Ministry of Agriculture
Causal agent: Phytophthora cactorum
Although crown rot is primarily a disease of apples in British Columbia, it has been identified on peaches, plums, apricots and cherries.
In apples, the first sign of disease is usually a purple-red colouring of leaves in late August and early September. The following year, infected trees are either dead or unthrifty with pale, sparse leaves. The inner bark of the crown and main roots of a diseased tree is orange to red-brown. In time the diseased bark becomes soft and dark brown in contrast to white bark in a healthy tree. On some trees, only the roots and not the crown show symptoms. Note that leaves on trees with M9 and M26 rootstock may turn red in late September and October without being infected with crown rot.
|Crown rot on apple|
Crown and root rot is caused by the soil-borne fungus, Phytophthora. This fungus requires wet soil for spore production and tree infection. Trees between the ages of three and ten are most commonly affected.
- Select rootstock for all new apple plantings from among the more resistant types. Losses have been less frequent on M4, M9 and MM111. In recent plantings of M26, crown rot losses have been more common than in past years. These losses may be due to winter injury predisposing the rootstock to crown rot. Intermediate susceptibility has been observed on M2 and seedling rootstocks. Losses have been most frequent on M7, MM104 and MM106, with MM104 and MM106 being the most susceptible.
- Inarching partially girdled mature trees with resistant seedlings may overcome the damage from infection.
- To stop enlargement of the lesions on partially girdled trees, expose the crown, scrape away infected bark and air dry. Replace earth around trunks before winter.
- To reduce the risk of collar rot in wet soils, plant apple trees with the graft union above, but no more than 5 cm above, the soil surface.
- Avoid prolonged soil saturation or standing water around the base of the tree.
- Control weeds around the trunk of apple trees. They can serve as alternate hosts for the crown rot fungus.
- Reduce nitrogen application on trees with excessive growth as they are more susceptible to crown rot.
BEARING APPLE TREES
Apply Aliette WDG (fosetyl al) as a foliar spray during the tight cluster or pink stage when there is sufficient leaf area present to take up the spray. Use a maximum rate of 5.0 kg/ha (2.0 kg/acre) in a maximum volume of 1000 L/ha (400 L/acre) on standard sized trees. Adjust the spray volume according to the tree row density and canopy (Tree Row Volume). Repeat approximately 6 weeks later. Treat again in the fall soon after harvest. Do not apply more than 3 sprays (15 kg Aliette/ha) per year, and do not treat within 30 days before harvest. Aliette should be sprayed to wet, not to run-off. Do not use a drench treatment for the application of Aliette on bearing trees.
NON-BEARING APPLE TREES
Apply Aliette WDG as a high volume coarse spray (5 L of water per tree) using a handgun to drench the trunk and soil surrounding the tree. The application rate varies from 5 to 10 grams per tree (see label), to a maximum dose of 5.0 kg/ha (2 kg/acre). Make the first application in spring any time after silver tip and again in early fall. Do not apply as a drench more often than twice per year. Aliette may also be applied to non-bearing trees as a foliar spray (see bearing tree section, above) if there is sufficient foliage present for good uptake of the chemical.
Apply Ridomil Gold 480 EC (metalaxyl-m) at the rate of 1.0 mL in 5 litres of water per tree, thoroughly drenching the soil around the base of the tree. The first application should be made at the time of planting (preventative treatment), followed by a second application in late August. Treatments can be repeated in the spring (before new growth begins) and late August, in each subsequent non-bearing year depending on the occurrence of crown rot in the orchard. Do not mix Ridomil with herbicides.
CAUTION - Resistance to Ridomil has already been observed for related species of crown rot fungus. Indiscriminate use of Ridomil may result in the development of resistant crown rot fungus. Use only when necessary.