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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.
For more information on replanting fruit trees, download a copy of the 2015 publication Steps to Success in Replanting.
Prevention of replant problems is much easier and more successful than trying to control them after replanting. There is 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.
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.
Specific testing for replant disease can help to identify appropriate treatments for your planting site. The replant bioassay test assesses the potential for the development of replant problems and the response to soil treatments, such as soil fumigation and phosphorous application. The growth of apple seedlings in treated and untreated orchard soil is used as an indicator of the response to be expected in the field. Contact Danielle Hirkala at the BC Tree Fruits Lab (250) 826-5061 or firstname.lastname@example.org or packinghouse field service for more information.
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.
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.
Research in the greenhouse using potted apple seedlings has shown that growth can be significantly increased by phosphate fertilizer (11-52-0 or 10-52-10) in 80 per cent of the soils.
Ammonium phosphate fertilizer (11-52-0 or 10-52-10) at a rate of 1.0 g per litre of soil should be thoroughly mixed with soil before planting (one level measuring cup of 11-52-0 or 10-52-10 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.
Soil fumigation with chemicals such as Vapam and Basamid may be used as a preventative treatment for replant disease or as a corrective treatment for high levels of pathogenic nematodes before planting. They will also help to reduce the levels of soil-borne plant pathogens and viable weed seeds.
New regulations for soil fumigants came into effect in September 2014. These include the requirement for a detailed fumigation management plan, following good agricultural practices, implementing buffer zones and an emergency management plan. All soil fumigants are now restricted products and require certification of all fumigant handlers and applicators. Read and understand the entire product label before using. Labels contain detailed information on the new requirements for use, including what is needed in the fumigation management plan.
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.
Vapam or Enfuse or Busan 1020 or Busan 1236 (metam sodium) is a liquid that can be injected into soil or applied in water to the soil surface for small areas. It is converted into a fumigant gas in moist soil.
Application of fumigant:
Fumigants can only be handled and applied by certified applicators and can only be applied after making a detailed fumigant management plan. Follow all instructions on the label for application, safety precautions, personal protective equipment, buffer zones, etc. Immediately following application, the soil surface should be sealed by watering and rolling or with polyethylene sheets.
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 consult your field advisor.
Soil solarization is a non-chemical technique that will help to reduce the population of nematodes and other soil-borne pathogens. 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. It is usually necessary to take the land to be solarized out of production for a year. Soil should be tilled before solarization, and should also have a good soil moisture level. The area to be treated should be 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 until replanting to prevent re-contamination. Black plastic is less effective than clear plastic.
Other cultural management techniques include “biofumigation” via the incorporation of manure slurries or Brassica green manure cover crops. These practices are not as effective as chemical fumigation, but they can have the additional benefit of enhancing soil organic matter and fertility. Preplant incorporation of composts does not reduce nematode populations but enhances soil organic matter and fertility and may promote root growth enough to compensate for the effects of parasitic nematodes. Depending on which species of nematodes are present, it may also be possible to minimize their populations by rotating to non-host crops for a year before replanting. Rotation to a non-host crop will not guarantee a reduction in nematodes, as populations may be sustained on weeds.
Updated February, 2017