First Report of Silver Leaf Caused by Chondrostereum purpureum on Vaccinium corymbosum in Oregon

Since 2014, the Oregon State University Plant Clinic has received five blueberry (Vaccinium corymbosum L.) samples, two with basidiocarps, from ’Draper’ and ‘Liberty’ blueberries planted in three counties within the Willamette Valley, OR, all with concern of silver leaf disease. Affected canes had sparse or stunted silvery foliage, tip dieback, internal browning, and decreased fruit production. The basidiocarps were shelf-like with a buff to purplish tomentose adaxial surface and a smooth reddish to purplish abaxial surface. Basidiospores were white en masse, smooth, cylindrical, and measured 5 to 8 × 2 to 3.5 μm (n = 25). Pieces of discolored xylem were soaked in 10% household bleach for 2 min, rinsed, air dried, and placed on streptomycin-amended (100 ppm) potato dextrose agar (SPDA). Plates were stored in darkness at 20°C. Within 7 days, flat, white, cottony mycelia with clamp connections emerged, from which hyphal-tipped (HT) cultures were made. Genomic DNA was extracted from HT cultures, and the internal transcribed spacer region (ITS) was amplified using primers ITS1/ITS4 (White et al. 1990). GenBank BLAST results of five isolates from three counties (GenBank accessions MG774402 to MG774406) resulted in 99% nucleotide identity (555/558) with the Chondrostereum purpureum (Pers.: Fr.) Pouzar draft genome (GenBank KR264909). The pathogen was identified as C. purpureum based on morphological features and ITS sequence identity. To fulfill Koch’s postulates, both 3-year-old potted Draper blueberries and detached cuttings were used. Tissues were surface sterilized with 10% household bleach and rinsed. Two stems per plant on four whole plants were wounded ≈0.5 mm deep with a sterile 4-mm cork borer and inoculated with 4-day-old colonized SPDA plugs of C. purpureum (isolate 14-1496B) or an uncolonized SPDA plug. Inoculated wounds were then wrapped in Parafilm. After 6 months, all C. purpureum–inoculated canes exhibited stunted growth and internal browning; however, silvery leaves were not observed. In other studies, leaves from inoculated plants exhibited silver coloration 1 year postinoculation. The control canes remained asymptomatic. A white filamentous fungus was consistently recovered from cross sections with internal browning several millimeters from the inoculation sites and confirmed to be C. purpureum by ITS sequencing. Six unrooted cuttings, each with four to five nodes, from the same plants as above, were also inoculated. A 6- × 3- × 0.5-mm region of the bark and cambial layers was aseptically removed. Five cuttings received a 2-mm colonized plug and one an uncolonized SPDA plug that were placed onto the wound, wrapped in Parafilm, and stored at 20°C in 95% relative humidity. Within 2 weeks, the C. purpureum–inoculated cuttings exhibited canker formation extending from the inoculation sites, internal browning, chlorotic leaves, and premature defoliation. The control cutting remained asymptomatic. C. purpureum was consistently recovered from canker margins and verified by ITS sequencing. A second replicate yielded identical results. The expansion of the silver-leaf host range has implications not only for Pacific Northwest (PNW) blueberry growers but also for many other horticulturally important and susceptible commodities such as cherry, prune, and apple (Farr and Rossman 2017) also grown in the region. Silver leaf disease causes dieback of affected blueberry plants and branch dieback of tree fruits. Silver leaf was first described on V. corymbosum cultivars ‘Brigitta Blue’, ‘Bluecrop’, and ‘Duke’ in Chile during the 2005 to 2006 growing season (France et al. 2009) and later also detected on rabbiteye blueberry (V. virgatum Aiton) in Chile (France et al. 2017). Although the disease has been observed in the PNW since 2009, to our knowledge this is the first confirmed report of C. purpureum causing disease on blueberry in the United States.