First report of the quarantine pathogen Xanthomonas arboricola pv. pruni on apricot and plum in Switzerland

Xanthomonas arboricola pv. pruni (Xap), cause of stone fruit bacterial spot on many Prunus species, is a European and Mediterranean Plant Protection Organisation (EPPO) A2-list quarantine pathogen (Anonymous, 2006). It can cause significant yield loss and tree death. In Europe, Xap occurs in France, Italy, Bulgaria, Romania and Ukraine, but appears to be spreading due to changes in trade patterns, climate, host range and/or pathogen genotype. This is the first report of Xap in Switzerland, occurring with very limited distribution.

Xap was first detected on apricot trees and fruit (Prunus armeniaca) showing symptomsin 2005 in four orchards located in canton Valais, the primary Swiss production region. It was detected in 2009 on Japanese apricot (Prunus mume) inter-cropped with apricot in two of these orchards. Isolation from symptom-bearing fruit, leaf and canker samples using YCDA medium recovered colonies confirmed to be Xap based on colony morphology, quinate metabolism, serology(Neogen Europe, EXPRESS™), gyrB sequencing (Parkinson et al., 2009; GenBank Accession No. FJ719771 - FJ719773) and pathogenicity bioassays with detached peach leaves. Direct detection in symptom-bearing fruit, leaves and cankers was achieved in 100% of samples using Xap-specific PCR primers (Pagani, 2004) and in 40% using X. fragariae/arboricola primers (Weller et al., 2007). These primer sets gave 100% and 93% agreement respectively with standard identification methods when applied to Xap isolates. All Swiss isolates were copper sensitive.

During intensive surveys between 2006 and 2009 throughout the region where Xap was first detected (208 ha), no incidence has been observed outside the four original infected orchards. In three of these orchards Xap is considered eradicated, with all symptom-bearing trees removed in the fourth. This suggests probable introduction via planting material rather than fruit or aerosols. Xap is considered present with limited distribution but not eradicated.