First Report of Phoma exigua as a pathogen of hop in Slovenia

In late summer (August) 2005, necrotic lesions on cones and leaves were observed on hop (Humulus lupulus) plants of cultivars Bobek, Magnum, and Merkur in the Koroška and Podravje regions of Slovenia. The symptoms on the leaves were initially small circular spots, which later increased in size (1-3 cm in diameter) and developed into oval greyish white lesions (Fig.1b). The affected cones showed reddish-brown necrotic areas on the tips of the bracts and bracteoles, which in some cases progressed throughout the entire cone (Fig.1a). Microscopic examination of the affected tissue revealed the presence of irregularly scattered globose pycnidia (80–200 µm wide) with a distinct ostiolum (Fig.1c). Conidia were hyaline, ellipsoid, occasionally one-septate, and measured 6.5 (4.5-8) × 2.5 (2-4) µm. Necrotic lesions were surface disinfected (1 min in 2% NaOCl), plated onto oatmeal agar (OA) and incubated at room temperature. Isolated colonies produced scarce pycnidia of a Phoma species. On the basis of the characters in vitro on OA, malt agar (MA) and cherry-decoction agar (CA) (Boerema et al., 2004), the pathogen was identified as Phoma exigua Desm.

The identification was confirmed by sequencing the ITS region spanning ITS 1, 5.8S rDNA and ITS 2 of two isolates (2PEX and 3PEX) and one Phoma exigua var. exigua isolate (CBS 431.74). All isolates yielded the same 523 bp long sequences, which were identical (E=0.0) to Phoma exigua ITS sequences in GenBank (Altschul et al., 1997). The nucleotide sequences have been assigned GenBank Accession Nos. EF136399 (2PEX) and EF136400 (CBS 431.74).

Pathogenicity tests were performed by spraying leaves and mature cones of detached lateral shoots (cvs. Bobek and Merkur) with a spore and mycelial suspension (10⁶ CFU/ml). Four bunches, each containing 2 lateral shoots, were used for each treatment. Bunches were covered with plastic bags and incubated in a growth chamber at 20ºC with relative humidity of 80%. The first lesions developed on leaves and cones of both cultivars six days after inoculation. Controls sprayed with distilled water showed no symptoms. The fungus was re-isolated from the lesions on leaves and cones. To our knowledge, this is the first report of P. exigua causing severe damage to the leaves and cones of hop.