Isolation, Identification and Pathogenicity of Fungal Pathogen(s) Causing Black Sigatoka Disease of Local Plantain and Banana in Ghana

Abstract

A study was conducted at the Crop Science Department, University of Ghana, Legon, to isolate,identify and test pathogenecity of fungal isolates causing the reported black sigatoka disease of plantain in Ghana and compare the isolate with that isolated from local tall banana having leaf spots. Field symptoms of plantain and banana were compared and found to be similar. Initial specks indicating infection were minute, reddish-brown and were conspicuous only on the abaxial leaf surfaces on both plants. Specks turned dark-brown and later dark-black with adjoining yellow margins. The dark black areas dried out and turned grey on both plants. The fungi species were isolated from necrosed banana and plantain leaves by Ascospore Discharge Technique. Ascospores from necrosed banana and plantain leaves were hyaline, two-celled, with one cell larger than the other and measured 13.9 by 3-2um and 13.8um by 3.2um respectively. Ascospores of both isolates germinated in Sterile Distilled Water (SDW) after 1^/2 hours and on PDA and Carrot Leaf Decoction Agar (CDA) between 12-18 hours and in all cases by bipolar germ tubes. No sporulation was observed for both isolates in SDW. Colony growth of both isolates on PDA and CDA were extremely slow and mycelia compact, erumpent, appearing dome-shaped from side view, and with a hard rind on the surface. ABSTRACT Sporulation was not observed on PDA for both isolates,but profuse sporulation was observed on CDA six days after inoculation. Conidia in culture was Cercospora-type, with thickened hilum. Conidia of the plantain and banana isolates on the average measured 18 .lum by 3.6um and 78.4um by 3.'Sum, respectively. Greenhouse inoculations revealed that the plantain isolate on plantain had an incubation period of 18 days, whilst the banana isolate pathogenic on the local tall banana also had an incubation period of 23 days. Symptoms in both cases were typical for black sigatoka and progressed to the mature-spot stage with grey centres. Cross-infection studies in the greenhouse revealed that plantain isolate of M. fijiensis was pathogenic on local tall banana with incubation periods of 29 days, and which produced typical black Sigatoka symptoms until the mature spot stage. Plantain ascospore inoculation on local tall banana also produced black sigatoka symptoms with 21 days of incubation period, after which symptoms progressed to the mature spot stage. On plantain, the banana isolate of M. fijiensis took 26 days for specks to appear and symptoms which were typical for black sigatoka progressed to the mature-spot stage. Control plants in the greenhouse did not develop symptoms. Neither ascospores nor conidia were produced in the greenhouse on inoculated plants due to unfavourable environmental factors thus rendering reisolation of the fungus from diseased leaves impossible. The study therefore established that black sigatoka disease in Ghana is found on both plantain and the local tall banana, with the causal agent being Mycosphaerella fijiensis Morelet.