Paul W. Steiner, Professor and Extension Fruit Pathologist
Department of Natural Resource Sciences and L.A.
University of Maryland, College Park, MD

(Presented at the Maryland/Delaware Peach School, Wye Research and Education Center, Queenstown MD, February 10, 1998)

BACKGROUND

Old orchard sites and most small fruit plantings generally harbor a variety of soil-borne pathogenic fungi and nematode species at levels that can contribute to the early decline of new plants set on these sites if chemical fumigation is not used. While soil fumigation has long been recommended in Maryland, it has not been widely used for a variety of reasons:

• The additional cost of between $750 to $1,500 per acre for fumigation can be prohibitive, especially where custom applicators may insist on a minimum of four to five acres to justify their set up costs for the job.

• The number of chemical fumigants still available for use is decreasing. Of the three primary materials still registered -- methyl bromide\chloropicrin [Dow-Fume MC-2 and MC-33], metam sodium [Vapam], and 1,3-dichloropropene (Telone II) -- methyl bromide is scheduled to be withdrawn from most agricultural uses by the year 2,001 and metam sodium is not generally effective for deeply rooted trees and vines.

• The often rocky soils in western Maryland and parts of central Maryland make deep placement of most injected fumigants difficult and this can reduce the effectiveness of these materials.

• There is normally only a very narrow window of time in this area -- usually early fall -- when soil temperatures and moisture levels are ideal for effective fumigant activity.

Aside from all of the above mentioned "problems" with soil fumigation, there remains one overriding fact which is that few growers want to take good orchard sites out of production for any length of time for any reason. Remember, however, that the damaging populations of soil-borne pathogens present in old orchard soils developed over a period of 20 or more years and, during that time, the soil profile remained largely undisturbed.  While many older, well established trees might tolerate and continue to produce fruit under such conditions, this soil environment can be so hostile to new, bare rooted trees replanted too soon after the old trees are removed that many trees will decline early and, even when these are replaced, the full productive potential for the site may never be achieved. Given an estimate of between $5,000 and $7,500 per acre for trees and posts to establish a new, high density orchard it is imperative that this initial investment be protected by proper site preparation.

Simply leaving the land fallow for several years before replanting is not enough to return the site to its full productive potential is not a good option for many reasons. If broadleaf weeds are not excluded during the fallow period, the site many continue to harbor the tomato ringspot virus. Most grain, corn and forage crops that might be grown in the interim between orchards do little in the way of reducing many plant parasitic nematode species to less than threshold levels for new plantings. Old orchard soils also often have persistent residues of herbicides, are low in organic matter, may have problems with soil compaction and internal drainage and may be quite acidic in the lower rooting zone. In short, specific measures are needed to ameliorate such damaging effects as well as the time to implement these measures in a planned program. All of the elements of the pre-plant site conditioning program outlined here are based on research in the mid-Atlantic region over the last decade. What is new is that these elements have now been combined into a cohesive two-year program aimed at establishing a soil ecosystem that will support the long term productivity needed in fruit orchards.

Four years before replanting

• Stop all use of persistent herbicides such as simazine in orchards that are to be replanted.

• Map the orchard site in detail, noting the presence, age and relative health of all remaining trees on the existing site(s). A simple procedure using quadrille ruled paper (1/4-inch squares) and marking each tree location as: O = healthy tree; W = weak tree or one with 2 or more of tree structure missing; X = tree dead or dying; -- = tree missing.

• Based on your experience with the overall productivity of the orchards and depending on the size of the orchard or number of orchard blocks being considered for replacement, use the above map(s) to rank the various blocks or parts of a large orchard in three categories: 1 = least productive; 2 = average production history; 3 = most productive. Note that "most productive" is a relative time referring here only to those orchards being considered for replacement because their overall productivity is less than other orchards in the vicinity measured in expected bushels per year.

• For small orchard blocks, the above steps may be unnecessary. Where multiple blocks of different orchards or large orchards are being considered, however, the map-and-rank process helps set a priority on which sections need to be renovated first, second and third in a three year planned program so that costs can be distributed more evenly and less total acreage is taken out of production for the two-year renovation period.

Fall, Three Years Before Planting

• Remove old trees and roots. Rip soil thoroughly to expose additional roots and large rocks for removal. Submit soil samples from top 18 inches for pH and basic fertility determinations.

• Apply lime to adjust soil pH to 6.5 and incorporate by deep plowing. If more than 1,500 pound of total oxides per acre are required, apply half before plowing and incorporate the remaining half after plowing by discing.

• Plant barley, oats or rye as cover crop to reduce winter erosion.

Two Years Before Planting

Mid-April to Early May

• Broadcast 50 pounds of actual nitrogen per acre along with the required amounts of phosphorus and potassium needed for forage crops based on soil test results and incorporate these materials as winter cover crop is plowed or disced under.

• Plant Sudex (sorghum x sudan grass hybrid variety of Sorghum bicolor) at 20-25 pounds of seed per acre. Note: Sudex is the crop of choice because it produces a large amount of biomass quickly and the roots will penetrate four to six feet deep. This additional organic matter should also help reduce the availability of toxic herbicide residues in the previous orchard soil.

Mid-July through Late-August

• Mow down Sudex in mid- to late-July and add an additional 75-100 pounds per acre of ammonium sulfate to support regrowth of the Sudex crop and to begin the nutritional plan for the following rapeseed crop. In mid-August, an additional mowing with a flail mower may be necessary to reduce the bulk of plant residue before plowing it down thoroughly.

• Incorporate 50-75 pounds of ammonium sulfate per acre by discing. Note: The additional sulfur added during this season may acidify the soil slightly, but the additional availability of sulfur should increase the amount of toxic materials produced in the following rapeseed crop.

• In late-August, approximately two weeks after plowing down the Sudex plant residues, plant rapeseed (var. >Dwarf Essex=) at 8 to 10 pounds of seed per acre. Note: In addition to adding more organic matter to the soil, rapeseed produces chemicals that are toxic to plant-parasitic nematodes. Tests show two successive rapeseed crops are equivalent in nematode control to a Telone II soil fumigation treatment.

One Year Before Planting

Mid- to Late-April

• Mow rapeseed using a flail mower and plow down the residue immediately. Never mow down more area than can be plowed under within two hours. Note: Mowing injures the plants and initiates a process releasing nematicidal chemicals into the soil. Failure to incorporate mowed plant material into the soil quickly, allows much of these available toxicants to escape by volatilization.

• Two weeks after plowing down the first rapeseed crop, broadcast 50-75 pounds of ammonium sulfate and plant a second crop of >Dwarf Essex= rapeseed at 8 to 10 pounds of seed per acre.

August-September

• Collect and submit soil samples in early August for pH and basic fertility levels so that results can be available by early September)

• In mid-August, mow down the second rapeseed crop and plow down the residue immediately as done previously.

• In early September, approximately two weeks after plowing down the second rapeseed crop, broadcast any lime needed to readjust the soil pH to 6.5 along with 15 to 20 pounds of actual nitrogen (do not use ammonium sulfate) per acre along with other nutrients needed for fruit crop production and plow or disc these materials in deeply.

• Plant 20 pounds of certified Kentucky-31 tall fescue seed and 10 pounds of winter oats per acre. Note: Use only certified Kentucky-31 seed for uniformity and maximum performance and then only seed lots that are "endophyte infested". Bargain seed lost marked K-31 often are not true to variety and endophyte free seed may not suppress nematode populations as intended.

Spring, Year of Planting

• Two weeks prior to planting trees, apply glyphosate (Roundup) herbicide as a directed spray to kill the K-31 sod cover in four foot wide strips marking the planting rows. Where possible locate the new tree rows in the row spaces from the previous orchard.

• Leave the killed sod in place and plant trees through the sod with a tree planter where possible or a suitable auger if necessary. Note: Killed sod does not compete with the new trees, traps more rain than bare ground and reduces soil loss through erosion. In addition, by not disturbing the soil, fewer weed seeds are exposed for germination.