Specific Orchard Replant Disease
The most common cause of poor tree growth and low yields in replanted orchards is called Specific Orchard Replant Disease. This disease is caused by soil fungi and bacteria that built up during the growth of the previous orchard.These root pathogens build to high populations in an orchard soil, and inhibit root growth when new trees are planted.
All tree fruits are affected by this disease complex, and switching from one tree fruit crop to another will not solve the disease problem.
Mature trees can tolerate these root damaging organisms, but young trees are unable to develop adequate root systems when pathogen numbers are high. Young trees seldom die from this disease. Growth is slightly to severely slowed, and the trees appear to be nutrient and moisture deficient. Yields are reduced, even after the trees fill their space.
This disease is more severe on sites having poor soil quality. Careful management of soils, nutrients, weeds and irrigation will reduce, but not eliminate the effects of this disease. There is no adequate treatment once the young trees are affected. Rotating the site out of orchard and leaving it fallow for at least 5-8 years may reduce the severity of the disease, but tree root suckers maintain its presence, so years when root suckers are present do not count as part of the rotation. Adding soil or other products to the planting hole may improve tree growth in the first year or two, but will not adequately control the replant problem.
Broad spectrum soil fumigants are the most common and effective controls used by fruit growers who must replant quickly. Soil fumigation often leads to normal tree growth and production, while untreated areas of the same block grow and yield 20% to 50% less. Fumigants that contain 1,3-DCP and chloropicrin (example: Telone C-17 or Telone C-35) or metam-sodium (example: Vapam HL or Sectagon 42) usually will provide control of this disease.
In the next several years, there will be replant resistant apple rootstocks available from nurseries. These will be a critical choice when replanting apples. No cherry or pear rootstocks have yet been found to be resistant to this disease.