Fungal diseases of pigeonpea in South Africa

Abstract

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.

Afrikaans: In die literatuuroorsig van gasheer-, patogeen- en omgewingsverwante faktore, was dit duidelik dat ‘n studie van die individuele subkomponente van die siektedriehoek tot ‘n holistiese benadering vir die beheer van duifert-siektes kan lei. Subkomponente wat as belangrik geïdentifiseer is in die beheer van peulgewassiektes, is gasheerweerstand en verbouiingspraktyke. Twee swamspesies, wat gereeld vanaf blaarvlek geïsoleer is, is geïdentifiseer as Passalora cajani en Cercospora apii. Patogenisiteit kon nie vir hierdie isolate bewys word nie, wat op ‘n moontlike latente infeksietipe dui. Die C. apii en P. cajani isolate het betekenisvol verskil in hulle voedings- en temperatuurvereistes. Inhibisie van swamgroei deur fungisiede het ook grootliks verskil. Die fungisied flusilasol/karbendazim het die groei van beide spesies die doeltreffendste geïnhibeer, terwyl mankoseb ondoeltreffend was. Oor die algemeen het al vyf fungisiede swamgroei doeltreffend geïnhibeer teen konsentrasies bo 5 :g a.b./ml. Groeperingsanalise, gebaseer op fisiologiese data, het twee groepe uitgewys wat onderskeidelik C. apii en P. cajani isolate bevat. Resultate van ‘n molekulêre studie het met fisiologiese data ooreengestem. Die ontwikkeling van infeksiestrukture van die patogeen Uredo cajani, oorsaak van roes op duifert (Cajanus cajan), is beskryf. Daar is gewys dat U. cajani ‘n goed aangepaste patogeen is en dat blaartopografie die groeiwyse van kiembuise en infeksie kan beïnvloed. Deur infeksiestrukture op ses duifertvariëteite te vergelyk, is variasie in vatbaarheid, van ‘n nie-hipersensitiewe aard, bevestig. Hierdie meganisme mag voldoende wees om die verspreiding vanaf die infeksiepunt van U. cajani te beperk en is ‘n moontlike verduideliking waarom hierdie siekte nie as belangrik beskou word nie. Variëteit ICP 6927 was die vatbaarste, en ICPL 87119 die minste vatbaar. Vatbaarheidsvlakke het gevarieër in die medium-durasie (MD) variëteite, maar was meer konsekwent in die langdurasie (LD) variëteite. ‘n Evalueringsisteem, wat ontwikkel is om verskille tussen variëteite gebaseer op siektevoorkoms aan te dui, het eenvoudige en doeltreffende lesings tot gevolg gehad. Die waargenome siektevoorkoms was hoër in die MD as in die LD variëteite, as gevolg van ‘n korter groeiperiode. Al ses variëteite was vatbaar vir roes, met variëteit ICP 6927 die vatbaarste en ICEAP 0020 die minste vatbaar. Flusilasol/karbendazim en asoksistrobien, alternatief gespuit, het U. cajani effektief beheer, maar het nie die saadgehalte of -kwaliteit betekenisvol verhoog nie. ICP 6927 het die hoogste en ICPL 87119 die laagste opbrengs gehad. ‘n Negatiewe korrelasie tussen siekte en opbrengs in die LD variëteite, dui moontlik daarop dat hoë opbrengsverliese geassosieer kan word met toename in siektegraad oor tyd. Die beheer van roes op duifert met behulp van fungisiede is doeltreffend, maar nie ekonomies regverdigbaar nie.