Phytophthora root rot

The impact of this soil-borne fungus can be devastating to susceptible fruit trees, writes PHIL DUDMAN

This serious fungal disease can affect a wide range of fruit trees we know and love, including citrus, avocados, stone fruit and pome fruit. It’s caused by several Phytophthora fungi species. In favourable conditions, the fungi infect the tree’s microscopic root hairs and start rotting the feeder roots. These roots are essential for moisture and nutrient uptake. When a tree is infected, the leaves turn yellow and fall, and branches die back.

In some cases, the soil-borne fungus enters a wound in the trunk, near ground level, causing cankers to form. You may have seen this on citrus or macadamias, recognisable by flaking bark and oozing gum. Over time, these cankers have the potential to ringbark and kill a tree.

Phytophthora root rot flourishes in wet soils, so when planting fruit trees, it’s vital that the soil at your chosen site drains freely. In areas with heavy soils, it may be necessary to plant trees on mounds of introduced free-draining loam. Apply gypsum and organic matter regularly in the form of compost and organic mulch to help suppress spores.

The spores spread in running water, so avoid planting susceptible trees downhill from other disease-prone plants.

Infected trees often need to be removed and destroyed. In commercial orchards, avocados, which are highly susceptible, are managed with phosphorous acid, a systemic fungicide injected into the trunk or sprayed onto the foliage. It moves to the roots, where it deals with the disease. To be effective, it must be applied during root growth – late spring, or autumn after the summer leaf flush has hardened. You can do this too, using injection kits from produce stores.

Treating fresh, ground-level wounds with fungicide can help prevent the formation of cankers. Also, if the cankers are detected early, they may be controlled by cutting out the infected tissue and applying fungicide.