Cause Thekopsora minima and Naohidemyces vaccinii (both formerly Pucciniastrum vaccinii), fungi. Leaf rust is usually of minor importance being a problem on lowbush, rabbiteye, southern highbush or ornamental blueberries that do not shed leaves in the winter. This disease can impact yield due to early defoliation and reduction in return bloom in the following year.
Naohidemyces vaccinii has also been reported on the deciduous huckleberry (Vaccinium membranaceum) from all PNW states, ever-green huckleberry (V. ovatum) WA, dwarf bilberry (V. caespitosum) OR and WA, Alaska blueberry (V. ovalifolium) OR and WA, little leaf huckleberry (V. scoparium) OR and ID, and on cranberry in Oregon and Washington.
Hemlock (western or Pacific hemlock, Tsuga heterophylla, and the mountain or black hemlock, T. mertensiana) is the alternate host where circular aecia erupt from beneath the epidermis on the lower side of current-season needles in yellow spots. The OSU Plant Clinic has never received samples of hemlock rust but N. vaccinii has been reported from all states in the PNW, however, T. minima has not been reported on hemlock in the PNW. An alternate host is not needed for blueberries that do not lose their leaves.
In the spring, airborne aeciospores from hemlock infect newly emerging blueberry leaves. Uredinia form about midsummer on the lower side of leaves. Urediniospores reinfect blueberry in a repeating secondary cycle. This leads to rapid inoculum buildup and disease spread especially on plants that lose leaves late or not at all. The latent period may be completed in only 10 days at 68°F. The pathogen does not systemically invade blueberry. Telia form within the epidermal cells of infected leaves, which eventually drop to the ground. Basidia arise from germinating teliospores in dead leaves, and basidiospores infect young hemlock needles in the spring.
Symptoms The first symptoms observe include small, chlorotic to yellow spots. Spots on the upper surface of the leaf turn reddish-brown. The entire leaf may die if numerous spots coalesce and become necrotic. On the underside of the leaf, opposite upper leaf symptoms, yellowish-orange pustules (uredinia) develop that turn rusty-red with age. Infected leaves usually drop prematurely.
- Remove hemlocks upwind from where blueberry is grown.
- Grow less susceptible cultivars.
- Prompt removal and disposal of fallen leaves may be helpful.
Chemical control Specific timing is not known but it is suggested to start when leaves first begin to break bud and unfold. Continue at regular intervals during wet weather.
- Bordeaux 8-8-100 plus spreader-sticker. O
- Bravo Weather Stik at 3 to 4 pints/A can only be used after harvest. Group M5 fungicide. 12-hr reentry.
- Indar 2F at 6 fl oz/A plus a wetting agent. Make reapplication on shortest interval allowed on the label. Do not use within 30 days of harvest. Moderate control. Group 3 fungicide. 12-hr reentry.
- Inspire Super at 16 to 20 fl oz/A. Do not apply within 7 days of harvest. Group 3 + 9 fungicide. 12-hr reentry.
- Kocide 3000 at 1.75 to 3.5 lb/A. Not for use in the spring. 48-hr reentry.
- Nu-Cop 50 DF at 2 to 4.2 lb/A. Not for use in the spring. 48-hr reentry. O
- Proline 480 SC at 5.7 fl oz/A. Do not use within 7 days of harvest. Good control. Group 3 fungicide. 12-hr reentry.
- Propiconazole-based fungicides are registered. Do not use within 30 days of harvest. Good control. Group 3 fungicides. 12-hr reentry.
- Bumper 41.8 EC at 6 fl oz/A.
- PropiMax EC at 6 fl oz/A.
- Tilt at 6 fl oz/A.
- Propulse at 10 to 13.6 fl oz/A. Do not use within 7 days of harvest. Group 3 + 7 fungicide. 12-hr reentry.
- Quadris Top at 12 to 14 fl oz/A. Do not apply within 7 days of harvest. Good control. Group 3 + 11 fungicide. 12-hr reentry.
- Quash at 2.5 oz/A. Do not use within 7 days of harvest. Good control. Group 3 fungicide. 12-hr reentry.
- QuiltXcel at 14 to 21 fl oz/A. Do not use within 30 days of harvest. Sprayers should not be used on apples. Good control. Group 3 + 11 fungicide. 12-hr reentry.
- Regalia at 1 to 4 quarts/A plus. May be used day of harvest. Does not benefit from the addition of an adjuvant. Unknown efficacy in the PNW. Group P5 fungicide. 4-hr reentry. O
Note: Some registered products offer only suppression of this disease and thus are not recommended for use. These products include Pristine.
- Serenade ASO (Bacillus subtilis strain QST 713) at 2 to 4 quarts/A. Active ingredient is a small protein and may be used on day of harvest. Unknown effectiveness in the PNW. 4-hr reentry. O
- Sonata (Bacillus pumilis strain QST 2808) at 2 to 4 quarts/A plus a spreader-sticker. May be applied up to and including the day of harvest. 4-hr reentry. O
Reference Wiseman, M.S., Gordon, M.I. and Putnam, M.L. 2016. First report of leaf rust caused by Thekopsora minima on northern highbush blueberry in Oregon. Plant Disease 100:1949.