Moving Toward Integrated Management of Root Diseases in
Northern Forest Nurseries

Principal Investigators: Jennifer Juzwik, research plant pathologist, USDA Forest Service, and Raymond Allmaras, research soil scientist, USDA–ARS, St. Paul, MN 55108.

Soil-borne diseases may result in significant mortality of nursery tree seedlings and negatively affect growth and quality of live seedlings remaining for lifting (i.e., harvesting) and shipping. Seedlings that have been morphologically (e.g., shoot height or root volume) or physiologically impaired in the nursery and that are shipped for out-planting may die or grow poorly during the first several years on the planting site, be it a reforestation area, a farm field conversion, or a conservation planting.

Based on a 1992 national survey of U.S. forest nursery managers, over 80 percent reportedly relied on methyl bromide for pre-plant soil fumigation in efforts to grow quality seedlings. Other soil fumigants (e.g., metam sodium and dazomet) have been used successfully by a small minority of nurseries; while even fewer nurseries produce stock without any soil fumigation.

Nursery cultural practices, especially those involving soil management, are important in controlling root diseases. Practices that influence the occurrence and severity of these diseases include soil tillage, soil water management, mulching, sowing of infested seed, fertilization, and soil fumigation. In the past fumigation has been the preferred pest management option and the primary means of controlling root rot in bare-root nurseries. Unfortunately the heavy reliance on a single "tool" that focuses on the disease organism(s) may lead nursery managers away from considering the soil conditions that actually predispose seedlings to infection by pathogens and are a result of cultural practices.

From 1994 to 1996 investigations were conducted in five northern nurseries with varying cultural regimes to: 1) document physical and biological soil conditions resulting from each nursery's practices and consider their relationship to root rot development, and 2) interpret operational fumigation practices in view of these conditions. The results of the studies suggest a number of non-chemical control actions that could be readily implemented in an integrated manner with or without soil fumigants to control root rot.

Resistance of nursery soils to penetration was measured at 15-mm increments from the soil surface to a 42 cm depth in fields with 2-year-old pine seedlings at three nurseries. The soils in these fields ranged from loamy sands to sand soils. Significant increases in the force required to insert the cone penetrometer at a controlled speed is indicative of compacted soil layers or hardpans. The resulting graph of the cone indices through the soil depth revealed a profile with two peaks of increased resistance in the Minnesota and the Wisconsin study nurseries, while no such peaks were found for the Michigan nursery studied. The more shallow hardpan (10- to 15- cm depth) was attributed to the use of rotary tillers in fields just prior to sowing of the pine crop but after sub-soiling had been performed. The second pan (30- to 36-cm depth) was attributed to moldboard plow use for incorporation of cover crops at the nurseries. This pan was partially mitigated by sub-soiling that occurred after plow use. The absence of pans in the Michigan nursery was attributed to the use of a disk for all soil-disturbing operations.

The rate of water movement through portions of the soil profile was also determined. Undisturbed soil cores (5-cm long × 5-cm dia) were removed and rate of water flow through cores was determined in the laboratory using saturated hydraulic conductivity methods. Reduced rates were associated with the tiller-compacted zone (e.g., 14.4 cm/hr, Wisconsin nursery) compared to the rates for cores from the same depth zone in non-compacted field areas (e.g., avg. 18.4 cm/hr, same nursery). Root disease occurrence in white pine seedlings, based on visual rating of seeding roots, was also higher in portions of fields with tiller pans.

The number of Fusarium species propagules per gram of dry soil in 6-cm depth increments from the soil surface to 42-cm depth was also determined for the same three nurseries. The resulting graphs of the propagule numbers versus increasing soil depth revealed similar Fusarium population profiles for the Minnesota and the Wisconsin nurseries, but a different profile for the Michigan nursery. Population peaks observed in the 0- to 6-inch zone in Minnesota and Wisconsin fields were attributable to recolonization and increase by the fungus that occurred after soil fumigation (metam sodium in the first nursery, methyl bromide in the second). The deeper peak (18- to 24-cm zone) was attributed to Fusarium that increased on carbon sources from incorporated cover crop material and "survived" soil fumigation. Moldboard plows place 90 percent of surface residue just above the maximum working depth of the implement (i.e., 18 to 24 cm in these two nurseries). Previous studies in two other Wisconsin nurseries have found methyl bromide fumigation to be only partially effective in the 15- to 25-cm depth zone. In the Michigan nursery where no fumigation occurred, the Fusarium populations were highest at the soil surface and decreased to negligible levels below 17 cm. This profile corresponds to the surface residue burial pattern characteristic of a disk.

In two additional nurseries, the effect of fumigation depth on vertical distribution of Fusarium in soil was determined. In a 1994 Wisconsin nursery trial, 560 kg/ha of dazomet was applied to the soil surface with a Gandy spreader and immediately incorporated into the sandy loam soil (moisture content at 60 percent field capacity) using either a rotary tiller with 22-cm- long, bent tines or a spading machine with six 13-cm-wide by 18-cm-long blades. The operational incorporation depth of the latter was known to be twice that of the rotary tiller used. The spading machine incorporation resulted in excellent and sustained reduction in Fusarium propagules at a 6- to 30-cm depth through the second growing season. In contrast, the rotary tiller incorporation of dazomet resulted in similar and sustained reduction in Fusarium only between a depth of 6- and 18- cm. Because cover crop material had been incoporated by moldboard plow in this field, a Fusarium peak between 18 and 24 cm was only partially affected by the more shallow incorporation with a rotary tiller compared to nearly total control when deeper fumigant incorporation was achieved.

In comparison, a second similar trial was conducted in another Michigan nursery where a disk was used for cover crop incorporation and surface residue was placed in the 0- to 18-cm zone. Pre-fumigation Fusarium populations were significant only between 0 and 15 cm, and negligible below. All three dazomet incorporation implements tested at this nursery (rotary tiller, spading machine, and disk) were equally effective in significantly reducing Fusarium in the 0- to 15-cm zone.

In summary, nurseries could use tillage to control depth placement of cover crop residue and subsequent buildup of fungal propagules, adjust tillage practices to prevent hardpan creation within the seedling rooting zone, and maintain optimal soil moisture conditions for seedling growth in their integrated management of root disease in pine crops. Consideration of tillage practices effects on residue placement can also be the basis for more effective and wise use of soil fumigation.

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Last Updated: April 22, 1998