HEMIBIOTROPHIC INFECTION OF ROUND-LEAVED MALLOW BY COLLETOTRICHUM-GLOEOSPORIOIDES F-SP MALVAE IN RELATION TO LEAF SENESCENCE AND REDUCING REAGENTS

The infection of round-leaved mallow (Malva pusilla) leaves by Colleto trichum gloeosporioides f. sp. malvae was studied using light and conf ocal microscopy. Conidia germinated and produced appressoria within 24 h after inoculation. An infection peg arose from the base of the appr essorium and directly penetrated an epidermal cell. An intracellular i nfection vesicle appeared beneath the penetration site by 48 h after i noculation. Large primary hyphae (LPH, approx. 4 mu m diam.) emerged f rom the vesicle and grew intracellularly through several adjacent epid ermal cells and then intercellularly between mesophyll cells. Epiderma l cells infected by LPI-I maintained their viability, as shown by thei r ability to plasmolyse and accumulate neutral red stain. No visible d isease symptoms appeared during this biotrophic stage of infection. Th in secondary hyphae (TSH, approx. 2 mu m diam.) developed from LPH in 4-5 days after inoculation and were associated with the appearance of necrotic lesions. Host cell wall maceration was visible only during th e necrotrophic stage. The duration of the biotrophic stage decreased a s mallow leaves became older or were senesced by placing them in the d ark. TSH and host cell necrosis developed as soon as 48 h after inocul ation of the most senescent leaves, and TSH were observed directly eme rging from the infection vesicle. Application of thiol reagents, such as glutathione and dithiothreitol, lengthened the biotrophic stage and delayed symptom development, whereas an antioxidant ascorbate, enhanc ed infection and promoted symptom development.