Abstract :
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.
