Once again, many blue spruce samples showing signs of Rhizosphaera needle cast having been arriving in the Plant Disease Clinic. This fungal needle disease causes browning and early loss of needles. Trees of any age may become infected, especially those that are stressed. Symptoms are most obvious on second-year (inner) needles. Diseased needles turn purplish brown and fall from the tree during summer and autumn.. Small black fruiting structures (pycnidia) of the fungus can be seen in the pore-like openings (stomata) of infected needles. These fruiting bodies typically appear in rows along the length of the needle. A hand lens or magnifying glass is helpful in viewing these structures.
Infected trees can be sprayed with a fungicide such as chlorothalonil (e.g. Daconil 2787) or Bordeaux mixture when new shoots are 1/2 - 2" long (late May or early June), and 4-6 weeks later. Also, improve tree vigor through mulching and watering when needed. Promote good air circulation by adequate tree spacing and weed control. Do not shear trees when foliage is wet. Finally, inspect trees carefully for fruiting bodies before planting.
Rhizosphaera needle cast is pictured in Pm-1528 Common Diseases of Conifers in Iowa. This publication is available from your local Extension office .
This article originally appeared in the May 8, 1998 issue, p. 51.