Collar rot and crown rot of apple trees, and trunk rot of stone-fruit trees (but rarely pear trees) are caused by the fungus Phytophthora.
They are among the most serious soil-borne diseases in the world.
Diseased trees are mostly found where soil type, topography, climate, and/or irrigation have contributed to long periods of saturated soil.
Chemical control (continued from last issue)
Phosphorus acid or Ridomil can help control Phytophthora infections, especially in the early stages of the disease, but chemicals cannot resurrect trees that are badly damaged by Phytophthora.
It is always best to protect your fruit trees by routinely spraying phosphorus acid (sold as Agri-fos) once in spring, summer and autumn.
You must strictly follow the manufacturer’s recommendations for these chemicals, especially Ridomil.
Phosphorus acid sold as Agri-fos, is not to be confused with phosphoric acid. Both chemicals are commonly used in horticulture, but they have very different uses.
Phosphorus acid has some fungicidal activity and is often used to prevent or treat Phytophthora root rot in a range of crops.
On the other hand, phosphoric acid is used as a phosphorus (P) fertiliser, especially in hydroponic and fertigation systems, because it is very soluble in water. Each product has its specific use and these are not interchangeable.
See this article in Tree Fruit Jan 2018
