Replant disease

Old trees may be become unproductive, ill formed from broken wood or where diseased wood has been removed, and plagued with a collection of pests and diseases. Though there are certainly times and places where such old trees can be quite attractive as they descend into decrepitude, one may wish to replace an old tree with a new specimen. Younger trees can also need replacement. They may fail to thrive, especially if the chosen rootstock is not well suited to the local conditions, the cultivar may fail to produce fruit of the desired quality, or they can succumb to pests and diseases.

The first step in replanting is the physical removal of the old tree. A well established tree can have an extensive root system; as a rule of thumb one can estimate that the root system of a natural shaped tree, as opposed to one of the restricted forms, is about as large as the canopy. Removal of a young tree may be facilitated by digging around the root system with spade and fork, cutting through the roots where necessary, and by levering with wrecking bars or similar, but it can be hard work. For a large specimen, machinery is best brought in to dig around the roots and pull the stump. Although roots left in the soil will eventually decompose they can harbour pests and diseases and it is preferable to remove as much of the old root system as possible.

A replacement tree is likely to struggle to establish in the same planting site unless steps are made to address the impoverishment of the soil and the problem of replant disease. Replant disease is a generic term for the deleterious effects of a variety of bacterial and fungal pathogens and pests that built up within the vicinity of the root system of the previous tree and can remain in the soil for some years, causing potential problems when replanting. Rather than a specific cause, then, there are numerous contributing factors, including a build up of parasitic nematodes, harmful bacteria, and fungal pathogens such as Rhizoctonia, Pythium, and Phytophthora. With annual and biennial plants, crop rotation can be used to help prevent the build up of pests and diseases, but trees can be in place for decades. Colonies of pests and pathogens build up slowly around the root system. A newly planted tree in clean ground is typically well established by the time they reach potentially harmful levels, but a young replacement tree planted on the same site can be readily overwhelmed. Symptoms may include failure to establish, stunted growth, delayed bearing, reduced yield, and reduced fruit quality. Various pathogens can remain in the soil for many years, thus simply leaving the planting site fallow for a few years may not be an effective strategy. Replant disease can affect many plants but those of the rose family, Rosaceae, which includes the pome fruits, stone fruits, and various soft fruits, are particularly susceptible, the problem being well known amongst growers of the ornamental roses. Amongst the fruiting crops it is commonly associated with apples, cherries, and the like.

The first option, and the easiest where it is possible, is to plant in a different area. One might find a new location for a rose, but fruit trees are rather more substantial and the typical garden or allotment likely has few options as to alternative locations. Orchard trees may also need replanting from time to time and it would probably not be convenient to relocate to a different site. Where one must replant in the same location, old soil can be removed and replaced with clean top soil and organic matter. This can be a substantial undertaking, one that might necessitate heavy machinery. Where trees are growing together at close spacings this may be both impractical and ineffectual. Depending on where the roots have run and other nearby plants or structures, such as paths, walls or fences that one does not wish to disturb, this may not be a practical proposition.

In non-organic fruit production, the usual approach is the application of soil fumigants. Chemicals, such as chloropicrin, can be applied to kill pests and diseases that remain in the soil after removal of the old tree. These are hazardous substances, harmful to humans and the environment, destroying beneficial organisms along with the more harmful sorts; indeed, chloropicrin was used during World War I as a chemical weapon. This, or substances like it, are arguably not the sort of thing one ought to be unleashing on the environment.

The organic gardener is not without options, however. Some promising organic approaches are based on both the reduction of pathogens and the boosting of beneficial organisms that promote healthy vigorous growth. Research is continuing into the identification of the specific pathogens that contribute to replant disease and studying the effect of natural substances that can reduce their numbers. Two genera of plants have been shown to be beneficial: Brassica and Tagetes. In the former, glycoasinolates in the green matter and seeds appear to be effectively toxic to root diseases, whilst in the latter, root exudates are known to repel parasitic nematodes. Seed meal produced as a by-product of various brassicas grown for their oil has been shown to be quite effective.

The addition of copious quantities of manures and composts can be most helpful, restoring potentially impoverished soil by providing organic matter, a nutrient rich environment to support vigorous growth of the new tree, and feeding the beneficial soil life that supports healthy growth. Whilst a balanced feed may be preferred when planting and one would not normally add material particularly high in nitrogen, on replanting, the addition of nitrogen rich fertilisers such as hoof and horn or poultry manure may be beneficial in promoting more vigorous vegetative growth.

Many authorities suggest the addition of mycorrhizal fungi when planting a tree and it is thought to be helpful in respect of replant disease. The majority of plants enjoy a symbiotic relationship with fungi in the soil, whereby the fungi colonise the root system and take carbohydrates from the plant in exchange for water and nutrients. In effect, they form a potentially large secondary root system. Though their addition is by no means essential – after all trees have been planted for millennia without them, and the fungi exist in nature and will in due course take up residence – the deliberate addition in the form a dipping solution for the roots or granules sprinkled directly onto the roots when planting is thought to be beneficial. With replant disease, any advantage that can be offered to give the new tree a head start is worth considering.

Rootstock selection can have a bearing on the success of the replacement tree: some rootstocks are known to be less susceptible to replant disease and the more vigorous rootstocks are less likely to struggle than the weaker growing dwarfing sorts. It is often recommended to follow pome fruits with stone fruits and vice versa. They belong to the same family but quite distinct genera and may well exhibit somewhat different susceptibilities.

Changing the soil conditions can alter the balance of the organisms that colonise the soil. In particular, a change in soil pH might be helpful. Unless over a calcareous substrate, soil tends to acidify over time. It is not uncommon, therefore, to lime prior to replanting, a practice that is thought to reduce replant disease.

For the gardener, who may well encounter this problem on occasion, it seems that it would be sensible to apply at least three measures: the addition of copious amounts of manures and/or composts to rejuvenate the soil, preferably leaving it at least several months before replanting; the application of lime unless on an alkaline soil; and the addition of mycorrhizal fungi when replanting.

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