Needle cast (Rhizosphaera)

About

Rhizosphaera needle cast is the most common disease of Colorado blue spruce in Wisconsin and a common one across the Midwest. It kills older needles from the bottom of the tree up, leaving bare lower branches, but it rarely kills the tree outright.

Also called needle cast, Needle cast, Rhizosphaera needle cast, spruce needle cast, needlecast, Rhizosphaera

How to recognize it

Symptoms

Inner, older needles on the lower branches turn mottled yellow in late summer, then purplish-brown by late winter and spring, and fall off, leaving only the newest needles at branch tips. The damage climbs the tree year after year and can leave holes in the middle of the canopy. With a hand lens, rows of tiny black dots fill the needle pores, where healthy pores look white.

Often confused with

Cytospora canker also browns spruces from the bottom up, but it kills whole branches and leaves white resin on the bark, with no rows of black dots on the needles. Some non-infectious problems look similar, so check for the black dots or get a lab diagnosis.

Worse when

Long periods of wet needles in mild weather, shade, crowded trees with overlapping branches, and stress from drought, deep planting or root injury.

How it spreads

The fungus spends the winter in infected needles on the tree and on the ground; spores splash and blow onto new needles from spring through fall, and symptoms often take a year or more to show.

When

Infection happens in spring and through wet weather; needles brown and drop the following late winter to fall.

Where

Mostly a problem on spruces planted outside their native range, such as Colorado blue spruce in the Midwest and Northeast.

What to do, gentlest first

Prevention works best; most sprays only protect healthy leaves and can't cure infected ones. Always read and follow the product label.

1. Plant a resistant spruce or a dwarf form Prevent

   Norway spruce is mostly resistant and white spruce moderately so; Colorado blue spruce is the most susceptible. If you want blue spruce, a dwarf variety is easier to keep healthy.

2. Sun, space and dry needles Prevent

   Plant in full sun with room so branches of neighboring trees never overlap. Avoid sprinklers hitting the tree, especially at night, and water deeply in droughts.

3. Remove dead branches and fallen needles By hand

   Prune out dead lower branches in dry weather and clean up fallen needles and cones when you can. Burn where allowed, bury or hot compost them.

   On a badly damaged blue spruce, replacing the tree may make more sense than years of treatment.

4. Copper Organic spray

   Copper products labeled for evergreens protect new needles. Spray when new shoots are about half an inch long and repeat every 3 to 4 weeks while it stays wet; it usually takes two years of sprays in a row.

   Copper prevents new infections but does not fix needles already infected, and full coverage is hard on big trees. WSU's bee guide rates copper products toxic to bees. Follow the label.

5. Chlorothalonil and other labeled fungicides Conventional spray

   Chlorothalonil, mancozeb, azoxystrobin or thiophanate-methyl products labeled for spruce follow the same spring timing and two-year schedule.

   Fungicides do little once the fungus fills the canopy. UC IPM rates chlorothalonil high in hazard to people and to fish and other water life. Follow the label.

Plants it affects

Colorado blue spruce