Association of Spring Pruning Practices with Severity of Powdery Mildew and Downy Mildew on Hop

Gent, D. H., Nelson, M. E., Grove, G. G., Mahaffee, W. F., Turechek, W. W., and Woods, J. L. 2012. Association of spring pruning practices with severity of powdery mildew and downy mildew on hop. Plant Dis. 96:1343-1351. Downy mildew (caused by Pseudoperonospora humuli) and powdery mildew (caused by Podosphaera macularis) are important diseases of hop in the Pacific Northwest United States, and cultural practices may affect the severity of both diseases. The association of spring pruning quality and timing with severity of downy mildew and powdery mildew was assessed through analysis of survey data collected from commercial hop yards in Oregon and Washington. Among 149 hop yards surveyed, the most common pruning method was chemical desiccation (48% of yards), mechanical pruning (23%), or a combination of these practices (15%). The quality of pruning was assessed using a threecategory ordinal scale (“excellent”, “moderate”, or “poor”) based on the amount of foliage remaining on plants following pruning. Excellent pruning quality was attained more often in 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 increased approximately twofold 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, seasonal severity of powdery mildew on leaves and the incidence of cones with powdery mildew 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 with yards that had moderate or poor pruning 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 fivefold only in ‘Willamette’ in 2007. Severity of powdery mildew and cone yield was similar between plots that received the delayed or standard pruning timing treatments. Collectively, these studies emphasize that early spring sanitation measures are associated with reduced primary inoculum and are critically important for managing both downy mildew and powdery mildew. A savings of at least one fungicide application per year appears achievable when spring pruning is conducted thoroughly and slightly delayed compared with growers’ current practices. Cultural disease management practices are actions that involve the manipulation of the plant, its components, crop refuse, and the environment to reduce or prevent disease (5,26,31,34). In hop (Humulus lupulus L.), cultural practices are an important component of integrated management of the two most important foliar diseases of this crop: downy mildew, caused by Pseudoperonospora humuli; and powdery mildew, caused by Podosphaera macularis (21,22,27,29). Hop is a long-lived perennial plant that produces annual, herbaceous shoots from overwintering buds formed on new wood or from established wood deeper in the root system (30). Both pathogens have a phase of their lifecycle that involves perennation in or on buds formed on new wood during the previous season. In the case of Pseudoperonospora humuli, this is thought to occur via systemic infection of the root system and subsequent invasion of crown buds during autumn and winter (2,32). Buds invaded earliest and most severely can be killed, whereas buds with less severe colonization by the pathogen are reported to survive winter to produce systemically infected shoots (termed “spikes” due to their resemblance to a wheat spike) in spring (2). These spikes are a primary source of inoculum to initiate downy mildew epidemics. Oospores are frequently found in leaves, shoots, and cones with downy mildew, although the contribution of oosporic inoculum to epidemic development has largely been discounted because definitive evidence of oospore germination and infection in the field is lacking (29). Systemic infections are generally accepted as a key source of primary inoculum (2,16,29). In an analogous but biologically distinct process, Podosphaera macularis may infect crown buds during the preceding season, persisting as asexual mycelia or generally less common, sexually derived cleistothecia in and on buds (19). After dormancy and resumption of host growth in the following early spring, some proportion of these infected buds give rise to shoots colonized by the fungus and are referred to as “flag shoots” (19). Bud perennation is the only known means of overwintering of the pathogen in the Pacific Northwest region of the United States, presumably because only one mating type of the fungus is present or prevalent in the region (8). Early-season pruning of hop plants is conducted for horticultural purposes so that shoot growth is synchronized to optimize vigor Corresponding author: D. H. Gent, E-mail: [email protected] Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the United States Department of Agriculture and does not imply its approval to the exclusion of the products or vendors that may also be suitable. *The e-Xtra logo stands for “electronic extra” and indicates that a supplementary figure is available online. Accepted for publication 9 April 2012. http://dx.doi.org/10.1094 / PDIS-01-12-0084-RE This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2012. e-Xtra*