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After harvest in autumn is the time when most fruitgrowers pull out blocks of orchard no longer considered to produce commercially viable crops. Where future replanting is being considered it's good orchard practice to carefully check root health in the trees being pulled out for root disease.
The most effective way to pull trees out is to apply a vertical lift which has the affect of getting most of the main roots out in one piece.
Where root disease exists evidence can usually be seen on the main roots near, or around the crown of the root system.
Diseases which may be present include:
- Phytophthora
- Armillaria
- Root Canker
- Southern Blight
- White Root Rot
It is important to identify if any of these problems are present because they are capable of infecting the next generation of tree planted on the site.
The symptoms are as follows:
Phytophthora - this is the most common cause of tree death and decline in New Zealand orchards. This disease will show up as dark brown lesions in root crown area. These lesions contain a watery soft rot which when cut into will reveal an orange brown mottle with dark brown bands running through it and a distinct margin with healthy tissue. The lesions produce a characteristic sour odour. Phytophthora are unable to attack lignified tissue so the lesions are confined to the bark and cambium tissue with no invasive rotting of the woody tissue, or rotting of any lignified strands of tissue within the bark zone.
Incidentally fireblight can produce similar lesions in susceptible rootstocks such as M26. The major difference between the two diseases is that the fireblight lesion will usually be most extensive at the bud union whereas phytophthora tends to be below soil level with healthy tissue on the rootstock side of the bud union.
Phytophthora may also kill susceptible varieties above the rootstock union as collar rot. In this situation rootstock tissue immediately below the bud union is usually healthy.
Armillaria - the distinguishing feature of Armillaria is the development of leathery fan shaped mycelial mats spreading through the cambium tissue area between the bark and the wood. With careful examination of the root surface it may also be possible to find black or dark brown rhizo-morphs, sometimes known as bootlaces.
Armillaria infections tend to show their presence in an orchard as areas of dying trees which gradually expand out from the center in a radial fashion.
Infections are fairly rare and often associated with recent alluvial soils that have buried logs present.
Root Canker (Moutere Hills dieback) - roots infected with this disease will show lesions on root swellings which have longitudinal cracks through which masses of white mycelial growth can be seen in late summer or autumn.
During wet winters the masses of white mycelium tend to disappear so it's more difficult to recognize the disease on trees pulled out in late winter or spring.
Infections of this disease are largely confined to the drier soils of Nelson and Marlborough.
Southern Blight - this disease generally only attacks young trees up to about three years olds and is confined to very warm districts such as Gisborne. Trees show sudden mid-summer collapse and at that stage will show white strands of mycelium present on the surface of the lower trunk and crown area of the roots. As the infection ages the white mycelium may disappear to be replaced by radish seed-like brown sclerotes.
White Root Rot - affected roots will have masses of web like strands of mycelium on the surface of rotting roots, and evidence of white mycelial masses within and beneath the bark. White or grey mycelium strands may also be found in the soil adjacent to the infected roots. Masses of dark-coloured spherical fruiting bodies may be present on the surface of the bark that it has killed.
It's relatively uncommon in New Zealand orchards.
Armillaria, root canker and white rot may all appear similar to the untrained eye so if any of these diseases is suspected it may be necessary to send samples to a diagnostic laboratory for confirmation of the disease presence.
Management and Control
Problems with phytophthora are usually associated with soil water logging in the autumn or winter. Improving drainage by controlling water table levels or planting on ridges will mean that it's possible to replant orchard without too much risk of loosing trees from the disease. It is, however, prudent to avoid using very susceptible rootstocks such as MM106.
Southern Blight usually only occurs in soils with a long history of cropping with susceptible crops. Its' distribution is sporadic so the best management for it is to remove affected trees and the soil around them, then replace with fresh non-contaminated soil from an area where the disease is not causing trouble.
Armillaria, root canker and white root rot are much more difficult to eradicate. They will persist in soils as long as undecayed infected root material is present and in some cases with white root rot even longer.
The first step in their eradication is to remove as much of the previous orchard's root systems as possible by deep cultivation with tyre cultivators that bring root matter to the surface from which it can then be picked up and taken off the site for burning.
Following this, spelling the land from woody crops for several years by putting it into pasture or growing annual crops will give a chance for any fine roots left behind from the root raking to decay.
Soil fumigation treatments have not been all that successful in controlling these particular root diseases due to their inability to kill infection which may be protected within root tissue that's been left behind from the previous orchard crop.
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