Question
My hawthorns' leaves are a mottled yellow and green. Many leaves also have small brown dots on the back. Some leaves have larger rusty-looking patches. I recently had several areas where the plants died back.  Irrigation system seems OK.  What should I do?

Answer
Sounds like Hawthorn Rust

(pathogen: Gymnosporangium globosum):

Hawthorn rust is very similar to cedar apple rust, except that it attacks a wider host range in the Rosaceae family. Hawthorn rust alternates between eastern red cedar, common and prostrate juniper and apple, crabapple, hawthorn, and sometimes pear, quince and serviceberry.

On juniper, the reddish-brown galls are smaller, usually less than 12 mm (1/2 inch), and more spherical than those caused by cedar apple rust. The telia are chestnut brown. Galls produced by hawthorn rust may produce spores for 3-5 years. Significant twig dieback can result from this disease.

On hawthorn, leaf spots are similar to those caused by cedar apple rust. If infection is heavy, hawthorn leaves may yellow and drop prematurely. Fruit and green wood may also be affected.

Rust diseases take two hosts to complete their life cycle--in this case Hawthorn and Junipers. IF there is a jumper near the hawthorn I would remove it or you you continue have rust spots on the leaves and fruit of the hawthorn. You can use a fungicide for the hawthorn.

Fungicides can be applied to hawthorn in the spring to prevent rust infection. The first spray should be applied as soon as the gelatinous tendrils are noticed on the cedar galls. or as the leaves are budding out. Continue applications on a 7- to 10-day interval as long as the galls remain active (usually till the end of May). Several chemicals, including Ferbam, triforine (Funginex, labeled for apples only), chlorothalonil (Daconil 2787, crabapple only), myclobutanil (Immunox, Eagle, Systhane, Rally) and triadimefon (Bayleton) are effective in controlling rust diseases.  Captan and benomyl, labled for control of apple scab, are not effective in controlling rust diseases.
Check with your local garden type store for these products.