INFECTION BY CLOVER YELLOW VEIN VIRUS ALTERS EPIDEMIC COMPONENTS OF CERCOSPORA LEAF-SPOT ON WHITE CLOVER

Epidemic components of leaf spot caused by Cercospora zebrina were evaluated in repeated experiments on two clones of Trifolium repens 'Tillman', which vary in response to infection by clover yellow vein virus (CYVV). CYVV-free and CYVV-infected plants of clones T7 (low virus titer, virtually asymptomatic) and T17 (high virus titer, symptomatic) were grown at 28/23 or 22/17 C and monitored for 17 days after inoculation with C. zebrina. Altered epidemic components for CYVV-infected plants of T17 compared with those of CYVV-free plants of T17 included diminished lesion density, shortened latent period, larger lesions, greater proportion of leaves with sporulating lesions, reduced defoliation, and reduced disease incidence and severity. For CYVV-infected plants of T7, incubation period (at 28/23 C) and latent period were shortened compared with CYVV-free plants. A clone X virus status X temperature interaction was found for incubation period and lesion diameter. Incidence of infected leaves was greater at 22/17 C than at 28/23 C, regardless of clone/virus status combination. In a separate study, CYVV-infected plants of each clone had a reduced incidence of petioles infected by C. zebrina at 9 days after inoculation and more conidia per square millimeter of lesion at 10, 12, and 14 days after inoculation at 24 C than CYVV-free plants.