NAME OF THE ORGANISM: Xanthomonas arboricola pv. juglandis (XANTJU)

Yes

The disease can cause three types of symptoms: bacterial blight of walnut (BBW), brown apical necrosis (BAN) together with fungal pathogens and X. arboricola pv. juglandis is also related to a more recently reported disease, so-called vertical oozing canker (VOC). The diseases may decrease the effectiveness of nursery production and reduce crop quality and yield due to premature fruit drop, as well as causing shell staining and kernel browning of the nuts still hanging on the tree (Belisario et al., 2002; Bouvet, 2005; Hajri et al., 2010; Lang & Evans, 2010; Lindow et al., 2014, cited from Kałużna et al., 2021). It is expected that, due to global warming and the popularity of walnut nuts, the harmfulness of these complex diseases will increase (Kałużna et al., 2021).
BBW: The disease starts with small water-soaked spots on the leaves, these coalesce, and turn into brown necrotic lesions. On twigs necrotic lesions can develop, the twigs can die. The pollen produced in catkins may also be colonized with X. arboricola pv juglandis, thus serving as an efficient dissemination pathway for the pathogen (Ark, 1944). On the fruits, the disease also starts with small, water-soaked, dark lesions, which rapidly turn necrotic, deepen, and collapse. At high humidity and warm temperatures, droplets of bacterial slime may ooze from the lesions. The affected fruits shrink, and in most cases drop off prematurely. Late infections, during shell hardening, are usually limited to the epicarp of the fruit, with the infected nuts showing a necrotized husk. On twigs and shoots, necrotic cankers may occur (Lang & Evans, 2010; Scortichini, 2010).
Symptoms of BAN manifest themselves as apical necrosis originating at the stigmatic end of the fruit. On fallen fruit, a brown patch appearing exclusively at the blossom end can occur, as well as blackening and rotting of inner tissues. The symptoms observed differ from those of WBB (Belisario et al., 2002; Moragrega & Özaktan, 2010).
The symptoms of VOC develop in woody tissues. Initially, they include longitudinal deformations of the affected trunks, followed by vertical cankers developing on both the trunks and branches, with brown to black exudates, observed mainly in summer. In the final stage, severe distortion and cracking of the affected trunks become evident (Hajri et al., 2010).

Major

No

Chemical control: Once the disease is present in walnut groves, copper compounds are widely used, mostly in association with dithiocarbamates; where allowed, antibiotics (preferably kasugamycin) are sprayed. The emergence of strains highly resistant to copper currently represents the major threat for effective management of the bacterial blight of walnut (Kałużna et al., 2021).
Cultivar choice: Early leafing cultivars are usually more susceptible to walnut blight (Olson et al., 1997). In addition, cultivar susceptibility would seem to play an important role also for fruit apical necrosis, cvs Hartley, Chandler, Serr, Lara and Vina being very sensitive (Moragraga & Özaktan, 2010). Other important predisposing factors increasing the severity of fruit apical necrosis are sandy soils, acidic soils and phosphorous, calcium and magnesium deficiencies (Garcin & Duchesne, 2001).
Cultural practices: This may include removing infected plant material, managing irrigation practices to reduce water on the foliage (Manthos et al., 2024).

Remark: in responses to the questionnaire, DE commented that no feasible and effective measures were available to control the pest on plants for planting at incidence above a certain threshold.