Managing phytophthora root-rot

03 April 2017  Read: 211


Europe’s importing retailers are increasingly putting pressure on fruit producers to minimize the use of pesticides on their crops. Dr Elizabeth Dann of the University of Queensland, Australia, explains that an integrated management approach remains the best strategy to reduce avocado growers’ reliance on pesticides.

Phytophthora root-rot caused by the soil-borne water mould, Phytophthora cinnamomi, is the most prevalent disease for avocados in South Africa with the phosphonate group of pesticides being the most widely and successful preventative treatment. European importing retailers are insisting on phosphonate residue levels of no more than 50mg/kg and some insisting on less than 20mg/kg. As most of South Africa’s avocado crop is exported to Europe this is an on-going concern.

Dr Elizabeth Dunn of the University of Queensland, Australia told SAAGA research symposium held on 15 & 16 February that an integrated management approach remains the best practice strategy to reduce the reliance on phosphonates.

The key elements of this approach include:
Careful site selection and preparation, including adequate soil water drainage to reduce the build-up of free soil water. Selection of rootstocks which are tolerant to Phytophthora cinnamomi such as Dusa and Bounty. Planting Phytophthora-free trees sourced from accredited nurseries. Application of mulches and or composts to improve soil structure, encourage root regeneration and stimulate microbial activity to supress Phytophtora. Optimal irrigation management and tree nutrition, including calcium amendment to suppress Phytophthora cinnamomi, and silicon to improve tree health and fruit quality, and judicious use of chemicals, such as phosphonates and metalaxyl, applied correctly and support by root and fruit residue analyses where available.

Efficient use of phosphonates
When using phosphonate the following can ensure that phosphonates are used more efficiently.
The main application window for applying phosphonates is autumn/winter to coincide with root flushing. Another window is in summer but there is some debate on whether this second application is really necessary. Australian researchers are currently testing whether the autumn/winter applications might carry through to summer.  

Phosphonates can be applied by soil drenching, foliar spray, bark painting or tree injection.
‘Research done in Australia showed that applying phosphonate through fertigation or drenching the soil is not an effective way of getting it to the roots,’ says Dunn.

Paints or sprays for young trees are quite effective but Dunn warns that foliar spraying is not a suitable method for treating sick trees.  ‘Before spraying, check the pH, it should be 7.2,’says Dunn.
South Africa is at the forefront of tree trunk injection with Phosethyl-AI (a phosphonate compound which is the active ingredient in certain fungicides) research and this method has proven to be the most effective in preventing and treating Phytophtora.

Dunn had the following tips on how to apply Phosethyl-Al:
Irrigate the night before as it set the trees up for a nice sap flow.  Inject early in the day or later in the evening when the stomata is open. Inject every hand-span apart. Angle the drill downwards slightly to give maximum contact with conducting issue.

Dunn warns against using higher doses with fewer injection sites. ‘There is no lateral movement of the phosphonate around the trunk, it goes up and then comes down,’ she says. ‘You can run into problems of not protecting the roots correctly when using less injection sites and have a too high residue levels in the fruit on those particular branches injected with a higher than recommended dose.’

Less expensive formulations are not recommendable according to Dunn. ‘The actual components might not be up to standard and may vary from batch to batch.’

– Alita van der Walt
Contact: Queensland Alliance for Agriculture and Food Innovation, University of Queensland, Australia at



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