most important in the control of legume diseases, were host resistance andcultural practices.Two fungal species, consistently isolated from pigeonpea leaf spots, wereidentified as Passalora cajani and Cercospora apii. Pathogenicity could howevernot be proven for these species, suggesting that they are probably latent-infectingfungi. The C. apii and P. cajani isolates varied significantly regarding nutritionaland temperature requirements. Inhibition by fungicides varied, with flusilazole/carbendazim inhibiting growth of both P. cajani and C. apii most effectively, butmancozeb was found to be least effective. The general trend was that all fivefungicides inhibited fungal growth effectively at concentrations higher than 5 :ga.i./ ml. Cluster analysis using physiological data revealed two distinct groups, onegroup containing the three C. apii isolates, and the other group the eight P. cajaniisolates. Results of a molecular study were consistent with those of physiologicalstudies.Development of infection structures were studied for the pathogen Uredocajani, cause of pigeonpea rust. Uredo cajani was shown to be a well adaptedparasite, with leaf topography influencing the growth habit of germ tubes andinfection. A comparison of infection structures, at different sampling times and onsix pigeonpea varieties, showed some variation in susceptibility which was of anon-hypersensitive nature. This mechanism seems to be sufficient to restrict U.cajani infection, and probably explains why pigeonpea rust is considered of lesserimportance. Variety ICP 6927 was least and ICPL 87119 most susceptible to rust.Susceptibility levels in medium-duration (MD) varieties were variable, but moreconsistent in long-duration (LD) varieties.A rating system was developed which provided a simple and effective aidin determining varietal and treatment differences based on the severity of rust on pigeonpea. Higher disease incidence was observed in MD varieties, due to theshorter maturation period when compared to LD varieties. All varieties used in thistrial were susceptible to rust, with variety ICP 6927 most susceptible and ICEAP0020, least susceptible. Flusilazole/carbendazim and azoxystrobin, sprayedalternately, effectively controlled U. cajani, but fungicide treatments did notsignificantly increase seed yield nor quality. The highest yielding variety was ICP6927 and the lowest ICPL 87119. A negative correlation for LD varieties betweenyield and disease suggests that high yield losses are associated with increasingdisease severity over time and the control of rust on pigeonpea with fungicides isan effective, but not economically viable, control measure.

[发布机构] University of the Free State