Phytophthora root rot (PRR) is a disease that affects close to 5000 plant species across the globe and is the most serious avocado disease worldwide limiting production. This destructive pathogen has been included in the list of the 10 most destructive oomycetes globally and as one of the 100 worst invasive species worldwide by the Invasive Species Specialist Group.
WHAT CAUSES PRR?
PRR is the result of root infection caused by a soil-borne oomycete, Phytophthora cinnamomi (Pc). After infection by Pc the feeder roots start to decay and turn black and brittle as the root tissue rots, 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.
SYMPTOMS OF PRR
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 replacement. Often these stressed trees set a heavy crop but it is of little worth as the fruit remain small and being exposed, become sunburnt.
The sunburnt branches exacerbate the tree decline even further as the damaged cambium cannot translocate water and nutrients efficiently.
PC BIOLOGY AND LIFECYCLE
In order to control PRR, we must first have a thorough understanding of the disease. Pc, 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, Pc 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 find and infect the roots, it’s food source, until the whole root system is destroyed and the plant dies.
PRR grows as microscopic sized filaments (mycelium) within the host plant’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 Pc to survive or flourish i.e. when there are no hosts 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. The 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 genetic 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 the Pc is thriving.
Rootstock selection
The use of tolerant rootstocks to control PRR 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 PRR. 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, overall tree performance, etc. 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 e.g. Dusa, is the most effective and easiest control strategy.
Cultural controls
Site selection
If you are wanting to plant avocados, the most important consideration is site selection. Root rot thrives in poorly drained soils. 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.
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.
Certified disease-free trees
Only buy certified disease-free trees from registered, certified avocado nurseries. By planting trees that are already infected with Pc, 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 is also spread through infected soil, especially by vehicles and even footwear.
To prevent spread, ensure there is no water runoff from areas with diseased trees to healthy trees. When working with tractors or hydra ladders, work in the most diseased areas last. Sanitise implements regularly e.g. disinfect your spade with dilute bleach solution between holes when interplanting in an already established orchard.
Soil solarization
Prior to replanting where a diseased tree has been removed, soil solarization 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
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. Maximizing yields therefore starts and ends with a healthy root system.
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.
A successful strategy to protect plants from yield-reducing diseases is to enable development of healthier, stronger root systems through management and treatments, thereby helping plants grow to their maximum potential.
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 fuel mycorrhizal activities. Both plant and fungus benefit from this “symbiotic relationship”. 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 maximise 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 air-filled 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 Pc.
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 Trichodermas is a key constituent of integrated pest management. Trichoderma actively grow on roots and “protects” them from Pc. 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 vigour as well as improve nutrient uptake.
Water management
If irrigating, controlled, regulated use thereof is one of the most critical practises for managing Pc. 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.
Over irrigation or not adequately allowing the soils to dry out between irrigation will favour conditions in which Pc will thrive.
Soil moisture meters are an important management tool to monitor tree water withdrawal and field capacity and allow you to schedule irrigation only when necessary.
Remember to modify irrigation on trees with a poor health rating or trees defoliated by frost, Pc 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 Pc 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 or by ozone.
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 “disease triangle” to illustrate the components needed for disease to occur.
Equal importance is given to all three components of this triangle. 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 PRR.
Chemical control
Phosphonate fungicides (fosetyl-Al and phosphorous acid) can improve the trees’ ability to tolerate, resist, or recover from Phytophthora root rot infection but cannot eradicate the disease. They can only 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 defence responses.
However, once chemical activity has subsided over time, Pc once again resumes growth within infected roots. This is why an integrated approach to managing Pc 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 to be 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 ‘sink’). 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.
Research and phosphonate root analysis has shown that the best results are from autumn applications made after the summer flush has hardened off and when competition from other organs are no longer dominant. The autumn application, if timed correctly, also ensures that there will be root protection for a longer period i.e. when healthy roots are required to support the critical flowering period – a time when significant stress is imposed on the tree.
Autumn applications can result in high phosphonite root concentrations which should persist, remaining above the 25-40ppm threshold of effectiveness, until the following autumn.
Monitoring root phosphonate concentrations by taking root samples before and following phosphonite 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!
To get accurate test results, a “bait” method using leaf discs can be used by placing the root material in a container of water and floating the healthy leaf discs (the bait) on top. The Phytophthora-infected material produces the swimming spores (zoospores) which infect the bait. This is then sent to the lab. Chances of obtaining an accurate culture from this freshly colonized material are very high.
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 10:00am.
If any trees earmarked for phosphonate application are going to be pruned, wait three 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, injection sites should be spread evenly around the tree as the chemical does not move laterally around the tree and only moves to the leaves directly above the injection site then down to the roots below them. If injecting is not done properly, some of the root system will not be protected.
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, one to three foliar sprays may be required. This quick easy, non-invasive method has been outperforming injecting in trials since 2015 with results showing higher root phosphite concentrations.
Focusing on soil health and fertility as well as tree health enabled me as a grower to control Phytophthora without resorting to blanket chemical control that is costly, invasive and time consuming. Be selective when doing chemical treatments and limit it to sick trees or marginal areas that are prone to Pc infection.
To conclude, I cannot emphasise enough the importance of an integrated approach to controlling Phytophthora root rot. Your tree health will improve, production will improve and so will your bank balance!