Pitch canker has puzzled forest pathologists since it was first described in 1946 , and the pathogen continues to provide biological surprises as it moves around the world.
Hosts are primarily in the genus Pinus, although Douglas-fir is also infected in California. In California, where the disease has received the most attention, Monterey pine (Pinus radiata) is the most important host thus far. This is particularly worrisome because Monterey pine is the most widely planted forest tree in the world and is severely impacted in its limited native range as well as in urban plantings.
Pitch canker is caused by Fusarium circinatum. In older literature the pathogen may be called Fusarium subglutinans f. sp. pini or Fusarium moniliforme var. subglutinans. It produces microconidia, macroconidia on sporodochia, and, when crossed with a compatible mating type, perithecia.
Free moisture may be necessary for successful infection of wounds, even when vectors are involved. This could explain why the disease in California is restricted to the foggy coastal regions and is more severe closer to the coast. Otherwise, the disease progresses faster under warmer conditions; the pathogen grows poorly below 10 C.
The fungus may be airborne, soilborne, dispersed in rain splash, or it may be vectored by flying insects. In the southeastern U.S., and most other areas, no vector relationships have been demonstrated and the infection courts are various mechanical injuries. In California, infections are above-ground and there is evidence that insects vector the pathogen . Insects that carry the fungus include twig beetles (Pityophthorus spp.), bark beetles (Ips spp.), and cone beetles (Conophthorus spp.). Although some of these insects have not been known to visit healthy portions of trees, workers speculate that the insects may “taste” trees indiscriminately to determine which ones are suitable for colonization. In this exploratory feeding, they may deposit inoculum that they picked up in their brood tree.
Epidemics in the southeastern United States appear to be related to the occurrence of infection courts following weather events that cause wounds, pruning, etc. The epidemic wanes as the infection courts are no longer available. Also, in parts of the world where pines are not native, the disease may be restricted to nurseries because of inadequate above-ground infection courts in plantations . In California, on the other hand, the vectors are creating their own infection courts and disease progress is continuous.
On Monterey pine, the disease usually begins with cankers on small branches, usually near the top of the tree, that girdle and kill branch tips. Cankers may then occur on large branches and stems, indicating an advanced stage of disease and a likelihood of continued dieback and death. Cankers are usually resinous and, in the inner bark, are sharply delimited from healthy tissue. Stem cankers may lead to tree death.
Pitch canker has been found in Chile , Colombia, Uruguay, Brazil , South Africa, Japan, Mexico, and Haiti . In Europe it has been found in Spain  and Italy, and is expected to spread through southern Europe unless there is a separate introduction in northern Europe . In the United States, it occurs in two disjunct regions: the Southeast, where it was first discovered, and California, where it was found in 1986 . Because pines in other areas are susceptible in greenhouse tests, there is concern that the disease may spread to other important pine regions.
Currently, management efforts in California have focused on quarantines and public education to avoid spreading the disease.
Pitch canker is one of the most threatening tree diseases in the world today. Because Pinus radiata (Monterey pine or radiata pine) is the most widely planted forest tree in the world, its native range is a valuable genetic resource. These stands are also a unique and increasingly rare example of native vegetation in the central coast of California. In addition, the disease may have a wider potential host range, and potential distribution, than it has today. If it moves into other important pine areas of the world, and finds there a suitable environment, we may be seeing only the beginning of its destructive capability.
The origin of the pathogen is likely Mexico or Central America . Mexico is a center of diversity of the genus Pinus. The disease is widespread there and reportedly causes little damage in native stands. Limited study indicates very high genetic diversity in the pathogen. It appears likely that the pathogen was introduced to Florida a long time ago [1, 2]. Florida has greater genetic diversity in the pathogen populations than does California.
It suddenly began causing damage in California in the 1980s. Its genetic diversity is more limited in California than in Florida . As such it is one of a long and tragic list of examples of the devastation that can be caused by exotic pathogens in a new environment.
About 2% of native Monterey pines appear to be resistant to the disease and an additional 7% may have moderate levels of resistance . There is growing evidence for some degree of systemic acquired resistance, a phenomenon in which trees become resistant to pathogens after subsequent inoculations. Some remission of symptoms has been observed in infected trees after several years.
A New Twist
It seems that Fusarium circinatum has much more varied kinds of behavior than we knew. Early observations suggested that F. circinatum may not always be a simple necrotrophic pathogen as it seems. Recent work shows that it can grow biotrophically, as latent, nonsymptomatic infections, in the cortex of pine roots [3, 10]. Such latent infections can persist indefinitely, at least for a year. Seedlings with such cryptic infections can be killed after outplanting when stress triggers a change in pathogen behavior. It appears that seedlings remain symptomless until infections reach the root collar. There, they may become necrotrophic, resulting in above- and belowground symptoms.
This versatile fungus may also grow as symptomless infection in grasses, essentially as an endophyte, as well as causing ear rot of maize . How do these previously unknown behaviors contribute to disease epidemiology? How many other pathogens we thought we knew well also have a secret life they have not yet revealed to us?