Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 9, 2012
Publication Date: N/A
Interpretive Summary: This research demonstrates that the amount leaf and stem material remaining after performing spring pruning of hop plants affects the subsequent amount of disease development and the need for fungicide applications. Hop growers who had little leaf and stem tissue remaining after their spring pruning made 1.1 to 1.5 fewer fungicide applications for management of downey and powdery mildew at a commercial scale.
Technical Abstract: Downy mildew and powdery mildew are important diseases of hop in the Pacific Northwest USA, and cultural practices may affect the severity of both diseases. The association of spring pruning quality and timing with severity of downy and powdery mildew from commercial hop yards in Oregon and Washington was assessed. Among 149 hop yards surveyed, the most common pruning method was chemical desiccation (48%), mechanical pruning (23%), or a combination of these practices (15%). Pruning quality was rated using an ordinal scale of “excellent, moderate, or poor” based on the foliage remaining following pruning. An excellent rating was associated with yards pruned twice (74.6 to 82.1% of yards) versus once (33.8% of yards), independent of pruning method. Seasonal severity of downy mildew in Oregon almost had a 2-fold with reduction in pruning quality from excellent to moderate to poor. Pruning quality was not significantly related to levels of powdery mildew on leaves or cones in Oregon. Under more severe disease pressure in Washington, however, powdery mildew severity on leaves and incidence of cones were significantly greater in yards that had poor pruning compared with excellent pruning. Moreover, yards that had excellent pruning quality received, on average, 1.1 to 1.5 fewer fungicide applications per season for downy mildew or powdery mildew compared to other yards with moderate or poor quality. This savings was associated with delayed initiation of the first application by 7.5 to 14.2 days in yards with excellent pruning quality. Replicated experiments in commercial yards in Oregon quantified the effect of delaying pruning timing 5 to 21 days compared with growers’ standard practices on the diseases and yield. Downy mildew suppression by delayed pruning was dependent on cultivar and year of sampling, being significantly reduced 5-fold only in ‘Willamette’ in 2007. Collectively, these studies emphasize that early spring sanitation measures are associated with reduced primary inoculum and are critically important for managing both downy and powdery mildew.