Host Range of Three Strains of Colletotrichum gloeosporioides From Tropical Pasture Legumes, and Comparative Histological Studies of Interactions Between Type B Disease-Producing Strains and Stylosanthes scabra (Non-Host) and S. guianensis (Host)

Abstract

The Type A isolate of Colletotrichum gloeosporioides from Stylosanthes hamata and an isolate from Aeschynomenefalcata had relatively wide host ranges, causing disease on 10 and 11 respectively of the 22 plant species tested. Seven plant species were common hosts to the two strains. The Type B isolate from Stylosanthes guianensis had a relatively narrow host range, causing severe disease on S. guianensis only and slight disease on Desmodium barbatum only. Because of this, a special form is proposed for the Type B isolate, namely C. gloeosporioides f. sp. guianensis, to indicate its specificity towards Stylosanthes guianensis. The histopathology at the light microscope level of the non-host resistance of S. scabra to C. Gloeosporioides f. sp. guianensis was studied both on whole leaves that had been cleared and stained, and in transverse sections. The compatible interaction between S. guianensis and C. gloeosporioides f. sp. guianensis was also studied in parallel as a control. Quantitative whole leaf studies showed that the pre-penetration processes were similar for the two interactions, with higher levels of germination occurring on the non-host (35%) than on the host (27%). Of 2250 appressoria examined for evidence of penetration in whole leaves for each of the two interactions at 72 h after inoculation, 6.6% had penetrated for the compatible interaction and in all cases only direct penetration was observed. For the interaction with the non-host (S. scabra) only one penetration was observed, and this appeared to occur through a stomate. The studies on transverse leaf sections showed that in the non-host interaction, penetration pegs, if produced, remained localised in the cuticle and were surrounded by a densely staining reaction matrix. In the compatible interactions, the fungus produced a vesicle in the cuticle by 24 h. A dense reaction matrix was often observed in the cuticle beneath the appressorium but penetration of the host epidermal cell, followed by subcuticular, intercellular and intracellular growth, had occurred by 72 h. Cell collapse was not evident after this period, suggesting the fungus was biotrophic for the first 72 h of the interaction.