Solving Problems of Mountain Ash
Symptoms	Probable Causes
Holes in Lower Trunk	Borers
Leaves Curled; Discolored	Aphids
Holes in Leaves	Sawflies
Leaves and Branches With Small Bumps	Scale
Brown Spots on Leaves; Withering	Mites
Flowers and Shoots Wilt Suddenly in Spring	Fire Blight
Rust Colored Spots on Leaves	Rust
Dead Spots on Leaves; Shoots Blighted	Scab

Holes In Lower Trunk indicates Borers.
The biggest nuisance pest of mountain ash trees is borers such as the flat-headed apple tree borer or the round-headed borer. The latter is the larva of a beetle that is about 3/4 inch long, brown with white stripes down its back. It emerges to lay its eggs on bark in April. The larva, which are light yellow worms with black heads, hatch and burrow into the trunk of the mountain ash. Signs of their activity are galleries in the trunk bark near the base of the tree, round holes about the diameter of a lead pencil, and frass (sawdust) nearby.
To control borers examine the tree closely before the spring season arrives and cut and burn any dying stems below the borer holes. In June, crush any eggs that are visible on the bark. During the summer season, check to see if fresh frass is being pushed from small borer holes. Such holes should be cut out with a sharp knife. If the tunnels are fairly straight, the borer can be killed by probing with a flexible wire, or pulled out by means of a hooked wire to make certain it is destroyed. Borers can also be killed with nicotine sulfate. Dip a piece of cotton or soft cloth into a solution of 1 part nicotine sulfate to 4 parts water, and stuff it into the borer's hole or try injecting nicotine paste into the holes. Another approach is to shoot BT into each hole at ten-day intervals until no more frass appears. A special hypodermic needle for applying BT is available. In any case, coat or seal wounds with tree paint, putty, paraffin, or chewing gum. A black light trap may prove effective against the adult beetles in May or June. For heavy infestations spray the trunks and main branches of affected trees with methoxychlor in mid-May. Repeat the spray 3 times at 2-week intervals. Always practice clean cultivation and burn all weeds, stems and plant remains likely to harbor over wintering eggs.
For more information see file on Dealing with Borers.
Holes in Leaves due to Sawflies.
Mountain ash sawfly lays eggs on tree leaves in late May. The larva are green caterpillars with black dots. They hatch and set to work eating the foliage from early June to mid July. They skeletonize leaves, leaving only the midrib and veins. Handpicking can take care of mild infestations on smaller trees. For heavier assaults spray the foliage of vulnerable trees with BT (Bacillus thuringiensis) just before you expect the caterpillars to begin feeding. Repeat the spray in 2 weeks or if it rains. The young larvae will ingest the bacteria and die in a matter of days. If sawfly infestations are heavy and it is well into July, spray mountain ash foliage with Sevin.
Leaves And Branches With Small Bumps indicates Scale.
Several kinds of scale insects sometimes attack mountain ash trees. Under the protection of small, round waxy shells, they feed on small branches and leaf undersides sucking plant sap. These telltale shells may be whitish, light brown, tan or black in color. Scale over winters in a partially grown condition. It starts feeding in late March or early April, and lays eggs in June and July. There is usually only one generation a year. The symptoms of scale include reduced vigor, yellow spotting of the leaves, brownish bumps on leaves and twigs, and sooty mold growing on sticky honeydew secreted by the pests.
For more information see file on Dealing with Scale.
Brown Spots on Leaves, Withering means Mites.
The pear leaf blister mite will sometimes attack mountain ash. These tiny pests are only 1/125 inch long, virtually impossible to see. What is obvious is the brown spots (blisters) that they cause in leaves as they feed. Infested leaves wither and drop prematurely. Start control measures as soon as you notice the first stippling on the leaves. Spray tree foliage in the early morning with a forceful water spray to knock the mites off the leaf undersides. Repeat the water spray daily for three days. If that doesn't do the job, spray the mites with insecticidal soap combined with pyrethrum every 3 to 5 days for two weeks. Spray mountain ash trees with dormant oil in early spring before their leaves emerge to kill over wintering mites.
For more information see file on Dealing with Mites.
Flowers And Shoots Wilt Suddenly In Spring due to Fire Blight.
This disease, caused by a bacterium, is spread by insects and rain. It usually begins in blossoms, brought from host plants by bees as they search for pollen. The bacterium multiply and infect other tissues. New shoots may wilt suddenly in late spring, turn black or brown and die. The infection may spread down the mountain ash tree to involve large branches. Large cankers, or sores, develop on the trunk and main branches. Trees that have been over fertilized with nitrogen are very susceptible to this disease. If possible, destroy any nearby diseased and neglected pear, quince and apple trees, since they may harbor the fire blight organism. Between November and March, cut off affected branches on mountain ash at least 3 inches below the damaged area. Discard diseased branches in the trash. Before each cut, disinfect the pruning saw or shears with Lysol. Spray trees with an antibiotic wettable powder, such as Agri-Strep, when 25% of the blossoms are open and at 5 to 10 day intervals during bloom. In the east, three applications are necessary for control five or more may be needed in California. Applied in time, the antibiotic will prevent infection from becoming deeply established. It is not effective if this has already occurred. To deal with deep infection, surgically remove the cankers that develop as repositories for the infection, or paint them with specially formulated chemical paint which will penetrate to the gall and destroy the germs.
Dead Spots on Leaves; Shoots Blighted means Scab.
Apple scab is a fungal disease found in most types of apple trees and many ornamental trees and shrubs. It affects both surfaces of the newer, succulent leaves and stems of mountain ash trees causing spots that swell, and turn brown. This tissue dies and leaves and stems appear blighted. Olive brown colored spores develop on the dead tissues and over winter there. The disease develops when the temperatures are moderate, but the humidity is high in the spring and early summer. There are very few natural controls for this disease. It is essential that fallen leaves and debris around infected trees is cleaned up and put in the trash. Spraying or dusting a fungicide such as captan on new growth early in the season will help protect trees.