Layers of Intrigue: Physocarpus and Powdery Mildew
Commercial cultivars of eastern ninebark vary widely in susceptibility to fungal pathogens. In a study conducted at the University of Connecticut, evaluation of popular selections showed which plants are relatively resistant to powdery mildew and which are prone to infection.
Photos courtesy of Mark H. Brand
As recently as 15 years ago, eastern ninebark (Physocarpus opulifolius) could be summarized in two words: tough and uninspiring. Provided its modest requirements were satisfied by sun or light shade and ordinary well-drained soil, this large shrub reached 10 feet to 12 feet tall, 8 feet to 12 feet wide and persevered for decades with little attention anywhere in zones 3 to 8. The ornamental attributes of this eastern North America native were similarly modest, with white flower clusters in late spring, humdrum three-lobed green leaves, mediocre fall color and peeling stem bark (hence the common name).
The outlook for ninebark and the fortunes of cold climate gardeners everywhere changed dramatically with the 1968 discovery in a German nursery of a rogue, purple-leaf seedling and the subsequent efforts of nurseryman Gunter Kordes to nurture this upstart and see it successfully to market. Now widely recognized by its descriptive trade moniker Diabolo® (cv. 'Monlo' PP 11,211), this blazing beauty ignited a firestorm that launched shy ninebark into the horticulture limelight. Plant breeders on both sides of the Atlantic have exploited this infusion of purple genes and crossed Diabolo® with the dwarf cultivar 'Nanus' and the yellow forms 'Luteus' and 'Dart's Gold' to generate a circus of colorful cultivars in various sizes.
About 10 years ago, a research team led by Dr. Mark H. Brand at the University of Connecticut (UConn) established a replicated test plot of ornamental ninebark cultivars to investigate their suitability as alternatives for purple and yellow cultivars of Japanese barberry (Berberis thunbergii). We noted infection of the test plants by powdery mildew soon after the evaluation commenced, with cultivars showing different levels of symptom development.
While the susceptibility of eastern ninebark to the specific powdery mildew Podosphaera aphanis var. physocarpi (syn. Sphaerotheca aphanis var. physocarpi) and the generalist powdery mildews Phyllactinia guttata and Podosphaera macularis (syn. Sphaerotheca humuli and S. macularis) has been well-established, the potential for resistance among common commercial cultivars has not been documented.
Ninebark plants infected by powdery mildew commonly develop superficial patches of white fungal colonies on plant parts. In addition, the ninebark-specific fungus may produce bizarre witches' brooms of thickened stem tissue with stunted foliage discolored white or light pink. These brooms turn black and further detract from the plant's appearance by persisting through winter and beyond. Given the aesthetically destructive potential of these mildews, it is prudent to identify which ninebark cultivars, if any, possess resistance and are preferable for commercial promotion and breeding consideration.
Ninebark plants in the UConn test plot in Storrs, Conn. (Zone 6), were arranged in rows in a completely random design with three replicates each of 10 cultivars (see Table 1). Shrubs in No. 2 containers were installed in 2003 and maintained for three years with minimal cultivation until analysis for powdery mildew commenced in late July 2006. We evaluated the severity of mildew infection using a rating scale of 0 to 5 where 0 = no visible infection, 1 = 1 to 20 percent plant coverage, 2 = 21 to 40 percent plant coverage, 3 = 41 to 60 percent plant coverage, 4 = 61 to 80 percent plant coverage and 5 = 81 to 100 percent plant coverage. The study plants, whose cultivar identities were not disclosed, were evaluated and ranked by experienced members of the UConn horticulture community (15 in 2006 and 18 in 2007). Each year's data were analyzed separately and combined for both years across each cultivar to reveal statistical differences in mildew susceptibility.
Powdery mildew infection varied significantly among the 10 ninebark cultivars studied (see Table 2). Overall trends were consistent across the two years of observation. Plants scoring on the low end of the 0-to-5 scale displayed light to moderate infection with scattered leaves exhibiting patchy colonies of mildew and sporadic presentation of witches' broom. Plants that scored on the high end of the scale were characterized by widespread mildew colonies afflicting many leaves and developing fruit and the presence of profuse brooming across most shoots. Damage to shrubs in this group was so widespread and their ornamental appeal so severely impacted that they likely would be rejected outright by horticulture professionals and the general public.
Green cultivars of eastern ninebark are not popular ornamental shrubs due to their large footprint and modest ornamental assets. They are often reserved for ecological restoration or difficult landscape situations, such as very cold climates where the plant palette is limited. Nonetheless, the older green cultivar 'Nanus' was one of the stars of our evaluation, since it did not develop any signs of powdery mildew (a rating of 0.0) during the two trial years. The other green cultivar, 'Snowball', exhibited intermediate mildew susceptibility with an overall rating of 2.7. While fungal brooming on this selection was generally light, moderate colonies of foliar mildew were observed on both leaves and fruit with some leaves heavily afflicted.
Given the widespread powdery mildew susceptibility of the ninebark cultivars we evaluated and the strong mildew resistance of 'Nanus', it is tempting to wax poetic over this modest, semi-dwarf plant. To be clear, however, while 'Nanus' is a perfectly functional shrub its ornamental merits do not overwhelm. Its real utility rests in its potential as a breeding parent to confer compact stature and mildew resistance on more handsome progeny, a role that has been exploited by the breeders of cultivars such as 'Seward' Summer Wine®.
The yellow cultivars of eastern ninebark occupy a dubious niche in horticulture, since they are widely cultivated but have never achieved great commercial success. The lukewarm response of the gardening public to these plants may be attributed to several factors. The overbearing size of older yellow cultivars makes them unsuitable for intimate garden settings. Also unsettling is their transient expression of yellow pigmentation - which, depending on climate and cultural conditions, may either fade to shades of green or photobleach to a sickly white - leading to disappointment with cultivars such as 'Luteus' and 'Dart's Gold'. These plants are useful nonetheless as reliable color accents in mixed borders, especially when they are coppiced in early spring to remove excess biomass and force vigorous shoots bearing large, brilliantly colored leaves.
Yellow ninebarks in our trial exhibited highly variable susceptibility to powdery mildew. Of all the cultivars considered, 'Morning Star' and 'Nugget' produced the most severe symptoms of fungal infection. In 2007, for example, 'Nugget' yielded the highest possible rating of 5.0 with copious shoot brooming and heavy fungal colonization of leaves, developing fruit and shoots. Damage was so severe on 'Nugget' (combined rating of 4.8) and 'Morning Star' (combined rating of 4.4) that the plants suffered premature partial defoliation in both study years. Mildew incidence on 'Dart's Gold', with a rating of 3.7, was of intermediate intensity for the yellow group with high levels of brooming and foliar mildew.
The ninebark cultivar 'Nugget' experienced severe symptoms of fungal infection, including copious shoot brooming and heavy fungal colonization of leaves, developing fruit and shoots.
The surprise performer in the yellow category was 'Luteus', an older, large-growing cultivar that has fallen out of favor commercially and was only obtained by propagating a senior specimen at the Arnold Arboretum in Boston. 'Luteus' plants yielded a low to moderate combined rating of 2.2 with minimal development of mildew symptoms. Trial evaluators noted the attractiveness of this cultivar relative to other yellow forms due to its clean presentation. It should be noted, however, that 'Luteus' exhibits poor retention of yellow foliage color compared to 'Dart's Gold' and 'Nugget' and is very large growing. Additional breeding is required to develop viable dwarf yellow ninebarks. One possibility is Lemon Candy® ('Podaras 3'), a 2012 introduction with purported compact form and strong yellow foliage that may be a step in the right direction should it express good mildew resistance and match its catalog description.
Early signs of mildew are evident on a purple-leaved ninebark.
Purple cultivars are the clear stars of the ninebark stable and remain the focus of intense breeding to develop forms that retain the intense, velvety leaves of 'Monlo' Diabolo® without its lanky, large frame. Spring Meadow Nursery, Grand Haven, Mich., initiated the pursuit of pygmy stature in 2004 with its introduction of 'Seward' Summer Wine®, a healthy, 6-foot- to 8-foot-tall shrub with more finely textured, purple leaves. Additional dwarf purple cultivars will debut in 2012, including 'Donna May' (Little Devil™) from Bailey Nurseries, Newport, Minn., and Peter Podaras' Burgundy Candy® ('Podaras 1') and Caramel Candy® ('Podaras 2'). It remains to be seen whether these plants will realize their marketing promise and remain dwarf, purple and mildew-free.
Among the purple ninebark cultivars we studied, 'Seward' Summer Wine®, with an overall mildew rating of 1.1, was the clear statistical and visual standout with minimal infection on very few leaves. The reduced mildew susceptibility of this cultivar is presumably derived from its parent 'Nanus', the only cultivar in our study that did not exhibit any mildew symptoms. 'Monlo' Diabolo® also performed well (combined rating of 2.2) with moderate foliar mildew and minimal stem brooming. The mildew expression of 'Center Glow' and 'Mindia' Coppertina® was almost identical (combined ratings of 2.9 and 2.8, respectively), a coincidence that may be explained by their shared parentage as hybrids between 'Monlo' Diabolo® and 'Dart's Gold'. The significant mildew susceptibility of these cultivars may reflect the influence of 'Dart's Gold' and its relatively high mildew vulnerability. In our study, both selections presented widespread foliar mildew infection and light shoot brooming. The coppery orange new growth for which 'Center Glow' and 'Mindia' Coppertina® were selected does not make them superior ornamentals to 'Seward' Summer Wine® or 'Monlo' Diabolo®, which remain the clear choices when selecting a purple ninebark.
Mildew paints the leaves of an eastern ninebark.
The future of ninebark
Our study suggests that powdery mildew and eastern ninebark will remain inextricably linked for many years to come, since the species is long-lived and most cultivars are susceptible. According to Margery Daughtrey, senior extension associate at the Long Island Horticultural Research and Extension Center, landscape management of powdery mildew is difficult and expensive if the goal is perfectly clean plants. This pristine state may only be achieved when fungicides are rotated regularly by type and applied at least twice monthly throughout the growing season. Daughtrey also emphasized that the impact of modern systemic fungicides on the growth of ninebark has not been well documented.
Aside from manual removal of heavily infected plant tissue (both living and dead), practical and sustainable mildew control on ninebark begins with the selection of genetically resistant cultivars. Our study indicates that when considering mildew resistance, the green cultivar 'Nanus', the yellow cultivar 'Luteus' and the purple cultivar 'Seward' Summer Wine® are preferable landscape choices. None of these cultivars is perfect, however, since they each possess drawbacks related to plant size or foliage color retention. The best days for ninebark lie ahead as plant developers relentlessly pursue traits such as brilliantly colored leaves on truly dwarf shrubs. It is our hope that mildew susceptibility will not be forgotten as breeders stir the cauldron of ninebark genetics. 'Nanus' and 'Luteus' are imperfect landscape shrubs, but they remain reservoirs of precious mildew resistance that should be exploited.
Jonathan M. Lehrer, PhD, is the author of this article and an assistant professor of ornamental horticulture at Farmingdale State College on Long Island, N.Y. He may be reached at email@example.com.Drs. Mark H. Brand and Jessica D. Lubell are faculty in the Department of Plant Science and Landscape Architecture at the University of Connecticut.