Our hawthorn trees always have a heavy "parasite load." This year, I've been noticing strange, deformed fruits alongside the "normal" haws, and these eventually pop out in spiny growths which spew forth brilliant orange spores.
These rust fungi are both common and of horticultural interest, so I was able to identify the fruiting bodies as "Cedar-Quince Rust Fungus." Bizarre, are they not? My customary list of links follows.
- Cedar-quince rust (Gymnosporangium clavipes) kills hawthorn fruit and twigs. Cedar-quince rust affects quince (Chaenomeles), serviceberry (Amelanchier), hawthorn (Crataegus), mountain ash (Sorbus) as well as many other plants in the rose family and can cause a great amount of damage to the fruits, twigs and thorns of susceptible plants. The rust fungus requires two different tree species to complete its lifecycle. On the primary host, juniper or red cedar, the fungus infects leaves and soft shoots, becomes perennial in the living bark and causes swellings that girdle twigs and small branches. During damp weather in April and May, orange spore masses emerge from infected, swollen juniper twigs and may be splashed or blown to hawthorn, one of the alternate hosts. On the hawthorn, this fungus causes distortion of fruit, twigs, and buds. Fruits become shrunken and often die; twigs become enlarged and woody. Pinkish-orange tubes, about the size of a pencil lead, protrude from affected fruits and twigs and shed orange spores that are splashed or blown back to the juniper, completing the life cycle of this rust fungus.
- UMass Cedar-Quince Rust pdf has similiar information to the resource above, but better photographs.
- Cornell's Cedar-Apple Rust Fact Sheet has some information on cedar-quince rust, good photos, and a dandy life cycle diagram.