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Diseases in Container Tree Nurseries

Excerpt from: Landis, T.T. 1989. Disease and pest management. Pp. 1-99. In T.D. Landis, R.W. Tinus, S.E. McDonald, and J.P. Barnett (eds). The Container Tree Nursery Manual. Volume 5. U.S. Department of Agriculture Agric. Handbook. 674.

Damping-off

Damping-off is a common disease that affects seeds, germinants, and  young seedlings of many plant species, and woody plants are no exception.  Traditionally, two different types of damping-off  are recognized: pre-emergence damping-off, which affects seeds and germinants before they emerge, and postemergence damping-off, which affects young seedlings until their stems become woody.   Both forms of  the disease occur in container nurseries and are caused by the same group of fungi (fig. 1). Damping-off was ranked second among the diseases affecting container seedlings, with a relative occurrence of 25% based on responses during the Container Nursery Survey.

Hosts.  All species of seed and seedlings are affected.

Symptoms/Damage.  Pre-emergence damping-off is a difficult disease to diagnose because the affected seeds are not visible; consequently, the losses are often attributed to "poor seed" (Baker 1957).  If the germinants have not emerged after a reasonable period, the seed should be excavated and examined; if the seed contents are decayed, then damping-off fungi may be involved.  Sometimes, the germinated seed is killed after the radicle of the seed has emerged.

The classic symptoms of postemergence damping-off (fig. 2, 3) include decay of the seedling hypocotyl at the ground line, causing the seedling to topple over.  Other stresses can produce symptoms similar to fungal damping-off , but the distinguishing characteristic is the presence of decayed root tissue.

Another germinant disease that is usually classed with postemergence damping-off is cotyledon blight.  This decay of the cotyledons develops when seedborne fungi spread from the seedcoat during the "birdcage" stage of germination (fig.4).

Disease development.  Pre-emergence damping-off is a fungal decay of seeds and young germinants and several different fungi can be involved.  Traditionally, Rhizoctonia has been considered to be the major cause of damping-off in ornamental container nurseries (Baker 1957); Peterson (1974) states that four fungal genera (Pythium, Fusarium, Phytophthora, and Rhizoctonia) "are being encountered" in container tree nurseries.  A search of the literature, however, reveals that only Fusarium spp.  have actually been implicated as a cause of damping-off of conifer seedlings in containers: Fusarium oxysporum (Schlect.) for Douglas-fir (Graham and Linderman 1983); Douglas-fir, western larch, grand fir, subalpine fir, and ponderosa pine (James 1985a); and Douglas-fir and pinyon container seedlings (Landis 1976).  Although F. oxysporum is most commonly mentioned, other members of the genus may also cause damping-off disease of container seedlings.  James (1985a) recovered F. avenaceum from diseased conifer seedlings, and Pawuk (1978) demonstrated that four seedborne species of Fusarium could produce damping-off of longleaf pine, that is, F. moniliforme, F. solani, F. roseum, and F. tricinctum.  Pythium spp. have also been isolated from diseased container seedlings, and these fungal pests may be more common than generally realized; Peterson (1974) concluded that Pythium-infected

The reason that the more traditional damping-off fungi are not being reported from container tree nurseries is probably due to the use of soilless growing media, which are generally considered to be pathogenfree.  Rhizoctonia, Pythium, and Phytophthora spp. are primarily spread through contaminated irrigation water or growing media, especially from mixes containing soil (Baker 1957).  McCain (1978) reports that peat is often infested with Pythium and Rhizoctonia spp., but Stephens and others (1983) could recover neither fungus from non-soil components of artificial growing media including peat, perlite, and vermiculite.  Coyier (1978) assayed perlite and peat moss samples for bacteria and fungi and found that, while perlite was sterile, the peat moss samples contained many species of pathogenic and nonpathogenic fungi and bacteria.  James (1987) has regularly isolated F. oxysporum from the roots of weeds growing in greenhouses.

McElroy (1985) has isolated both Pythium and Fusarium spp. from unopened bags of container medium; James and Gilligan (1984) reported very high populations of both pathogens from one brand of growing medium and demonstrated that the recovered Fusarium isolates could cause damping-off.  Kim and others (1975) made fungal isolations from 52 randomly selected samples of peat and found that all samples contained Fusarium spp., 29% contained Pythium spp., but no Rhizoctonia spp. were isolated from any of the peat-based media.

Apparently, seedborne fungi are a primary source of inoculum for damping-off fungi in container nurseries. Pawuk (1981) found that seedlots of four species of southern pines were infected with Fusarium spp. at intensities of 54 to 90% and the same author (Pawuk 1978) found that some Fusarium spp. isolates from longleaf pine seed caused both pre-emergence and postemergence damping-off.  Graham and Linderman (1983) reported that Fusarium oxysporum isolated from Douglas-fir seed caused significant pre-emergence damping-off losses.  Three species of pathogenic Fusarium were isolated from spruce seed and seed debris (James 1985b).  In an earlier study, James (1983) recovered Pythium and two Fusarium species from Douglas-fir seed; the Fusarium spp. were isolated from within the seedcoat.  Fusarium spp. have also been recovered from several species of pine seed in container nurseries in the western United States, including pinyon and ponderosa pine.

Because container seedlings are spatially isolated in individual cells, secondary spread of damping-off is not as severe a problem as it is in bareroot nurseries.   When more than one seed is sown per cell, fungal inoculum introduced on seedcoats can spread to adjacent seedlings and cause damping-off or other diseases, such as root rot, later in the growing season.  Graham and Linderman (1983) report that Fusarium spp. grew and sporulated on Douglas-fir seedcoats and considered this to be a potential source of secondary disease spread.  Fungal spores from infected seeds could be splashed from container to container during irrigation.

Other damping-off fungi have been isolated from growing medium on used containers (Baker 1957) and from dust and soil particles from the floors of container nurseries (Stephens and others 1983).  James (1987) has isolated Fusarium spp. from several different types of reusable containers, which had already been "sterilized" by conventional techniques.

Figure 1.
Figure 2.
Figure 3.
Figure 4.

Disease management.

Cultural. Damping-off  is actually not as common in container nurseries as it is in bareroot nurseries, and when it does occur, some environmental or cultural factor is usually involved (table 1).  The most important of these factors is seed quality: fungal contamination is more common in dirty seed lots, and poor quality seed produce weak germinants that are particularly susceptible to damping-off.   Reusable containers should be carefully cleaned to prevent fungal inoculum from carrying over to the next crop.  Contaminated growing media are a source of fungal inoculum, and fine-textured mixes often compact and provide an ideal environment for damping-off fungi.  High pH, either in the growing medium or irrigation water, can favor damping-off, but the low pH of most sphagnum peat should inhibit this fungal disease (Carlson 1983).  Oversowing leads to weak germinants that are more susceptible to disease.  Fertilization with high nitrogen levels and over-irrigation can also predispose seedlings to damping-off as can a growing environment of high humidity, low light,  and extremely high or low temperatures.

Chemical.  Because many of the fungi responsible for damping-off are seedborne, seeds can be sanitized prior to sowing.  Seed treatments include standing water soaks, running water rinses, and chemical treatments  with bleach, hydrogen peroxide, or fungicides.  If a contaminated growing medium is suspected, chemical fumigation or pasteurization can be attempted.   Fungicidal drenches can be applied after the disease symptoms become evident.  This practice is rarely curative: most of the damage has already been done by the time the chemical is applied.  However, drenches can stop secondary spread of the disease.  Sutherland and Van Eerden (1980) conclude that drenches are seldom effective against damping-off, besides being expensive and potentially hazardous to the environment.

Table 1.  Environmental conditions and cultural practices affecting damping-off in container tree nurseries

Environmental condition or cultural practice

Effect on Disease Development

Encouraging

Discouraging

Seed quality

Dirty or contaminated; slow, weak germinants

Clean and sterile; vigorous germination

Growing medium

Contaminated
Fine-textured
Over-compacted

Pest-free
Mixture of particle sizes
Good porosity

pH

High (>6.5)

Acid (4.5-6.0)

Growing density

Oversowing

One seedling per cavity

Nutrition

High nitrogen

Well-balanced fertilization especially phosphorus, potassium, and calcium

Irrigation

Frequent, heavy applications

Frequent, light applications

Growing environment

High humidity
Low light
Extreme temperatures

Moderate humidity
Adequate light
Ideal temperatures

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Last updated on Tuesday, July 09, 2002 at 01:02 PM
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