NAME OF THE ORGANISM: Xanthomonas arboricola pv. corylina (XANTCY)

Xanthomonas arboricola pv. corylina damages both cultivated and wild hazelnut (Corylus sp.) plants (Scortichini et al., 2002). Bacterial blight is most significant on European hazelnut (Corylus avellana) which is commercially grown for nut production, but Corylus pontica, C. maxima [both now C. avellana], C. rostrata [C. cornuta], and C. colurna are also susceptible to the pathogen (Miller et al., 1949; EPPO, 1986; Ibarra et al., 2012; Lamichhane & Varvaro, 2014). The latter are considered minor hosts (EPPO, 1986; EPPO, 2004).

Two factors are involved with dispersal and movement of X. arboricola pv. corylina: (i) nurseries, and (ii) hazelnut production orchards. Traditionally hazelnut is propagated by layering. Layering produces large, well-rooted trees but the risk of contamination is high, since it allows a direct passage of the pathogen from latently infected mother plants to nursery material. Symptoms of plants affected by bacterial blight can be different in orchards and nurseries as a result of different growing systems. In orchards, suckers are pruned away every year, while suckering is encouraged in nurseries on the mother plants, to produce shoots for layering (Wu, 2013). Mother plants bearing densely crowded, long, young shoots create high moisture conditions, ideal for the increase and spread of the pathogen (Miller et al., 1949; EPPO, 2004; Lamichhane & Varvaro, 2014).

Once the latent-infected propagation material is introduced into new orchards, new infections occur rapidly. Buds in spring give first expression of symptoms and provide the main source of inoculum for subsequent more widespread infection. Penetration of buds, leaves and new shoots is through stomata and wounds. The presence of moisture on the host is a prerequisite for infection. Tissue is more susceptible when young and succulent. The bacterium multiplies on the young plants and spreads from plant to plant by pruning tools and/or by rain splash. The occurrence of secondary infection depends on the relative abundance of primary sources, and on the frequency and the amount of rainfall during the infection period, which extends late summer (buds are 75% grown) until they open in the following spring (Miller et al., 1949; CABI, 2022). An epiphytic phase of the bacterium occurs on leaf surfaces during the growing season. There is a possibility of transmission by pollen to female flowers. The bacterium has an epiphytic phase (Gardan & Devaux, 1987; EPPO, 2004; CABI, 2022). X. arboricola pv. corylina can survive in dormant buds and in cankers from 1 year to the next, though survival in cankers probably has little epidemiological importance. The bacterium can survive on fallen leaves but cannot overwinter in soil (Gardan, 1982, 1983; cited in Lamichhane & Varvaro, 2014).

Stress predisposes young Corylus trees to the disease (Moore et al., 1974; Lamichhane et al., 2013). Especially Adverse growing conditions, i.e., poor soil drainage, sunscald, cold and drought during the first few years of the life of an orchard predispose trees to infection (Miller et al., 1949). Moore et al. (1974) demonstrated that mortality of inoculated young hazelnut trees significantly increased without irrigation. In the presence of irrigation, plants grew vigorously and were tolerant to X. arboricola pv. corylina infection.

Seeds from fruits picked on infected trees can produce infected seedlings (EPPO, 1986).

Although many publications highlight importance of plants for planting as a pathway, the Fruit SEWG could not conclude on the relative importance of natural spread compared to plants for planting. Plants for planting was not considered as a significant pathway for Xanthomonas arboricola pv. juglandis because of high risk of infection from the surrounding area. In the case of X. arboricola pv. corylina, production practices of plants for planting (layering, no rotation, humidity conditions) may increase the risk with plants for planting. X. a. corylina is less widespread because less present and Corylus productions are less distributed.