|
Phytophthora root and trunk rot of pome and stone fruits | AG0191 |
T.L. Lim, Tatura
December, 1997 |  |
To view the Adobe Acrobat file, you will need the Adobe Acrobat reader. |  PDF 33 kb | |
Phytophthora root and trunk rot is widespread in the fruit growing districts of Victoria, causing significant losses of production by killing both young and mature trees. It most often attacks apple, peach and apricot trees, but also occurs on nectarines, plums and cherries.
Symptoms
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 is 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 and 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. In both instances, these bark lesions are invariably traceable to one or two 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 or a major part of the root system is invaded. They vary with the time of girdling or root destruction. Stunted leaves, yellow discolouration, cessation of shoot growth, wilting, and dieback are common symptoms. Affected trees sometimes bear fruits which ripen early. Some trees girdled in autumn may survive until the following spring, when they die before the shoots elongate. In an artificial inoculation experiment, a four year-old apple tree dies within 18-24 months following an infection induced at the collar.
Cause
This disease may be caused by any one of three species of Phytophthora (P. cactorum, P. cinnamomi and 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 which commonly attacks apples grafted on the popular but highly susceptible MM 106 rootstock.
These fungi exist in the soil as resting spores. The resting spores germinate when the soil is wet and warm to form sac-like sporangia which 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 fungus 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.
Influence of environment
Optimum soil temperatures for infection occur from October to April. Recent artificial inoculation experiments made on peach trees suggest that trunk infection also occurs during autumn and winter. Apart from a slower rate of fungal growth along the trunk, the bark infected at this time of the year 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. Over-watering, exceptionally heavy or extended periods of rainfall, and soils with slow or impeded drainage favour development of the disease.
Control
Fungicide treatments
Currently, the phosphorous acid-based fungicide, Aliette WP (Rhone Poulenc Australia Pty Ltd) is registered for use as a foliar spray onto healthy peach and apple trees for protection against infection in susceptible areas. Each year, a total of 2-3 sprays are applied, the first in early spring, the second 12 weeks later then followed by a third and final round after harvest in early autumn before the leaves senesce. This multiple spray program, using the spray dosages as recommended on the labels, gives the desired year-round protection against Phytophthora species named above. Contact DPI for further chemical information.
Management practices
The application of fungicides represents only one part in the total management strategy used with good effect against the root and trunk rot. Irrigation management is obviously important in reducing the incidence of the disease. Infection is favoured if the amount of water applied exceeds losses through transpiration, evaporation and drainage. 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 may also become diseased. Avoid movement of soil and infected plant material from diseased to healthy areas. Sod culture is preferred to clean cultivation for this and other reasons.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
|
