An emerging fungal disease on Lactuca sativa in Bulgaria
Lettuce (Lactuca sativa) is the most popular among the salad crops grown on a commercial scale. It is commonly cultivated as a leaf vegetable but sometimes for its stem and seeds. Lately, an emerging fungal disease on its flowering stem has been noticed in Bulgaria especially in hot wet weather during bolting and flower stalk formation. The aim of this investigation was to describe the symptoms of the new disease and to isolate, identify and characterize the causal agent. The symptoms consisted in large necrosis on flower stalks, which totally destroyed the whole reproductive plant parts in a short time. A fungus belonging to the genus Phomopsis was consistently isolated from all damaged parts (flower stalk, inflorescence). Many black pycnidia bearing mainly α-, β- and seldom γ-conidia appeared in the diseased tissue. Pin pricked lettuce stems were inoculated with agar plugs containing fungal mycelium. The symptoms were entirely reproduced. Reisolations from the diseased tissue were successful. Total genomic DNA was extracted from 10 axenic cultures of the pathogen. Universal ITS5 and ITS4 primers, based on the internal-transcribed spacer (ITS) of ribosomal genes were used for PCR amplification. Single product of 602 bp was amplified, purified and sent for sequencing. Phomopsis accessions from GenBank (NCBI) and our own accessions isolated from different host plants were involved into evolutionary studies. Phylogeny was performed by MEGA5. Phomopsis isolates from L. sativa represented a strongly separated clade with bootstrapping value 99% in phylogenetic tree. On the basis of morphological, cultural, pathogenic and molecular data the fungus was identified as Diaporthe (Phomopsis) lactucae.
Rodeva, R., Stoyanova, Z. and Kacergius, A. 2016. An emerging fungal disease on Lactuca sativa in Bulgaria. Acta Hort. (ISHS) 1142:135-140
ITS sequence, rDNA, lettuce, molecular phylogeny, pathogenicity, Diaporthe (Phomopsis) lactucae, stem necrosis