Pests & Diseases » Phytophthora Root Rot

Phytophthora root rot has been recorded as a problem in various olive growing regions throughout the world. Usually the disease occurs in situations
where periodic waterlogging of the soil has taken place. Phytophthora problems, however, have been known to occur in olive trees growing in well-drained soils, if the pathogen is present and environmental conditions are favourable.

Leaves of infected trees become yellow and drop, stems wilt and die back from the tips. The tree usually shoots from dormant buds lower down on the tree, either from the main trunk or lateral branches. Vascular browning primarily of the xylem tissue is usually present in affected parts of the tree. Cracking and distortion of the trunk can also occur.

In a number of fruit tree crops (avocado, citrus) Phytophthora is adequately controlled with applications of potassium phosphonate (also referred to as Phosphite or phosphorous acid). Potassium phosphonate is available under a number of trade names including: Fosject 400, Agri-Fos 400, Foli-R-Fos 200, Anti Rot Phosacid and CM Phosacid 200. Potassium phosphonate is a preventive treatment, it works primarily by enhancing the plants defence mechanisms and therefore is best applied to the plant before infection occurs. On woody perennial crops potassium phosphonate is generally applied as a foliar application approximately every 6 weeks throughout the growing season. Label rates are in the vicinity of 5ml-10ml per L (air blast sprayer) depending on the concentration of the product (200 or 400g/L). Where there is little foliage left on the tree, a 10% solution can be applied directly to the trunk. It is a relatively cheap fungicide and has low human toxicity. As well as its protectant properties potassium phosphonate has been shown to enhance root development in some plant species.

As Phytophthora is widespread in soil and water throughout Australia, especially in warmer regions that are subject to summer rainfall patterns. Trees being planted into these regions could be treated with potassium phosphonate as a preventative measure until they are well-established.

A supplementary treatment involves the application of calcium and boron. These two nutrients are associated with the young growing tips of plants. We have seen plants showing moderate tip dieback symptoms respond favourably to calcium and boron treatments. Some olive varieties appear to have a relatively high requirement of these two nutrients in comparison to other fruit tree crops. Calcium nitrate 10g/L and Solubor 1.5g/L can be combined and applied to the foliage of the trees on a regular basis (every 6-8 weeks)

General Recommendations
* Please note before applying potassium phosphonate, growers should check their local horticultural regulations as to the legality of doing so.