AS PINUS THUNBERGII BECOMES INCREASINGLY PLAGUED BY PROBLEMS, ALTERNATIVE PINES FOR COASTAL REGIONS ARE NEEDED.
Japanese black pine (Pinus thunbergii) has long been a favorite in coastal plantings because of its excellent salt spray and soil salt tolerance and its function as a wind screen. But in the last few years, it has been showing up in plant clinics across the region far too frequently to be considered a sustainable plant. This article will examine the pests responsible for the decline of the Japanese black pine and will explore more sustainable alternatives.
The main pest causing Japanese black pine to die in large numbers is the turpentine beetle (Dendroctonus terebrans) which frequently carries with it the bluestain fungus (Leptographium spp.). These beetles typically feed on the inner bark of the lower four feet of the trunk and may even girdle the tree. Characteristic signs of turpentine beetle injury include long clumps of hardened sap around the lower trunk and reddish sawdust around the very small entry holes. In addition, the introduction of the bluestain fungi can destroy cambial tissue and compound the injury caused by the beetles. A pine affected by this pest complex will show a yellowing or rusty browning throughout the entire tree before the foliage begins to die and needles begin to drop, often within the year.
Another pest equally capable of killing these pines is the pinewood nematode (Bursaphelenchus xylophilus). This nematode is transmitted by the pinewood sawyer, a long-horned beetle (Cerambycidae), which feeds on healthy trees and breeds in dying trees or logs. The nematodes enter through feeding wounds, reproduce quite rapidly, feed on the tree from the inside out, and cause localized foliage dieback before the tree quickly begins to yellow, wilt and die. Trees can often die within months of initial nematode infestation.
Japanese black pines are most susceptible to either of these pest complexes if they are older than 15 years of age or are growing in stressful sites, such as near the coast. Austrian pine (Pinus nigra) is another very salt-tolerant pine, which is also susceptible to the turpentine beetle complex. Other two-needled pines (such as red pine) are experiencing heavy pest/disease pressure as well, but the insects and treatment involved are different.
The greatest attraction to Japanese black pine in the northeast is its salt tolerance. Yet it seems to be more susceptible to disease in salty environments. Finding a substitute for Japanese black pine in coastal areas is a top priority in the University of Rhode Island (URI) Sustainable Landscapes Program.
Current research at URI has identified two possible alternatives for salt-tolerant windscreens in coastal plantings. Bosnian pine (Pinus heldreichii var. leucodermis) has similarly stiff but longer needles and a more upright conical habit. Foliage will appear a bit denser than on Japanese black pine. It is known for its excellent salt-tolerance and provides a beautiful evergreen screening. Although Bosnian pine is not very common, our plant protection specialists have never seen a case of turpentine beetles associated with this pine. And, according to Mori Nurseries in Canada, Bosnian pine is also resistant to diplodia, which commonly attacks many other pines in this area.
Bosnian pine grows extremely slowly and to date it hasn't been financially attractive for nurserymen to grow substantial quantities. However, at Pikes Peak Nurseries in Pennsylvania, young transplants seem to be growing more vigorously than expected. Bosnian pine is available locally in limited quantities from Bald Hill Nursery in North Kingstown, RI.
Another promising alternative to black pine is the Southwestern white pine (Pinus strobiformis). This tree has long had a bit of an identity crisis as taxonomists try to decide whether it is more closely related to limber pine (Pinus flexilis) or Mexican white pine (Pinus ayacahuite). Now considered a species in its own right, it is still not widely known outside its native range in Arizona and New Mexico.
The Southwestern white pine has stiff needles, longer than those of Japanese black and with a bluish hue. It also shows more uniform growth characteristics. The growth rate is comparable with Japanese black pine and almost approaches the fast growth rate of Eastern white pine (Pinus strobus). Salt tolerance research by Townsend and Kwolek (1987) found Pinus strobiformis to be the most salt-tolerant, five-needled pine, ranking in between the very susceptible Pinus strobus and the kings of salt-tolerance, Pinus thunbergii and Pinus nigra. And, like the Japanese black, P. strobiformis is tolerant of high ozone levels, which typically occur several times each growing season.
Although we haven't come across any formal studies on P. strobiformis problems, it appears that it is susceptible to the common diseases that attack other white pines (such as white pine blister rust and the shoot weevil) and is not susceptible to turpentine beetles or diplodia. No Rhode Island nurseries are currently growing Southwestern white pine, but there are several nurseries in Pennsylvania, which have grown it for a long time, both as a Christmas tree and for ornamental use. Through URI's close partnership with the Rhode Island Green Industry, we hope that this pine will be locally available in coming years.
For now, we recommend caution in planting Japanese black pines. If you live away from the coast, you would save yourself may maintenance headaches by planting any of a number of other more sustainable evergreens. If you live on the coast where your choices for salt-tolerant wind screens are limited, you may enhance the survival potential of your Japanese black and Austrian pines by watering them deeply during dry periods, fertilizing correctly once a year and avoiding unnecessary pruning/wounding of the trees. Also, if you catch the beginnings of beetle injury early enough and before the introduction of bluestain fungi, well-timed insecticides may be effective. As this often involves fairly toxic chemicals and direct injection into the tree, we recommend this option only for trained and licensed arborists. For coastal areas, you could also plant Eastern red cedar (Juniperus virginiana), though its texture and form are quite different from pine.
Renee Stoops and Brian Maynard, Department of Plant Sciences, University of Rhode Island, North Knogstown, RI 02881
published in Yankee Nursery Quarterly. Winter 1998. Vol. 7. No. 4. pp.15-16.