First Report of Hop Powdery Mildew in the Pacific Northwest

Abstract

Hop powdery mildew (HPM) was first observed in commercial hop (Humulus lupulus L.) fields in Washington State on 10 June 1997 near Toppenish in the Yakima Valley. The disease appeared throughout the valley in 1997; by mid-July, scattered fields throughout the Yakima growing area reported HPM. Approximately 2,000 of 30,000 acres in production were not harvested in 1997 due to HPM. The pathogen apparently perennated in buds, and flagshoots originating from infected buds were observed during March and April 1998 at various locations throughout the Yakima Valley. During the 1999 growing season, the majority of hop acreage in Washington State was affected, and most fields planted to susceptible cultivars contained at least one infected plant. HPM was initially discovered in southern Idaho during early July 1998, in two adjacent fields of hops in Canyon County. HPM was found ≈644 km (400 miles) north in another hop-growing region of Idaho, Boundary County, during mid-July 1998. HPM eventually was observed in more than 20 Idaho hop fields. The initial discovery of HPM in Oregon's Willamette Valley was made during late-July 1998, in two neighboring hop fields. By the end of the growing season, HPM was observed in nine commercial fields representing 3.7% of the hop production acreage in Oregon. Affected cultivars include Brewer's Gold, Chinook, Cluster, Columbus/Tomahawk, Eroica, Fuggle, Galena, Golding, Liberty, Olympic, Perle, Symphony, Tettnanger, Willamette, and Zeus. Infected basal leaves of bines had small whitish circular spots on adaxial surfaces. In some cases, blisters preceded direct observation of the fungus. Cones also were infected, appearing stunted and malformed. The pathogen usually was visible on infected cones but sometimes was found only under overlapping bracts. Cleistothecia have not been observed in the field to date. Conidia were transferred to leaf disks (12 mm diameter) excised from greenhouse-grown cv. Galena hop plants. Inoculated leaf disks were incubated on moistened filter paper in glass petri dishes at 20°C with illumination provided for a 12 h day by two cool-white fluorescent bulbs. HPM lesions with chains of unicellular, barrel-shaped conidia (30 to 36 × 15 to 18 μm) were visible within 7 days. The causal agent was identified as Sphaerotheca macularis (Wallr.:Fr.) Lind (synonym S. humuli (DC.) Burrill) on the basis of conidial shape and size as well as host range (1). Reference: (1) D. J. Royle. 1978. Powdery mildew of the hop. Pages 381–409 in: The Powdery Mildews. D. M. Spencer, ed. Academic Press, New York.

Distribution and Characterization of Podosphaera macularis Virulent on Hop Cultivars Possessing R6-Based Resistance to Powdery Mildew

“…R6-based resistance provided a level of disease suppression that was equivalent to immunity to powdery mildew in the Pacific Northwest for 14 years. This was a crucial component of disease management programs developed following the destructive epidemics that followed the introduction of P. macularis to the Pacific Northwest in the mid 1990s (Ocamb et al 1999). This research demonstrates that isolates of P. macularis capable of overcoming R6 resistance are now widespread in the region and outbreaks of powdery mildew occur annually on formerly resistant cultivars.…”

“…Breeding for host resistance to powdery mildew on hop, caused by Podosphaera macularis, began in the early 1900s and continues to be a primary breeding focus in many breeding programs worldwide (Neve 1991;Salmon 1917). This is especially true for the U.S. Pacific Northwest (Washington, Oregon, and Idaho), where the disease became a major constraining factor for hop production after the introduction of the pathogen to the region in 1996 (Ocamb et al 1999). To date, seven R genes have been recognized in hop (named RB, R1, R2, R3, R4, R5, and R6) that can impart immunity to select isolates of P. macularis (Neve 1991;Seigner et al 2001).…”

A phased, diploid assembly of the Cascade hop (Humulus lupulus) genome reveals patterns of selection and haplotype variation

“…A recent outbreak of powdery mildew affecting Cascade in the U.S. Pacific Northwest (PNW) was the result of a Cascade-adapted isolate of P. Macularis [18]. Very little management was required to grow Cascade until powdery mildew was introduced in the PNW in the mid-1990s [28], and the rise of PM infection corresponds to an increase in Cascade acreage [18]. Hop breeding with resistant cultivars [26, 27, 29] can result in new resistance, but given the inevitability of pathogen adaptation, an understanding of the genomic features underlying resistance and susceptibility is necessary to breed for hop that can withstand pathogen adaptations.…”

Identification and distribution of mating‐type idiomorphs in populations of Podosphaera macularis and development of chasmothecia of the fungus

“…In the case of Podosphaera macularis, the causal organism of powdery mildew on hop, the ascigerious stage is known to exist in Europe (Neve, 1991) and eastern North America (Blodgett, 1913), but has not yet been reported in the Pacific Northwest (Gent et al, 2006). Despite the hop industry being present in the Pacific Northwestern region of North America since the early 1900s (Neve, 1991;Barth et al, 1994), powdery mildew was not reported in hop yards of the Pacific Northwest until 1997 (Ocamb et al, 1999).…”