First Report of Leaf Rust of Blueberry Caused by Pucciniastrum vaccinii in Hawaii.

Blueberries (Vaccinium corymbosum L.) are a potential high-value, niche market crop for Hawaii. In May of 2007, rust-like symptoms were observed on multiple blueberry plants in a private nursery in Waimea, HI. In September of 2007, a similar leaf rust was observed on one bush of V. corymbosum cv. Sharpblue in the corner of a 36.6 × 9.1-m experimental plot at Mealani Research Station in Waimea. Within a month, rust was observed throughout the plot on 'Biloxi', 'Emerald', 'Jewel', 'Misty', 'Sapphire', and 'Sharpblue'. Preliminary field observations suggest that 'Sharpblue' and 'Sapphire' are highly susceptible to the rust and 'Biloxi' shows some tolerance. Leaf lesions began as approximately 1-mm chlorotic flecks that expanded and developed into reddish brown, necrotic spots with a chlorotic halo. New lesions and uredinia kept appearing over the course of 4 months. Defoliation occurred on plants where infection was severe. Yellowish orange pustules containing urediniospores first appeared on the abaxial side of older leaves and later appeared on new leaves. Urediniospores were elliptical to obovate (19.4 to 24.8 × 15.2 to 19.8 μm) with a thick, slightly roughened wall and a well-developed pore. Urediniospore morphology and dimensions were consistent with the description of Pucciniastrum vaccinii (G. Wint.) (1). A pathogenicity test was conducted with two 18-month-old 'Sharpblue' plants. Fully expanded leaves were sprayed with freshly collected urediniospores (3.8 × 10 spores per ml) suspended in a 0.05% solution of Tween 20 in water. The control plant was sprayed with sterile distilled water (SDW). Plants were covered with plastic bags for 48 h and held in a growth chamber at 20 to 22°C under continuous fluorescent lighting. The plastic bags were then removed and the plants were maintained in the growth chamber. Yellowish orange pustules that were identical to the original symptoms developed on 100% of inoculated leaves after 10 days. The plant inoculated with SDW remained symptomless. While leaf rust caused by P. vaccinii has been reported on Ohelo berry (V. reticulatum) (2), it has not been reported on V. corymbosum in Hawaii. To our knowledge, this is the first report of P. vaccinii on blueberry plants in Hawaii. This rust disease may pose a threat to the potential blueberry industry in Hawaii. References: (1) P. R. Bristow and A. W. Stretch. Page 20 in: Compendium of Blueberry and Cranberry Diseases. F. L. Caruso and D. C. Ramsdell, eds. The American Phytopathological Society, St. Paul, MN, 1995. (2) D. F. Farr et al. Fungal Databases. Systematic Botany and Mycology Laboratory. Online publication. ARS, USDA, 2008.