Seasonal Variation in Host Susceptibility to Fusarium Canker in Young Almond Trees.

Loss of water that reduces the relative water content (RWC) of bark can occur during processing, cold storage, and planting of bare-root stone fruit trees. In California nurseries and newly planted orchards, this stress can predispose young almond trees (Prunus dulcis) to a canker disease caused primarily by Fusarium species. While reduced bark RWC contributes to disease development, anecdotal observations suggest a seasonal effect on host physiology may also influence disease severity. We evaluated the effect of season and the impact of drying and reduced RWC on susceptibility of almond branch segments excised from orchard trees (cv. Nonpareil) to Fusarium acuminatum, Fusarium avenaceum, Fusarium brachygibbosum, and Fusarium californicum sp. nov. With lesion size as the criterion, excised inoculated branch segments were most susceptible in spring, of intermediate susceptibility during winter dormancy, and least susceptible during summer and fall. Consistent with an earlier study, branches with RWC between 80 and 85% yielded lesions that were significantly larger than lesions from branches with bark that was above or below that range. However, the effect of reduced bark moisture on lesion size was only apparent in the spring. These results affirm the importance of avoiding conditions that diminish moisture status in bare-root almond trees in Fusarium canker disease management, especially during transport and planting operations in the spring, a period of high physiological vulnerability. California nurseries apply fungicides to bare-root trees prior to cold storage to reduce "mold" growth. Of eight fungicides currently registered for use on almond trees, fludioxonil (Scholar), fluopyram/trifloxystrobin (Luna Sensation), and fluxapyroxad/pyraclostrobin (Merivon) were most inhibitory to in vitro mycelial growth of F. acuminatum, F. avenaceum, and F. brachygibbosum. However, our almond branch disease assay did not demonstrate preventive or curative fungicide action against infections by F. acuminatum or F. avenaceum.