Moulding of seedlings in storage has become a major problem in British Columbia because of the growing length in storage periods. The need for longer storage has occurred primarily because the recent increase in seedling production has prevented the lifting and shipment of all trees precisely when they are needed for outplanting; and the greater demand for high elevation provenances often means storing the stock until snow melt is complete in late spring. Another contributing factor has been the increased production of container-grown stock, which may contain incipient gray mould that can develop further in storage. Moulding of stored seedlings is a concern because stored trees are the nurseryman's final product, and they are at their maximum pre-shipping value.
Moulding of bareroot seedlings affects all species, although pines apparently less so than others. Initial evidence (Figure 42) of the disease includes cottony, mould on the lower needles, especially on seedlings within the storage bundles. These symptoms gradually progress upward on the shoots. Mould may be most noticeable around string holding seedlings in bundles. As the disease moves upward, stems and needles become watery and decayed, and affected needles fall off. Sometimes, after prolonged storage, moulding appears at numerous points scattered over entire seedlings.
Other symptoms that may appear on all or part of the stem and branches include water-soaked lesions, bark that strips off easily, and dead, butterscotch-colored cambium. Bundles of diseased seedlings may emit a musty odor and small clouds of mould spores. Symptoms can appear any time after storage begins, but usually the amount and probability of damage occurring are proportional to the length of the storage period. Sometimes mycorrhizal fungi proliferate on roots of stored seedlings, but they should not be confused with storage moulds that rarely occur on roots.
The moulding of stored container-grown seedlings differs from that of stored bareroot seedlings in that only one or two fungi are involved (i.e., usually the gray mould pathogen Botrytis cinerea and sometimes Rosellinia). These fungi can become established during the growing season and continue to develop after the seedlings are stored. Hosts, symptoms, and damage of these diseases are described in separate chapters. Certain root fungi such as Cylindrocarpon and Fusarium, can be acquired by container-grown seedlings in the nursery and carried over into storage; see earlier sections on these diseases.
To date, in British Columbia, the amount of moulding damage on either bareroot or container-grown seedlings has not been correlated with subsequent outplanting survival. However, trees with advanced stem and branch decay and defoliation (Figure 43) probably survive poorly, and seedlings with low to moderate damage may be more affected by site, weather, and other factors than would healthy seedlings.
Fungi such as Fusarium, Rhizopus, Aspergillus, Penicillium, Epicoccum, Cylindrocarpon, and numerous non-sporulating forms, are common on mouldy bareroot stock. Ordinarily, they live on dead or dying organic matter and only become pathogenic under favorable conditions. These fungi are ubiquitous, thus inoculum is on foliage prior to storage, in soil particles deposited onto shoots during lifting, or in soil adhering to seedling roots. None of these fungi occurring on bareroot seedlings have any specialized spores for survival, reproduction, or inoculation related specifically to their role in moulding.
* Because of the variation in the life histories of the pathogens involved, a diagram is not included here.
Storage moulds of both container and bareroot-grown seedlings are better prevented than controlled. Some practices that help reduce losses include: (i) storing stock for the shortest period possible, (ii) periodically examining a sample of each stored seedlot, and (iii) immediately shipping stock with initial moulding. The latter should receive special care to prevent further moulding in transit and prior to outplanting.
Seedlots containing significant quantities of dead organic matter (e.g., frost-killed or fertilizer-burned foliage) should be monitored closely because moulding fungi become established on this material before moving to healthy tissues. Transplants are very prone to moulding, especially if they have been lifted and stored as 2+0 stock - not shipped, transplanted late, then lifted and re-stored as 2+1 transplants. Such seedlings are likely to be in a weakened condition and thus more susceptible to moulding.
There is some evidence that moulding risk decreases as seedling frost hardiness increases. The best control for storage mould on container seedlings is to prevent establishment of gray mould or Rosellinia blight on the stock prior to storage. Infested stock should be inspected frequently once it is stored. Storing seedlings at 1-2°C reduces damage by severely limiting growth of most storage moulds. Their growth can be completely stopped by dropping the storage temperature to -2 to -3°C. Storing stock at these temperatures provides excellent control for stock that can withstand frozen storage. To date, species originating from the Interior seem best suited to sub-freezing storage. Frozen stock is usually thawed gradually, thus it should be watched closely for mould development during thawing and also afterward during shipping and prior to outplanting.
Most attempts at fungicidal control of storage moulds have involved either systemic or protective fungicides, or both at various pre-storage intervals. However, the results, especially with the protectants, have been erratic. Removing trees from storage, dipping them in fungicide, and re-storing them is not recommended because it is too time-consuming, expensive, and potentially hazardous to nursery workers.
Hocking, D., and R.D. Nyland. 1971. Cold storage of coniferous seedlings: a review. State Univ. Coll. For., AFRI Res. Rep. 6, Syracuse, N.Y.
Hopkins, J.C. 1975. A review of moulding of forest nursery seedlings in cold storage. Environ. Can., Can. For. Serv., Pac. For. Res. Cent., Report BC-X-128. Victoria, B.C.
|
Other Fungi |
Insects |
Environmental |
|---|---|---|
|
Bacterial decay |
|
Freezing damage |
|
Principal, locally grown hosts |
Host age and season when damage appears |
|
Nursery type and location |
|
|
|
|---|---|---|---|---|---|---|
|
Bareroot |
Container |
|||||
|
Age |
Season |
Coastal |
Interior |
Coastal |
Interior |
|
|
All species, pines least affected |
1+0 |
When stored |
Yes |
Yes |
Yes |
Yes |
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Figure 42. Bareroot Douglas-fir affected by storage moulding.
Figure 43. Foliage showing advanced symptoms of moulding in storage.