Ministry of Agriculture
Updated February 2010
Replant disease is the term used to describe the commonly observed poor growth and delayed cropping of apple trees planted in old orchard sites. Other tree fruit may also be affected. Replant disease is not a specific disease caused by a single agent. It can be caused by certain pathogenic fungi, nematodes, or soil factors such as improper pH, moisture stress or insufficient available phosphorous. Any of these factors are capable of affecting early tree growth alone, but research has shown that multiple factors are often involved. Overcoming replant disease is critical for the successful establishment of high density orchards. Early cropping and high productivity are essential to recover the high cost of replanting.
Prevention and Control
Prevention of replant problems is much easier and more successful than trying to control them after replanting. There is very little that can be done to correct replant problems once trees have been planted.
The causes of apple replant disease on different sites are highly variable. Not all soils respond in the same way to the various pre-plant treatments. Thus a treatment that is beneficial in one orchard may have no effect in another. Preliminary soil testing can help to identify some potential causes of poor replant performance and is highly encouraged.
1. Soil Testing Before Planting
Preparation for replanting orchards should start at least one year ahead of planting. Soil analysis is necessary to determine fertilizer requirements, and also if lime is required to adjust pH prior to planting. It is recommended that soil be tested for pathogenic nematodes the year before replanting.
2. Cultural Practices
Special attention to all cultural practices is important to obtaining good growth of young trees. Irrigation and mineral spray requirements are essential, as is good weed management. Trees must also be handled carefully and planted as early as possible.
3. Soil Replacement or amendment
- Soil replacement with 20 L or more of new soil, or a well prepared steamed planting soil mixture, can be satisfactory alternatives to fumigation for small sites. New top soil should be tested for pH and salinity before use. It should not come from old apple plantings, and should not contain residual herbicides.
- Soil replacement or amendment with a ratio of 1 part peat to 2 parts planting-hole soil can also be a beneficial treatment.
- Compost, which is typically a rich source of phosphorus and beneficial microbes, can be a very effective preplant soil amendment. Composts vary in their properties, and some can be high in salts (electrical conductivity). Consequently, it is important to obtain an analysis of any compost and adjust amendment rates to ensure that the resulting level of salts in the soil is not too high.
4. Preparation of Site for Chemical or Fertilizer Treatments
In preparing old orchard sites for replanting, remove as many old roots as possible. The area to be treated should be cultivated thoroughly before applying the soil chemical or fertilizer treatments.
5. Fertilizer Treatments
Research in the greenhouse using potted apple seedlings has shown that growth can be significantly increased by phosphate fertilizer (11-55-0) in 80 per cent of the soils.
Ammonium phosphate fertilizer (11-55-0 or 11-51-0) at a rate of 1.0 g per litre of soil should be thoroughly mixed with soil before planting (one level measuring cup of 11-55-0 or 11-51-0 weighs 240g). If soil is very coarse, this rate may be reduced slightly. Great care must be taken to avoid fertilizer concentrations close to the roots or burning and death may result.
Fertigation is an alternative method of applying phosphate to the root zone with less risk of root injury. When using fertigation, lower amounts of phosphate can be applied to achieve similar improvements in tree vigour and phosphate nutrition.
6. Soil Fumigation
Chemical soil fumigation may be used as a preventative treatment for replant disease or as a corrective treatment for high levels of pathogenic nematodes before planting. Fumigation will also kill germinating weed seeds. Metam-sodium (Vapam, Busan 1020, Busan 1236) or dazomet (Basamid Granular) may be used for control of nematodes and soil-borne diseases in orchard soils prior to planting. Plan ahead for fumigation for best results.
Timing of fumigation:
- After harvest is the most effective time to fumigate. It must be done before soil cools off too much in the fall, and before water is turned off. Consult product labels for information on optimal soil temperatures.
- Spring fumigation can also be effective if soil is allowed to warm up. This may cause unacceptable delays in planting. Allow sufficient time (usually several weeks) after fumigation for the chemical to dissipate before planting, or new trees will be injured or killed.
- If planting can be delayed for a year, this will provide more time to prepare the land and more optimal conditions for fumigation while soil is warm and water is available.
Preparation of the land:
- Prepare the land by removing old trees, stumps and roots. Work and level the soil for best results.
- Fumigant must be applied to moist soil to be effective. Ensure soil is moist to the depth you plan to treat for at least a week before fumigating. Irrigate if necessary.
Selection of fumigant:
- Basamid (dazomet) is a granular that must be tilled into the soil. Soil must be in seedbed condition. Fumigant is released when the granules contact moisture in the soil. Soil should be sealed after treatment with a roller or polyethylene sheets.
- Vapam (metam sodium) is a liquid that can be injected into soil or applied in water to the soil surface. It is converted into a fumigant gas in moist soil. Vapam is more easily used on difficult or rocky sites. It can be applied with a weed sprayer with irrigation running to help move the fumigant into the soil.
Application of fumigant:
- Follow label rates, instructions and safety precautions. Full protective gear including a respirator is required.
- Consult the OVTFA publication Soil Fumigation for Orchards: an Overview (pdf 1.6 mb) for detailed information on fumigation, including calibration of sprayers.
- Be careful not to mix soil from unfumigated areas into fumigated areas.
- Wait the recommended amount of time before planting. You may need to aerate soil to allow any trapped fumigant to be released. Do not work the soil deeper than fumigant was applied.
For more information on fumigation, see the OVTFA publication Soil Fumigation for Orchards: an Overview (pdf 1.6 mb), or consult your field advisor.
7. Alternatives to Fumigation
Soil solarization can be an effective, organic-friendly alternative to fumigation, but requires taking the land out of production for a year. Soil solarization is a non-chemical technique that will control many soil-borne pathogens and pests, including nematodes, root and foliar diseases and some weeds. Solarization involves capturing the heat of the sun by covering the soil with transparent polyethylene plastic sheets during warm sunny months. The soil temperatures under the plastic increase to levels lethal to many soil-borne plant pathogens, weed seeds, seedlings, and nematodes. Soil should be tilled before solarization, and should also have a good soil moisture level. The area to be treated should be level and free of weeds, plant debris, and large clods which would raise the plastic off the ground. Cover the area with a double layer of clear polyethylene sheet, seal the edges with soil and leave it in place for 4-6 weeks during the heat of the summer (mid-June through mid-August). If possible, leave the poly in place over the winter to prevent re-contamination. Black plastic is less effective than clear plastic.
Also see: Nematodes