Paul Oester, OSU Extension Service, Dave Shaw, OSU Extension Service, Lia Spiegel, US Forest Service, and Rob Flowers, Oregon Department of Forestry
Have you noticed scattered dead branches on our ponderosa pine trees? If so, you might be wondering what can that cause tip or branch dieback?
Recently, we have seen an increase in this type of damage in NE Oregon especially on the fringes of the Grande Ronde and Wallowa Valley, and in the Troy area of Wallowa County. Two diseases are associated with the branch mortality; western gall rust and pine shoot blight. Both are common on hard pines throughout Oregon.
Western gall rust (Endrocronartium harknessii) which is very common on lodgepole pine, e.g. in Central Oregon, but also can be important locally on ponderosa pine (Figure 1). This disease can cause branch flagging (dieback), top kill, stem malformation, stem breakage and mortality of young trees. It is most commonly found in riparian areas or locations where moist air collects.
Western gall rust can be identified by the small to large, round to pear-shaped swellings on branches and stems. Bright yellow-orange spore pustules are produced in cracks in the galls in the late spring and early summer. If you look closely at the dead branches on ponderosa pine you may find one or more of these galls on the branch. The gall is restricting or slowing the flow of water through the branch and during hot, dry periods branch tips can die from lack of moisture.
Management for this disease includes favoring uninfected or lightly infected trees during thinning or seed collection. Pruning may help reduce spore production but is not practical except in landscapes around homes or with 300 yards of nurseries. Spores are spread during shoot elongation and it is these new shoots that become infected. Weather patterns during spore dispersal appear to control infection success.
Pine shoot blight or Diplodia (Diplodia species), caused by a fungus whose hosts are 2-3 needle pines, especially ponderosa pine (Figure 2). The first obvious sign is stunted, discolored needles on the current year’s growth. Cankers, which are sunken dead areas, may form on the branches, but are most at the shoot tip. Needles killed by this disease stay on the branch all winter. Look for stunted new shoots or flagged branches with drooping tan, brown or gray needles occurring anywhere on the crown. Infected needles are stunted and may have a resinous droplet near the needle base. The cambium of infected shoots is resinous and discolored. Minute, round, black fruiting bodies (pycnidia) are produced in the spring on twigs, bases of needles, and on cone scales. Infections are promoted by wet springs and areas where cool, moist conditions persist, such as along water courses, valley edges and low areas.
This fungus is a weak parasite and usually only affects trees planted off site or trees weakened by drought or other agents. Wounding will permit spores to enter. Diplodia alone usually will not kill trees except under massive infections. Theoretically, a tree weakened by this disease could be killed by bark beetles.
Management considerations for this disease are generally cultural in nature, such as improving tree vigor to help reduce pine shoot blight infections and allow infected trees better recovery potential. Thinning will help as well as favoring less infected trees. Trees at higher risk are those planted on sites they are not well adapted to or on sites with unfavorable conditions, such as poor soils. Mechanical injury should be avoided. Pruning high value trees will not decrease disease spread because fruiting bodies can be on a number of structures that are difficult to remove, e.g. plant debris including cones and twigs. For trees in landscape settings keep trees well watered and stress free. Several fungicides are registered for use for these two diseases in landscape settings. For information on pesticide recommendations refer to the PNW Plant Disease Management Handbook or contact your local Oregon State University Extension office.
PNW Plant Disease Management Handbook. Senior editors: J.W. Pscheidt and C.M. Ocamb. 2012. To order: Toll free: 1-800-561-6719. Email: firstname.lastname@example.org. A single book is $50.00. Web: http://extension.oregonstate.edu/catalog
Schnepf, C.C. 1992. Diplodia tip blight on ponderosa pine. Publication 946. University of Idaho Extension Service, Moscow, ID
Goheen, E.M. and E. A. Willhite. 2006. Field guide to common diseases and insect pests of Oregon and Washington conifers. R6-NR-FID-PR-01-06. Portland, OR: USDA Forest Service, Pacific Northwest Region. 327 p.
Shaw, D..C., P.T.Oester and G.M. Filip. 2009. Managing Insects and Diseases of Oregon Conifers. Oregon State University Extension Service, EM 8980. 98 p.