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Common Name: White Pine Blister Rust
Notes:

Introduction

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

Figure 1: Close-up view of typical blisters (aecia) on white pine stem. Spores appear as a powdery yellow mass.

Figure 2: Close up of hair like telia on underside of infected Ribes leaf.

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 (Fig. 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 (Fig. 2). 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 Blister Rust Hazard Map shows areas in New York State where the climate is favorable for infection of white pine. 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 infection. 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

On pine, 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, susceptible Ribes should be removed from the vicinity of valuable white pines. There has been a recent change to the law regarding planting of Ribes in NYS. Formerly, Ribes were not allowed to be planted in several entire counties and in specific towns within other counties. In NYS, a new ruling was recently adopted regarding the "Use of Disease-Resistant or Immune Cultivars of the Genus Ribes".The details have been published in the NYS Register and can be found at:http://www.dos.state.ny.us/info/register/2006/nov29/pdfs/rules.pdf.

Please note: planting, propagating, possessing, or bringing Ribes into counties or towns considered to be part of the White Pine Blister Rust Quarantine district is still forbidden unless written permission is obtained first from the NYS Department of Environmental Conservation. Certification for planting, if given, will apply only to the cultivar(s) specified and to the specific location on the certificate. If you wish to plant Ribes in New York State, and you are uncertain how this ruling applies to you, or if you have any other questions, please contact Jason Denham, NYS DEC, 625 Broadway, Albany, NY 12233-4253, Phone: (518) 402-9425, e-mail jbdenham@gw.dec.state.ny.us.Contact the Department before purchasing or planting to determine ifyou can plant in your area, and what cultivars you may be able to plant. Within , While reducing Ribespopulations 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.

Source:  Cornell University

Submitted by: pepper23