Histological investigation of infection processes of Discula destructiva on Cornus florida leaves and influence of pH on growth of Discula destructiva and an undescribed Discula species

The infection process and colonization of Discula species in greenhouse grown Cornus florida L. leaves were studied histologically. Penetration of Discula destructiva Redlin isolate (TN 1) hyphae through natural openings, wounds, or directly through the leaf surface was not observed. Infection, detected by fungal colonization of leaf tissue, occurred between one and eight days postinoculation in wounded and non-wounded leaves inoculated with TN 1 conidia. Fungal colonization was not observed in wounded or non-wounded leaves misted with water (controls). Leaves inoculated with the D. destructiva isolate and an undescribed Discula species isolate (NC 2) developed symptoms of dogwood anthracnose after two wks. Disease symptoms induced by both isolates were similar to those previously described for dogwood anthracnose caused by Discula destructiva. This is the first report that the undescribed Discula sp. is pathogenic on C. florida.

The influence of pH on the growth of three D. destructiva isolates (GA 1, TN 1 and MA 11) and an undescribed Discula species isolate (VA 17B) in buffered liquid culture media was studied. Citrate-phosphate buffer was used to adjust culture media to 4.0, 5.0, 6.0 or 7.0. The pH of the media remained at the initial level two wks after inoculation with the fungus. Growth of the undescribed Discula sp. isolate and D. destructiva isolate (GA 1) decreased significantly from pH 4.0 to 7.0; whereas, D. destructiva isolates (MA 11 and TN 1) grew at similar rates regardless of the pH of the growth media.