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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.

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).

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.

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.

Figure 2. Figure 3.

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