Risk Factors for Bud Perennation of Podosphaera macularis on Hop

The hop powdery mildew fungus Podosphaera macularis persists from season to season in the Pacific Northwestern United States through infection of crown buds because only one of the mating types needed to produce the ascigerous stage is presently found in this region. Bud infection and successful overwintering of the fungus leads to the emergence of heavily infected shoots in early spring (termed flag shoots). Historical data of flag shoot occurrence and incidence in Oregon and Washington State during 2000 to 2017 were analyzed to identify their association with the incidence of powdery mildew, growers’ use of fungicides, autumn and winter temperature, and other production factors. During this period, flag shoots were found on 0.05% of plants evaluated in Oregon and 0.57% in Washington. In Oregon, the incidence of powdery mildew on leaves was most severe and the number of fungicide applications made by growers greatest in yards where flag shoots were found in spring. Similarly, the incidence of plants with powdery mildew in Washington was significantly associated with the number of flag shoots present in early spring, although the number of fungicide applications made was independent of flag shoot occurrence. The occurrence of flag shoots was associated with prior occurrence of flag shoots in a yard, the incidence of foliar powdery mildew in the previous year, grower pruning method, and, in Washington, winter temperature. A census of hop yards in the eastern extent of the Oregon production region during 2014 to 2017 found flag shoots in 27 of 489 yards evaluated. In yards without flag shoots, 338 yards (73.2%) were chemically pruning or not pruned, whereas the remaining 124 (26.8%) were mechanically pruned. Of the 27 yards with flag shoots, 22 were either chemically pruned or not pruned and 4 were mechanically pruned in mid-April, well after the initial emergence of flag shoots. The prevalence of yards with flag shoots also was related to thoroughness of pruning in spring (8.1% of yards with incomplete pruning versus 1.9% of yards with thorough pruning). A Bayesian logistic regression model was fit to the data from the intensively assessed yards in Oregon, with binary risk factors for occurrence of a flag shoot in the previous year, occurrence of foliar mildew in the previous year, and thoroughness of pruning in spring. The model indicated that the median and 95% highest posterior density interval of the probability of flag shoot occurrence was 0.0008 (0.0000 to 0.0053) when a yard had no risk factors but risk increased to 0.0065 (0.0000 to 0.0283) to 0.43 (0.175 to 0.709) when one to all three of the risk factors were present. The entirety of this research indicates that P. macularis appears to persist in a subset of chronically affected hop yards, particularly yards where spring pruning is conducted poorly. Targeted management of the disease in a subset of fields most at risk for producing flag shoots could potentially influence powdery mildew development regionwide.

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Adaptation to Partial Resistance to Powdery Mildew in the Hop Cultivar Cascade by Podosphaera macularis

Plant Disease ◽

10.1094/pdis-12-16-1753-re ◽

2017 ◽

Vol 101 (6) ◽

pp. 874-881 ◽

Cited By ~ 8

Author(s):

David H. Gent ◽

Stephen T. Massie ◽

Megan C. Twomey ◽

Sierra N. Wolfenbarger

Keyword(s):

Powdery Mildew ◽

Partial Resistance ◽

Quantitative Resistance ◽

Washington State ◽

R Genes ◽

Powdery Mildew Fungus ◽

Field Surveys ◽

The Pacific ◽

Qualitative Resistance ◽

Podosphaera Macularis

The hop cultivar Cascade has been grown in the Pacific Northwestern U.S. and elsewhere with minimal input for management of powdery mildew (Podosphaera macularis) for nearly 15 years due to the putatively quantitative resistance in this cultivar. While partial resistance is generally thought to be more durable than qualitative resistance, in 2012, powdery mildew was reported on Cascade in Washington State. Field surveys conducted during 2013 to 2016 indicated increasing prevalence of powdery mildew on Cascade, as well as an increasing number of fungicide applications applied to this cultivar in Washington State. Nearly all isolates of P. macularis tested were able to infect Cascade in laboratory inoculations. However, the greatest number of colonies, most conidia produced, and the shortest latent period was only observed with isolates derived originally from Cascade, as compared with other isolates derived from other cultivars. Further, the enhanced aggressiveness of these isolates was only manifested on Cascade and not six other susceptible cultivars, further indicating a specific adaptation to Cascade by the isolates. There was no evidence of a known major R-gene in Cascade, as seven isolates of P. macularis with contrasting virulence all infected Cascade. Among 158 isolates obtained from hop yards planted to Cascade, only two (1.3%) were able to infect the cultivar Nugget, which possesses the resistance factor termed R6, indicating that isolates of P. macularis virulent on Nugget are largely distinct from those adapted to Cascade. Further, race characterization indicated Cascade-adapted isolates of P. macularis were able to overcome R-genes Rb, R3, and R5, but not other known R-genes. Therefore, multiple R-genes and other sources of partial resistance are expected to provide resistance to Cascade-adapted strains of the fungus. Given the plasticity of the powdery mildew fungus, breeding strategies for powdery mildew need to consider the potential for adaptation to both qualitative and partial resistance in the host.

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Erysiphe trifolii Causing Powdery Mildew of Lentil (Lens culinaris)

Plant Disease ◽

10.1094/pdis-93-8-0797 ◽

2009 ◽

Vol 93 (8) ◽

pp. 797-803 ◽

Cited By ~ 22

Author(s):

Renuka N. Attanayake ◽

Dean A. Glawe ◽

Frank M. Dugan ◽

Weidong Chen

Keyword(s):

Powdery Mildew ◽

Pacific Northwest ◽

The United States ◽

Morphological Characters ◽

Grain Legumes ◽

Its Sequences ◽

Powdery Mildew Fungus ◽

The Pacific ◽

Soybean Genotypes ◽

Erysiphe Trifolii

The taxonomy of the powdery mildew fungus infecting lentil in the Pacific Northwest (PNW) of the United States was investigated on the basis of morphology and rDNA internal transcribed spacer (ITS) sequences. Anamorphic characters were in close agreement with descriptions of Erysiphe trifolii. However, teleomorphs formed chasmothecial appendages with highly branched apices, whereas E. trifolii has been described as producing flexuous or sometimes loosely branched appendages. Branched appendages have been described in Erysiphe diffusa, a fungus reported from species of Lens, Glycine, and Sophora, raising the possibility that the PNW fungus could be E. diffusa. Examination of morphological characters of an authentic specimen of E. trifolii from Austria determined that it included chasmothecial appendages resembling those seen in PNW specimens. Furthermore, ITS sequences from five powdery mildew samples collected from lentils in PNW greenhouses and fields from 2006 to 2008 were identical to one another, and exhibited higher similarity to sequences of E. trifolii (99%) than to those of any other Erysiphe spp. available in GenBank. Parsimony analysis grouped the lentil powdery mildew into a clade with Erysiphe baeumleri, E. trifolii, and E. trifolii–like Oidium sp., but indicated a more distant relationship to E. diffusa. In greenhouse inoculation studies, the lentil powdery mildew fungus did not infect soybean genotypes known to be susceptible to E. diffusa. The pathogenicity of E. trifolii on lentil was confirmed using modified Koch's postulates. This is the first report of E. trifolii infecting lentil. E. diffusa and E. trifolii have different host ranges, so the discovery of E. trifolii on lentil has implications both for determining species of powdery mildews on cool-season grain legumes, and in disease management.

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Responding to an Introduced Pathogen: Podosphaera macularis (Hop Powdery Mildew) in the Pacific Northwest

Plant Health Progress ◽

10.1094/php-2003-1113-07-rv ◽

2003 ◽

Vol 4 (1) ◽

pp. 21 ◽

Cited By ~ 10

Author(s):

Walter F. Mahaffee ◽

Carla S. Thomas ◽

William W. Turechek ◽

Cynthia M. Ocamb ◽

Mark E. Nelson ◽

...

Keyword(s):

Powdery Mildew ◽

Pacific Northwest ◽

Management Practices ◽

Risk Index ◽

Lessons Learned ◽

Infection Model ◽

The Pacific ◽

Sphaerotheca Macularis ◽

The Cost ◽

Podosphaera Macularis

Powdery mildew of hop (Humulus lupus L.), which is caused by Podosphaera macularis (formerly Sphaerotheca macularis) was found in the Yakima Valley, WA in 1996 and subsequently spread to the growing regions in Oregon and northern and southern Idaho. To rapidly assist growers in reducing the cost associated with the preventive fungicide program, the Gubler/Thomas grape powdery mildew risk infection model was adapted for hops. In addition, field surveys were utilized to identify other management practices that impacted disease development. Weather networks were established and utilized to deliver daily regional maps indicating the risk index. These maps were posted to the web for daily access. Lessons learned from this experience will be useful in addressing future pathogen introductions. Accepted for publication 28 March 2003. Published 13 November 2003.

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First Report of Powdery Mildew of Eschscholzia californica (California Poppy) caused by Erysiphe cruciferarum in North America

Plant Health Progress ◽

10.1094/php-2006-1213-01-br ◽

2006 ◽

Vol 7 (1) ◽

pp. 45 ◽

Cited By ~ 1

Author(s):

Dean A. Glawe

Keyword(s):

North America ◽

Powdery Mildew ◽

Pacific Northwest ◽

Ongoing Study ◽

Annual Species ◽

Powdery Mildew Fungus ◽

First Report ◽

California Poppy ◽

The Pacific ◽

Erysiphe Cruciferarum

California poppy is an annual species grown widely in the Pacific Northwest. Once established, populations are self-seeding and require little care. During an ongoing study of Erysiphales, a powdery mildew fungus was collected repeatedly on this species in Seattle, WA. The fungus was determined to be Erysiphe cruciferarum Opiz ex Junell, a species not reported previously on this host in North America. This report documents the occurrence of the disease and provides information on the morphology and identification of the causal agent. Accepted for publication 8 November 2006. Published 13 December 2006.

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Hop Powdery Mildew Control Through Alteration of Spring Pruning Practices

Plant Disease ◽

10.1094/pdis-10-15-1127-re ◽

2016 ◽

Vol 100 (8) ◽

pp. 1599-1605 ◽

Cited By ~ 8

Author(s):

Claudia Probst ◽

Mark E. Nelson ◽

Gary G. Grove ◽

Megan C. Twomey ◽

David H. Gent

Keyword(s):

Powdery Mildew ◽

Pacific Northwest ◽

Low Cost ◽

Specific Effect ◽

Disease Development ◽

Quality Factors ◽

Early Season ◽

Yield And Quality ◽

The Pacific ◽

Podosphaera Macularis

Podosphaera macularis, the causal agent of hop powdery mildew, is a recurrent threat to hops in the Pacific Northwest because of the potential to reduce cone yield and quality. Early-season pruning is a common practice in hop production for horticultural reasons. Studies were conducted over a 3-year period in a commercial hop yard to quantify the effect of pruning method and timing on disease development, yield, and cone quality factors. A 4-week delay in pruning reduced the incidence of leaves with powdery mildew from 46 to 10% and cones from 9 to 1%, with the specific effect being season dependent. Pruning using chemical desiccants rather than by mechanical means had similar effects on disease levels on leaves. On cones, though, chemical pruning had a small but significant reduction in the incidence of powdery mildew compared with mechanical pruning. Cone yield, levels of bittering-acids, and color were not negatively affected in any individual year or cumulatively over three seasons when pruning treatments were applied repeatedly to the same plots during the study period. Delayed pruning may offer a low-cost means of reducing both the incidence of powdery mildew and early-season fungicide inputs in certain cultivars.

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Characterization of Resistance to Powdery Mildew in the Hop Cultivars Newport and Comet

Plant Health Progress ◽

10.1094/php-br-13-0129 ◽

2014 ◽

Vol 15 (2) ◽

pp. 55-56 ◽

Cited By ~ 8

Author(s):

Sierra N. Wolfenbarger ◽

Emily B. Eck ◽

David H. Gent

Keyword(s):

Powdery Mildew ◽

Pacific Northwest ◽

Host Resistance ◽

Resistant Cultivars ◽

Virulent Strains ◽

The Pacific ◽

Potential Sources ◽

Important Disease ◽

Podosphaera Macularis

Hop powdery mildew, caused by Podosphaera macularis, is an important disease in the Pacific Northwest. Resistant cultivars of hop have been developed and have provided field immunity to the disease until virulent strains of P. macularis emerged. Due to the increase of powdery mildew on formerly resistant cultivars, studies were conducted to characterize potential sources of host resistance. Accepted for publication 5 February 2014. Published 27 March 2014.

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First Report of Powdery Mildew Caused by Podosphaera macularis on Hemp in Oregon

Plant Health Progress ◽

10.1094/php-04-21-0071-br ◽

2021 ◽

Author(s):

Taylor Bates ◽

Mary Holzberger-Block ◽

Michele Wiseman ◽

Andrea Garfinkel ◽

David Gent ◽

...

Keyword(s):

Powdery Mildew ◽

Cannabis Sativa ◽

Natural Conditions ◽

Powdery Mildew Fungus ◽

Management Implications ◽

First Report ◽

Podosphaera Macularis

In Oregon, hemp (Cannabis sativa) production has increased substantially after cultivation was legalized in the 2014 and 2018 Farm Bills. Typically, hemp species are affected by powdery mildew caused by Golovinomyces species. This paper reports on hop powdery mildew caused by Podosphaeria macularis, found colonizing hemp in natural conditions. The occurrence of the hop powdery mildew fungus on hemp may have management implications for both the hemp and hop industry.

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

Plant Disease ◽

10.1094/pdis-10-17-1530-re ◽

2018 ◽

Vol 102 (7) ◽

pp. 1316-1325 ◽

Cited By ~ 5

Author(s):

David H. Gent ◽

Briana J. Claassen ◽

Megan C. Twomey ◽

Sierra N. Wolfenbarger ◽

Joanna L. Woods

Keyword(s):

Powdery Mildew ◽

Field Experiments ◽

Extended Period ◽

Early Summer ◽

Powdery Mildew Fungus ◽

Initial Inoculum ◽

Potted Plants ◽

Preventative Measures ◽

Horticultural Practice ◽

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.

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First Report of Powdery Mildew of Convolvulus arvensis (Field Bindweed) Caused by Erysiphe convolvuli var. convolvuli in North America

Plant Health Progress ◽

10.1094/php-2003-1021-01-hn ◽

2003 ◽

Vol 4 (1) ◽

pp. 31

Author(s):

Dean A. Glawe ◽

Gwenyth E. Windom ◽

Gary G. Grove ◽

Jennifer S. Falacy

Keyword(s):

North America ◽

Powdery Mildew ◽

Pacific Northwest ◽

Morphological Features ◽

Convolvulus Arvensis ◽

Powdery Mildew Fungus ◽

First Report ◽

The Pacific ◽

Field Bindweed ◽

Noxious Weed

Field bindweed (Convolvulus arvensis L.) is widespread in the Pacific Northwest where it is considered a noxious weed. During 2002, collections of a powdery mildew fungus attacking C. arvensis were made from Pierce, Spokane, Whitman, and Yakima counties, WA. Based on morphological features we determined the fungus to be Erysiphe convolvuli. Accepted for publication 22 September 2003. Published 21 October 2003.

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Population Diversity and Structure of Podosphaera macularis in the Pacific Northwestern United States and Other Populations

Phytopathology ◽

10.1094/phyto-12-19-0448-r ◽

2020 ◽

Vol 110 (5) ◽

pp. 1105-1116

Author(s):

David H. Gent ◽

Briana J. Claassen ◽

David M. Gadoury ◽

Niklaus J. Grünwald ◽

Brian J. Knaus ◽

...

Keyword(s):

United States ◽

Powdery Mildew ◽

Population Diversity ◽

Western United States ◽

Wild Plants ◽

Eastern United States ◽

The Pacific ◽

Differential Cultivars ◽

Pathogen Populations ◽

Podosphaera Macularis

Powdery mildew, caused by Podosphaera macularis, is one of the most important diseases of hop. The disease was first reported in the Pacific Northwestern United States, the primary hop-growing region in this country, in the mid-1990s. More recently, the disease has reemerged in newly planted hopyards of the eastern United States, as hop production has expanded to meet demands of local craft brewers. The spread of strains virulent on previously resistant cultivars, the paucity of available fungicides, and the potential introduction of the MAT1-2 mating type to the western United States, all threaten sustainability of hop production. We sequenced the transcriptome of 104 isolates of P. macularis collected throughout the western United States, eastern United States, and Europe to quantify genetic diversity of pathogen populations and elucidate the possible origins of pathogen populations in the western United States. Discriminant analysis of principal components grouped isolates within three to five geographic populations, dependent on stringency of grouping criteria. Isolates from the western United States were phenotyped and categorized into one of three pathogenic races based on disease symptoms generated on differential cultivars. Western U.S. populations were clonal, irrespective of pathogenic race, and grouped with isolates originating from Europe. Isolates originating from wild hop plants in the eastern United States were genetically differentiated from all other populations, whereas isolates from cultivated hop plants in the eastern United States mostly grouped with isolates originating from the west, consistent with origins from nursery sources. Mating types of isolates originating from cultivated western and eastern U.S. hop plants were entirely MAT1-1. In contrast, a 1:1 ratio of MAT1-1 and MAT1-2 was observed with isolates sampled from wild plants or Europe. Within the western United States a set of highly differentiated loci were identified in P. macularis isolates associated with virulence to the powdery mildew R-gene R6. The weight of genetic and phenotypic evidence suggests a European origin of the P. macularis populations in the western United States, followed by spread of the pathogen from the western United States to re-emergent production regions in the eastern United States. Furthermore, R6 compatibility appears to have been selected from an extant isolate within the western United States. Greater emphasis on sanitation measures during propagation and quarantine policies should be considered to limit further spread of novel genotypes of the pathogen, both between and within production areas.

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