Swiss Needles Cast Disease of Douglas Fir – A Review from a New Zealand Perspective
Project: 1998-06. Report - Swiss Needles Cast Disease of Douglas–Fir – A Review from a New Zealand Perspective.
Date: June, 1999
Author: I. A. Hood
Publication: Forest Research Unpublished Report No. 7544
Project reference: 1998-06
Swiss needle cast disease is caused by Phaeocryptopus gaeumannii, a microscopic fungus which parasitises needles of Pseudotsuga menziesii (Douglas fir) and Ps. macrocarpa from North America. P. gaeumannii occurs unevenly throughout the natural Douglas fir distribution range in North America, incidence ranging from high, where spring rainfall is substantial, to zero where it is negligible. Infected trees in the natural range are generally very healthy.
P. gaeumannii was first found in Switzerland in 1925, infecting foliage in diseased stands of planted Douglas fir. Swiss needle cast disease gradually spread eastwards across Europe from the initial infection centres in Switzerland and Great Britain during the next three decades. P. gaeumannii reached Douglas fir plantations in the eastern United States in 1938, and has also been found in Australia, and possibly Japan. Recently Swiss needle cast disease appeared unexpectedly within part of the natural Douglas fir range along the coast in Oregon, where it is currently causing considerable concern.
P. gaeumannii was first found in New Zealand, near Taupo, in 1959. Infection gradually spread through the country and could be found in most of the North Island by 1970. P. gaeumannii was detected in the South Island, near Nelson, in 1969, and continued to spread southwards over the next two decades. It took more than 30 years for P. gaeumannii to establish in Douglas fir plantations in most parts of the country.
Douglas fir trees and stands infected by P. gaeumannii produce variable symptoms of defoliation and crown yellowing that grade from green and healthy, through mildly chlorotic, to badly diseased. The inconsistent nature of the disease has periodically led to questions about the role of the fungus. However, there is now no doubt that P. gaeumannii is a primary pathogen: infection has been closely associated with defoliation and chlorosis in all regions where it has been found; significant disease–related growth decline set in when the disease first appeared in stands in Denmark, Germany, Michigan, New Zealand, and now Oregon; and rigorous inoculation experiments have proved that P. gaeumannii directly causes defoliation. Under natural conditions the fungus is a parasite in balance with its host, but where these are disturbed disease frequently develops.
Symptoms of chlorosis and defoliation appeared during the 1960s in a number of older, latethinned Douglas fir stands, shortly after P. gaeumannii had spread and established in forests in the central North Island. Losses in one year in Kaingaroa Forest amounted to 140, 000 m3 . Some of the declining stands experienced further stresses from insect defoliators. Although younger central North Island stands appeared healthier, they were also heavily infected, shed needles prematurely, were mildly chlorotic in appearance, and were found to be growing more slowly (ca. 2/3–3/4 full capacity). Younger central North Island Douglas fir stands are heavily infected and continue to show the same symptoms today. In the cooler south, plantations apparently still appear healthy, but a systematic evaluation of infection and needle retention is needed for a more complete picture of South Island Douglas fir throughout the range of latitude and elevation. Although defoliation has occurred in some infected stands, no growth decline has been reported in the South Island.
Phaeocryptopus gaeumannii is recognised by its tiny fruitbodies (ca. 1mm diameter) which grow out from the stomata and appear as two parallel "sooty" bands along each side of the midrib on the undersides of the infected needles. Spores are produced in spring (in New Zealand between September and February), spanning the period of flush and expansion of new foliage. Spores are released, and infect new needles during periods of rainfall. Most infection occurs on new foliage, but one year old needles are also infected to varying degrees. There is no asexual stage, and only one infection cycle occurs each year. This, and the apparently small dispersal distance, appear to explain the slow rate of spread between and within plantations. In New Zealand new fruit bodies become detectable in march, and gradually grow to maturity by September. Successive crops of fruitbodies are produced each year, so that more are present on older needles.
Infection is favoured by moist conditions, and the disease is more prevalent in areas with high spring rainfall, during the period when infection occurs. Such conditions, which are found over most of New Zealand, provide for a longer cumulative period suitable for infection than occurs in the natural Douglas fir range (except close to the coast), and encourage high inoculum pressure. In younger central North Island stands typically more than 80% of needles are infected.
Douglas fir populations show significant variation in resistance and tolerance to infection by P. gaeumannii. Through natural selection, seedlots and clones from locations where higher infection occurs naturally are more resistant than those from places where there is less natural infection. However, in the North Island and northern South Island, high rates of infection override intrinsic resistance, and all seedlots tend to be heavily infected. Nevertheless, variation in needle retention between– and within–provenances is still substantial under these conditions. Selection of improved genetic material is probably the best opportunity for correcting the impact of Swiss needle cast disease in future North Island Douglas fir stands. Infection and needle retention should therefore be monitored along with other attributes such as growth, form, and wood density, in the ongoing Douglas fir breeding programme, as was prudently done in the past. The current problem in Oregon has possibly originated in part through the planting of nursery stock from outside the region, when local seed may have been in short supply, into an area of high spring rainfall. If so, this affirms the importance of suiting the appropriate seed to the location. Swiss needle cast is today not seen as a serious problem in Europe, except on certain sites, and this situation may have arisen partly through genetic improvement.
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