The Japanese blueberry (Elaeocarpus decipens) is an evergreen tree that thrives in USDA hardiness zones 8 to 10. It grows between 30 to 40 feet high and equally wide at maturity, and produces tiny white flowers every spring that make it stand out in the landscape. Depending on personal taste, prune the tree into a shrub or allow it to grow naturally. The drought-resistant tree prefers a sunny spot with well-drained soil. Although considered hardy for the most part, it is susceptible to a particular disease that affects its health and vigor if left untreated.
Japanese blueberry is susceptible to leaf rust caused by Naohidemyces vaccinii fungus. Young or new foliage is more susceptible to damage by the fungus than old growth. Leaves exposed to water for extended periods are more vulnerable to developing the disease.
Initial symptoms of the disease include yellow spots on upper and lower surfaces of the otherwise green leaves. Left untreated, the spots turn brownish-red, indicating the progress of the disease. Symptoms are visible as early as 10 days after the infection. In case of extreme infections entire leaves drop, turn brown or die.
The fungus responsible for leaf rust causes spores that resemble orange pustules on the lower surface of infected leaves. The pustules spread from one part of the infected tree to another part, or to a different tree altogether through water, air or animals. Telia that resemble dark, flat crust-like structures develop on the lower surface of leaves later in the season. Leaves infected with telia drop to the ground, giving the fungus a chance to overwinter.
According to the Cornell University Extension, leaf rust in Japanese blueberry trees is reported from Asia, Mexico, Argentina, Europe, Canada, United States, Australia and New Zealand. It is most prevalent in areas in the southern United States because of the warm weather that promotes its spread and development.
Even under controlled environmental conditions, a leaf wetness period of 48 hours makes the tree susceptible to the disease. In order to counter this, irrigate the tree only when needed and water at soil level so you do not wet the foliage. Spray the tree with a registered systemic fungicide to control spread. Follow label directions as application rates of fungicides vary. Apply it in early spring so the disease does not infect a healthy Japanese blueberry tree.