Evaluation of Fungicidal Canopy Sprays for the Management of Cytospora plurivora in Western Colorado Peach Orchards

Cytospora canker, primarily caused by Cytospora plurivora D.P. Lawr. L.A. Holland & Trouillas is one of the most destructive diseases of peach (Prunus persica [L.] Batsch), a crop with significant economic and cultural importance in Colorado. Canker pathogens including Cytospora spp. are a major driver of reduced productivity and longevity of orchards worldwide, yet management options are limited. Fungicides can be used to protect pruning wounds from infection but have primarily been tested as hand sprays which are labor intensive and therefore expensive. Air blast sprayers are currently used by growers to manage other diseases and pests and for nutrient applications; however, their efficacy on canker pathogens is not well understood. Additionally, methods for application on woody tissue may differ from foliar sprays. This study evaluated the efficacy of canopy spray applications of captan and lime sulfur in preventing infections on scaffold branches and examined the effects of application timing and axial fan use on spray coverage and uniformity. In spring trials, 3% lime sulfur significantly reduced lesion size by 58% on middle branches and 87% on upper branches, whereas captan (3.5 liters per hectare) showed mixed results. The rate of application may have been insufficient. Minimal lesion development occurred in fall trials, and neither treatment was significantly different from untreated controls. Discrepancies in efficacy across heights were linked to uneven distribution of spray coverage. In coverage trials, spray coverage, as measured by water sensitive paper cards, varied significantly by season, height, and orchard. Greater coverage was achieved on bare trees in spring compared to foliated trees in summer. Axial fan use improved coverage in some cases but decreased it in others. The results from this study will help inform future trials and aid peach growers in developing management programs for Cytospora canker.