Resumen
- Symbioses among ambrosia beetles and fungi are one of most common and successful relationships in any forest ecosystem. Ambrosia beetle–fungus interactions with trees are typically benign, and only dead or stressed trees are susceptible to be colonized. In recent decades, many of these relationships transformed into unexpectedly destructive tree diseases, especially when they challenge a new noncoevolved host (Kasson et al. 2013; Ploetz et al. 2013). Tree colonization by xylomicetophagous Scolytinae and its ambrosia fungi provoking branch dieback, wilting, and mortality have been reported in avocado-growing regions around the world (Freeman et al. 2013). Some ambrosia fungi can be laterally transferred by several species of xylomicetophagous beetles, increasing their host range and spatial distribution (Carrillo et al. 2014). During the 2013–14 dry and rainy seasons, avocado trees exhibiting branch dieback and wilting were observed in San Diego, La Paz, and Jagua del Pilar municipalities located in the Serrania de Perija mountain range. Close examination revealed white powdery exudate over the outer bark associated with beetle holes on the trunk and main branches of avocado trees. Old trees (>15 m height) exhibited black exudate extensively covering the whole trunk (black gum pitchout). Wood under the exit hole revealed brown necrosis. Adults and larvae of ambrosia beetles (Xyleborus volvolus and Premnobius spp.) were found within galleries. Sapwood tissues were plated onto V8 agar amended with amikacin sulfate (0.1 g/liter). After 1 week incubation, white flat filamentous fungal colonies with filiform margin and aseptate hyphae were observed. Conidia were hyaline, short, aseptate, straight-cylindrical, and rounded at both ends. To confirm identity, partial sequencing of the β-tubulin (TUB) gene from three isolates (TUR5.159, TUR5.192, and TUR5.181) was performed. Sequences were deposited into GenBank (KX981917, KX827304, and KX827303). A BLASTn search revealed high similarity (>97%) to Bionectria pseudochroleuca strain CML 2526 (accession no. KX185045.1). Ten additional ID loci were included but PCR amplifications were not successful. Healthy 6-month-old avocado trees were wounded to a depth of 3 to 5 mm and 5-mm mycelial plugs from 5-day-old cultures were placed onto the freshly wounded surfaces and then wrapped with Parafilm. Control shoots were inoculated with sterile agar plugs and three replicates per isolate were used. Expanding lesions were observed on all inoculated plantlets except for the control (lesions length: 4.21 ± 0.62 cm). The fungal pathogen was reisolated from lesions of inoculated avocado plantlets and the identity was confirmed through cultural characteristics and conidial morphology. Bionectria species associated with ambrosia beetles colonizing avocado trees has been reported in California (Eskalen et al. 2012; McDonald and Eskalen 2011). To our knowledge, this is the first report of a Bionectria species causing dieback and wilting on avocado in Colombia.