Needle cast

Rhizosphaera kalkhoffii & Dothistroma septosporum

Conifer needles turning purple-brown and dropping, leaving branches bare from the inside out and the bottom up. Look for the tiny black dots in the stomatal rows.

Fungus Serious Rain splashAirborne spores Spreads to neighbors

What you'll see

Needles turning a dull purple-brown, then falling, leaving only the current year needles at the branch tips.
Bare inner branches and a thin, see-through look, starting at the bottom of the tree and moving up.
Rows of tiny black dots along the length of a fallen needle, replacing the normal white stomatal specks — visible with a hand lens and diagnostic.
On pines, red bands across the needle with the tip dead beyond the band, which is Dothistroma rather than Rhizosphaera.
A whole side of the tree affected where a hedge or a wall keeps the air still.

Why it happens

Fungi that infect the current year new needles in spring, sit in them for a year or more without symptoms, and then kill them. This delay is why the tree looks suddenly bad in a year that was fine and why one spray season never fixes it. Spores are splashed and blown from the older infected needles onto the new ones.

Wet springs and long needle wetness. Crowded plantings, poor air movement, overhead irrigation, and Colorado blue spruce planted outside its dry native range are the standing conditions; a run of wet Mays is what brings an outbreak.

Treating it

Prune out the worst lower branches and rake up the fallen needles, which are the spore source.
Thin the surrounding planting to get air and light onto the tree.
Stop any irrigation that wets the foliage.
A copper or chlorothalonil spray at bud break and again three weeks later protects the new needles — but it must be repeated for at least two consecutive years to show, because of the delay.
Replace a tree that is bare more than halfway up; it will not refurnish those branches.

Keeping it away

Plant conifers with full spacing and do not use blue spruce as a tight screen.
Choose Norway spruce, white fir or an eastern white pine instead where needle cast is common locally.
Keep sprinklers off the foliage.
Mulch and water in drought so the tree is not stressed on top of the infection.
Clear fallen needles from under a valued specimen each autumn.

Who gets it

Colorado blue spruce above all, then Norway spruce, Austrian and ponderosa pines, Douglas fir and true firs.