Plant Health Problems
Our native mountain ash and the European species (commonly called rowan) are frequently planted as ornamentals and are susceptible to many of the diseases of apple, particularly apple scab, fire blight, black rot, and rust. See Apple for a detailed discussion of these diseases.

Insect Problems:
Mountain ash sawfly, Pristophora geniculata.
Young larvae are greenish white, with black head and legs and black dots on the body. Fully grown larvae are almost 3/4" long, yellowish, with orange-yellow heads and legs, black eyes and black spots on all but the last segment. The adult sawflies emerge late in the spring. Larvae feed during the summer and drop to the ground to pupate over the winter. There may be a partial second generation. Spray the larvae with either carbaryl, malathion or Spinosad, which are among the compounds registered for control of this pest in Connecticut, when they are feeding. Consult the label for dosage rates and safety precautions.

Pear leaf blister mite.
See Pear.

Roundheaded borer.
Mountain ash trees are sometimes severely injured by this insect. See Apple.

San Jose scale.
This scale infests mountain ash. See Pear.

Scurfy scale.
This scale also occurs on mountain ash. See Pear.

Woolly aphid.
The bluish-white, woolly colonies of this aphid are often seen on the leaves and branches of mountain ash. See Apple.