Studies on Cercospora Leaf Spot

Abstract

Fresh Pseudocercospora cornicola specimens were compared to an isotype specimen to confirm species identity. The two specimens proved to be the same taxonomic entity. The fungus was then redescribed and illustrated from these specimens while consideration was given to reclassification of the genus. This anamorph has some characteristics in common with both Pseudocercospora and Pseudocercosporella. However, possession of intermediate characteristics between the two genera warrants its placement in Pseudocercospora. The epidemiology of Cercospora leaf spot was examined with respects to maximum radial growth and sporulation in vitro based on nutritional and environmental parameters and maximum lesion development based on environmental parameters. No sporulation occurred in culture during the experiments. Optimal radial growth occurred on water agar and V-8 agar, the optimum temperature for growth was 30 C for both media, and optimum growth occurred with a 12 hour diurnal light cycle. Optimum lesion development occurred at 35 C and with a 12 hour diurnal light cycle. Humidity did not affect lesion development. Flowering dogwood cultivar resistance to Cercospora leaf spot was evaluated at two locations for each of two years. At location one, seven cultivars were evaluated in the first year and eight cultivars the second year; at location two there were five cultivars. Three cultivars at location one were determined to be disease free both years: ‘Pumpkin Patch’, ‘Pygmy’, and ‘Red Pygmy’. ‘Cherokee Brave’, ‘Cherokee Chief’, ‘Cherokee Princess’, and ‘Cloud 9’ showed high levels of resistance both years at both locations. ‘Little Princess’ and ‘Stellar Pink’ showed lower levels of resistance. Flowering dogwoods were also evaluated for infection occurrence of P. cornicola by applying a protectant fungicide during each of six months. Trees in landscapes were divided into sections and each was sprayed in a different month under the assumption that the section(s) with the lowest level of disease at the end of the season would indicate which month(s) infection occurs. Preliminary results suggest that infection occurs from May to July and this period would be the appropriate time to apply protective fungicides to prevent Cercospora leaf spot.