Damping-off is a general term applied to the rotting of seeds, germinants, and succulent seedlings. There are two stages: pre-emergence damping-off kills seeds and germinants before they emerge, post-emergence damping-off affects young seedlings after emergence. Both diseases occur in bareroot and container nurseries throughout the province. In containers, damping off pathogens are often introduced as seed-borne inoculum, or via contaminated water or growing media. In bareroot nurseries, losses are mainly caused by soil-borne fungi, especially Fusarium and Pythium.
Losses in local bareroot nurseries are usually less severe than elsewhere in North America, probably because of the absence or scarcity of well-known damping-off fungi such as Rhizoctonia, Phytophthora, and Cylindrocladium. The presence of antagonistic micro-organisms in bareroot nurseries in British Columbia may also account for the relatively low damping-off losses. However, some losses do occur each year in all nurseries throughout the province and they are occasionally severe in bareroot fields with heavier soils and long histories of nursery or agricultural use. Part of the germination failures usually attributed to poor seed viability in bareroot and container nurseries may be caused by seed-borne fungi (see next chapter).
Seeds and seedlings of all locally grown species suffer from both pre- and post-emergence damping-off. Failure of germinants to emerge is the only above-ground evidence of pre-emergence damping-off; indeed, damage may go unnoticed unless it is severe or confined to patches. Both affected seeds and germinants that have not yet emerged are rotted. Cool, wet, or compacted soil or growing medium all reduce germination and emergence rates and increase losses from this disease. High humidity, dense sowing in bareroot seedbeds, and multiple sowing in container cavities also favor the disease.
The main symptoms of post-emergence damping-off (Figure 10) are rotting of the stem slightly above and below groundline, and subsequent toppling of the shoot. Dead shoots sometimes dry out and blow away. If the seedling has been attacked during emergence, only the hypocotyl crook will be visible and the germinant will not develop further. Seedlings are susceptible to this disease until their stems become woody, usually 4-6 weeks following emergence. Ordinarily, only random seedlings in bareroot drill rows (Figure 11) are affected but, under ideal conditions, the pathogen may spread and kill small patches of seedlings. Post-emergence damping-off in containers, although uncommon, often occurs in patches resulting from water-spread inoculum.
Most damping-off fungi are unspecialized pathogens with similar life histories. Frequently, the same pathogen can be isolated from several seedling species affected by either pre- or post-emergence damping-off. The pathogen overwinters and survives other adverse periods as thick-walled spores, usually in small root pieces and other organic matter. In spring, in proximity to seeds or growing roots, they germinate and infect. Several types of sexual and asexual spores may be produced, and serve in disease spread while the seedlings are susceptible. Most damping-off fungi have only limited ability to grow through soil. As mentioned earlier, some pre-emergence damping-off fungi may be seed-borne.
Management is better based on prevention rather than on cure. The first and best prevention for bareroot nurseries is to select a site with a light soil because disease is invariably less prevalent and control, if required, is easier. Rapid germination, which reduces pre-emergence damping-off losses because the germinants escape the pathogen, may be achieved by: (i) spring sowing stratified seeds, (ii) covering seeds with non-compacting sand or grit, and (iii) sowing when soil or growing medium temperature and moisture are optimum for germination.
Some practices place the pathogen at a nutritional or survival disadvantage (e.g., maintaining or adjusting pH (by adding lime or sulfur) to between 4.5 and 6.0, thus hindering growth of some pathogens but not of seedlings; and bare fallowing between bareroot crops, which promotes depletion of pathogen food bases). Addition of nitrogenous fertilizers to young germinants promotes succulent growth and increases susceptibility, but adequate soil or growing medium fertility levels - especially of phosphorus, potassium, and calcium - promotes development of woody tissues resistant to post-emergence damping-off.
Overcrowding of seedlings, high humidity, and poor air-circulation or drainage all favor post-emergence damping-off; thus, regulation of sowing densities and watering alleviates disease development and spread. Increasing sowing densities to compensate for anticipated damping-off losses is not recommended.
In the past, pre-plant pelleting or dusting of seeds with a fungicide was used for pre-emergence damping-off control in bareroot nurseries. This practice is seldom recommended today because when disease incidence is low or moderate, as in most British Columbia nurseries, the fungicide's phytotoxic effects frequently exceed the losses from damping-off. Fungicide treatment of seeds may be ineffective because: (i) the active spectrum of most fungicides is too narrow to be effective against the numerous kinds and strains of damping-off fungi, (ii) resistant fungus populations develop as the pesticide eliminates the more susceptible strains, and (iii) the fungicide is soon leached off the seed. Thus, if conditions favoring damping-off are prolonged, protection against attack is too brief.
The harmful effects of seed-applied fungicides are often more prominent in container than in bareroot nurseries because the biological and chemical "buffering capacity" of container growing media is very low. Their use on container-sown seeds is usually not recommended because they can sometimes be phytotoxic. Such damage can be confused with post-emergence damping-off.
Fungicide drenches are sometimes applied to control pre- and post-emergence damping-off. For reasons mentioned above, they are seldom effective and are expensive to apply. Soil fumigation is a standard practice in many North American bareroot nurseries, but because of its expense and harmful effects on mycorrhizal fungi, it probably can be justified only in bareroot nurseries with multiple pest problems (e.g., damping-off, insects, and weeds). It is more practical to follow a fallow rotation, choose a better nursery site, or grow seedlings in containers.
Bloomberg, W.J. 1971. Diseases of Douglas-fir seedlings caused by Fusarium oxysporum. Phytopathology 61: 467-470.
Lock, W., J.R. Sutherland, and L.J. Sluggett. 1975. Fungicide treatment of seeds for damping-off control in British Columbia forest nurseries. Tree Planters' Notes 26(3): 16-18, 28.
Salisbury, P.J. 1954. A review of damping-off of Douglas-fir seedlings in British Columbia. For. Chron. 30: 407-410.
|
Other Fungi |
Insects |
Environmental |
|---|---|---|
|
Many fungi responsible |
Cutworms |
Heat |
|
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 |
1+0 |
Late Spring and early summer |
Yes |
Yes |
Yes |
Yes |
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Figure 10. Symptoms of post-emergence damping-off. Note the rotted stem at groundline and compare with sunscald damage on the two germinants at right.
Figure 11. Post-emergence damping-off of yellow pine. Note random distribution of diseased seedlings.
Figure 12. Life history of pre- and post-emmergence damping-off.