Landscape - White Pine Blister Rust

I had an email recently on why Currants and Gooseberries are regulated for planting in Delaware. The issue is White Pine Blister Rust. Contact the Delaware Department of Agriculture Plant Industries Section for more information on the restrictions. The following is an article from Cornell on White Pine Blister Rust.

White Pine Blister Rust (WPBR), caused by the fungus Cronartium ribicola, is one of the most important diseases of white pine in the northeastern United States. White pines, especially young trees, and plants belonging to the genus Ribes (currants and gooseberries) are susceptible to the disease. 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.Symptoms

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 (1/4 inch) 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. 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. Large numbers of these filaments give the lower leaf surface a fuzzy brown appearance.

Disease cycle

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 pine trees provides a place where the rust fungus may safely overwinter; it cannot survive in the Ribes leaves or outside a living host plant.

Control Strategies

Branches with cankers should be cut off where they join the next healthy branch. This cut should be made at least 15 cm or 6 inches 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 greatly reduced.Infections on trunks can be eliminated by removed all bark 5 cm or 2 inches on each side and 10 cm or 4 inches 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.Within the blister rust hazard areas, all susceptible Ribes should be removed from the vicinity of valuable white pines.

Ribes are regulated in Delaware. If you wish to plant Ribes in Delaware, contact the Plant Idustries Section of the Delaware Department of Agriculture 302-698-4500 before purchasing or planting to determine if you can plant in your area, and what cultivars you may be able to plant. While reducing Ribes populations will help, it will not completely prevent possible infection since the infectious spores may be carried by air for up to several miles. Cultivated currants and gooseberries may be planted in areas where infection probability is low.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.

Adapted from "WHITE PINE RUST, Cronartium ribicola" a Cornell University Plant Disease Diagnostic Clinic Factsheet with added information for Delaware.