ISBN 0-662-25008-7 Cat. No. Fo 29-6/38-1996E 71 FOREST Foliage Diseases in Western Larch Pest in British Columbia LEAFLET By R. Garbutt Pacific Forestry Centre

Introduction Premature needle discoloration of western larch, Larix occidentalis Nutt., is a common phenomenon in south- eastern British Columbia (B.C.). The discoloration can be caused by one of a number of abiotic factors such as drought or frost, or a defoliating insect such as the larch casebearer, Coleophora laricella (Hubner), but is most often caused by a fungus dis- ease. The most common diseases affecting the foliage of western larch in B.C. are: Larch needle blight—caused by Hypodermella laricis Tub. (Ascomycete); and Larch needle cast—caused by Meria laricis Vuill. (Hyphomycete). Hypodermella laricis is native to North America, whereas M. laricis is native to Europe and was introduced to North America early in this century. Fig. 1. Premature needle cast of western larch seedlings infected by Meria laricis. Both pathogens occur in Europe on European larch, Larix decidua Mill. northwestern U.S. in 1942 (4). In 1981 H. laricis, but has caused no signifi- Hypodermella laricis occurs on it was found for the first time on west- cant damage. It has, however, long tamarack, Larix laricina Du Roi, in ern larch in the Nelson and Kamloops been recognized as the single most Ontario and the eastern United States, forest regions of B.C., and on alpine important disease of European larch and on western larch in southeastern larch, Larix lyallii Parl., in the Rocky in British nurseries (7). In British B.C., where it is a common cause of Mountains of western Alberta (6). The Columbia it has caused intermittent widespread premature foliar discol- broad distribution of infections at the damage to containerized and bare- oration. It has not, however, been time suggested that it had become root western larch seedlings in south- identified from tamarack in northern established somewhat earlier. Since ern interior nurseries since 1981. British Columbia or Yukon. then, M. laricis has frequently infected Meria laricis has not been reported on Meria laricis was first identified in the needles of western larch in native tamarack. North America on western larch in the stands, sometimes in association with

Natural Resources Ressources naturelles Canada Canada Canadian Forest Service canadien Service des forêts needles turn brown and are cast. Spores are produced in colorless, nearly invisible clusters on conidio- phores, which project through the stomatal openings on the undersides of the needles (4). Conidiophores seem able to produce a succession of spores, budding new ones as the mature ones are shed (7). The cycle from infection to sporulation requires only two to four weeks (4). In combination, the above charac- teristics enable M. laricis to continue reinfecting and intensifying disease damage throughout the growing sea- son, as long as moist conditions pre- vail. However, the capacity of the fun- gus to reinfect trees in the forest envi- ronment is limited by the early casting of dead needles prior to spore matura- tion. This is less limiting in seedling Fig. 2. Hypodermella laricis fruiting on western larch needles retained for up to two years. nurseries where cast needles remain close to healthy susceptible ones. Life history Most reinfection, however, seems to the mature needle forms a physical arise from the few needles that are Though M. laricis can infect western barrier to penetration. retained in the fascicles, and possibly larch needles throughout the growing About four weeks after infection, from those caught and retained in the season, both fungi characteristically needles begin to discolor. In early crowns (7). infect in May, within two weeks of nee- August elongate black hysterothecia dle elongation. Of the several climatic begin to form on the undersides. Recognition factors which influence the chance of These hysterothecia house the fruiting Hypodermella laricis infection by both fungi, moisture, pro- structures or asci which produce the vided by rainfall or high humidity at the spores. Hypodermella laricis is Needles infected with H. laricis time of budbreak, is the most impor- thought to produce the growth regulat- change from green to a yellow-orange tant. Conversely, dry weather is unfa- ing hormone indoleacetic acid (auxin) color, normally in June. In severely vorable to the spread of these dis- which inhibits the formation of an diseased stands, the scorched eases, and new infections are abscission layer at the base of the appearance of the foliage is sudden checked by summer drought. needles and prevents them from being and striking. Soon after needles die cast (1). Dead needles are retained on from H. laricis infection, oblong or Hypodermella laricis the trees for one or more years, elliptical (0.5Ð0.8 mm long) black hys- The maturation of H. laricis spores is ensuring that the disease inoculum terothecia appear, generally on the timed to coincide with spring budbreak remains in proximity to its host. undersides of the needles. These fruit- on the larch trees. Abundant moisture Meria laricis ing structures are readily seen from is not only the major vehicle for spore early August onward, and remain dispersal, it is also instrumental in trig- Most M. laricis infections also occur prominent on the straw-colored nee- gering spore release. During spring during the period of initial needle elon- dles that can be retained in the fasci- rains, the walls of the fruiting bodies, gation. Though moisture does not play cles for up to two years. These fea- or hysterothecia, absorb moisture, a direct role in the release of spores tures are diagnostic of H. laricis. swell and split along a suture, expos- as with H. laricis, wind and rain splash Meria laricis ing the spores (1,4). When the spores are the primary mechanisms by which are dispersed by rain splash, some spores are transferred from dead nee- The first symptoms of disease caused land on young needles where they dles to adjacent healthy needles. by M. laricis are yellowing and wilting germinate and penetrate the meris- Germinating conidia gain entry of the infected portion of the needle, tematic tissue at the needle base (1). through the tips or mid-portions of and the formation of characteristic Needles remain susceptible to infec- developing needles, probably because necrotic bands. The discoloration then tion by H. larcis only during this period the older needle tissue is less resis- rapidly darkens to reddish brown and of elongation, after which the cuticle of tant to infection (4). Shortly thereafter spreads over the remainder of the

2 disease of container-grown western larch in B.C. (12). Cooley reported a 32% decrease in stocking at an Oregon nursery in 1980 due to severely diseased, dead or stunted 2-year-old western larch, and most of the remaining stock was infected as well (3). The greatest hazard was rec- ognized to be in the 2-year-old stock in which the disease had intensified. Similar damage occurred at the Skimikin Forest Nursery at Salmon Arm B.C. in July of 1981, and con- tainerized seedlings were moderately to severely affected at three B.C. nurseries in 1995 (8). Diseased seedlings sometimes develop stem lesions which contain no pathogens, but are thought to result from fungus-produced toxins (12). Kooistra described how seedling stem lesions Fig. 3. Meria laricis fruiting on western larch needles cast in the year of infection. resulted in significant stem breakage during seedling lift. According to his needle. Symptoms often appear in Damage guidelines, seedlings with stem May and June, but infection can occur lesions girdling more than one-third of at other times during wet weather. As Neither H. laricis nor M. laricis are the circumference are now culled (8). M. laricis produces no fruiting struc- known to cause mortality in natural Because of yearly moisture fluctu- tures, it cannot be identified with any stands, and are not considered seri- ations, successive years of severe certainty in the field. Spore clusters ous forest pests. Needle infections by needle disease caused by either fun- which appear on the underside of the H. laricis often cause entire crowns to gus are uncommon, and trees normal- needles as white spots on the stomata discolor, and the resulting loss of ly regain full vigour after a year of little can only be seen with the aid of a growth potential may be significant in or no infection. strong hand lens or a dissecting some severe cases; however, there is Control microscope. To avoid confusion very little experimental data to support between spores and white waxy ele- this (4). Among the more severe Chemical control has not been ments of the stomata, samples are instances of reported damage was the attempted in the forest environment usually treated with a stain such as death of spur shoots after repeated because of the limited damage cotton blue (7), which facilitates detec- severe needle infection by H. laricis in caused by either fungus. For control of tion by staining the fungal structures Oregon (1). This type of damage has H. laricis on ornamentals or in nurs- deep blue. not been observed in B.C. eries, Bordeaux mixture or lime sul- Because of the similarity of symp- Meria laricis has long been recog- phur is effective if applied from toms, M. laricis damage may be con- nized as a nursery pest in England approximately one week before to fused with damage caused by late and Europe on European larch (7), three weeks after budbreak. Benomyl spring frosts. Frost damage discolors and more recently in B.C. and the (Benlate 50 WP), Captan and Zineb1 whole needles, often killing the new northwestern United States where provide some control of M. laricis. First shoots as well (11). Frost-killed nee- western larch has increasingly been applications should be made at bud dles are also retained by the tree for sown in forest nurseries. Nursery swell, with second and third applica- several weeks. Moreover, the effects seedlings are particularly susceptible tions at one-month intervals, and fur- of frost can usually be detected on to the disease because of the proximi- ther applications at two-week intervals more than one species of plant. ty of succulent young needles to infec- through July, or until dry weather pre- Initially, M. laricis only discolors the tious needles that have been cast. vails. Regular applications of fungicide portion of the needle around the point The risk of reinfection is increased by in the first year may prevent sporula- of infection, and most killed needles regular irrigation, which facilitates tion and subsequent proliferation of are quickly cast. spore transfer and disease intensifica- the disease in the second year (10,2). tion (12). Though not normally life threatening, needle cast caused by 1 All three are registered in Canada for field M. laricis is the single most important and greenhouse ornamentals, but not for use against M. laricis in forest situations.

3 Some cultural control of M. laricis References 7. Peace, C.R.; Holmes, C.H. 1933. can be obtained by transplanting stock Meria laricis, the leaf cast of larch. at the end of the first year, to beds in a 1. Cohen, L.I. 1967. The pathology of Oxford at the Clarendon Press. distant part of the nursery, and culti- Hypodermella laricis on larch, 8. Radley, R. 1995. Larch needle vating the old beds to bury infected Larix occidentalis. Am. J. Bot. 54: cast. British Columbia Ministry of needles (2). The most effective control 118Ð154. Forests, Extension Service, Green for both diseases in forest nursery sit- 2. Cooley, S. 1981. Meria laricis: Timbers Nursery. Proj. uations, however, is obtained by grow- Fungicide control and outplanting No. 3 01 167. ing larch seedlings outside their natu- survival of infected seedlings. 9. Rehfeldt, G.E. 1991. Breeding ral range, where the disease does not U.S.D.A. Forest Service, Portland strategies for Larix occidentalis: occur. Oregon. adaptations to the biotic and abio- In studies designed to determine 3. Cooley, S. 1984. Meria laricis on tic environments in relation to the effects of improved growth and nursery seedlings of western larch improving growth. Can. J. For. growth rhythm (the synchronization of in Washington. Plant Dis. 69(9): Res. 22: 5Ð13. growth with climate), genetically 826. 10. Schonhar, S. 1958. Control of enhanced shoot elongation correlated 4. Dubreuil, S.H. 1982. Western larch larch needle cast caused by Meria with increased resistance to M. laricis needle blight and needle cast in laricis. Empirical report No.6 from infection (9). Selective breeding, the Northern Region. U.S.D.A. the Forest Experimental Station, therefore, could increase the disease Forest Service. Northern Region Stuttgart, Germany. resistance of nursery stock while Forest Pest Management. 11. Sinclair, W.A.; Lyon, H.H.; improving growth. 5. Lephant, C.D; Denton, R.E. 1961. Johnson, W.T. 1987. Diseases of Needle discolorations of western trees and shrubs. Cornell larch. U.S.D.A. Forest Service. University Press. Forest Pest Leaflet #21. 12. Sutherland, J.R. 1995. Diseases 6. Maruyama, P.J. 1984. A new host and insects in British Columbia for- distribution record of a larch nee- est nurseries. Addendum to FRDA dle blight, Meria laricis. Can. Plant report #065. Canadian Forest Dis. Surv. 64: 1Ð19. Service, Pacific Forestry Centre, Victoria.

Additional Information

Additional copies of this and other leaflets in this Forest Pest Leaflets series, as well as additional scientific details and information about identification services, are available by writing to:

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© Her Majesty the Queen in Right of Canada, 1996

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