Glomerella acutata on Highbush Blueberry (Vaccinium corymbosum L.) in Norway

In August 2004, Glomerella acutata Guerber & Correll was detected on fruits from highbush blueberry (Vaccinium corymbosum L.) for the first time in Norway. Both the conidial (Colletotrichum acutatum J. H. Simmonds) and the ascigerous (G. acutata) stage developed on naturally infected blueberry fruits. Perithecia also readily formed on blueberries and strawberries inoculated with a culture from highbush blueberry, and on artificial, solid medium (both on strawberry leaf agar and potato dextrose agar). To our knowledge this is the first report worldwide of the teleomorph of Glomerella acutata on a naturally infected host. Introduction Highbush blueberry (Vaccinium corymbosum L.) is a minor crop in Norway, and is grown commercially only on approximately 30 ha. Climatic conditions for highbush blueberry are only suitable in coastal regions in southern Norway (10). Anthracnose rot of blueberry is a problem worldwide, causing preand postharvest rots (1,5,6,9). Fertile perithecia of the perfect (sexual/teleomorphic) stage of C. acutatum were discovered for the first time by Guerber and Correll (3), during crosses of self-sterile monoconidial strains of C. acutatum isolated from apple, blueberry, kiwi, pecan, strawberry, avocado, and papaya. The species was named Glomerella acutata Guerber & Correll. Here we report the first occurrence of anthracnose in highbush blueberry in Norway and the in vivo discovery of the teleomorph of G. acutata. Location, Sampling, and Symptoms A grower in Vest-Agder County in southern Norway reported extensive damage on highbush blueberry fruits in August 2004. Damaged berries were collected from randomly-selected plants in a field with 500 bushes. A mixed sample of both ripe and unripe berries from the cvs. Toro, Duke, and Patriot were investigated. Symptoms included some berries with brown, sunken, dry spots and some shrivelled berries. In general, the berries were small, and callus had formed in the dry spots. The dry spots did not host any fungi after incubation, and there were no signs of insect damage. The grower reported that there had been a hailstorm at an early stage of fruit development and that might explain the dry spots. Incubation, Isolation, and Identification All incubation of fruit or other plant material described below took place in moist chambers (sealed plastic boxes with 100% RH) at room temperature. After three days of incubation of the plant material from Vest-Agder, many berries were covered with gray mold (Botrytis cinerea), and such berries were removed from the incubation chamber. Two weeks after the incubation started, a few berries were partly covered with orange, conidial spore masses of C. acutatum emerging from densely situated acervuli. 9 May 2007 Plant Health Progress The fungus was isolated on acidified potato dextrose agar (PDA) by transferring conidia from a naturally infected berry to the agar with a sterile inoculating needle. See Table 1 for morphological details. According to EPPO (2), conidial spores from C. acutatum should be in the range 8-16 (20) × 2.5-5 μm. Thus, the size of the conidia we initially found on the incubated fruit were in the upper range. Conidia obtained after an inoculation test in August 2006 were smaller (see Table 1). Setae were never observed in acervuli on berries. Table 1. Characteristics (signs) of the imperfect (conidial) and perfect (ascigerous) stage of Glomerella acutata from highbush blueberry. After five weeks of incubation of the infected berry, numerous perithecia (small black spots) of G. acutata had emerged in and between the conidial spore masses (Fig. 1). The isolate obtained from the conidia was self-fertile, and perithecia were also readily produced on potato dextrose agar (PDA) and strawberry leaf agar (SLA) (Fig. 2). SLA was prepared as described by Gunnell Imperfect stage Perfect stage