Phytophthora Control by Gus Gorissen

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Phosphonic Acid Injection for Phytophthora and other root diseases was compiled by Ted Newton from a talk given to the Sub-Tropical Fruit Club of Qld by Gus Gorissen of Chemjet Tree Injectors. website:

History of the Chemjet tree injector

The company Chemjet started in the early 1980s. We had an avocado farm in Caboolture. A tree would be green one day, and yellow the next. The DPI confirmed that there was phytophthora on our place. Phytophthora is a fungus soil disease spread by feet – humans, birds, anything – and it’s all over the world. Overuse of pesticides in the past killed good bacteria along with the bad, so now the only way to control phytophthora is to treat it with fungicide. Our challenge was to get the fungicide into the tree. There were trees with no leaves left, so spraying was not an option. So the DPI suggested that we could inject it into the tree. We drilled a hole in a tree that we had been ready to take out and put in some fungicide using veterinary injectors. It worked; in three to four weeks the tree had new life and in six months was fully restored. However, with five thousand trees, injection using veterinary injectors had practical limitations. This led to the development of our first prototype injector, with a spring-loaded plunger. Over the years it developed into the little machine we sell now all over the world. It is a simple gadget and it lasts forever. It is also environmentally sound as each injection uses only 20ml of fungicide. With spraying, it’s proven that just ten percent of the fungicide is effective and the rest is wasted. The leaves do not absorb fungicide that is applied by spraying as effectively as they do when it is delivered by systemic injection.

Facts about phytophthora

Phytophthora stops the feather roots taking up moisture and nutrients. It is often thought that the tree needs nitrogen or something else. Often it doesn’t need anything; all it needs is a healthy root system. If you have a healthy root system, you can do with half of the nutrients that you have applied. The tree will look after itself, and the fruit will tell you if other nutrients are needed. Prevention of phytophthora is the key. Phytophthora affects tree health and when a tree is sick, every disease it gets hits hard. It will affect the size and numbers of your fruit. If you wait until you get that yellow colour on your leaf, you’re too late because you will lose your crop. If you have just one or two trees in your orchard with phytophthora, then it is in your place and you will spread the disease as you walk between healthy and infected plants. It is better to prevent it by injecting your trees at least once a year and you can be sure that you will keep your crop for that year. Often it is lack of water that has transferred the fungus through the soil. There is a way of measuring phytophthora in your root system. One way is to take all the dirt under the tree away, and cover the area with newspapers. Under the newspaper you will find new growth. The roots should be pure white. As soon as they show brown, that’s phytophthora. Phytophthora is in every agricultural crop. They spray for it, in potatoes, strawberries, and many others.

Other types of injectors

There are some other injecting machines around. Some have developed a system where the fungicide is shot in under high pressure. When you shoot it in under pressure, you can imagine what happens; you blow a hole in the tree. Just as when you get a splinter in your finger, nature responds by sealing it off, so fungicide fired into a tree under high pressure does not get taken up and is wasted. It won’t go anywhere, because the cells identify it as a foreign object and close it off and leave it there. Don’t be fooled by the idea that if you pump the fungicide in quickly and seal the hole to stop it running out, that it will be taken up over time. That is not the case. The fungicide has to be fed into the tree’s system under low pressure, 35 psi, otherwise capillaries can be damaged.

The fungicide

The fungicide is phosphonic acid, commonly known simply as phosphorous acid. Sometimes it is referred to as phosphonate. It is important to distinguish it from phosphoric acid which is a different chemical with a range of commercial uses such as rust and scale removal, and in dentistry. In diluted form it is an additive (E338) in carbonated beverages to give them a distinctive cola flavour. To be certain you get the right one, it is recommended that you buy a recognised brand of the fungicide from a retailer of gardening or agricultural products, ensuring that the label on the product specifies its use for the treatment of fungal root diseases in plants. When you read the labels of the various fungicides, bear in mind that all those manufacturers are aiming for volume use. Very small portions of their labels, if any, will provide guidelines on injecting, as they all want you to either spray the chemical or trench it. That is not necessary at all.

Injecting trees using the Chemjet tree injector

We can inject trees from as little as 50 cm (2 inches) diameter. On a young small tree less than 50 cm (2 inches) diameter, you may brush diluted fungicide on to the stem where it has a green surface and it will be absorbed. You may do this until the tree gets to a size when you can start injecting. The fungicide should be diluted according to the instructions on the label, but usually the ratio is about 25% fungicide to 75% water. When you inject a tree, you drill a hole, making a wound that takes time to heal. Not every tree is the same; one tree will be able to take up fungicide fairly quickly while another is unable to do that as fast.  In a young healthy tree, you need only one injector, because it will spread around and cover the whole tree. Stay away from the branches of young trees. If you drill a hole in a small branch, you may weaken the tree and it may snap. More injectors are needed on larger trees. Step out the canopy diameter of the tree, and the rule-of-thumb is that for every metre of canopy diameter, you put in 20ml of fungicide. Each injector holds 20ml of fungicide so this will determine the number of injectors you will need for that tree.

Drilling the holes

Always drill injection holes into fresh wood; if the sawdust is dark, you are drilling into old wood. The wood (the sawdust) should be white on the inside. If you drill into old wood, there may be a cavity inside and the fungicide will not be taken up. When you don’t drill deep enough, you don’t have enough space in front of your nozzle. You need surface area around the hole for the fungicide to penetrate the wood. Capillary action is better as you drill deeper. Also, it is better to drill at a downward angle. If you go in horizontally, you are likely to reach the core wood, whereas if you drill at an angle, you will stay in the sap wood. It is the sap wood that takes up the fungicide. You don’t have to do anything with the hole; being small, it heals itself.

How fungicide injection works

The fungicide goes into the sap flow and the leaves of the tree, photosynthesises with sunlight, then goes back through the sap and settles into the root system. There is where it stops the phytophthora going into the feather roots. It goes up, comes down, and will cover that particular area where you have injected. With growth rings, the phosphonate will not spread laterally; it will go up vertically. Then it comes down and it settles in that particular root system on that particular side of the tree. In the end it’s a matter of judgement. Particularly in drought conditions, the more roots you can create the more chance the tree will survive. Using the canopy ‘rule-of-thumb’ described earlier, you divide that around the trunk. You don’t have to put it all into the trunk. You can put it into the branches coming from the main trunk. However, if you drill into too small a limb, it burns because it overdoses. For example, a limb of 100cm (4 inch) diameter or less could be too small. Sometimes, in 24 hours the tip of the leaves will turn brown. It means that you might have overdone it a little bit, but it is an indication that the fungicide is at least in there. That’s good; the leaf will fall off and quickly re-grow and the tree will be protected for the next 12 months. The uptake on the side of a tree away from the sun may be a lot slower than on the side facing the sun. Therefore individual injectors are needed because you have to cover 360 degrees around the tree to cover the whole root system. Stay away from a tree when it is flowering. Also water it well when flowering. It will develop the growth cells at that particular time. If you don’t water, it won’t have the growth cells when it gets water later on, and it will split.

Treating already sick trees

You need to inject sick trees at least twice a year to get them back to where they should be, and then you may be able to get away with once a year after that once the tree is fully restored. There is a way of measuring fungicide present in the soil, but it is a very technical process and you would need to ask for professional help. For small growers, that might be too costly. The simplest way is to inject once a year for healthy trees, and twice a year for the sick trees.

Using avocadoes as an example

With avocadoes, first you have your fruit-set. After your fruit-set, you get your new shoots coming out. As soon as the young red leaves turn into green, then your root system starts developing. That is the time you start injecting because all the new roots will develop from that month on, and you want them protected. That is your first flush. Your next flush – the summer flush – will be in January, so if you have sick avocado trees you will give them another injection then. If you don’t see much improvement, inject them again before winter time. The good thing about injecting is that provided the injector containing the fungicide doesn’t fall out, which it won’t if you put it in properly, then once it is empty the fungicide is in the tree and no other place. If you spray it on, you have to wonder how much of it will be taken up, and you have no idea. When you do it with this injector, it’s guaranteed to be in the tree. It also pays to keep up with developments from DPI. They will let you know latest information, for example, they are currently researching a theory that just one injection eight weeks before flowering will protect avocadoes from phytophthora for the whole year. However this research has not yet been finalised.

Further information

Comprehensive information on the Chemjet injector and its uses may also be found on the company’s website at  A product brochure in PDF format may also be downloaded from this site.

Members’ questions

Member: Can you inject anything other than phosphonate?

Answer: Yes. Companies have developed a systemic insecticide – Dimethoate – you may already be spraying it in your orchard. That is for borers and similar pests. However because it is systemic, you have to be careful that there is no residue coming in to your fruit because you injected at the wrong time. But in the off-season, particularly with citrus where you have problems with borers, by all means, inject it.

Member: What about a Neem extract? That should be perfect for it.

Answer: Exactly. In America they did very successful trials of it.

Member: Does this systemic fungicide find its way into the fruit?

Answer: No – it won’t go into the fruit; it affects the roots, and therefore the health of the tree. That will improve the fruit.

Member: When is the best time to inject?

Answer: The best time is in the morning before the day heats up. You also put all the injectors in at once, because the holes ‘communicate’ with each other.

Member: The barrel of the device has a graduated scale for measuring the phosphonate. Do you always use 20ml?

Answer: Yes, always 20 ml. The scale is there because in the early days we used to use coal tar to dwarf the tree, to stop it growing too fast. That was an ICI product, and the DPI in Nambour discovered that with 10ml injected each year, you could dwarf them and slow down all the energy going into growth. However ICI never registered it for injecting; they said you have to pour it or drip it on.

Member: How can you tell if a tree has phytophthora?

Answer: You can see the yellow nerves in the leaf – that’s an indication.

Member: Have you experienced phytophthora in trees other than avocadoes such as mangoes?

Answer: Yes. We do mangoes, citrus, chestnuts and ornamentals – they all suffer from phytophthora. It is often said that they are blown over by wind. They do that because they have no roots left. The only state in Australia that recognises die-back is Western Australia and they do something about it. We suffer it everywhere. The Sydney Harbour Trust did a full study of it because of gum trees that fell over. If you have an unhealthy tree, borers and other insects have a feast. They affect the whole growth cycle of a tree. The roots are not expanding. Years ago I know we had roots on trees that went looking for water. Nowadays they say that if there is no water the roots don’t go looking for it, but they used to. They would go under asphalt, under anywhere to find water. But if they haven’t got that root system, they can’t do it.

Member: So how long does the phosphonate take to go in?

Answer: Temperature is a factor. If the temperature is low, there is no respiration in the tree; respiration is the amount of water that is flowing up and evaporating through the leaves, and that’s what it’s dependent on. If there is not much evaporation, then there is very slow uptake. The tree is just saving its energy for other times of the day, when it will be growing. It can take anything from 25 minutes to 2 hours.

Member: There are a lot of beneficial fungi. What happens to those when the phosphonate goes down to the root system?

Answer:  You are only putting in a minute amount compared to spraying or trenching. Your soil fungi actually do need phosphorous in small amounts, so this may actually help them.

Member: If we use injectors, does that mean we can grow avocadoes in flat, poorly drained country, because we are able to combat the phytophthora?

Answer: Possibly! Actually the avocado tree was originally a swamp tree. It came out of the Guatemalan rain forest.

To inhibit the spread of Phytophthora the following should be adhered to:

Sanitation of tools and machinery – tools must have all traces of soil washed off then be regularly drenched in a solution of disinfectant. (A solution of one per cent bleach is sufficient for disinfecting machinery.) When planting several plants, disinfect tools in a portable container of disinfectant before and after planting each one.

Boots and tyres – soil clinging to boots and tyres is a common vector in transporting Phytophthora. To limit the spread of this fungus, ensure all soil is scrubbed clean and the surface is disinfected (using a one per cent solution of bleach, or a 70 per cent solution of methylated spirit. Using disinfectant according to the manufacturer’s directions is also suitable.)

Infected vegetationPhytophthora can persist for many years in the dead organic tissue of any trees it has infected. Infected vegetation must be disposed of carefully. Never woodchip any vegetation suspected of being infected by Phytophthora. Prevention and limitation of the spread of Phytophthora is the most effective means of control (Royal Botanic Gardens Trust 2007).

Sheryl:  Many Thanks to Ken & Muriel Webb for hosting this field trip at their place.

Terminology explained:

Phosphonic acid: Phosphonic acid is one of two forms of an organic compound commonly known as “phosphorous acid”. See the explanation below under “Phosphorous Acid”.

Phosphoric acid:  Phosphoric acid is a mineral (inorganic) acid having the chemical formula H3PO4. It has a range of industrial uses including: rust removal; a food additive in processed beverages such as cola drinks; an etching solution used in dentistry and orthodontics to roughen the surfaces of teeth where dental appliances or fillings will be placed; electrolyte in some fuel cells; an industrial-strength cleanser, and a pH adjuster in cosmetics and skin-care products and in hydroponics. It is not to be used as a fungicide.

Phosphorous Acid: Phosphorous acid is a compound generally described by the formula H2PO3, although it is an organic compound that exists in equilibrium and reacts as if its molecule has more than one structure. One form is called phosphorous acid, and the other phosphonic acid. As an aqueous solution, the acid is trunk-injected or foliar-sprayed to control a variety of microbial plant diseases such as phytophthora, dieback/root rot, and downy mildew. It is also used to prepare phosphite salts used for similar applications. Phosphorous acid and its salts are more toxic than phosphoric acid and should be handled with caution.