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Horticulture and Home Pest News
Horticulture & Home Pest News is filled with articles on current horticulture, plant care, pest management, and common household pests written by Iowa State University Extension specialists in the Departments of Entomology, Horticulture and Plant Pathology.

When Should I Spray Conifers for Needle Diseases?

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Several fungal needle diseases of conifers require fungicide sprays for satisfactory control. Fungicides do not cure needles already infected, but they protect the newest growth as it emerges. Because the new growth emerges in the spring, the ideal time to spray conifers is coming up soon.

The most common disease of blue spruce is Rhizosphaera needle cast, a fungal disease that usually requires fungicides for management when it occurs. Rhizosphaera needle cast causes needles to turn purplish brown and fall from the tree, usually from the inside of the tree working out and from the bottom of the tree working up. For effective control, infected trees should be treated once in mid-May and again four to six weeks later.

Austrian pines are susceptible to two fungal needle diseases, Diplodia tip blight and Dothistroma needle blight. Diplodia tip blight causes new growth to appear as tufts of stunted, brown needles on the tips of branches. Dothistroma needle blight causes spots to appear on the needles, which turn brown above the spots. Both diseases can progress over the years to make trees very unsightly. Both can be managed with fungicide sprays, but the timing of application differs for each. To manage Diplodia tip blight requires three fungicide applications, once when buds begin to swell (late April), again in early May, and again in mid-May. Dothistroma needle blight can be managed by spraying once in mid-May and again four to six weeks later. Note that trees that are infected with both diseases require a total of four sprays.

Don't apply fungicides unless you are certain your tree has a fungal needle disease. Many factors other than disease can cause browning of needles, and fungicides are no help for these noninfectious problems. Send a sample in for diagnosis before you decide to spray.

Follow label directions exactly. The label will tell you whether a fungicide is labeled for your disease on your tree, application rates, and protective equipment to wear while applying the fungicide. Adequate coverage throughout the canopy is important, and this requires high-power spraying equipment. For most homeowners, it is more feasible to hire a tree care company to make the application than to do it yourself.

Remember that it's not easy or cheap to spray large trees, so it's better to avoid planting disease-prone plants in the first place. Colorado Blue Spruces are very susceptible to Rhizosphaera needle cast, but other types of spruce are more resistant. Options include Norway spruce or Black Hills spruce. White pines have fewer disease problems than Austrian or Scots pines. Other evergreen options include concolor (white) fir, Easter red cedars or other junipers, and yew.

Rhizosphaera needle cast on blue spruce
Rhizosphaera needle cast on blue spruce.
Diplodia tip blight
Diplodia tip blight on an Austrian pine branch.
Dothistroma Needle Blight on Austrian Pine
Dothistroma needle blight on Austrian pine.
Page References: 
36-39
Year of Publication: 
2007
Issue: 
IC-497 (8) -- April 18, 2007