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Severe Outbreak of Crown Rot and Root Rot Caused by Cylindrocladium pauciramosum on Strawberry Tree in Italy

From 2006 to 2009, a new disease was noticed in several commercial nurseries in Sicily (Italy) on ~13,000 potted plants of strawberry tree (Arbutus unedo L.) ranging from 3 months to 4 years old. Symptomatic plants showed a crown and root rot and internal brown discoloration of the basal stem. No leaf spots were detected. The percentage of symptomatic plants was variable in the different stocks (1 to 28%). A Cylindrocladium sp. was isolated consistently from symptomatic tissues on potato dextrose agar (PDA). Although many crown rots are caused by Phytophthora or Rhizoctonia spp., plating small pieces of diseased tissue from rotten roots and crowns onto corn meal agar or PDA yielded these pathogens sporadically. Twenty Cylindrocladium isolates obtained from infected basal stems, crowns, and roots were selected and subsequently cultured on carnation leaf agar (CLA). Macroconidiophores consisted of a stipe, a penicillate arrangement of fertile branches, and stipe extension terminating in an obpyriform-to-ellipsoidal vesicle (6 to 10 μm in diameter). Conidia cylindrical, straight, and 1-septate ranged from 40 to 60 × 4 to 5 μm. All single-conidial isolates were mated with tester strains DISTEF-G87 (MAT1-1) and DISTEF-G128 (MAT1-2) of Cylindrocladium pauciramosum (3) on CLA and produced fertile perithecia after 2 months. Perithecia were solitary or in groups, orange to red-brown, subglobose to ovoid, and ranged from 260 to 400 × 180 to 290 μm in diameter. On the basis of the combination of morphological and perithecial characters and mating type the isolates were identified as C. pauciramosum C.L. Schoch & Crous, teleomorph Calonectria pauciramosa C.L. Schoch & Crous (1,4). One representative isolate was deposited at Centraalbureau voor Schimmelcultures (CBS; No. 123918) open fungi collection. Pathogenicity tests were performed by adding sterile water to CLA cultures of C. pauciramosum from a single-conidial isolate and incorporating the resulting spore suspension (10⁵ conidia per ml) on the soil surface of 20 3-month-old strawberry tree potted plants. The same number of plants served as a control. After inoculation, plants were maintained in a growth chamber at 25 ± 1°C and 90 to 95% relative humidity. All inoculated plants developed crown and root rot identical to one observed in the nursery 2 months after inoculation. Control plants remained symptomless. C. pauciramosum was always reisolated from infected plants. This fungus represents a serious threat for ornamental nurseries in Europe. Diseases related to infection of C. pauciramosum are different in relationship to the host tissues and the affected species. On strawberry tree, C. pauciramosum was previously detected in Italy as causal agent of leaf spots (2). To our knowledge, this is the first record of crown and root rot of strawberry tree caused by C. pauciramosum. In contrast with leaf spots, this report could indicate an important limiting factor for cultivation of strawberry tree potted plants in nurseries. The knowledge of the susceptibility of this host to crown and root rot caused by C. pauciramosum can help growers apply specific control strategies in nurseries.

References: (1) P. W. Crous. Taxonomy and Pathology of Cylindrocladium (Calonectria) and Allied Genera. The American Phytopathological Society, St. Paul, MN, 2002. (2) G. Polizzi and V. Catara. Plant Dis. 85:803, 2001. (3) G. Polizzi and P. W. Crous. Eur. J. Plant Pathol. 105:407, 1999. (4) C. L. Schoch et al. Plant Dis. 85:941, 2001.