Cause Rhizosphaera kalkhoffii is the most prevalent cause although several other fungi (such as R. pini, Lophodermium sp., Lirula sp.) can cause needle cast diseases on spruce. The fungus overwinters in infected needles attached to the tree or on the ground. Rain-splashed spores spread to current-season needles. Shearing crews also can spread spores if foliage is wet. Infection requires wet periods over 48 hours at optimal temperatures. Young trees may die if weather favors the disease several years in a row.
The disease has been described on Christmas trees in the Eastern United States and has been a problem in Pacific Northwest nurseries and landscape trees. Colorado blue spruce and Engelmann spruce are highly susceptible; white spruce is intermediate; Norway spruce is relatively resistant. Other conifer hosts include pine, Douglas-fir, true fir, and western hemlock.
Symptoms At first, current-season needles mottle yellow late in the summer. They soon turn brown or, on blue spruce, purplish brown. Brown needles are shed the next spring and summer. The fungus produces a fruiting body (pycnidia), which emerges from stomata. These small, black pycnidia are in rows on the underside of the needle. The disease is first on lower branches and gradually moves up the tree year after year.
- Use pathogen-free planting stock.
- Avoid shearing and other operations if foliage is wet.
- Avoid overhead irrigation, or water such that plants are not wet for extended periods of time.
- Remove and destroy dead needles underneath and within the tree.
Chemical control Begin applying when new shoots are 0.5 to 1.5 inches long. Continue at 2- to 3-week intervals if weather stays very wet.
- Avelyo at 3 to 5 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Bravo Weather Stik at 5.5 pints/A. Group M5 fungicide. 12-hr reentry.
- CuPRO 5000 at 3 to 5 lb/A. Group M1 fungicide. 48-hr reentry.
- Echo 720 at 5.5 pints/A. Group M5 fungicide. 12-hr reentry.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker. Group M3 fungicides. 24-hr reentry.
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
- Tourney 50 WDG at 2 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
Reference Skilling, D.D. and Waddell, C.D. 1975. Control of Rhizosphaera needlecast in Blue Spruce Christmas tree plantations. Plant Disease Reporter 59:841-843.