Rhizosphaera needle cast

What is Rhizosphaera needle cast?

Rhizosphaera needle cast is caused mainly by the fungus Rhizosphaera kalkhoffii. It infects spruce needles through their breathing pores. Colorado blue spruce is the most susceptible, white spruce is moderately affected, and Norway spruce is mostly resistant.

The disease usually starts on the lower inside branches. Infected needles turn mottled yellow, then purple to brown, and drop. Branches end up with only the newest needles at the tips, and lower limbs go bare. Illinois Extension says small trees are more likely to die from serious yearly infection.

Symptoms show up slowly. Illinois Extension says browning takes 12 or more months after infection, so this year's damage started last year. Spores spread by wind and splashing rain. UMass says the disease is more destructive on spruces in shade or in tight hedgerows, and that drought, deep planting and root injury make it worse.

Symptoms and signs

• Inner, older needles on lower branches turning purple or lavender, then brown
• Older needles dropping, leaving only the newest needles at branch tips
• Bare lower branches
• Rows of tiny black dots along the needles (best seen with a 10X hand lens)

Rhizosphaera needle cast identification: what to look for

• Symptoms: Current-year needles turn mottled yellow in late summer (Illinois). Later they turn purple, then brown to straw-colored (UMass).
• Signs: Tiny black fruiting bodies sit in neat rows along the needle, following the lines of breathing pores. They can be seen with a 10X hand lens.
• Damage: Discolored needles drop. Lower branches keep only the newest needles at the tips.
• Timing: Penn State says young needles are infected in May and June, with a second infection period in September and October in some places.

Disease cycle

The fungus lives in infected needles. Wind and splashing rain carry spores to healthy needles, and UMass says spores can survive on dry needles for several weeks. Spores enter through the needle's breathing pores. Penn State says new needles are infected in May and June, with a second period in September and October in some places. Symptoms take about a year or more to show, and the infected needles then make more spores.

When to look: Check spruces for purple-brown inner needles and black dots any time. New infections happen in late spring. Spray timing, if used, is in spring as new shoots grow.

Rhizosphaera needle cast look-alikes

A few things get mixed up with it:

• Stigmina needle blight (Stigmina lautii): Penn State says it looks very similar, with black structures in rows, but the dots sometimes look fuzzy-topped. Managed the same way.
• Cytospora canker (Leucostoma kunzei): Penn State says it makes sunken dead areas of bark on lower branches, with resin flowing out. Scattered lower branches die, and branch death moves up the tree. It is a branch canker, not a needle disease.
• Spruce needle rust (Chrysomyxa weirii): Penn State says year-old needles turn rust-colored in spring and drop.

How to manage Rhizosphaera needle cast

Cultural control

• Plant Norway spruce or other more resistant species where needle cast is common.
• Give spruces room. Space them so branches do not overlap at full size (UW-Madison).
• Keep weeds down so air moves freely around the tree (Penn State).
• Prune out dead lower branches and remove diseased needles.
• Reduce stress from drought, deep planting and root injury (UMass).
• For badly damaged blue spruce, UMass says replacement may be wiser than years of treatment.

Chemical control

Fungicides can prevent new infections but will not cure existing ones (UW-Madison), and UMass says they likely do little once the fungus is well established. Penn State says to spray when new shoots are about 1½ inches long and again about 3 weeks later; UMass says to start at about 1/2 inch of new growth and repeat while wet weather lasts. Illinois Extension says treatment must continue two years in a row. Extension sources name chlorothalonil, copper products, mancozeb, azoxystrobin, thiophanate-methyl and phosphites. UW-Madison stresses covering all needles, which is hard on large trees. Use only products labeled for this use; the label is the law. Have a licensed applicator treat large trees (see pesticide applicator license guide).

Use only products labeled for this disease and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Avoid planting Colorado blue spruce where it will be shaded or crowded.
• Choose dwarf blue spruce or leave enough spacing for air flow.
• Limit drought stress on spruce (UMass).

When to call a pro

Call an ISA Certified Arborist or licensed applicator if a valued spruce is losing needles from the bottom up for more than a year. A correct diagnosis matters, since several other spruce problems look similar.

Related guides: Tree database: mature size and planting distance · Pennsylvania tree identification · Firewood rules by state · All pests and diseases

Questions about Rhizosphaera needle cast

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