Fusiform rust is caused by the fungus Cronartium quorum f. sp. fusiforme. Like many rust species, this fungus produces five different spore types and completes its life cycle on two different hosts: pine and oak.
During the spring, the fungus produces bright orange spores like the ones I observed on the galls. Typically, these spores are produced from late March to mid-April. Wind-blown spores infect newly formed leaves of several oak species, especially water, willow, and laurel oaks. Symptoms on oak are not conspicuous and usually the tree is not harmed. The fungus produces a different type of orange spore on the under-surface of oak leaves from late April through the middle of June. These spores are then wind-blown to nearby pines where they create new infections, causing new galls and continuing the cycle.
Pine trees are highly susceptible to Fusiform rust when young and infections that occur within the first 5 years of growth usually result in death. Later, main stem cankers can girdle and kill the tree or reduce its value. Stems with cankers are weak and susceptible to wind and ice breakage, and galls easily catch fire and stay afire. Secondary infections by the pitch canker fungus (Fusarium moniliforme var. subglutinans), black turpentine beetles (Dendrodoconus frontalis) and coneworms (Dioryctria spp.) aggravate the tree’s weakened condition, causing further damage.
Over the years, fusiform rust has become an increasing problem, particularly in pine plantations. Loblolly and slash pines are the most susceptible tree species. Longleaf is fairly resistant and shortleaf is highly resistant.
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