Eyes Wide Open
According to Scot Nelson of the University of Hawaii at Manoa’s Department of Plant and Environmental Protection Sciences, leaf rust is caused by Naohidemyces vaccini, formerly named Pucciniastrum vaccinii, and is heteroecious, meaning it has two hosts. It also occurs on many hosts and has two forms in North America — an eastern form and a western form. Hosts of the pathogen include Vaccinium spp. (blueberry, cranberry, huckleberry), Tsuga (hemlock, hemlock spruce), Rhododendron (including azalea), Lyonia, Menziesia (mock azalea), Pernettya, Hugeria, Pieris, Leucothoe, and Oxycoccus.
“Blueberry leaf rust first appears as tiny yellow spots on the upper surface of young blueberry leaves about 10 days after inoculation,” writes Nelson in a 2008 plant disease report. “Spots later turn reddish brown and may be surrounded by a slight yellow halo. The lower leaf surface gathers a yellowish-orange dust, which is associated with the leaf spots. This dust is the spores of rust, and produced in the uredinia, which is at the center of each spot. Yellowish-orange rust pustules soon become visible, scattered over the surfaces of lower leaves. Affected leaves may redden and on heavily diseased plants, leaves often turn brown, curl up, and drop. This defoliation is the principal direct cause of harm to the plant, The indirect effect of leaf loss is reduced plant vigor and poor fruit production.”
In many climates, like California’s Central Coast, leaf rust is active the first four months of the year during fruit production, and young fruit are very susceptible to infection, according to a 2006 “Blueberry Leaf Rust” report by Franklin Laemmlen and Mark Gaskell at the University of California. This combination of fruit production and rust spore activity causes leaf rust to be dangerous economically because infected fruit is unmarketable. While a few leaf spots are not a problem, when fruit infection occurs, berries are a total loss.
In the northern U.S., the fungus overwinters in infected leaves and reinfects hemlock needles in early spring; however, it needs both species — blueberry and hemlock — to complete its life cycle in cold climates. In the southeastern U.S., where hemlocks do not grow, the fungus overwinters in uredia on evergreen blueberry leaves.
Solutions For Rust
In a Michigan State University Blueberry Fact Sheet released in 2009, experts recommended implementing a preventive control program to control rust, including removing hemlock trees within a third of a mile in northern climates, avoiding susceptible cultivars, limiting overhead irrigation, and applying effective fungicides. Even with frequent fungicide sprays, however, this disease may prove too difficult to manage. Fungicide sprays may also cause phytotoxicity to blueberry leaves, so they must first be tested and tankmixes of incompatible products must be avoided.
Nelson says one of the best ways to control blueberry leaf rust is to plant a resistant variety. A Cornell University online report lists Northern highbush varieties resistant to blueberry rust to include: Bluecrop, Burlington, Collins, Dixie, Earliblue, Gem, Ivanhoe, Olympia, Stanley, and Weymouth. Moderately susceptible varieties are: Jersey, Herbert, Berkeley, Blueray, and Pacific. Susceptible varieties to blueberry rust include: Coville, Pemberton, Washington, and Atlantic.