Red Band Needle Blight
This disease is also known as Dothistroma needle blight. I favor the "red band" name because it is more descriptive and means something to people. We should avoid using Latin names of pathogens in disease names. They make disease names more difficult for nonspecialists. Also, what happens when the name of the pathogen changes, which happens so frequently? Most people then start changing the disease name as well, which is undesirable.
Hosts include at least 82 species of pines and a few other conifers that are only slightly susceptible (®). There does not seem to be a relationship between sections of Pinus and susceptibility. Among the most susceptible pines, at least according to some reports, are the well-known species P. contorta, P. ponderosa, P. jeffreyi, P. radiata, P. sylvestris, and P. strobus.
Formerly only one species was recognized as causing the disease world-wide. Based on a comprehensive study (®), it now appears there are two closely related species, one widespread and the other less common and with a more restricted distribution. The pathogens have a complex and tortured systematic and nomenclatural history. At one time the name Dothistroma pini was used throughout the US and D. septosporum in Europe, then more recently the latter name was applied to the pathogen everywhere. They were also separated into varieties which do not appear to be valid phylogenetically. Here are the currently used names:
|Dothistroma septosporum||Mycosphaerella pini||Worldwide in at least 63 countries, including most of North America|
|Dothistroma pini||None known||North-central United States (Minnesota, Illinois, Michigan, Nebraska, probably Kansas and Kentucky) (®) and the Ukraine (®)|
The teleomorph of Dothistroma septosporum is in the class Loculoascomycetes; it is generally uncommon and does not appear in many areas where the pathogen occurs. Since D. pini has no known teleomorph, and that of D. septosporum is uncommon, we generally use the names of the anamorphs. The two species are closely related and very much alike ecologically and morphologically, but they are distinct phylogenetically. Conidial sizes differ (those of D. pini being somewhat shorter and wider), but they are highly variable and the ranges overlap.
As the disease is well distributed in Eurasia, Africa, Oceania and the Americas, it is apparently not highly restricted by environmental conditions. The fungus can survive for up to 11 months without rain (®), but it does require continuous moisture for some period to sporulate and establish new infections.
Spores germinate on needle surfaces during wet weather. The fungus penetrates via stomata and colonizes a portion of the needle. Conidiomata develop under the needle surface, which splits to expose them. They usually mature the following spring, but can mature in the same year in some areas, such as the west coast of North America. Conidia are produced in a mucilage and are thought to be dispersed primarily by rain-splash.
Conidia may be released and cause infections any time during the growing season when there is wet weather and temperatures above 5 C. During wet years, the increase in disease can be explosive. In the Great Plains of the US, fruiting usually begins the spring after infection, so there is only one complete cycle per year. Along the Pacific coast, sporulation occurs more quickly, often in the year of infection, so there may be multiple cycles per year.
Infected needles usually fall prematurely. Current-year needles usually do not become susceptible until midsummer, symptoms appear in autumn, and the needles may fall during the following summer. Older needles may drop in autumn after infection or persist through the following summer. Needles on the ground may continue producing conidia for two to four months (®).
The teleomorph of Dothistroma septosporum is uncommon. In North America, it has been found along the Pacific coast from California to British Columbia. The role of ascospores is not clear. Undoubtedly they cause some infections, introducing recombined genomes into the population and facilitating adaptation.
Early symptoms are often deep green bands and yellow to tan spots on needles. The spots and bands turn brown to reddish brown. The bands are brighter red and more numerous in the western than in the eastern US. The needle dies distal to (beyond) the infection, progressively turning light green, tan, and brown, while the needle base may remain green.
Infection is usually most severe:
However, under ideal conditions, it can attack even young foliage on mature trees throughout the crown. Such infections can be lethal, and on a large scale can be devastating. Growth reduction is usually proportional to level of infection. For example, a mean crown infection of 70% can cause 68% reduction in wood volume growth (®). Consecutive years of high infction can lead to tree mortality.
This is a story not only about nonnative pathogens, it is also a story about nonnative trees. Tree species that coexist with the pathogen in their native range, suffering little damage, or do not have the pathogen in their native range, are often severely affected when they are planted elsewhere. For instance:
Monterey pine has proven to be a very productive tree elsewhere and has been planted all over the world, primarily in the Southern Hemisphere: Australia, New Zealand, many countries of Africa, Chile, and other South American countries. Dothistroma septosporum has eventually found its way to these plantations, in many cases sooner rather than later. Epidemics have devastated the plantations, resulting in abandonment or substantially increased costs to manage the disease.
In much of the Southern Hemisphere, only one mating type of D. septosporum occurs (®). This is important because, without a second mating type, sexual reproduction, genetic recombination and adaptation to local conditions and host resistance are much less likely. Therefore, even though the pathogen already occurs in those countries, it is important to prevent further introductions, diversifying the gene pool and making adaptation more likely.
In Canada and Europe, where the teleomorph of D. septosporum does occur, both mating types are present (®). However, both types are also present in South Africa and the United Kingdom, but the teleomorph has never been found there. Similarly, both mating types of D. pini are present within its range in the US, but no teleomorph has ever been found.
The native ranges of the pathogens are unknown, but it is speculated that D. septosporum originated in the Himalayas or in the mountains of South America. Movement of the pathogens around the world, especially D. septosporum, occurred early. Long-distance dispersal is apparently most common through movement of infected seedlings.
In plantations of the Southern Hemisphere, the disease has caused great damage, as noted above. Where sufficient resources were available, several approaches have reduced the impact of the disease. These include alternative tree species, selection and breeding for resistance, managing planting and crown density to reduce the time foliage stays wet, and aerial applications of fungicides. In some of these areas, these approaches have allowed the continued production of Monterey pine.
Red band needle blight may be the only major forest disease that is actually controlled by fungicides in forest settings. Plantations are protected this way until they get older, then they are safe.
Drying of foliage can be hastened by keeping other vegetation around pines low and by planting at low density so that crowns do not close early in development. This reduces the likelihood of infection during marginal climatic conditions, especially in the lower crown, where inoculum first builds up.
Nurseries should protect seedlings by avoiding large pines around the nursery and by spraying fungicides when necessary. Seedlings with symptoms should not be outplanted.
Although the severe epidemics of the 1960s and 1970s in the Southern Hemisphere are more or less managed now, severe epidemics of red band needle blight have begun to occur elsewhere more recently. Even in the current climate, the disease is expected to expand its world-wide range (®), but climate change appears to be making the disease much more severe in some areas than it has been in the past.
In Britain, dramatic increases in disease levels were noticed beginning in the late 1990s, especially on Corsican pine, but lodgepole pine is occasionally damaged severely also (®,®). Earlier the disease was generally rare in Britain. Now it is so severe that there is a five-year moratorium on planting of Corsican pine. Evidence suggests that earlier, warmer springs and increased spring and summer precipitation are likely causes of the increased prevalence and severity of the disease.
Increased epidemics have also been noted in other European countries, including Austria, Czech Republic, France, Germany, Hungary, Netherlands, Poland and Portugal (see ® for references).
In northwest British Columbia, an epidemic of red band needle blight was first detected in 1997, and has developed to a size and severity never before seen in North America (®). A survey of 41,000 ha in three forest districts in 2002-2004 showed 38,000 ha were impacted, with mortality occurring on 2,700 ha. Even mature lodgepole pine trees were severely affected and dying. Two factors seem to have contributed to the epidemic. First, lodgepole pine, although it is native to the area, has been planted widely since the early 1980's, increasing host abundance. However, convincing evidence has been assembled pointing to a directional climate change as a key factor. The key climatic factor appears to be increased summer precipitation in the late 1990s.