Phytophthora root and trunk rot of fruit trees
Phytophthora root and trunk rot is widespread in the fruit-growing districts of Victoria. It causes significant production losses by killing both young and mature trees.
It can attack the following fruit trees:
Symptoms of root and trunk rot
The following symptoms can indicate Phytophthora root and trunk rot:
- On stone fruits, pale amber, cloudy drops of gum exude from the bark near the base of infected trees.
- The gum darkens with age until it's almost black, while new drops exude further up and around the trunk as the infection progresses.
- The bark has a characteristic sickly-sweet odour when removed.
- The inner bark and cambium in the lesion are banded or mottled with shades of cream and brown.
An infection occurring beyond the growing season during autumn or winter, when sap-flow along the trunk ceases, shows no gum exudates on the affected bark. In contrast, the infected bark of apple appears as a dry, grey to dark depressed area with no gum exudation even with an infection occurring during the active sap-flow periods (see image).
In both instances, these bark lesions are invariably traceable to 1 or 2 of the roots. The infection commonly originates in the susceptible region near the root crown.
Symptoms in the foliage are secondary and are rarely obvious until:
- girdling of the trunk is well advanced
- a major part of the root system is invaded.
They vary with the time of girdling or root destruction.
Common symptoms in the foliage include:
- stunted leaves
- yellow discolouration
- cessation of shoot growth
Affected trees sometimes bear fruits that ripen early. Some trees girdled in autumn might survive until the following spring, when they die before the shoots elongate.
In artificial inoculation experiments, a 4-year-old apple tree dies within 18 to 24 months following an infection induced at the collar.
Fungal cause of disease
This disease can be caused by any of the 3 species of Phytophthora:
- P. cactorum
- P. cinnamomi
- P. cambivora
The symptoms caused by each of these soil fungi are identical and laboratory techniques are needed to identify them.
The most widespread species is P. cactorum. This species commonly attacks apples grafted on the popular but highly susceptible MM 106 rootstock.
These Phytophthora species exist in the soil as resting spores. The resting spores germinate when the soil is wet and warm to form sac-like sporangia that release many zoospores (swimming spores) into the soil water.
The zoospores swim to the underground plant parts and infect them. The cycle is completed when the Phytophthora growing in the plant produces more resting spores, which are released when the dead tissue rots.
Infections of roots at a distance from the trunk are less likely to result in trunk rot than are direct infections of the root crown region.
Optimum soil temperatures for infection occur from October to April. Recent artificial inoculation experiments made on peach trees suggest that trunk infection also occurs in autumn and winter.
Apart from a slower rate of Phytophthora growth along the trunk, the bark infected in autumn and winter shows no copious gum exudation associated with an infection in spring or summer.
The dependence of Phytophthora zoospores on wet soil for infection is reflected in the association of outbreaks of the disease with frequent or extended periods of high soil moisture. This means:
- heavy or extended periods of rainfall
- soils with slow or impeded drainage.
Controlling root and trunk rot
Chemicals are available to control Phytophthora.
For information on currently registered and or permitted chemicals, check the Australian Pesticide and Veterinary Medicine Authority (APVMA) website.
Always consult the label and Safety Data Sheet before using any chemical product.
Applying fungicides is only a part of the management strategy to control root and trunk rot.
Irrigation management is important to reduce the incidence of the disease. This is because infection is favoured if the amount of water applied exceeds losses through:
Apply water quickly to minimise the period of saturation of the soil and to allow enough time for drainage before the next irrigation cycle begins.
Hilling up the topsoil along the tree line and shallow planting on the hill improve surface drainage and expose less of the susceptible trunk tissue to the infected soil. An infected stand of trees is difficult to manage because the replacements for trees killed by Phytophthora are smaller and use less water than the surrounding trees.
If no adjustments are made for this, the replacements will be over-watered and can also become diseased.
Avoid moving soil and infected plant material from diseased to healthy areas. Sod culture is preferred to clean cultivation for this and other reasons.
Image from H. J. Larsen, Bugwood.org
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