Bright orange rust pustules are now evident on leaves, stems, and petioles of elderberry plants. These symptoms are caused by the fungus, Puccinia sambuci. In central Missouri, symptoms were first observed on elderberry plants on April 1 (Figure 1). With heavy dew, rainfall, high humidity, and temperatures over 50°F, infection will continue and flowers may also become infected.
The life cycle of P. sambuci includes five spore types, with pycniospores, aeciospores, and urediniospores occurring on elderberry plants. During summer, aeciospores travel from elderberry plants to the sedge (Carex sp.) plants, which is the alternate host. After infection, rust symptoms, appearing as necrotic lesions, are visible on sedge leaves. During late summer, teliospores develop and overwinter on sedge plants. The following spring, basidiospores from sedge plants are carried by the wind back to elderberry plants where the disease cycle continues.
In Missouri, Frank's sedge (C. frankii) is a common alternate host for elderberry rust. It typically is found in low, wet areas in a planting. However, eleven other species of sedge are known to serve as an alternate host including Bebb's sedge (C. bebbii), button sedge (C. bullata), longhair or bottlebrush sedge (C. comosa), fringed sedge (C. crinita), raven or crow-foot sedge (C. crus-corvi), greater bladder sedge (C. intumescens), false hop sedge (C. lupuliformis), hop sedge (C. lupulina), shallow or lurid sedge (C. lurida), blunt broom sedge (C. tribuloides), and hairy fruited sedge (C. trichocarpa).
Research conducted at the University of Missouri found that 'Bob Gordon' elderberry yield was reduced by 31% when young plants had an average of six foliar rust pustules. When unpruned elderberry canes averaged 137 rust pustules on foliage and stems of 'Wyldewood' plants, infected canes lost nearly twice as many leaves as uninfected canes during the growing season and fruit yield was decreased by 47%. Any rust found on flowering cymes reduced fruit yield (Figure 2). Also, when many pustules caused stem or petiole distortion, flowering and fruiting was also reduced. After harvest, the soluble solid content (i.e., measurement of sugar content) of berry puree was slightly decreased at any level of rust infection, which would require the addition of a greater amount of a sweetener in the final processed product.
Rust control by non-chemical means is challenging. In small plantings, when there are few rust pustules present, infected elderberry leaflets and stems can be pruned and removed from the site. Since many sedge species are perennial, they tend to be difficult to control. Sedge plants have parallel veins, produce seed on culms (stalks), and spread by rhizomes. Mowing sedge plants before they produce culms will help limit seed dispersal. Repeated applications of glyphosate, especially in the fall, will suppress or control this weed.
REVISED: February 25, 2022