European hazelnut or filbert (Corylus avellana L.) is a small to medium-sized tree or shrub that is widely distributed in Europe from Portugal to the southern part of the Urals and from Norway (68° north latitude) to Spain, Italy, and Greece. It also is native to eastern and central Asia, including Turkey, Syria, and Iran, and to Algeria in northern Africa (10). European hazelnut produces a high-quality nut prized for its flavor. In prehistoric times, hunter-gatherer populations in eastern Asia (13,000 B.C.) and Scandinavia (11,000 B.C.) used the nuts as an important nutrient source (2,17). Remarkably, European hazelnut was one of the dominant trees in northern Europe after the end of the latest glacial period between 8,000 and 6,000 B.C. In some European peat bogs, the amount of hazelnut pollen exceeds that of all other tree species combined by 75% (39). Currently, European hazelnut is grown commercially for nut production primarily in Turkey, Italy, and Spain. The tree was introduced into Oregon in the United States in the mid-1850s (8,11), where it also is grown commercially (Table 1). Cultivation of European hazelnut is expanding to other countries in the northern and southern hemispheres that have a temperate climate (1,7,16). The main diseases of European hazelnut are eastern filbert blight, caused by Anisogramma anomala (Peck) E. Muller, in the United States (14), Apple mosaic virus in Spain (23), and bacterial canker and decline, caused by Pseudomonas avellanae, in Italy (25). Bacterial Canker and Decline Bacterial canker and decline on European hazelnut was first observed in northern Greece in 1976 (22). Within a few years, young plantings of the Turkish cultivar Palaz were almost completely destroyed by the disease (20). Subsequently, the same disease, referred to locally as “moria,” was observed in plantations throughout a 20,000-ha area in the Latium region of central Italy (i.e., Viterbo province) (37). Since it was first discovered, bacterial canker and decline has resulted in the mortality of more than 40,000 trees in central Italy. It continues to damage trees on approximately 1,000 ha in this area. The estimated loss per year is approximately $1.5 million, and the disease is considered a serious problem. There is a national law to partially compensate farmers whose trees have been seriously damaged by this disease (4). Bacterial canker also has been found in wild European hazelnut trees growing in forests adjacent to commercial orchards in Italy (31). The possibility of the pathogen spreading to the apparently highly susceptible wild European hazelnut population is a serious concern (24).