Pine rusts—Various fungi
Several dozen species of native or introduced rust fungi attack conifers in California. Western gall rust and white pine blister rust are important canker and gall diseases, but most pine rusts are usually of minor importance.
Damage resembling that from needle rusts can be caused by sucking arthropods such as mites and scale insects. Abiotic factors, such as air pollution, drought or water deficit, herbicide damage, low temperatures, over-fertilization, and salt injury also kill needles or parts of them. Other causes of cankers and causes of galls include abiotic (nonliving e.g., environmental) factors and certain bacteria, insects, mistletoes, and other types of fungi.
The host species and characteristics of the damage and fungal spores are used to identify rusts.
Needle rusts. Coleosporium spp. and others cause brown, reddish, or yellow bands or spots on needles. Needles often turn brown or yellow and develop tiny, pale or dark spore-forming bodies, usually in winter or spring. Most Coleosporium spp. affect only the youngest needles and sporadically so during years when abundant rainfall coincides with new growth.
Aster rust, Coleosporium asterum =C. solidaginis, is one of the most common pine needle rusts. It is found mostly on the foliage of lodgepole pine and its alternate hosts.
Stalactiform rust. Peridermium stalactiforme causes rough bark (blisters) and cankers mostly on the lower branches and lower trunk of yellow (two- or three-needle) pines, including Jeffrey, lodgepole, and ponderosa pines. On the alternate hosts, stalactiform rust causes discolored blotches and orangish spores on leaves.
To complete their life cycles, most rusts must alternate between the pine and a host in a different plant family. An exception is western gall rust, which spreads from pine to pine.
The aster rust alternate hosts are asters and goldenrods (Asteraceae). The alternate hosts of stalactiform rust are figworts (Scrophulariaceae), especially Indian paintbrush, Castilleja spp.
Most rust fungi have four or five types of spores, which appear in a definite succession. The pycniospore and aeciospore are usually produced on the conifer needle. The urediospore (or uredospore), teliospore, and basidiospore are produced on the alternate hosts. The fruiting (spore-forming) structure of the fungus occurs below the cuticle or epidermis, or in blisters or fungal tissue covering the host tissue. The infective spores are windborne to the alternate host, where they germinate, penetrate the leaf tissue, and cause disease.
Needle rusts occur only sporadically in landscapes and forests. Damage to pines is usually negligible, except in nurseries and Christmas tree plantations. Most outbreaks involve a single season, and then years may pass with no new infections occurring.
Stalactiform rust causes trunk cankers, which can kill trees by girdling. Tree death is uncommon except in seedlings or where two or more cankers coalesce. Infected branches are occasionally killed, but this usually has no significant affect on the growth of otherwise healthy trees.
An interaction of host, pathogen, and environmental factors determines rust outbreaks. Although unsightly, most rusts usually do not seriously harm their hosts. Preventing pine rust damage is difficult or infeasible in many situations.
See western gall rust and white pine blister rust for management of these important diseases. Consult the California Forest Insect and Disease Training Manual and Diseases of Pacific Coast Conifers for more information.