Interaction of Basal Foliage Removal and Late-Season Fungicide Applications in Management of Hop Powdery Mildew

Identification of quantitative trait loci associated with R1-mediated resistance to powdery mildew and sex determination in hop (Humulus lupulus L.)

KEY MESSAGE

Two QTL were identified using linkage mapping approaches, one on hop linkage group 3 (qHl_Chr3.PMR1) associated with powdery mildew resistance and a second on linkage group 10 (cqHl_ChrX.SDR1) associated with sex determination. Hop (Humulus lupulus L.) is a dioecious species cultivated for use in beer. Hop powdery mildew, caused by Podosphaera macularis, is a constraint in many growing regions. Thus, identifying markers associated with powdery mildew resistance and sex provides the opportunity to pyramid R-genes and select female plants as seedlings, respectively. Our objectives were to characterize the genetic basis of R1-mediated resistance in the cultivar Zenith which provides resistance to pathogen races in the US, identify quantitative trait loci (QTL) associated with R1 and sex, and develop markers for molecular breeding-based approaches. Phenotypic evaluation of the population indicated that R1-based resistance and sex are inherited monogenically. We constructed a genetic map using 1339 single nucleotide polymorphisms (SNPs) based upon genotype-by-sequencing of 128 F₁ progeny derived from a Zenith × USDA 21058M biparental population. SNPs were assigned to 10 linkage groups comprising a map length of 1204.97 cM with an average density of 0.94 cM/marker. Quantitative trait locus mapping identified qHl_Chr3.PMR1, associated with R1 on linkage group 3 (LOD = 23.57, R² = 57.2%), and cqHl_ChrX.SDR1, associated with sex on linkage group 10 (LOD = 5.42, R² = 25.0%). Kompetitive allele-specific PCR (KASP) assays were developed for both QTL and assessed against diverse germplasm. Our results indicate that KASP markers associated with R1 may be limited to materials that are pedigree-related to Zenith, whereas markers associated with sex may be transferable across populations. The high-density map, QTL, and associated KASP markers will enable selecting for sex and R1-mediated resistance in hop.

Temperature Influences on Powdery Mildew Susceptibility and Development in the Hop Cultivar Cascade

The hop cultivar 'Cascade' possesses partial resistance to powdery mildew (Podosphaera macularis) that can be overcome by recently emerged, virulent isolates of the fungus. Given that hop is a long-lived perennial and that brewers still demand Cascade, there is a need to better understand factors that influence the development of powdery mildew on this cultivar. Growth chamber experiments were conducted to quantify the effect of constant, transient, and fluctuating temperature on Cascade before, concurrent to, and after inoculation as contrasted with another powdery mildew-susceptible cultivar, 'Symphony'. Exposure of plants to supraoptimal temperature (26 and 32°C) before inoculation led to more rapid onset of ontogenic resistance in intermediately aged leaves in Cascade as compared with Symphony. Cascade was overall less susceptible to powdery mildew when exposed to constant temperature ranging from 18 to 32°C directly after inoculation. However, cultivar also interacted with temperature such that proportionately fewer and smaller colonies developed on Cascade than Symphony at supraoptimal yet permissive temperatures for disease. When plants were inoculated and then exposed to high temperature, colonies became progressively more tolerant to temperatures of 26 to 30°C with increasing time from inoculation to exposure, as moderated by cultivar, the specific temperature, and their interaction. Subjecting plants to simulated diurnal temperature regimes at the time of inoculation or 24 h later indicated Cascade and Symphony responded proportionately similarly on days predicted to be marginally unfavorable or marginally favorable for powdery mildew, although Cascade was quantitatively less susceptible than Symphony. In sum, this research indicates that Cascade is overall less susceptible to powdery mildew than Symphony, and supraoptimal temperature before, concurrent to, or after infection may interact differentially to moderate disease risk in Cascade. Therefore, cultivar-specific risk assessments for powdery mildew appear warranted.

A Comprehensive Characterization of Ecological and Epidemiological Factors Driving Perennation of Podosphaera macularis Chasmothecia on Hop (Humulus lupulus)

Hop powdery mildew, caused by the ascomycete fungus Podosphaera macularis, is a consistent threat to sustainable hop production. The pathogen utilizes two reproductive strategies for overwintering and perennation: (i) asexual vegetative hyphae on dormant buds that emerge the following season as infected shoots; and (ii) sexual ascocarps (chasmothecia), which are discharged during spring rain events. We demonstrate that P. macularis chasmothecia, in the absence of any asexual P. macularis growth forms, are a viable overwintering source capable of causing early season infection two to three orders of magnitude greater than that reported for perennation via asexual growth. Two epidemiological models were defined that describe (i) temperature-driven maturation of P. macularis chasmothecia; and (ii) ascosporic discharge in response to duration of leaf wetness and prevailing temperatures. P. macularis ascospores were confirmed to be infectious at temperatures ranging from 5 to 20°C. The organism's chasmothecia were also found to adhere tightly to the host tissue on which they formed, suggesting that these structures likely overwinter wherever hop tissue senesces within a hop yard. These observations suggest that existing early season disease management practices are especially crucial to controlling hop powdery mildew in the presence of P. macularis chasmothecia. Furthermore, these insights provide a baseline for the validation of weather-driven models describing maturation and release of P. macularis ascospores, models that can eventually be incorporated into hop disease management programs.

The hop downy mildew pathogen Pseudoperonospora humuli

Pseudoperonospora humuli is an obligate biotrophic oomycete that causes downy mildew, one of the most devastating diseases of cultivated hop, Humulus lupulus. Downy mildew occurs in all production areas of the crop in the Northern Hemisphere and Argentina. The pathogen overwinters in hop crowns and roots, and causes considerable crop loss. Downy mildew is managed by sanitation practices, planting of resistant cultivars, and fungicide applications. However, the scarcity of sources of host resistance and fungicide resistance in pathogen populations complicates disease management. This review summarizes the current knowledge on the symptoms of the disease, life cycle, virulence factors, and management of hop downy mildew, including various forecasting systems available in the world. Additionally, recent developments in genomics and effector discovery, and the future prospects of using such resources in successful disease management are also discussed.

TAXONOMY

Class: Oomycota; Order: Peronosporales; Family: Peronosporaceae; Genus: Pseudoperonospora; Species: Pseudoperonospora humuli.

DISEASE SYMPTOMS

The disease is characterized by systemically infected chlorotic shoots called "spikes". Leaf symptoms and signs include angular chlorotic lesions and profuse sporulation on the abaxial side of the leaf. Under severe disease pressure, dark brown discolouration or lesions are observed on cones. Infected crowns have brown to black streaks when cut open. Cultivars highly susceptible to crown rot may die at this phase of the disease cycle without producing shoots. However, foliar symptoms may not be present on plants with systemically infected root systems.

INFECTION PROCESS

Pathogen mycelium overwinters in buds and crowns, and emerges on infected shoots in spring. Profuse sporulation occurs on infected tissues and sporangia are released and dispersed by air currents. Under favourable conditions, sporangia germinate and produce biflagellate zoospores that infect healthy tissue, thus perpetuating the infection cycle. Though oospores are produced in infected tissues, their role in the infection cycle is not defined.

CONTROL

Downy mildew on hop is managed by a combination of sanitation practices and timely fungicide applications. Forecasting systems are used to time fungicide applications for successful management of the disease. USEFUL WEBSITES: https://content.ces.ncsu.edu/hop-downy-mildew (North Carolina State University disease factsheet), https://www.canr.msu.edu/resources/michigan-hop-management-guide (Michigan Hop Management Guide), http://uspest.org/risk/models (Oregon State University Integrated Plant Protection Center degree-day model for hop downy mildew), https://www.usahops.org/cabinet/data/Field-Guide.pdf (Field Guide for Integrated Pest Management in Hops).

Susceptibility of Hop Crown Buds to Powdery Mildew and its Relation to Perennation of Podosphaera macularis

In the Pacific Northwestern United States, the hop powdery mildew fungus, Podosphaera macularis, survives overwintering periods in association with living host tissue because the ascigerious stage of the pathogen is not known to occur in this region. Field experiments were conducted over a 5-year period to describe the overwintering process associated with crown bud infection and persistence of P. macularis. Surface crown buds increased in abundance and size beginning in early July and continuing until mid-September. Buds of varying sizes remained susceptible to powdery mildew until late September to early October in each of 3 years of experiments, with susceptibility decreasing substantially thereafter. Potted plants were inoculated sequentially during early summer to autumn, then evaluated in the following year for development of shoots colonized by the powdery mildew fungus (termed flag shoots) due to bud perennation. Emergence of flag shoots was asynchronous and associated with shoot emergence and elongation. Flag shoots emerged over a protracted period from late February to early June, year dependent. In all 4 years of experiments, some infected buds broke and produced flag shoots after chemical desiccation of shoots in spring, a common horticultural practice in hop production conducted to set training timing and eliminate initial inoculum. Flag shoots were most numerous when plants were inoculated with P. macularis in early summer and, consequently, when powdery mildew was present throughout the entire period of crown bud development. The number of flag shoots produced was reduced from 6.8- to 46.6-fold when comparing the latest versus earliest inoculation dates. However, all inoculation timings yielded flag shoots at some level, suggesting that bud infection that occurs over an extended period of time in the previous season may allow the fungus to perennate. In studies in two commercial hop yards in Washington State, fungicide applications made after harvest reduced the level of powdery mildew on leaves in the current year but did not significantly reduce flag shoots in the following year. Given that bud infection occurred over a 10-week period, flag shoots developed even when plants were exposed to inoculum in October and some flag shoots survived chemical pruning practices, management efforts seem best directed to both preventative measures to reduce the likelihood of bud infection and remedial practices to physically eliminate infected crown buds in the ensuing year.