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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.
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Water Molds
The water mold fungi, species of Pythiumand Phytophthora,
are commonly occurring pathogens that cause serious root diseases in many plants.
Although both of these fungal genera have been consistently linked to root
rot of ornamental container stock, only Pythium is a serious pest of
forest tree container seedlings. Peterson (1974) predicted that
phytophthora root rot will not be a serious pest of container tree seedlings
because most nurseries use well-drained growing media.
Hosts. All seedlings are susceptible to root rots
caused by water molds.
Symptoms/Signs. Water mold fungi cause wilt
symptoms, followed by chlorosis and stunting. Pythium-infected roots
are black and water-soaked and are often hollow and collapsed (Nelson 1978).
Because symptoms develop from the root tips, container seedlings
affected with pythium root rot often have a root system with few lateral roots (fig. 1).
Phytophthora root rot is characterized by a distinctive reddish brown discoloration of the
cambial region of the infected root; in some hardwood species, the stain is blue-black or
inky colored (Kuhlman and Smith 1975).
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| Figure 1. |
Disease development. Water molds are so named because they have
motile spores that swim in water and therefore thrive in damp soils. Unlike many other
fungi, they have no airborne spore stage. Both Pythium and Phytophthora are
favored by wet, poorly drained media and cool temperatures. They are able to withstand
periods of drying by forming thick-walled resting spores (Baker 1957).
Disease Management. These root rots are more easily
prevented than controlled. Although water molds can be seedborne, they are
most often introduced in contaminated irrigation water or growing media.
Growers, therefore, should check their water sources and media.
Irrigation water can be tested for Pythium and Phytophthora by a "baiting"
procedure in which baits of unripe fruit (apples or pears) are suspended near the
water surface. These baits attract motile zoospores, which penetrate the
fruit and can be subsequently isolated and identified on a selective medium
(McIntosh 1966). Water molds thrive in wet conditions, and so growing media
should be formulated to provide good aeration and drainage.
Fungicidal
drenches can be used to control water molds, but many of these chemicals are only
fungistatic, merely stopping the spread of the disease, not eradicating it.
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Waterlogged media. One of the drawbacks of using
containers is that the natural drainage patterns found in filed soils are not
present. Containers develop a perched water table that creates a layer of
saturated medium at the bottom of the container.
The peat-vermiculite media
used in most container tree nurseries are particularly liable to compaction,
which can further aggravate the problem. Waterlogged media reduce the
necessary gas exchange between the roots and the atmosphere and can lead to
oxygen deficiency.
A growing medium that has been overwatered will often develop
excessive growth of mosses and algae and may smell sour when removed from the
container. Cauliflower-shaped growths may be present on the roots (fig. 2);
these hypertrophic structures are swollen lenticels that develop in response
to low levels of soil oxygen (Boyce 1961). Lieffers and Rothwell (1986)
report that black spruce seedlings grown in waterlogged media produced a large
number of swollen lenticels, which they consider an adaptive response to
saturated conditions. Tamarack seedlings grown under the same conditions
did not develop the swollen lenticels, which may indicate that some species have
a higher tolerance to waterlogged conditions than others
(Tripepi and Mitchell
1984). Another common symptom of waterlogging injury is dark, swollen roots
that feel soft and spongy (fig. 3); these roots are often infected with
pathogenic fungi.
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| Figure 2. |
Figure 3. |
Contents
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