Arild Sletten


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Ås H7

Visiting address
Høgskoleveien 7, 1433 Ås


Fire blight, caused by Erwinia amylovora, was detected for the first time in Norway in 1986. It was a limited outbreak on the south-western coast, only on ornamentals, and particularly on Cotoneaster spp. An action group handling the eradication and containment of the disease was quickly established. Comprehensive statutory powers and resources were given by the government to do surveys and eradicate diseased or symptomless but highly susceptible plant species from contaminated areas. These activities have likely restricted fire blight to the western and southern coastal areas. Eastern and northern parts of Norway are considered free from fire blight. The disease has not been observed in important fruit-growing areas. Uncontrolled movement of beehives from areas with fire blight to areas free from the disease has contributed to its introduction to new areas. From 1969 to 2016 import of most host plants of E. amylovora from countries with fire blight was prohibited. A yearly program for annual surveys in parts of the country with commercial fruit-growing and nurseries, using digital maps on internet connected tablets with GPS and software for in situ registrations, proved to be an efficient method for discovering new outbreaks at an early stage, and to start eradication and thus limit further spread.

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European ash (Fraxinus excelsior), also known as common ash, occurs naturally inland in lower areas of southeastern Norway and along the southern coast of the country. It is important both as a forest and ornamental tree. During the last decade, dieback has become a disastrous disease on F. excelsior in many European countries. The anamorphic fungus Chalara fraxinea T. Kowalski (1), described for the first time from dying ash trees in Poland, is now considered the cause of ash dieback (2). In May of 2008, C. fraxinea was isolated from 1.5 m high diseased F. excelsior in a nursery in Østfold County in southeastern Norway. Symptoms included wilting, necrotic lesions around leaf scars and side branches, and discoloration of the wood. From symptomatic branches, small pieces (approximately 1 cm3) were excised in the transition area between healthy and discolored wood. After surface sterilization (10 s in 70% ethanol + 90 s in NaOCl), the pieces were air dried for 1 min in a safety cabinet, cut into smaller pieces, and placed on media. The fungus was isolated on potato dextrose agar (PDA) and water agar (WA). On PDA, the cultures were tomentose, light orange, and grew slowly (21 mm mean colony diameter after 2 weeks at room temperature). Typical morphological features of C. fraxinea developed in culture. Brownish phialides (14.8 to 30.0 [19.5] × 2.5 to 5.0 [4.1] μm, n = 50) first appeared in the center of the colonies on the agar plugs that had been transferred. The agar plugs were 21 days old when phialides were observed. Abundant sporulation occurred 3 days later. Conidia (phialospores) extruded apically from the phialides and formed droplets. Conidia measured 2.1 to 4.0 (3.0) × 1.4 to 1.9 (1.7) μm (n = 50). The first-formed conidia from each phialide were different in size and shape from the rest by being longer (6 μm, n = 10) and more narrow in the end that first appeared at the opening of the phialide. Internal transcribed spacer sequencing confirmed that the morphological identification was correct (Accession No. EU848544 in GenBank). A pathogenicity test was carried out in June of 2008 by carefully removing one leaf per plant on 10 to 25 cm high F. excelsior trees (18 trees) and placing agar plugs from a 31-day-old C. fraxinea culture (isolate number 10636) on the leaf scars and covering with Parafilm. After 46 days, isolations were carried out as described above from discolored wood that had developed underneath necrotic lesions in the bark and subsequently caused wilting of leaves. All the inoculated plants showed symptoms, and C. fraxinea was successfully reisolated. No symptoms were seen on uninoculated control plants (eight trees) that had received the same treatment except that sterile PDA agar plugs had been used.


This report assesses the plant health risk for the bacterial plant disease fire blight for the Pest Risk Assessment area of Norway. In addition to providing an updated pest categorisation for fire blight in relation to Norway, the report provide new results on fruit tree blossom infection risk based on examination of historical climate and phenology data. The report conclude that if E. amylovora is introduced into the main fruit growing districts of Norway, it is expected that the damage and losses to commercial fruit production and nurseries will be minor, under the current phytosanitary regime in Norway. Relaxation of the regulations in force for fighting fire blight in Norway will increase the expected damage and losses to commercial fruit production and nurseries to a moderate level.