Hawthorns do tend to be vulnerable to some insect pests and some fungal disease problems from time to time. In most cases, the problem can be ignored and it will not return the next year. In serious attacks, you may wish to get some help from a certified arborist.
Distorted growth, honeydew on the leaves, and sooty mold growing on the honeydew
Aphids - Aphids are soft-bodied, pear-shaped insects that cluster on growing tips, new leaves and flowers of many plants. While they are usually about the size of a pinhead, a larger aphid attacks tree leaves, sucking their sap and causing them to turn yellow and curl. Aphid colonies on the lower branches can be dislodged with a strong stream of water from the garden hose.
Brown blotches on the leaves
Leaf miners - Found in the spring, the tips of leaves turn brown, and brown internal larvae trails become visible. Sometimes leaves are skeletonized and rolled up. The leafminers don’t kill birches, but an attack can leave the foliage looking charred and dead. However leafminers leave the tree very vulnerable to attack by the Aphids - Leaves Become Curled And Distorted - Aphids - Aphids are fond of birch trees, and the sticky honeydew these insects secrete may drip off the leaves. Aphids are soft-bodied, pear-shaped insects that cluster on growing tips, new leaves and flowers of many plants. While they are usually about the size of a pinhead, a larger aphid attacks tree leaves, sucking their sap and causing them to turn yellow and curl. Aphid colonies on the lower branches can be dislodged with a strong stream of water from the garden hose. Avoid planting birch where drips can fall onto parked cars, patios or decks.
Chlorotic flecks on the upper leaf surfaces. The lower sides of the leaves are covered with small, brown, sticky flecks.
Lacebugs - Lacebugs are only about one-eighth inch long and hard to see. The clear wings of the adults are marked with a dark pattern, giving them a lacey look. The bugs feed by sucking sap from the underside of the leaves, and their excretions characteristically creat many small, shiny spots. As their population builds through the summer, the foliage looks more and more stippled and bleached. Adult lace bugs overwinter in bark crevices on their host plants, which include cotoneaster, pyracantha and quince besides Hawthorn.
Branch Tips Turn Brown
Fire blight - The tips appear to be burned or scorched and the dead, brown leaves droop but hang on the tree. Cankers form and the bacteria is washed farther down the branch by rain. The problem is Fire Blight. This disease can be severe in some parts of the country. The bacteria, Erwinia amylovora , are spread from diseased to healthy twigs by rain, bees, and other mechanical means. There is no satisfactory chemical control. The disease is less of a problem if trees are not located near apple or pear orchards. Prune out blighted branch tips by cutting a foot or two beyond the diseased wood. Over-fertilizing with nitrogen fertilizer may increase tree susceptibility to fire blight.
Small reddish brown spots on the leaves which may run together.
Leaf Blight - Infected leaves drop in August and severely infected trees may be completely bare. Leaf blight attacks most Hawthorns but especially English Hawthorn
Orange Or Rust Colored Spots On The Leaves Leading To Early Defoliation
Cedar Hawthorn Rust - The fruits and twigs are also attacked. Juniper is an alternate host. Cedar-rust attacks fruits. Washington, Lavelle and Cockspur Hawthorn are resistant to rust diseases. Rust needs two kinds of plants to survive, so chances of it being around are increased if there are eastern redcedars (^Juniperus virginiana^) in the area. The disease starts on the Hawthorn leaves--red or brown spots ringed with yellow that develop horns that protrude from the underside of the leaf, pop and release spores. They infect nearby junipers which do not show any harm, but host the spores over the winter. The following spring they pop over to the Hawthorn again to cause further harm.
Branches Die Back And Sometimes The Plant Dies
San Jose Scale - Scale are small insects carrying their own shell as protection. Scale form groups of small bumps or blister-like outgrowths on tree stems and leaves, from 1/10 to 2/5 of an inch across. These white, yellow, brown or black bumps are waxy shells that protect the insect beneath. When the scale infestation is heavy, many branches may turn yellowish brown, foliage turns gray green and wilts. The scales that remain in a dormant stage on twigs and branches throughout the winter season constitute a population susceptible to a dormant-season application of horticultural oil. Apply oil just before buds begin to open in early spring.