Rust on Native Azaleas in Nursery

This week, Rust disease was identified on native deciduous azaleas and smooth hydrangea in a nursery in western North Carolina. Most guidebooks list it as either Rust, hemlock-blueberry rust (Pucciniastrum vaccinii) or hemlock-hydrangea rust (Pucciniastrum hydrangeae). Apparently it affects hemlock needles in early spring and summer, then the infected needles inoculate both wild hydrangea (Hydrangea arborescens)(photo) and panicle hydrangea (Hydrangea paniculata) during the summer (Sinclair et al 1987). Because Rhododendron are in the Ericaceae family along with blueberry, these plants also seem to be infected. Additionally, blueberry grown as an ornamental in nurseries has been a "hot" trend lately, which could contribute to the renewed presence. On native deciduous rhododendron, for example, R. canescens (piedmont azalea), R. periclymenoides (pinxterbloom azalea), R. runifolum (Plumleaf azalea), R. viscosum (Swamp azalea), and R. calendulaceum (flame azalea), symptoms resemble these photos. Symptoms of leaf rust first appear in mid-summer as circular yellow flecks about an 1/8 inch in diameter on the upper leaf surface.

The fungus can be found forming spores in rust-colored pustules on the undersides of the leaf below the yellow flecking (photo below).

The orange-colored spores in the pustules are disseminated by air currents to other azalea leaves. If the leaf remains wet overnight, new infections (pustules) can form repeating the cycle throughout the remainder of the summer and fall. When conditions are favorable for severe disease development with numerous pustules early in the summer, extensive defoliation of infected leaves can occur. In mild climates, the fungus may over winter as spores in pustules of fallen leaves. So removing leaves from deciduous azalea blocks or plantings in the spring can help reduce the severity of disease later in the season. However, the fungus also can be re-introduced to azaleas in the nursery/landscape each year from nearby hemlocks (Tsuga canadensis) that serve as a second (alternate) host of the fungus. Normally, fungicides are not needed to control leaf rust (Benson and Creswell). It is possible to let the foliage die back and fall off this year without control, especially if plants are not going to be sold until spring 2010. In a study conducted by Jones, Bir and Benson in 1981, Plantvax 75W (oxycaboxin; Class: Carboxamide), Mancozeb 80W (Manganese ethylenebis; Class: carbamate), and Ferbam 76W (Ferbam; Class: carbamate) provided the best control for rust on the hybrids mentioned above when applied from mid-August through September according to label recommendations. Heritage (Azoxystrobin; Class: QOI-strobilurins), is only labeled for Puccinia sp. of rusts and not Puccinastrum spp. of rusts. Please read the labels of all fungicides applied and always wear personal protective equipment required on the label.

For more information contact your local Cooperative Extension Center and ask for the Commercial Horticulture Agent.