Avocado root rot control program

This work was motivated by the presentations given by Elizabeth Dann at the SAAGA research symposium and phosphonate workshop held in February 2017.

Compiled by Nico Claassens & Elsje Joubert

Reviewed & improved by Stephan Schoeman & Henk Kuperus

Avocado fruit Elsje Joubert Avocado root flush Elsje Joubert Compost heap Elsje Joubert Cover crops Sunhemp Elsje Joubert Healthy avocado nursery tree Elsje Joubert Healthy avocado roots Elsje Joubert Size of Cover crops Sunhemp Elsje Joubert Tree injection Elsje Joubert Windbreaks Elsje Joubert

1. New Plantings
1.1 Soil preparation 1.1.1 Do a deep cross-rip of the new orchard.
1.1.2 Do soil pH (KCl: 5.0 – 5.5 or H2O: 5.8 – 6.2) correction according to soil analyses.
1.1.3 Adjust kation balances at least 3-6 months prior to planting. Ca(60-75%): Mg(15-20%): K(5-12%)
1.1.4 Do a phosphate correction to a depth of at least 60 cm.
1.1.5 Constructing ridges of 40 to 70 cm high x 2.5 m wide at the top on a 8 m row spacing.
1.1.6 Plant legumes (nitrogen fixing plants) or certain mustards as biofumigants.
1.1.7 Aim to improve the soil organic matter content by planting green manure and working it into the soil at least 3 months prior to planting or by the addition of compost, manure or continued application of liquid carbon. Mix compost / kraal manure into the soil to maintain good soil health through the continuous introduction of organic matter in the vorm of organic acids / manure or chips. Ground covers will help to stabilize the soil. Try to establish cover crops between the rows and do not apply raw manure to young trees.
1.1.8 Plan rows to allow for drainage, respect and use natural water sources, and if necessary, apply underground drainage, make watertrenches to avoid any surface water accumulation.
1.2 Planting of trees 1.2.1 Plant holes should be squired off by a spade. Ensure trees are not planted too deep / shallow.
1.2.2 Mix composted organic matter with the planthole soil before closing the hole again.
1.2.3 Paint the stems of the trees before planting with a potassium-, aluminium- or ammoniumphosphonate (or foliar application) and then paint the tree stems with a diluted white PVA paintmixture.
1.2.4 Always follows the products applied recommendations and only use registered products
1.2.5 Plant according to avocado planting recommendations supplied by the nursery.
1.2.6 Only plant certified trees with root rot resistant rootstocks such as Dusa or Bounty as clonal rootstocks and Velcick as a seedling rootstock.
1.2.7 Apply an organic mulch at least 1m around the tree without closing up the tree stem. The organic mulch may also form part of the retaining wall to consentrate the irrigated water around the tree which keeps the soil cool, reduce weed growth, prevent crusting and improve the soil.
1.2.8 First irrigation water should consist of a mixture of cytokinins and bascillus simbiotics to stimulate root growth and improve root and soil health. Repeat the  treatment every three months for the first year of planting.
1.3 After planting care 1.3.1 Irrigation water should be concentrated in the basin of the trees for at least the first 6 months to ensure the planting medium is wetted as well during irrigation without over irrigating the surrounding soil. If a irrometer is used to schedule  the irrigation there should be one placed into the surrounding soil and into the potting mix for the first six months or until the roots has grown into the surrounding soil. Make sure not to manage young trees too wet or not too dry.
1.3.2 Apply potassium- or ammonium phosphanate in the soil (as per label as soil drench, leaf or stem paint treatment) in the first two weeks after planting. Repeat every three months or replace it with a suitable biological product.
1.3.3 The trees should be sprayed monthly with products that contain both all relevant plant nutrients as well as plant stimulants such as cytokinins and auxins.
1.3.4 Spray or paint the stems monthly with Fosetyl-Al at 3.75g/? water for at least the first six(6) months.
1.3.5 Protect the tree against adverse enviromental conditions (sunburn and frost damage) by sparaying it regularly with a coaline product and or anti-stres products.
1.3.6 Fertilise the tree on a regular basis with fertiliser that contains N.P.K +Mg and with micro-nutriënt products such as zink and boron. There are products such as Ghwamis / controled release from Haifa that contains all this nutrients and is applied only ones a season for the first three years. Alternative: SAAGA mix – dry mixture of 3 parts KAN, 2 parts MAP and 1 part KNO3. Apply 20g SAAGA mix per tree for the first 18 months.
1.4 Pest & disease control 1.4.1 Enclose the trees individuely or the total planted area to protect the trees against antilope and other vertibra annimals.
1.4.2 Monitor regularly for the  locasts, beetles, termites and thrips that might damage the trees. Use a suitable registered product when neccassary.
2. Replant of Avocado’s: Adhere to all above mentioned points and give special attention to the following points.
2.1 Apply kraal- or chicken manure at least two months before planting on the plant rows and work it in.
2.2 Apply mulch under the trees after planting with a good quality mulch at least 1meter diameter around the tree.
2.3 Use a microbial soil conditioner to stimulate the soil microbes and to protect the trees against phytophthora infestations.
2.4 NB! The trees must be established on ridges to ensure good drainage and aeration of the soil. It is essential for root growth and health.
2.5 Intensive care must be taken to ensure good root growth and soil health for the first year after planting.
3. Non Bearing trees
3.1 General 3.1.1 Treat the soil against P.c. spores with metalaxyl at the beginning of the season and repeat the treatment six months later or use a biological soil conditioner six weeks after the metalaxyl treatment and repeat after three months.
3.1.2 Paint or spray the tree stems with a potassium-or ammonium phophate product every three months.
3.2. Trees with P. c. 3.2.1 Apply metalaxyl at the start of the season to the soil (according to tree age). After every metalaxyl treatment a biological soil conditioner should follow.
3.2.2 Paint or spray the tree stems with a registered potassium- or ammonium phosphanate product every two weeks
3.2.3 If the stems are thicker than 30mm it can then be injected with a single 20ml dosage of a suitable potassium- or ammonium phosphanate product, every 6 months. Injections should only be done as a last resort.
4. Bearing Trees
4.1. P.c. Already present 4.1.1 Inject the trees twice a season with a registered potassium- or ammonium phophanate prduct. Take care to ensure the prescribe MRL levels would not be exceeded at time of harvest.
4.1.2 First injection of the season should occur directly after fruit set.
4.1.3 Second injection four months later. The second injection should not be closer than 100 days before harvest.
4.1.4 Pick all the set fruit from infected trees to reduce the stress on the tree and treat it. Heavy invested trees should not be allowed to have fruit on because it slow down the recovery proses.
4.2. Maintenance 4.2.1 Only one series of injection per season after fruit set, usually in spring
4.2.2 Follow up with two leave sprays with a registered potassium- or ammonium phosphante product with a 90 day interval between sprays. Three months after the injection. Leaves flush must be semi matured. No more reddish leaves. Use with a penetrater product for best results.
5. General practises to maintain good root health
5.1 Soil should always be well aerated and drained with no compacted layers up to  a depth of 1000mm.
5.2 Always plant on ridges if possible to ensure good drainage and soil erasion.
5.3 Irrigation scheduling should always aim to induce oxygen into the soil profile as well, not just for applying water.
5.4 Increase the organic compound of the soil for improved aeration and microbial activity. Apply organic mulch around trees where there is no natural organic layer present for instance young newly planted trees or trees that lost its normal leave mulch deu to P.c infestation.
5.5 Keep Calcium(Ca) levels optimum. Base saturation should be between 65% to 75% depending of the soil texture.
5.6 Boron is also essential for root health and growth. Apply regularly small amounts through the irrigation or twice a season every six months with a slow release boron product.
6. Water & irrigation systems
6.1 Do a water test to determine the mineral as well as the pathogenic content. Attached is the water standards regulations.
6.2 Ensure the irrigation system is suitable for avocado irrigation and fertigation so that fertigation can be done through the system.
6.3 Use soft acid products to clean the system from salt and mineral layers and particles on a regular base.
6.4 Hydrogen peroxide shoiuld be used on a continious base to ensure the water and irrigation pipes stay clear of organic films and can be pathogen free.
6.5 Use irrometers to do irrigation scheduling.  Also use a soil auger to monitor the irrometers and irrigation shedule.

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