ALTERNARIA ALTERNATA CAUSING LEAF SPOT ON CERCIS SILIQUASTRUM IN GREECE

Abstract

Small, circular, necrotic spots with yellow border, present on ca. 50% of the leaves were observed in 2012 on Judas trees (Cercis siliquastrum L.) in the urban area of Thessaloniki (Greece). Tissue fragments excised from the margin of the lesions were surface- disinfected for 1 min in 1% sodium hypochlorite and plated on potato dextrose agar (PDA). Two single-spore cultures grown on potato carrot agar (PCA) and incubated at 24°C with a 12 h photoperiod, gave rise to initially white colonies which turned grayish-black due to abundant sporulation. Conidiophores were short, septate, branched or unbranched, green to brown. Mature conidia, produced in long, single or more often branched chains, were obpyriform, with a conical or cylindrical beak, ovoid or ellipsoidal, had 1 to 5 transverse and 0 to 3 longitudinal septa and measured 9.4-30.8 x 5.6 -15.4 μm (average 20.1 x 10.7 μm). These morphological traits tally with those of Alternaria alternata (Fr.) Keissl. (Simmons, 2007). The ITS1-5.8S-ITS2 region of the two PCA isolates was amplified with primers ITS1 and ITS4 and sequenced (GenBank accession Nos. KP780092, KP780093). A BLAST search revealed 100% homology with the sequences of various A. alternata isolates (e.g. KM233278). Spraying leaves of 10 healthy 2-year-old greenhouse-grown C. siliquastrum plants with a 106 conidia/ml spore suspension resulted in the production, 20 days post inoculation, of leaf spots similar to those observed in the field. Controls plants sprayed with sterile distilled water remained healthy A. alternata was reisolated from artificially inoculated leaves fulfilling Koch’s postulates. This is the first report of A. alternata as the cause of a leaf spot disease on C. siliquastrum in Greece and elsewhere in the world.