The latest seems to be that phytophthora growth is pushed by high nitrogen levels with low carbon and low calcium
Supplementing with calcium and carbon and avoiding nitrogen is indicated if you have a problem. Ref: Dwight Carter
How do I know if I have root rot in my trees? The best way to determine if trees have root rot is to scrape away the mulch layer under the trees and examine the feeder roots. Root rot shows up as black roots. Check the picture of root rot in the Problem Solver section of the information kit. If you are in doubt, submit a sample of roots and soil for pathological examination. This is best done in spring and summer when the soil is warmer and the trees are growing more actively.
What should I do to protect newly planted trees from root rot? The best way to ensure the trees are healthy is by buying only ANVAS accredited trees. Then apply metalaxyl granules around the tree straight after planting. Repeat this six to eight weeks later. Also avoid planting new trees in areas where you have previously had major root rot problems, or on slopes below existing orchards. If the new trees are being planted in a new block of land, try to isolate it from existing blocks and disinfect machinery before it is brought into the new block.
Do I need to treat healthy trees for root rot every year? Yes. We recommend that even apparently healthy trees be injected with phosphonate fungicide at least once a year in early summer to keep them healthy. Alternatively, apply six foliar sprays of phosphonate fungicide between spring and autumn. Trees with mild to severe symptoms should be injected twice a year. Foliar sprays are also recommended for trees with a small trunk diameter (less than 4 cm), where trunk injection is impractical.
When should I inject my trees? Timing of injection depends on how severely trees are affected by root rot. If trees have no visible symptoms, inject once in early summer, six weeks after the end of the spring leaf flush. It trees have any visible symptoms (yellowing, droopy leaves, leaf drop), inject twice a year. The timing of these two injections depends on the severity of root rot symptoms. For trees with mild to moderate symptoms, inject as soon as most of the spring flush has matured (late November in south-east Queensland), and again in mid-March. For trees with severe root rot symptoms, inject at the start of spring and then again in mid-March.
Should I plug the injection holes after I have injected the phosphonate fungicide? No. We recommend leaving the holes as they are and allowing them to callus over. However, if high pressure injection devices are being used, holes may need plugging after the injector has been removed to prevent leakage of fungicide from the hole.
Can I spray the phosphonate fungicide on the ground to control root rot? No, because phosphonate fungicide breaks down very quickly in the soil and consequently doesn’t work well. It is also likely to encourage the development of strains of the Phytophthora fungus that are resistant to phosphonate fungicide. The fungicide works best when it is applied to the leaves or injected directly into the sap.
Can I spray phosphonate fungicide on my trees rather than injecting? Yes, but sprays of phosphonate fungicide should only be applied to apparently healthy trees. Don’t use foliar sprays on affected trees, as they are less effective because of insufficient uptake. Six sprays are recommended between spring and autumn, with no more than about four to six weeks between sprays. Spraying the fungicide can cause a problem. Leaf and fruit burn is likely if phosphonate fungicide is sprayed seven to ten days before or after sprays containing dimethoate, if it is applied with copper hydroxide, or if copper hydroxide residues are present on leaves. A risk may exist with other chemicals as well, depending on the quality of the water used in the spray mixture. As a result, it is often difficult to find at least six ‘safe’ spray points between spring and autumn. An additional problem is that phosphonate fungicide cannot generally be mixed in tanks with endosulfan insecticide because of spray incompatibility.
How do I apply phosphonate fungicide to trees whose trunks are too small to inject? Use foliar sprays instead (as outlined previously) or use metalaxyl granules on the ground.
What should I do if a tree dies from root rot and I want to plant another in its place? Move a little distance away from the site of the dead tree and spend time preparing the site. Dig it over, check the pH and apply limeor dolomite as required, and put on some organic manure. Mulch the site and leave for a few months. Then apply metalaxyl granules around the tree at planting and eight weeks later. Remember to adjust the irrigation emitter and fertiliser rate for the smaller size of the newly planted tree.
Sometimes I get leaf burn after injecting trees. Is this a problem? Leaf burn is not a problem if it is only on a small part of the tree. If it is more severe, it could affect growth. Check the phosphonate fungicide rates that you are using, inject trees only when the maximum daily temperature is above 23°C, and use a phosphonate product that is buffered to a pH of 7.0.
Phosphorous Acid As I’ve mentioned many times I’ve seen very good responses in growth on a variety of fruit trees. Pitombas increase in growth was amazing. A lot of trees suffer from this disease and show these symptoms in the form of crippled growth, tip die back & also leaf tip burn, of course some plants inevitably die, such as Avocados and Jarrah Eucalyptus marginata (a Myrtaceae as is Pitomba). The amount of plants that are registered for use with this spray around the world is enormous. Soluble silica is quoted on the web as having controlling effects on plant health and phytophthora resistance. Ref: George Allen