Phytophthora cinnamomiย is the scientific name of the causal organism of phytophthora root rot, sometimes only called root rot. It is the single greatest threat to the effective production of healthy and productive avocado trees.

Phytophthora root rot

On all varieties of avocado, this pathogen attacks the feeder roots, which can result in death of the tree. Although the disease has been studied for more than 60 years, definitive control measures
have not been found. However, many control strategies have been discovered that will reduce the impact of avocado root rot. An integrated management approach to root rot will allow theย  continued economical production of avocados in the presence of Phytophthora cinnamomi.

Phytophthora cinnamomiย is a soil-borne water mold that is believed to have originated from South East Asia and has now spread to over 70 countries. Avocado trees did not evolve in conjunction with root and so have no natural defences to it.

Phytophthora root rot?

Phytophthora Root Rot is a disease that affects close to 5000 plant species across the globe and is the most serious avocado disease world wide limiting production. PRR is the result of root infection caused by a soil-borne oomycete, Phytophthora cinnamomi (Pc).

After infection by Phytophthora cinnamomi, the feeder roots start to decay and turn black and brittle as the root tissue rots โ€“ giving rise to the name Phytophthora Root Rot. This restricts water and nutrient uptake by the roots and leads to branch-dieback, tree decline, and if left untreated or is severe enough, eventual tree death.

The visual symptoms of a tree with PRR include small, pale green or yellowish leaves that often appear wilted during high temperatures. The canopy is sparse and as branches die back and leaves defoliate, the fruit and branches become exposed to sunburn. The severity of the outward symptoms depends on the balance between feeder root death and feeder root regeneration.

Often these stressed trees set a heavy stress crop but it is of little worth as the fruit remain small and with a sparse canopy are exposed and become sunburnt. The exposed branches also become sunburnt which exacerbate the tree decline even further as the damaged cambium cannot translocate water and nutrients efficiently.

Phytophthora cinnamomi

In order to control Phytophthora root rot, we must first have a thorough understanding of the disease. Phytophthora cinnamomi, the cause of PRR, is not a fungus although much about its biology and life cycle is fungus-like. It belongs to the group of micro-organisms known as water moulds or oomycetes, which are related to algae. Water moulds were once included in the fungi kingdom and, as a result, Phytophthora cinnamomi has been incorrectly classified as a fungus in earlier years.

As the name water mould suggests, it requires moist conditions to thrive. Zoospores swim in the soil water and in this way find and infect the roots โ€“ its food source, until the whole root system is destroyed and the plant dies. Phytophthora root rot grows as microscopic sized filaments (mycelium) within the host plantโ€™s roots. It consumes the root tissue causing lesions (areas that appear rotten). This weakens or kills the plants by reducing or stopping the movement of water and nutrients within the plant.

During conditions that are unfavourable for Phytophthora cinnamomi to survive or flourish, i.e., when there are no host to infect or soil conditions are too dry to spread, two types of thick-walled resting spores are formed: chlamydospores and oospores. Both can survive for several years. This resilience of these chlamydospores and oospores, contributes to its persistence in soils and difficulty to control.

CONTROL STRATEGIES

In order to manage PRR effectively an integrated approach of cultural and chemical control as well as rootstock selection needs to be adopted. If you are using chemical control alone, you are only treating the symptoms and not changing the conditions under which Pc is thriving.

Rootstock selection

The use of tolerant rootstocks to control Phytophthora root rot is proposed as being the ultimate method for managing this disease. Research on developing Pc resistant or tolerant rootstocks has been a major focus of avocado research worldwide.

Since the early 1950s, researchers have scouted for avocado trees surviving Phytophthora root rot. These selections, are subjected to rigorous screening and eventually placed in field trials throughout the world, in infested orchards. In these field trials other characteristics such as yield, nutrient uptake, excessive vigour, scion and rootstock compatibility and overall tree performance are also monitored.

Due to the genetic variability of seeds, the only way to retain the resistance or tolerance and characteristics of these rootstock selections is to propagate them clonally. The predictability of genetically uniform, root rot tolerant, productive trees is a distinct advantage over the variability of seedling rootstocks. Using a rootstock that has a high tolerance to Pc eg : Dusa, is the most effective and easiest control strategy.

Cultural Controls

Site Selection

If you want to be a hass avocado farmer, the most important consideration is site selection. Root rot thrives in poorly drained soils as these saturated soils not only provide a perfect environment for spore release and dispersal but are also not favourable to plant growth and can predispose plants to infection.

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Well drained, fertile soils with good organic matter content and balanced soil microbiology will ensure healthy root development and natural suppression of the disease. Poor drainage can be improved by planting on ridges, installing drainage or deep cross ripping. Soil layers such as hardpans also impede drainage and often allow free water to accumulate above the hardpan. Preventing excess soil compaction or deep ripping these areas can also help to improve water drainage.

Certified disease-free trees Only buy certified disease-free hass avocado seedlings from registered, certified avocado nurseries. By planting trees that are already infected with Phytophthora cinnamomi, not only will you infect your orchard but you will be fighting a constant battle against the disease.

Limit the spread of the disease

PRR is not only spread through water and root-to-root contact between trees but also spread through infected soil, especially by vehicles and even footwear. Ensure there is no water runoff from areas with diseased trees to healthy trees. When working with tractors or mobile elevating work platforms, work in the most diseased areas last. Sanitise implements regularly e.g. disinfect your spade with dilute bleach solutions between holes when interplanting in an already established orchard.

Soil solarisation

Prior to replanting where a diseased tree has been removed, soil solarisation can be effective for treating infested soil. Leave the new planting hole open to the sun or cover it with plastic polythene sheeting.

Root health

Roots are important plant organs. They absorb water and nutrients from the soil and translocate them to the rest of the plant. Roots also give mechanical support to plants and synthesize growth substances and hormones that affect many processes associated with growth and production. Because roots are out of sight, they are often out of mind and are widely overlooked for their significance in plant health. It has been estimated that 80% of all plant problems start with soil/root problems.

A successful strategy is to protect plants from yield-reducing diseases by enabling development of healthier, stronger root systems through management and treatments, thereby helping plants grow to their maximum potential. Maximizing yields therefore starts and ends with a healthy root system. When there is an overlap of roots from different plants (including grasses and weeds) the roots with the most surface area wins. It is therefore important to ensure that there is no competition from grasses and weeds.

There is an interdependence of shoots and roots for growth and development. The shoots rely on the roots for water and nutrients, while the roots depend on the shoots for carbohydrates and photosynthates. Therefore, anything that interferes with photosynthesis, or transport of photosynthates, will reduce root growth.

Such factors may include cold damage, sunburn damage, inadequate nutrition, leaf pathogens and mechanical or insect-related damage to the canopy. Mycorrhizal fungi excrete powerful chemicals that dissolve mineral nutrients, absorb water, retard soil pathogens, and glue soil particles together into porous structure. In return, the mycorrhizal fungi receive sugars and other compounds from the roots to fuelย  mycorrhizal activities. Both plant and fungus benefit from this โ€œsymbiotic relationshipโ€. Applications of Mycorrhizal fungi are therefore beneficial in maintaining a strong and healthy root system.

Root health and soil fertility remains a relatively low priority for growers yet in order to maximize plant performance and yields we need to look beyond just soil pathogens!

Organic matter

Increase organic matter e.g. mulches and composts to enhance biological suppression of Pc. This will create an active and diverse microflora. Composted bark increases the airfilled porosity of soil, releases inhibitors as it decomposes, and allows antagonistic soil fungi such as Trichoderma sp. to build up. Mulching also stimulates plant root growth, increases nutrient uptake, decreases evaporation from the soil, increases soil-water holding capacity, reduces surface water run-off, facilitates drainage, regulates soil temperature, and provides a high level of nutrients for soil microbes.

Soils with high organic matter generally support higher numbers of bacteria, fungi and actinomycetes and contain higher percentages of micro-organisms antagonistic to Phytophthora. Soil micro-organisms use soil organic matter as food so in order to maximise their benefits that improve root growth and function we need to ensure that we maintain their food source by supplying a continued source of organic matter. It is as important as fertiliser!

Biological fungicides

Use of biological fungicides such as certain Trichoderma is a key constituent of integrated pest management. Trichoderma actively grow on roots and โ€œprotectsโ€ them from Phytophthora cinnamomi. They compete with plant pathogens for nutrients and space, by producing antibiotics, by parasitizing pathogens, or by inducing resistance in the host plants. The ability of these fungi to sense, invade, and destroy other fungi has been the major driving force behind their commercial success as biopesticides.

Trichoderma defend the plants by their direct and indirect effect on plant-pathogen-soil interaction. These fungi not only protect plants by killing pathogens but also induce resistance against plant pathogens, impart abiotic stress tolerance, improve plant growth and vigor as well as improve nutrient uptake.

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Water management

If irrigating, controlled, regulated use thereof is one of the most critical practices for managing Phytophthora cinnamomi. This includes both the amount, frequency, and duration of irrigation as well as control of the runoff.

Phytophthora species generally require free water for a certain duration in order to infect plants. They are not active until the soil is at or above field capacity. In other words, when water does not move down through the soil with the force of gravity. Soil moisture meters are an important management tool to monitor tree water withdrawal and field capacity and to schedule irrigation only when necessary. Over irrigation or not adequately allowing the soils to dry out between irrigation will favour conditions in which Phytophthora cinnamomi will thrive.

Also, remember to modify irrigation on trees with a poor health rating or trees defoliated by frost, Phytophthora cinnamomi or mites. When interplanting in established orchards, reduce irrigation to the smaller trees by using a micro sprinkler reducer or changing to an emitter with lower water output. Regularly check for leaks in the irrigation system that will result in saturated areas prone to Phytophthora cinnamomi infection. Phytophthora can also contaminate irrigation water. Ensure that either your water source is tested and is free from Phytophthora and Pythium or install a dosatron to treat the water chemically.

Nutrition

One of the fundamental strategies for maintaining plant health and suppressing plant diseases is managing nutrition. Proper nutrition can often influence the fine line between host susceptibility and resistance. Plant pathologists refer to the โ€œdisease triangleโ€ to illustrate the components needed for disease to occur. Equal importance is given to all three. Altering the balance will affect whether the disease occurs or the severity of the disease. Complete and balanced nutrition is a powerful tool against Phytophthora.

Applying foliar nutrients can make up for loss of nutrient uptake due to root rot or when there are otherย  constraints affecting uptake e.g. inadequate soil moisture.

Chemical controls

Phosphonate fungicides can improve the treeโ€™s ability to tolerate, resist, or recover from Phytophthora root rot infection but cannot eradicate the disease. They can prevent establishment of the organism before it gets into the plant or prevent continued growth if the organism is already inside the plant. The result is that they can delay symptoms that might have developed by the combined effects of direct inhibition of the pathogen and enhanced host defense responses.

Once chemical activity has subsided over time however, Phytophthora once again resumes growth within infected roots. This is why an integrated approach to managing Phytophthora is so important.

It is important to time the application of phosphonate accurately in order for the concentration in the roots to be high enough and effective in controlling the disease. When any chemical is applied to the tree it sinks to the part of the tree that is growing most actively at the time (the strongest โ€˜sinkโ€™). If the leaves are flushing the phosphorus acid will sink to the leaves and not the roots. Good levels of phosphonite in the roots are achieved by following the phenological cycle and timing injections or sprays to coincide with when root growth flushes are occurring.

There are two main root flush periods, one after the spring leaf flush has hardened off and one in autumn after the summer leaf flush has hardened off. There is only a small window of opportunity in spring as fruit growth occurs soon after the root flush and the phosphonate will mostly sink to the flowers and any sizing fruit.

Do not do any phosphonate application during dry weather or when the trees are water stressed. Wait for good soil moisture levels or irrigate well before application to get better uptake as treatments are always more effective when applied during periods of active sap flow.

During warm to hot weather, treatments should be applied before 10am. If any trees earmarked for phosphonate application are going to be pruned, wait 3-4 weeks after application to commence pruning to allow the chemical to be drawn up by the leaves and translocated down to the roots.

If choosing the injecting method, space syringes evenly around the base of the trunk avoiding the vicinity of old injection sites. Generally, you can work on spacings of a hand width apart (10-15cm) or one syringe per metre canopy diameter. Spacing the syringes correctly is important as the chemical does not move laterally around the tree and only moves to the leaves directly above the injection site then back down to the roots below them. If injecting is not done properly, some of the root system will therefore not be protected.

To recap: there must be adequate soil moisture, the summer leaf flush must be hardened off, there must be a good proportion of healthy roots and the roots must be actively flushing for control to be most effective.

Monitoring root phosphonate concentrations by taking root samples before and following phosphonate applications, are as important as monitoring soil and leaf nutrient concentrations. If we are not measuring, we cannot manage effectively or improve on what we are doing! Group similar trees together into a composite sample. 10-50g of healthy roots are required per sample. Aim for phosphonate root concentration of 25-40mg/kg which is considered the threshold of effectiveness for NZ.

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Foliar spray application can be effectively utilised when the trees have sufficient canopy to take up the chemical. Foliar sprays of 0.5% ammonium phosphonate or potassium phosphonate (Phosguard) can be sprayed in Autumn once the summer flush has hardened off. Depending on the existing root phosphite concentrations and the severity of the disease, 1-4 foliar sprays may be required. This quick easy, noninvasive method has been outperforming injecting in research trials since 2015 with results showing higher root phosphite concentrations.

You may opt for a combination of both chemical methods depending on your tree health and just elect to inject the poor health trees that do not have sufficient canopy volume for uptake of the foliar applied phosphonate.ย For young trees (0-4 years) I prefer to use a Ridomil or Terracin drench or/ and an Aliette foliar spray.

Remember, any chemical control for Phytophthora will affect beneficial microbes as well so follow up 3-4 weeks later with biostimulants such as Mycorrcin etc.

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