Foamy bacterial canker of almond caused by <i>Lonsdalea britannica</i> in Iran

Almond cultivation in southwestern Iran, accounting for over 30% of the national production, is currently threatened by an emerging bacterial disease. During 2023-2025, a comprehensive survey was conducted across 11 districts in Chaharmahal va Bakhtiari and Isfahan provinces to investigate the distribution and etiology of this condition. Affected trees exhibited foamy bacterial canker, characterized by the profuse exudation of viscous, amber-colored gum containing numerous air bubbles from bark cracks and pruning wounds. Symptoms typically emerged in late spring and progressed rapidly during high summer temperatures, leading to sunken necrotic lesions, and extensive internal woody discoloration. Infected trees often showed canopy dieback and succumbed to the disease within two to three years. From 259 symptomatic samples, a Gram-negative bacterium was consistently isolated (yielding 118 isolates) and identified as Lonsdalea britannica based on phenotypic traits and multilocus sequence analysis (MLSA) of the 16S rRNA, gyrB, and flhD genes. Pathogenicity was confirmed through detached shoot assays on almond cultivars 'Ferragnès' and 'Mamaee' and greenhouse inoculations on seedlings, fulfilling Koch's postulates. This study constitutes the first confirmed report worldwide of L. britannica causing foamy bacterial canker on almond, highlighting a significant biosecurity threat to the almond industry.