Keep Phytophthora out of your orchard (part 2)

Written by   Bas van den Ende and Judy Tisdall

Phytophthora trunk rot most often attacks peach and apricot trees, but sometimes also nectarine, plum and cherry trees.

Cloudy, pale–amber coloured drops of gum are exuded from the bark near the base of infected trees. The gum darkens with age until it is almost black, while new drops are exuded further up and around the trunk as the infection progresses. When removed, the outer bark smells sickly-sweet, and the inner bark and cambium in the lesion are discoloured.
The old dead tissue is dry and uniformly rusty brown, whereas new dead tissue is sticky with gum and banded or mottled with shades of cream and brown. The tree trunk is girdled and the tree dies, but, because the fungus grows faster along branches than around them, the lesion may extend one metre or more above ground-level before girdling is complete.
Preventative measures
Prevention is much better than cure, especially with Phytophthora.
In many cases, Phytophthora is a secondary disease. The prime cause is saturated soil followed by root damage.
Diseased trees are most likely to be found where soil type, topography, climate, or irrigation scheme have contributed to periods of excessive soil wetness in the past.
The cheapest and best methods for control of Phytophthora are:

  • Before the trees are planted, prepare the soil well, with good surface-drainage.
    Laser-grade the block with a slope of at least 1:80 along the traffic lanes, so that excess water will drain from the surface of the soil under the trees
  • Careful management of soil and water.
    Make good use of the surface soil taken from the traffic lanes by hilling (ridging) it up along the tree-rows. This will increase the depth of good free-draining soil available to the tree roots.
    Hilling-up can increase the volume of surface soil in the tree-row up to four times the volume without hilling-up.
    The surface soil should be deep, soft, stable, well-structured and well-drained.
    Regardless of the method of irrigation, you must avoid too frequent or long periods of saturated soil, especially around the trees. In well-drained soil, zoospores of Phytophthora are not released and cannot swim.

Trees under net
Fruit trees under hail-net need a different irrigation management from trees in the open.
Soils dry more slowly under net and can be watered less often. The net prevents water loss mainly because of lower evapotranspiration.
Fruit trees under net with a 20% shading factor often need up to 20% less water. This means it is easy to over-water trees under net with a greater risk of infecting the trees with Phytophthora.

Chemical control (cont. next issue)

See this article in Tree Fruit Dec 2017

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