Rhizosphaera needle cast is a needle disease of spruce that causes one-year-old and older needles to fall or to be “cast” from the tree. The fungus invades new growth in the summer; but symptoms do not show until fall, when new growth appears yellowed or possibly brown. In the following year, homeowners usually notice that one-year-old needles turn purplish brown and fall. The disease has been discussed in issues no. 2 and 6 of this newsletter.
Now is the time to look for damage on your spruce. You will observe apparently healthy new growth on the stems, but last year’s needles are brown or purplish. How do you know the difference between this year’s growth and last year’s growth? The new growth has a lighter color, and the new stem has not yet turned brown and woody.
We have seen many positive Rhizosphaera needle cast samples at the Plant Clinic lately. If you have this problem on your tree, do not resort to sprays at this time. Fungicides work only as preventives against this disease. You can spray the foliage with a fungicide next spring to protect next year’s growth. Chlorothalonil or the copper fungicides work well when applied as soon as bud caps fall off and again when the new needles are fully elongated. Improper spray timing results in poor disease control. If the disease is left untreated several years in a row, branches may die. This year, rake and remove infected needles now or certainly in the fall as a part of fall cleanup. Should we enter a period of drought, provide additional water to help tree vitality.