Hemlock-Blueberry Rust

Naohidemyces vaccinii (Jørst.) S. Sato, Katsuya & Y. Hirats. 

Host(s) in Alaska:

western hemlock (Tsuga heterophyla) and blueberry (Vaccinium spp.)

Habitat(s): western hemlock needles and blueberry leaves

Current Status & Distribution in Alaska (2023 Update)

Hemlock-blueberry rust was previously considered a disease of minor importance, difficult to find on both blueberry leaves and hemlock needles. However, there has been notable disease on both new western hemlock needles and blueberry leaves in Southeast Alaska since 2019 (see Detection Map). This year, we observed this disease at 29 locations during ground surveys, 23 on western hemlock and six on blueberry species near Juneau and on Mitkof, Zarembo, Wrangell, Prince of Wales, Revillagigedo, and Annette Islands. In Sitka, one research grade observation was contributed via iNaturalist. In 2021, we sequenced DNA from samples near Juneau, Wrangell Island, and Mitkof Island. Our sequences all showed consistent base pair differences with the Naohidemyces vaccinii voucher specimen in GenBank. With continued collection and evaluation of spore morphology, we hope to learn more about the causal fungus in Alaska compared to related species elsewhere in North America and Europe. 

Symptoms, Biology & Impacts

Hemlock-blueberry rust is caused by a heterocyclic rust fungus (Naohidemyces vaccinii) with one lifecycle stage on hemlock needles (rarely on cones) and the other stage on several species of blueberry (Vaccinium spp.). Another needle rust of western and mountain hemlocks, Melampsora epitea f. sp. tsugae, is also present in Alaska, which has lifecycle stages on willow.

In summer, current-year hemlock needles become infected by spores produced on blueberry leaf-litter (for deciduous Vaccinium spp.) or on persistent leaves (for evergreen Vaccinium spp.). Orange or yellow pustules form on the underside of hemlock needles. Pustules occur in two rows, on either side of the midrib vein of the needle. Infected needles turn yellow. Hemlocks are not usually heavily infected, thus new shoots often have a scattering of diseased yellow needles among healthy green needles. Infected needles turn yellow and die, and are shed prematurely. Injury to infected trees is generally negligible, perhaps slightly reducing growth. The generally low occurrence of this rust is surprising given the great abundance of western hemlock and blueberries in coastal Alaska. Control is not usually warranted. On occasion, heavier infection is observed on hemlock shoots, infecting 50-75% of new hemlock needles.

Life cycle: The fungus overwinters on infected blueberry foliage. In early-summer, basidiospores are released from infected blueberry leaves on the ground (and from everygreen blueberry leaves, where present) to infect elongating hemlock shoots. Next, pycnia/spermagonia (sexual spores stage) and aecia (fruiting bodies) are formed on new hemlock needles, releasing aeciospores that infect blueberry leaves. A repeating spore (urediospores) stage on infected blueberry leaves, facilitating disease buld-up during the growing season. If everygreen blueberry hosts are present, the disease can persist without the hemlock host, persisting with the repeating urediospore stage. 

Detection Map