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Apple (Malus spp.) - Replant Disease
Cause A complex of fungi, oomycetes, and nematodes are the biological factors contributing to replant disease. Although more research has been done on apple replant disease, pear replant disease is considered to be very similar in its overall epidemiology. In addition, nonbiological factors including poor soil structure, moisture stress, low or high pH, insufficient available phosphorus, and cold stress while not primary causes of the disease, may exacerbate overall symptom development. Sometimes different causal factors are reported in different geographical areas. This is a serious, common cause of poor growth of trees established on sites previously planted to apple or pear orchards or nurseries. Tree growth is suppressed the first year and for the life of the orchard and on occasion of severe disease pressure tree death may occur. Compared to healthy trees, yields are significantly reduced and fruit quality is lowered.
Soil analysis is recommended to detect nutrient deficiencies and to determine whether lime is required to adjust pH. There is no effective treatment of replant disease once trees are planted.
Mixing monoammonium phosphate fertilizer (11-55-0 or 11-51-0) with the planting hole soil at a rate of 1 g/liter of soil is no longer recommended since the high salt concentration can burn roots. Although there is an initial growth increase, the practice has no overall benefit after 5 years.
Symptoms Replant disease has no definite symptoms other than trees' poor growth the first few years after transplanting. Vigorous young trees planted in a problem site stop growing in early summer. Affected trees leaf out each spring but produce little or no shoot growth. Leaves are often smaller and lighter green than leaves on vigorous trees. Few new lateral or feeder roots are produced, and existing roots become discolored and deteriorate.
Cultural control
- Avoid by not planting apples or pears on the same ground where an apple or pear orchard or nursery has recently been removed. Rotations out of pome fruit for 5 to 8 years are advised.
- Adjust soil pH if too high or low.
- If planting on old orchard ground, establish trees in the drive row (aisle) of the previous orchard rather than the old tree row to minimize disease pressure. Field observations indicate tree growth response to this aisle-planting method is better than planting in the old row but not as good as fumigation. Plant as early as possible in the spring taking care not to skip important preplant operations.
- Use 1.78 tons/treated acre of equal parts yellow mustard (Brassica juncea) and white mustard (Sinapis alba) seed meal as a preplant soil amendment in the fall prior to planting in combination with a postplant application of mefenoxam may yield effective control of this disease complex.
- Provide proper management practices from plant nutrition to irrigation.
Chemical control Preplant fumigation is very effective and, although expensive, will eventually pay for itself during a typical orchard lifetime. Consult or hire a professional applicator to be sure it is done safely and effectively.
- Metam-sodium products such as Metam CLR (42%), Sectagon 42, or Vapam HL. Vapam HL can be used at 56 to 75 gal/A. Use in enough water to penetrate only 3 ft. Remove as much tree root debris as possible before application. Trees adjacent to the treatment site may be injured. 5-day reentry. Restricted-use pesticide.
- Telone C-17 at 32.4 to 42 gal/A depending on soil type and depth of penetration. Do not use Telone alone; it has not been effective in eastern Washington. 5-day reentry. Restricted-use pesticide.
Biological control Soil replacement with new soil or well-prepared, steamed planting soil mixture, or soil replacement with a ratio of 1 part peat to 2 parts planting-hole soil. Data indicate that a hole 6 ft square or more and 3 ft deep gives best long term control.
Reference Wang, L., and Mazzola, M. 2019. Field evaluation of reduced rate Brassicaceae seed meal amendment and rootstock genotype on the microbiome and control of apple replant disease. Phytopathology, 109:1378-1391.