Varieties of eastern ninebark (Physocarpus opulifolius), an eastern U.S. native shrub, have become popular landscape plants in recent years, beginning with the development of the cultivar Diabolo® (P.o. 'Monlo'). This now widely available cultivar was bred by German nurseryman Gunter Kordes from a chance purple-leaved seedling. Since then, other purple and yellow leaved cultivars have come onto the market. Gardeners appreciate these plants for their sturdy constitutions and cold hardiness.
But ninebark and some of its cultivars have an Achilles heel -- a susceptibility to powdery mildew, a fungal disease that coats the leaves with white and causes witches' brooms, sections of stems with distorted growth and discolored leaves, to form. To assess which cultivars were most susceptible to this problem, researchers at the University of Connecticut carried out long-term evaluations of a number of varieties.
Among the purple-leaved cultivars evaluated, the researchers found that Summer Wine® (P.o.'Seward'), a 4-6 foot tall and wide shrub with deep reddish-purple leaves, was the most mildew-resistant and was the only cultivar in the study that didn't show any signs of infection. Diabolo® (pictured) also performed reasonably well, with only moderate infection and minimal brooming.
Yellow-foliaged cultivars of ninebark are not as popular as the purple-leaved selections, since most don't retain their yellow color well as the season progresses. The yellow cultivar that showed the greatest mildew resistance was 'Luteus', a large, older cultivar that shows poor color retention.
A number of new dwarf ninebark cultivars have just been released, including the purple-leaved Little Devil™, Burgundy Candy®, and Caramel Candy®, and the yellow-leaved Lemon Candy®. Continuing evaluation as these new varieties are planted in landscapes will be needed to determine how well they resist infection by this troublesome fungal disease.
For more information ninebark and powdery mildew, go to: American Nurseryman.