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Two needlecast fungus diseases- Rhabdocline (Rhabdocline pseudotsugae)
and Swiss needlecast (Phaeocryptus gaumanni)- have become serious
problems in Douglas-fir plantations across the United States, particularly in
the Northeast and more recently in the Pacific Northwest. These two diseases
cause premature needle loss resulting in trees with thin foliage. This
condition adversely affects all Douglas-fir plantations, but is especially bad
in Christmas tree plantations where thin-foliaged trees may not be salable.
Both diseases seem to be more severe on the Rocky Mountain variety of
Douglas-fir than on the Pacific coast variety.
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Rhabdocline infected needles are yellowish brown to reddish brown in
spring. |
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These diseases can be controlled-but early detection is important. Described
and illustrated here are the symptoms with special emphasis on the most
distinguishing features. Disease development will vary somewhat with geographic
location.
Look for:
Needlecast Stage
Rhabdocline Needlecast
Current years needles develop small yellow spots in late fall or early winter.
Yellow spots enlarge and needles appear mottled. By spring the needle spots are
a yellowish brown to reddish brown. Sometimes spots coalesce to discolor the
entire needle. The discolored needles are cast in the spring after spore
discharge.
Swiss Needlecast
Foliage symptoms include yellowing and browning of needles in late fall of the
first year or during spring and summer of second year. Symptoms are most severe
on older needles on lower branches. Diseased needles are often uniformly brown
at the tips, but never mottled. infected needles are usually cast during late
fall and winter.
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| Needles
infected by Swiss needlecast often turn brown at the tips in early
spring. |
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Trees with severe
infection by P. gaumanni often have very few needles remaining prior to
spring bud break. |
Spores
Hyaline ascospores are released during wet weather and are wind disseminated.
Rhabdocline Needlecast
1-celled bone shaped 7x19 microns. |
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Swiss Needlecast
2-celled spores 4x14 microns |
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| Ascospores of R. pseudotsugae
are windblown and infect the young needles of opening
buds. |
Fruiting Stage
Fungus fruiting bodies develop on infected needles on the tree.
Rhabdocline Needlecast
Depending on the sub-species of the fungus, fruiting bodies develop on
1-year-old needles either on the upper or lower surface of the brown mottled
spots. At first they appear as round cushions. In May-July (depending on
location) the needle epidermis covering these cushions ruptures with an
irregular slit exposing the pale orange spore bearing areas. Ascospores are
released during wet weather and are wind disseminated to other trees. Spores
can only infect elongating young needles at bud break. Fruiting bodies will
develop and mature on these infected needles during the following winter and
early spring.
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| The fruiting bodies of R.
pseudotsugae rupture the needle epidermis in spring exposing the spore
bearing areas. |
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Fruiting bodies of
P. gaumanni (Swiss needlecast) protruding from stomata appear as small
black dots. |
Swiss Needlecast
Fruiting bodies appear only in rows of stomata on the underside of infected
needles as early as August of the first year (more commonly Nov-March) in the
Lake States. When magnified, these fruiting bodies appear as lines of black
dots emerging from the normal white stomates. Fruiting bodies are very similar
to those found on blue spruce trees infected with Rhizosphaera needlecast.
These fruiting bodies also produce airborne ascospores that are released in wet
weather. Major spore release and infection coincides with bud-break and new
shoot growth, but some spores are released throughout the summer. Infection is
mostly on new needles but some are infected during the second year. The disease
spreads mostly within the infected tree and to adjacent trees, but spores can
be disseminated over long distances. This pathogen is mainly disseminated
through shipment of infected nursery stock.
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| The Douglas-fir on the right is
infected with both Rhabdocline and Swiss needlecast. The tree on the left shows
resistance to these two diseases. |
CONTROL
Cultural
- Control vegetation around base of trees to increase air circulation and
reduce moisture conditions necessary for infection.
- Plant healthy stock.
- Identify disease early to minimize losses.
- Shear trees in healthy plantations first to avoid contamination of these
plantations by workers' clothing and equipment.
- Sterilize tools by dipping in denatured alcohol for 3 minutes after
shearing infected plantations.
Genetic
- Most Douglas-fir plantations contain some trees with genetic resistance to
Rhabdocline and/or Swiss needlecast. These trees should be favored whenever
possible and should be used for seed collection programs.
Chemical
- Rhabdocline
Trees should be sprayed with chlorothalonil (Daconil 2787 WP) at 21/2 pounds
per 100 gallons of water in hydraulic spray equipment or 5 1/2 pounds per 100
gallons in high-pressure mist blower. Apply at 1112 to 23/4 lbs/acre for aerial
spray. Foliage should be completely covered. The first application must be made
as soon as some of the trees start to break bud. Treatment should be repeated
2-3 times at 2-3 week intervals.
The timing for fungicide control of Swiss needlecast is fairly close to that
required for Rhabdocline. However, control of Rhabdocline will require an early
spray applied before Swiss needlecast control is needed.
- Swiss Needlecast
Trees should be sprayed with chlorothalonil (Daconil 2787 WP) at 2 1/2 pounds
WP per 100 gallons of water in hydraulic spray equipment or 51/2 pounds per 100
gallons in high-pressure mist blower. Use 1 1/2 to 2 3/4 lbs/acre for aerial
application. Make first application in the spring as soon as new shoots are 1/2
to 2 inches long (this would be too late for control of Rhabdocline); make the
second application 2-3 weeks later. If rainfall is high a third application may
be necessary. Two years of fungicide protection will restore moderately
infected trees to full foliage. Heavily infected stands will require longer.
Nursery managers should follow a more intensive spray program to avoid the
chance of shipping infected stock. In the Lake States the stock should be
protected at 2-week intervals from bud break until mid-August. Swiss needlecast
can be controlled using the above recommendations when spray application is
based on foliage development.
References
For Rhabdocline
Collins, D.G. Rhabdocline needlecast of Douglas-fir in British
Columbia. Forest Insect and Disease Survey Pest Leaflet 32. Canadian Forestry
Service, 1971.
Morton, H.L.; Miller, R.E. Chemical control of Rhabdocline
needlecast of Douglas-fir. Plant Dis. 66: 999-1000; 1982.
O'Brien, J.G. Studies on Rhabdocline spp. occurring in Michigan
Christmas tree plantations. Masters thesis. Ann Arbor, MI: University of
Michigan; 1981. 77 p.
For Swiss Needlecast
Ford, K.F. The etiology, distribution, and impact of Swiss
needlecast on Douglas-fir in Michigan. Masters thesis. Ann Arbor, MI:
University of Michigan; 1973. 32 p.
Morton, H.L.; Patton, R.F. Swiss needlecast of Douglas-fir in the
Lake States. Plant Dis. Rep. 54: 612-616; 1970.
Skilling, D.D. Control of Swiss needlecast in Douglas-fir Christmas
trees. Am. Christmas Tree J. 25: 34-37; 1981.
Darroll D. Skilling
Principal Plant Pathologist North Central Forest
Experiment Station
St. Paul, Minnesota |
Harrison L. Morton
Professor
School of Natural Resources University of Michigan
Ann Arbor, Michigan |
Copies Available from-
North Central Forest
Experiment Station
1992 Folwell Avenue
St. Paul, MN 55108 |
1983 |
| UNITED STATES GOVERNMENT PRINTING
OFFICE HT-59 |
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Identify and Control
Rhabdocline and Swiss Needlecasts of Douglas-Fir 