In April 2010, pink-orange spore masses that later turned brown were observed on 7 to 50% of the transplant lots during a routine screening of Belgian strawberry (Fragaria × ananassa, cv. Elsanta) for the latent presence of Colletotrichum acutatum using the petiole freeze method (4). These spore masses contained hyaline, canoe-shaped to allantoid conidia (mean size 7.5 × 1.8 μm), which is not consistent with C. acutatum spore morphology. Subsequently, a spore mass was transferred onto potato dextrose agar (PDA) and a gray-to-brown colony with whitish, aerial mycelium was produced, which is also not consistent with C. acutatum isolates. To identify the fungus, the ITS1-5.8S-ITS2 rDNA region was amplified by PCR and sequenced. The 485-bp region was 100% identical to that of Pilidium concavum specimen voucher BPI 1107275 (GenBank Accession No. AY487094). P. concavum (Desm.) Höhn. (synanamorph Hainesia lythri; teleomorph Discohainesia oenotherae) is a pathogen of strawberry causing tan-brown rot of fruit and is a common secondary invader of roots and dead strawberry plant parts (3). A recent strain of P. concavum from strawberry, isolate UPL 50, obtained from Brazil (L. Zambolim, Univ. Fed. de Viçosa, personal communication) showed similar colony, microscopic (mean spore size of 6.8 × 1.8 μm), and molecular (ITS sequence 98% identical to that of P. concavum specimen voucher BPI 1107275) features as the Belgian isolate. Pathogenicity tests were conducted on mature strawberry fruits by submerging 15 fruits per isolate for 3 min in a conidial suspension (2 × 106 conidia ml–1 of water) obtained from a 2-week-old colony on PDA. Controls were submerged in sterile distilled water. The inoculated fruits were incubated in a moist chamber at 25°C. Sunken, yellowish brown lesions with pink and later orange-brown spore masses were observed starting 3 days after inoculation on 88 and 94% of the fruit for the Brazilian and Belgian isolate, respectively. The control fruits remained healthy. The fungal isolates were reisolated from symptomatic fruits and their identity was confirmed based on morphological features. During a strawberry field survey in July 2010 in Sint-Truiden (Belgium), lesions typical of those described above were observed on eight strawberry fruits (cv. Elsanta). The fungus was isolated from the symptomatic tissue of two fruits and characterized as described above. Since P. concavum was latently present on strawberry transplants and caused disease on the fruits in the field, we conclude that P. concavum is a potential threat for Belgian strawberry production. Moreover, no strawberry cultivars with resistance to the pathogen have been reported. The disease has previously been reported on strawberry in South America and Poland (1,2), but to our knowledge, this is the first report of P. concavum on strawberry in Belgium. Although the spore and colony morphology of P. concavum is different from C. acutatum, the spore masses of P. concavum can easily be confused with the spore masses of C. acutatum when using the freeze method. This suggests the need for microscopic analysis of these spore masses during routine analyses.
References: (1) L. Cedeno et al. Interciencia 26:113, 2001. (2) U. P. Lopes et al. New Dis. Rep. 21:7, 2010. (3) J. L. Maas. Compendium of Strawberry Diseases. The American Phytopathological Society St. Paul, MN, 1998. (4) J. C. Mertely and D. E. Legard. Plant Dis. 88:407, 2004.