Meria needle cast, caused by the fungus Meria laricis Vuill. (Fungi Imperfecti), is the most serious disease of container-grown western larch, Larix occidmtalis Nutt., in British Columbia. Where grown, other species of Larix, both in and outside nurseries, are also affected and the fungus has been reported on seed-orchard Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, trees at Duncan, British Columbia. Locally, the disease affects western larch in both bareroot (Figure 118) and container (Figure 119 ) nurseries, but damage is mainly confined to 1+0, container-grown seedlings, especially in the Interior of the province where most larch seedlings are grown. To date the sole exception has been at Chilliwack where severe needle cast occurred on 1+1 larch transplants, which had been grown their first year in containers.
Meria needle cast affects many Larix species including western larch, alpine larch, L. lyallii Parl., and European larch, L. decidua @., but so far it has not been reported on tamarack, L. laricina (Du Roi) K. Koch. Only western larch has been affected in local nurseries, and there mainly in the Interior where symp- toms on 1-year-old, container-grown seedlings have been observed as early as the first week in July. The disease, which is favored by high moisture, intensifies throughout the summer and fall and can continue to develop until late December. Initial symptoms include yellowing and wilting of the infected part of the needle. As the disease progresses, the infected part of the needle becomes reddish brown and the discoloration spreads over the remainder of the needle. Initial symptoms occur on needles on the lower part of the seedling shoot. From there the disease advances upward. On severely diseased seedlings, needles on the lower two-thirds of the shoot can be affected (Figure 120). Affected needles droop, become dry and fall off, beginning at the base of the shoot and progressing upward as the disease develops (Figure 121 ). Sometimes diseased seedlings have stem lesions, which although they do not contain the pathogen, may result from fungus produced toxins. Since high moisture favors the disease, its development may cease during periods of low moisture resuming when moisture returns. Meria symptoms can develop very quickly and can, consequently, be confused with frost damage. However, Meria-affected needles have lesions whereas frost kills entire needles plus succulent stems. To date, no other foliage pathogens of locally grown larch seedlings, which could be confused with Meria needle cast, have been found. A remote exception is gray mould, caused by the fungus Botrytis cinerea. However, with gray mould, affected tissues are covered by a profusion of grayish-brown mould.
Depending upon the degree of defoliation, diseased seedlings may be culled, perhaps mostly for aesthetic reasons as the disease seldom affects either the growth of fast growing larch seedlings or kills them. Culling may also be justified where diseased needles remain attached to seedlings as they could provide inoculum for disease development on seedlings outplanted to reforestation sites. Them is no evidence supporting the speculation that Meria needle cast decreases seedling frost tolerance.
The life history of Meria needle cast for forest nursery seedlings has not been determined in detail. It can be assumed to be quite similar to that which occurs on forest trees, except that nursery conditions are ideal for the disease in that infection occurs through young, succulent needles, which am always present on fast growing nursery seedlings. Frequent irrigation of nursery seedlings provides an extremely favorable environment for the disease. These two factors alone must account for the rapid development of the disease on larch seedlings. Spores of the pathogen may be blown great distances or carried in rain or irrigation water. Therefore, in nurseries, initial inoculum is assumed to come from diseased larch trees, or perhaps other conifers such as Douglas-fir in and around the nursery.
Another source of spores is undoubtedly infected needles in debris from previous larch crops, or diseased seedlings of previous crops that have not been discarded.
Infection is assumed to originate from such primary inoculum and to infect larch needles in spring or early summer. Following primary infection, secondary inoculum from within the diseased crop almost certainly accounts for spread and intensification of the disease. Under ideal conditions, especially when moisture is abundant, disease severity can increase through to lifting in late fall or early winter
Since primary inoculum originates from diseased trees within and around the nursery, the potential for damage is greatest in bareroot and container nurseries with histories of Meria needle cast on larch. Undoubtedly larch is the prime source of inoculum, but Douglas-fir and perhaps other conifers can become infected. When feasible, such trees should be removed. Another source of inoculum is diseased needles from the previous years' larch crop.
In container nurseries, greenhouses and other growing areas should be thoroughly sanitized before sowing a new larch crop. In bareroot nurseries, plowing or cultivating in diseased needles should help alleviate the problem.
In bareroot nurseries it is helpful to transplant diseased larch, leaving behind diseased needles. In both container and bareroot culture where the disease has become established, it is important to reduce watering as much as possible as blight development is, closely linked to high moisture level, Because of the high moisture conditions associated with container nurseries, it is especially important, to prevent initial infection. Once the disease is epidemic, it is almost imperative that fungicides be used to control it.
Another threat to Meria-affected seedlings is the build-up of gray mould on the fallen needles
Garbutt, R.W 1984. Foliage diseases of western larch. Agric. Can., Can. For. Serv., Pacific For. Res. Cent., For. Pest Leafl. 71, Victoria, B.C.
Sinclair, W.A., H.H. Lyon, and W.T. Johnson. 1987. Diseases of trees and shrubs. Comstock Publishing Assoc., a division of Comell Univ. Press, Ithaca, N.Y. and London.
|
Principal, locally grown hosts |
Host age and season when damage appears |
|
Nursery type and location |
|
|
|
|---|---|---|---|---|---|---|
|
Bareroot |
Container |
|||||
|
Age |
Season |
Coastal |
Interior |
Coastal |
Interior |
|
|
Douglas-fir, all hard pines, all larches |
1+0 |
Early to mid-summer |
Yes |
Yes |
Yes |
Yes |
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Figure 118. Bareroot, western larch transplants affected by Meria needle cast.
Figure 119. Container-grown, 1+0 western larch affected by Meria needle cast.
Figure 120. Severe Meria needle cast on 1+0 container-grown western larch. note drooping and lack of decay of diseased needles.
Figure 121. Meria needle cast on 1+0, container-grown, western larch. Note how some needles have been shed as the result of the disease (at arrows).
Figure 122. Life history of Meria needle cast.