Advice
RHS Help & Advice
Replant problems
When some trees and shrubs, particularly members of the family Rosaceae (roses, apples, cherries, pears and plums) are planted into soil in which the same species was previously growing, they may fail to thrive and even die. The subsequent above-ground symptoms of stunting and poor growth are, in fact, the result of poor root development. This problem has been known by several names, including 'replant disease', 'soil sickness', 'specific apple replant disease', or 'rose sickness.' The plants will often recover if they are moved to fresh soil.
There is thought to be more than one cause, but the general opinion is that during the life of the first planting, there is a significant build-up of root pests and diseases, such as nematodes and pathogenic fungi. These increase gradually and do not reach high levels until the plants are well established, by which time the root systems are large and robust enough to withstand any adverse effects. The pathogens therefore have no effect on the vigour of the initial planting. However, when the original plants are removed, a high population of these harmful organisms will remain in the soil in the root zone. When new, young replants with only small root systems are planted in this 'contaminated' soil, they struggle to cope. Evidence for this theory is not conclusive, but is based on:
- Detection of high levels of damaging fungi and nematodes on some affected sites.
- Removal of the problem by soil sterilisation.
- Amelioration of the problem in some cases by the use of additional fertiliser to promote rapid root growth.
Plants that are particularly prone to this problem include apples, pears, plums on St Julien A rootstocks, peaches, cherries, roses (especially on some Rosa canina roostocks), quince and citrus.
Apples on rootstock M27, cherries on 'Colt' and roses on Rosa x dumetorum 'Laxa' are said to be more resistant.
Although replant problems have been thought traditionally to be host specific - apples should not be followed by apples, etc - it is now suspected that the problem may be more general; e.g. occasionally even non-related species of fruit may be affected.
In view of the uncertainties surrounding this subject, only rather general advice can be given to gardeners. Points to consider include:
- The effects are reported to persist in the soil for between four and 20 years, so waiting for the problem to subside before replanting is not usually an option.
- Wherever possible, avoid planting a young replant within the root zone of the previous plant.
- There are no chemical soil sterilants available to gardeners. Hiring a contractor to steam sterilise is possible, but would be costly.
- As indicated above, some rootstocks are more resistant and are to be preferred.
- Changing the soil is probably the safest option when only a few plants are involved.
- There are suggestions that with roses, boosting soil fertility and hence root growth with extra slow-release nitrogenous fertiliser and additions of composted bark will reduce the severity of the problem.
- The use of products containing mycorrhizae (symbiotic fungi) has been recently highlighted in the media. Anecdotal evidence varies from no-effect to good results, but until there is significant scientific evidence as to their benefit, the RHS is unable to make a recommendation. However, these products are unlikely to harm the plants if used as directed and may be beneficial.