by Graeme Thomas.

It was discovered that a simple chemical called phosphorous acid (H3PO3) was more effective in restoring health of diseased trees when measured amounts were injected directly into tree trunks. Due to the acidic nature of phosphorous acid significant damage occurred around injection sites, so a safer formulation of potassium phosphonate was developed and is now commercially available from several companies who market the product in Australia. The knowledge that has accumulated since this time, both on tree physiology and its relationship with movement of the fungicide within the tree is significant. However, it is disappointing that many avocado growers do not correctly apply current technology in the management of root rot today.

Efficient and cost-effective Phytophthora control using phosphonate fungicides is based on understanding and working with the phenology or growth phases of the tree (Fig. 1). When potassium phosphonate is applied to trees it enters the tissues and becomes completely systemic moving within the vascular system to all parts of the tree. However, the quantity found in various organs will depend on the stage of growth at the time of application. This is due to different parts of the tree having stronger or weaker access to resources. For example, small, fast-growing fruit has the highest priority for available nutrients, shoots and leaves the second highest while roots have the lowest priority. If young fruit are developing on the tree at the time of treatment (during October early November in Fig. 1) most phosphonate applied will move

to these fruit with very little reaching the roots. Indeed research has shown that fruit may have phosphonate concentrations in excess of 280 mg.kg-1 while roots only finish with 7-10 mg.kg-1. Since we are controlling a root disease, the fungicide needs to be applied when the tree is diverting most of its resources to roots.

As can be seen, there are two distinct times when avocado trees will move phosphonates to roots. These being after the spring and summer shoot flushes. If phosphonates are applied outside these times, a significant percentage will be translocated to the non-target organs. When making an application after the spring flush, there will be reduced competition from leaves. However, in many regions the summer flush follows on very quickly and will compete more strongly for the phosphonate so the amount making it through to the roots may be quite low. Experience supported by phosphonate root analysis has shown the best results are from applications after the summer flush, when competition from other organs is basically non-existent. Recent research by QDPI has shown that elevating the phosphonate levels in the roots prior to a new root flush can reduce final root volume. It is therefore advised to wait for root development to complete prior to applying phosphonates. In most regions and with most varieties, this is late autumn to early winter (May to June).

It should also be remembered that even though phosphorous acid is systemic, moving from roots and leaves to other organs, it does NOT move laterally around the tree. This should be remembered when injecting. Many growers are now using concentrated product to reduce the number of injection points. This is truly a false economy as you are overdosing a small area of roots and leaving large areas of roots untreated. Always distribute the injection sites evenly around the tree. To do this effectively, the best practise is to dilute the current 600g. / litre product back to 200g. / litre phosphorous acid. This situation has been clearly demonstrated with phosphorous acid levels in the roots under a single tree varying from 5mg./ kg. to over 200mg./kg.

When phosphonates were first registered as a foliar spray application results were inconsistent. As research continued, it was found that spray volumes were critical. Low volume applications were not lifting root phosphonate levels to the same degree as high volume applications. It has since been found that it is essential to get thorough coverage not only to the leaves, but to branches and trunk as well. In mature orchards the volume needs to be as high as 3000 litres/ha. Younger orchards with smaller trees will require less.

Root Phosphorous Acid Levels

Prior to the development of the test measuring avocado root phosphorous acid concentrations (SGS, Toowoomba, QLD), recommended applications were based on tree response to treatment. There were many unknown variables that made results less predictable. Integrating root phosphonate analysis with application strategies increased confidence in protecting trees and controlling Phytophthora root rot in the orchard. Recommendations are based on maintaining a concentration in excess of 25 mg.kg-1 of phosphorous acid in roots to manage the disease.

Talking Avocados

Other Management Factors

Fungicide management is only one factor in the control of Phytophthora root rot. The integrated management technique as best described in the “Principles of Phytophthora root rot management” must also be incorporated in your management program.

In mature trees, the key factors are:

•    Irrigation management – water consumption of diseased trees is dramatically reduced compared with healthy ones in the same block. Where diseased trees are present in an otherwise healthy block reduce water application by installing lower volume sprinkler heads.

•   Nutrition –strategic use of gypsum while avoiding application of large quantities of fertilisers containing ammonia.