Authors: Karen Snover-Clift and Sandra Jensen-Tracy, Cornell University Plant Disease Diagnostic Clinic
White Pine Blister Rust (WPBR), caused by the fungus Cronartium ribicola, is one of the most important diseases of white pines (Pinus sect. Quinquefolius). White pines, especially young trees, and plants belonging to the genus Ribes (currants and gooseberries) are susceptible to the disease. The fungus is native to Europe and Asia, and white pines native to Europe and Asia are generally resistant to the disease, while those native to North America are more susceptible. Although WPBR is occasionally a severe foliar disease on Ribes plants, on white pines it is lethal if allowed to spread from an infected branch into the trunk.
The fungus occurs throughout Europe and Asia at low levels wherever white pines occur, causing little damage to the species which have evolved alongside it. It was introduced accidentally to eastern North America early in the 20th Century, and soon spread across the range of Eastern White Pine, causing extensive mortality. By the 1930s it had appeared in the Pacific Northwest, introduced accidentally on Eastern White Pine nursery stock. Resulting mortality in Western White Pine, Sugar Pine and Whitebark Pine has been even higher than in Eastern White Pine, though the spread of the fungus into southern California has been slowed by the hot dry summer climate there. Spread is continuing south into Arizona and New Mexico, causing high mortality on Southwestern White Pine.
On white pine, the initial symptoms appear in late summer or autumn as small, yellow spots on needles. The infection spreads down the needle and into the twig, where slight swelling and yellowing develops during the next growing season. Numerous pale yellow blisters (called aecia) may be as large as 3 mm across and break through the infected bark in mid-April to mid-May a year or more after the bark first becomes infected. These blisters rupture and release large numbers of dry, yellow-orange spores (figure 1). Blisters disappear after spore discharge and form again the next year. As the bark dries out it appears roughened. The sporulation pattern continues over years until the stem is girdled.
Rodents frequently feed on rust-infected bark because of its high sugar content. Bark injured by the rodents yields copious amounts of resin, often obscuring the typical symptoms of rust infection.
On Ribes, the symptoms develop throughout the growing season and are comparatively mild. The lower leaf surface, when infected, becomes pale. This is followed within a few days by the development of tiny orange pimple-like fruiting bodies (uredinia) in which yellow-orange rust spores are produced. These spores cause repeated new infections on Ribes leaves from May through late summer, when another spore-bearing structure of the rust fungus appears. This structure, called a telium, is a short, yellow-brown, hair-like filament(figure 2). Large numbers of these filaments give the lower leaf surface a fuzzy brown appearance.
During moist weather in August and early September, after seasonally cool weather has prevailed for about 2 weeks, telia on leaves of Ribes plants produce spores that cause new infections on pine needles. The rust fungus grows slowly within the pine needle and twig; aecia (blisters) first rupture the bark in April-May of the second or third growing season after a pine needle becomes infected. The spores from these blisters (aeciospores) cause new infections on the growing leaves of Ribes plants but are not capable of causing infections on pine. This alternation of host plants is essential for the perpetuation of the fungus; it cannot complete its life cycle on the pine or Ribes alone.
The pimple-like uredinia that develop on infected Ribes leaves produce orange spores (urediniospores) that cause new infections on Ribes leaves throughout the growing season. These spores, however, are not capable of causing infections on pines. The telia that develop on infected Ribes leaves in late summer produce spores (called basidiospores) that cause new infections on pines. The infected pines provide a place where the rust fungus may safely overwinter; it cannot survive in the Ribes leaves or outside a living host plant.
- Pinus armandii - Chinese White Pine
- Pinus cembra - Swiss Pine
- Pinus koraiensis - Korean Pine
- Pinus parviflora - Japanese White Pine
- Pinus peuce - Macedonian Pine
- Pinus pumila - Dwarf Siberian Pine
- Pinus sibirica - Siberian Pine
- Pinus wallichiana - Blue Pine
- Pinus albicaulis - Whitebark Pine
- Pinus ayacahuite - Mexican White Pine
- Pinus flexilis - Limber Pine
- Pinus lambertiana - Sugar Pine
- Pinus monticola - Western White Pine
- Pinus reflexa - Southwestern White Pine
- Pinus strobiformis - Durango White Pine
- Pinus strobus - Eastern White Pine
Branches with cankers should be cut off where they join the next healthy branch. This cut should be made at least 15 cm beyond the yellowish margin of the canker. This margin can be easily detected by rubbing the area with a wet cloth. Lower branches are most commonly infected. If lower branches are removed, the probability of infection is reduced.
Infections on trunks can be eliminated by removed all bark 5 cm on each side and 10 cm above and below the canker margin. After excision of the infected bark or removal of a branch, the area may be treated with a tree wound dressing for cosmetic purposes.
The currant cultivars 'Cornet', 'Consort', and 'Crusader' are resistant to white pine blister rust. In one study, the cultivars 'Red Lake', 'Jumbo', 'Cherry', and 'White Current' were less susceptible and the cultivars 'Welcome', 'Redjacket', 'Green Hansa', 'Poorman' and 'Pixwell' were the most susceptible.
- White Pine Blister Rust, www.forestpathology.org
- How to Identify White Pine Blister Rust and Remove Cankers, USDA Forest Service
- Pine Blister Rust, Plant Management Article