Histological investigation of infection processes of Discula destructiva on leaves of Cornus florida

Abstract

The infection process of Discula destructiva Redlin on Cornus florida L. leaves was studied using histological and microscopic techniques. Penetration of fungal hyphae through natural openings and wounds was not observed, while direct penetration without appressorium formation was demonstrated 3 days after inoculation (DAI). Leaves inoculated with D. destructiva developed symptoms of dogwood anthracnose after 7 to 8 days. At 8 DAI, hyphae were observed in aggregations located between the cuticle and epidermis and also growing intracellularly towards epidermal, palisade, parenchymal and spongy mesophyll cells. At 16 DAI, typical chlorotic and necrotic halos, with a red to purple external border, were formed on the inoculated leaves. Within leaf tissues, at 16 DAI, chloroplasts were intact but decompartmentalized and infection sites were clearly defined. Sporulation and ruptured acervuli (cuticle ruptured and spores released) were first detected at 20 DAI, and had fully developed to rupture the plant cuticle on both adaxial and abaxial leaf surfaces by 24 DAI.