Epiphytic microbial communities on foliage of fungicide treated and non-treated flowering dogwoods

Microbial populations on leaves of Cornus florida (flowering dogwood) in two state parks, a commercial nursery (CN) and at Tennessee State University (TSU) were assessed monthly from May through September. Trees at the TSU location were treated with propiconazole or thiophanate-methyl, at the CN with thiophanate-methyl in rotation with propiconazole at 12–14 days intervals. The highest numbers of colony forming units (CFU) were detected in Rock Island State Park (RISP) and in the CN while the lowest numbers were in Fall Creek Falls State Park (FCFSP) and TSU. Incidence of powdery mildew caused by Erysiphe pulchra was severe in non-treated flowering dogwood at CN and TSU, not in the state parks. Fungicides treatments were highly effective in controlling powdery mildew, but did not significantly affect the microbial populations as indicated by total CFU counts. Because E. pulchra is a biotroph and does not grow on artificial media, it was not included in the tabulation. Experimental data suggest that fungicide treatments did not kill all epiphytes; if they did, there was a rapid re-colonization and replacement with new organisms. Bacteria and yeasts CFU counts were higher than for filamentous fungi. No difference in microbial diversity was detected between fungicide-treated and non-treated trees. In addition to Cladosporium spp. isolated from fungicide treated plots at CN and TSU, several epiphytes known to suppress foliar diseases were isolated from the two state parks where fungicides were not applied.