First Report Of Alternaria Leaf Spot On Gerbera (Gerbera Jamesonii H. Bolux Ex J.D. Hook) In Bulgaria

Alternaria leaf spots of gerbera (Gerbera jamesonii H. Bolus ex J. D. Hook) were observed on plants from different greenhouses on commercial plants in Bulgaria. The symptoms of the disease on the leaves were characterized by the development of brown, small, scattered dots, which gradually enlarged and coalesced to form large, oval, circular or irregular, brown to black lesions with concentric rings. Affected plants showed lower vitality, suppressed development and fewer, smaller, distorted in shape flowers. Alternaria isolates, obtained from infected leaf tissues were grown in pure culture and the morphological characteristics of the colony and sporulation apparatus were determined. DNA, extracted from the fungal isolates was subjected to polymerase chain reaction (PCR) with primers ITS1/ITS4, amplifying the internal transcribed spacer (ITS) regions. The resulting products were sequenced and compared for homology with other species in the GeneBank. The isolates showed 94% homology of the ITS region with either Alternaria alternata , A. arborescens , A. tenuissima , A. longipes , A. lini or A. smyrnii . None of the studied isolates was amplified with the A. alternata specific primers AAF2/AAR3, indicating that they are pathogenic varieties of it or belong to another species. Pathogenicity tests on 10 gerbera cultivars revealed that all of them were susceptible to Alternaria leaf spot. Additional tests on nine other crops (Solanum lycopersicum , Calendula officinalis , Capsicum annuum , Celosia argantea , Pelargonium spp., Petunia hybrida , Nicotiana tabacum , Cucurbita moscata and Raphanus sativus var. radicina ) and on tomato or pepper fruits, potato tubers and carrot roots also indicated that all tested plant species were potential hosts of the disease. This is the first report of highly virulent isolates of Alternaria spp. in Bulgaria that cause leaf spots on gerbera in greenhouses.