Apple trees that do not establish well or fail to establish when planted on a site previously grown with apples are often considered to be suffering from apple replant disorder. Although thought to occur in sites replanted after removing very old fruit trees, replant disorder has been documented to occur within three years of establishing an orchard on new ground.
The causes and symptoms of replant disorder vary from region to region and even from site to site. While not well understood, biological factors are thought to play a major role in this disorder including a complex of several fungal pathogens (Cylindrocarpon, Phytophthora, Pythium spp. and Rhizoctonia), bacteria and parasitic nematodes. In addition to biological factors, soil pH, moisture stress (too much and too little), soil compaction, toxins, soil structure, heavy metals and insufficient availability of nutrients (particularly phosphorous) are also implicated as contributing factors to replant disorder. However, research showing dramatic tree growth in response to soil pasteurization and fumigation to eliminate harmful microorganisms suggests this disorder is primarily a biological phenomenon.
Symptoms of replant disorder include:
- stunting of the tree with short internodes
- small and light green rosette leaves
- small root systems and decayed or discolored roots
- few new lateral or feeder roots
Vigorous young trees affected by apple replant disorder often stop growing in early summer. Affected trees leaf out in the spring but often produce little or no shoot growth. Severe replant disorder results in the death of young trees and entire orchards. Trees in orchards not killed by replant disorder have delayed fruit bearing and reduction in overall yield.
Prevention of replant problems is much easier and more successful than control. There is very little that can be done to correct replant problems once the trees are planted. The causes of apple replant disorder on different sites are highly variable. Not all soils respond in the same way to the various pre-plant treatments, and a treatment that is beneficial in one orchard may have no effect in another. Take a soil sample for nematodes and fungal pathogens at a site before establishing a new young orchard, particularly if the site was planted with apples or other fruit trees in the past.
The following cultural controls help avoid apple replant disorder:
- Avoid planting apples on the same ground where an old apple orchard has recently been removed. Rotating out of pome fruit for several years (two to eight years) is advised.
- Adjust soil pH if too high or low prior to planting with a lime or sulfur application.
- Plant as early as possible in the spring, taking care not to skip important pre-plant operations.
- Provide adequate nutrition and irrigation as indicated by soil and tissue tests, and soil moisture monitoring equipment.
- Use rootstocks resistant to Phytophthora (e.g. CG.30, CG.6210 and CG.16) at sites where this pathogen is a contributing factor. See Table 3–10. Insect and Disease Susceptibility Ratings of Common Apple Rootstocks for more information on Phytophthora-resistant rootstock.
- Consider replant-resistant rootstock if available. Various Geneva rootstocks such as G.41, G214, G.935, G.202, G.30, G.210 and G.969 claim tolerance to the classic replant pathogens.
- Stagger planting rows to avoid planting directly in old tree sites.
- Grow nematode-suppressing cover crops in the years prior to orchard establishment.