Evaluation of the pathogenicity of Phoma exigua var. exigua on Cirsium arvense : A dissertation submitted in partial fulfilment of the requirements for the Post Graduate Diploma in Horticultural Science

Abstract

Seven isolates of Phoma exigua var. exigua were evaluated for the effects of spore concentration on germination rates. Differences in germination rates were found between isolates at two spore concentrations (1 x 10⁵ and 1 x 10⁷ spores/ml), for example at 1 x 10⁵ spores/ml after 12 h incubation isolates had significantly (P < 0.05) different germination: DSB (86.6%) and HA (75.6%) had higher germination than ARW (49.0%) and DSC (44.3 %), ARW was significantly higher than HW ( 20.3%) and HB (11.3%), DSC was significantly higher than DSA (6.6 % ). At a spore concentration of 1 x 106 spores/ml after 12 h incubation, germination ranged between 2.3 to 44% with no significant differences between isolates and for 1 x 10⁷ spores/ml concentration, germination ranged between 1.3, to 18.6% with ARW (18.6%) and DSB (15.3%) germinating significantly (P < 0.05) better than other isolates. The pathogenicity of isolates was determined by measuring lesion development on detached wounded Cirsium arvense leaves after inoculation with conidial solutions of the isolates. Significant but variable differences in pathogenicity were found between isolates indicated by mean percentage lesion coverage in four detached leaf assays. Two isolates produced significantly (P < 0.05) greater lesion development than the other isolates and the control after 6 days in three of these assays: DSA (range of 31.2 to 86.6%) and HB (12.5 to 65%). Wounding did not significantly enhance lesion development on detached leaves. Inoculation of wounded C. arvense leaves on whole plants also revealed significant differences in pathogenicity (as indicated by mean percentage lesion coverage) for isolates DSC, and DSB in Assay 1 and isolate DSA in Assay 2, at the early stage of lesion development. Comparison of the ranking of pathogenicity of isolates between detached and attached leaves found general similarities, thus indicating that detached C. arvense leaves may be suitable for screeningcollections of P. exigua var. exigua isolates for pathogenicity on C. arvense. These findings are discussed in the context of the requirements of microbial weed biological control agents.