Introduced fungal diseases and trees have a sad history in the United States. One cannot read of Dutch elm disease or chestnut blight without feeling a sense of loss. Now a fungal disease called pitch canker is threatening pines in the western United States. The iconic Monterey pine (Pinus radiata) of the central California coast is the most susceptible species.
Pitch canker is caused by the fungus Fusarium circinatum. The fungus causes swollen lesions that girdle branches, trunks, and exposed roots. Water flow is obstructed in the affected tissue and needles die, turning yellow and then reddish brown. The tree may ooze large amounts of resin in response to the disease (see thumbnail photo). The fungus is not known to move within the trees, so each canker is the site of a separate infection.
Though the disease can be wind borne, beetles are the usual vectors. Monterey pine cone beetles (Conophthorus radiatae) feed on twigs. The infected twigs die. This weakens the tree and provides a breeding ground for twig beetles (Pityophthorus spp.). The newly emerged beetles travel to other trees and spread the disease. The main trunk of the tree can also be attacked by engraver beetles (Ips spp.). If the beetles are carrying the disease, the resulting bole cankers weaken the tree and leave it vulnerable to further attack by beetles. If the tree dies, this provides more breeding habitat for beetles, which then spread the disease further. The disease is able to survive in the leaf litter and soil, and with this comes the potential to infect seedlings. Fire can sterilize the soil and may have the potential to slow the disease in habitats where fire is allowed to occur.
Pitch canker is native to the eastern United States and Mexico. In its natural range, the disease is kept in check by the balance of nature, though sometimes tree farms may be infected. The native eastern pine species and pitch canker have been co-existing for ages, so the eastern trees have developed some resistance, at least to the point that whole species are not wiped out. Anything that was susceptible and did not build resistance would have been wiped out millenia ago. Monterey pine as a species has no resistance to this foreign disease. The disease was first detected in California in 1986. This strain is thought to have reached California via the eastern United States. It is not known exactly how it arrived in California but infected nursery stock is suspected. The first trees infested were ornamental plantings but within eight years the disease spread to all three wild American populations of Monterey pine.
Canker on a branch, note reddish, dying needles in the background
Early estimates were that as many as 85% of Monterey pines could become infected and very few would survive. A loss of more than a small percentage is bound to have a great impact on the ecology of the Central Coast. Monterey pines provide food and habitat for many creatures, including providing winter roosting sites for monarch butterflies. The trees provide a moist, shady habitat in summer-dry country. Forest makeup varies from location to location, but the forests of the Cambria population, for example, are almost entirely Monterey pine. There is also the loss in urban and suburban areas of street, park, and privately owned trees. There is economic loss, as trees add to property values. There is the aesthetic loss as the picturesque trees disappear. There is emotional loss as people lose their yard trees to which they have become very fond. The economic loss has been great at Christmas tree farms in warmer regions, where the Monterey pine has been the tree of choice.
Cankers on trunks of several trees
Monterey pines are native to a relatively small region, but they have also been planted as ornamentals across mild-winter California. The disease has also shown up along almost the entire length of the state, from Mendocino County to San Diego County. Though Monterey pine has been the most susceptible species, the disease has also shown up in Coulter (Pinus coulteri), ponderosa (Pinus ponderosa), Bishop (Pinus muricata), knobcone (Pinus attenuata), Torrey (Pinus torreyana), shore (Pinus contorta), gray (Pinus sabiniana), Italian stone (Pinus pinea), Aleppo (Pinus halepensis), and Canary Island pines (Pinus canariensis) and douglas-firs (Pseudotsuga menziesii). Many of these are important landscaping and lumber trees. Some are relatively rare native trees. At this time, the disease has shown up only in coastal California and has not appeared in the Sierra Nevada.
The is no known cure or practical preventative for the disease. The best hope is that trees will be found that are resistant to the disease. This is happening. Unaffected trees are sometimes found in severely infected groves. A small number of infected trees recover and even develop resistance. Because diseased trees may recover and may even provide valuable genetic material, it is recommended that diseased trees not be removed unless they become a hazard. However, every effort should be made to prevent the spread of the disease to new areas. Wood, bark, soil, seeds, and seedlings from infected areas should not be moved to new areas. If working with diseased trees, pruning tools should be disinfected. To prevent or at least slow the spread of the disease to other species, it is recommended that Monterey pines not be planted outside of their native region.
Pitch canker has the potential to greatly alter western pine forests and woodlands, at least in coastal California. However, nature is proving to be resilient and hope is not lost. Hopefully the scenic, shady, and diverse Monterey pine forest lands will continue to grace the Central Coast for ages to come.
Perhaps these healthy Monterey pines will prove to be disease resistant
All photos were taken of the Cambria population of wild Monterey pines in December 2011.
About Kelli Kallenborn
I have lived in California for 20 years and really enjoy the climate and all of the varied natural ecosystems.