Chemical control
A range of chemicals is registered for control of Phytophthora. Apart from the fumigants listed above, no chemicals are capable of eradicating Phytophthora. Some chemicals do have a curative action and may reduce spore numbers in the soil but none eliminate the pathogen from either the plant or the soil.
Registered chemicals include phosphorus acid or phosphite which is sold under a range of trade names, fosetyl aluminium (Aliette®), metalaxyl-m (Ridomil Gold®), furalaxyl (Fongarid®), etridiazole (Terrazole®), captan (Captan®) and copper products.
Phosphite
Phosphite is the anionic form of phosphonic acid, also known as phosphorus acid – a highly effective and relatively non-toxic chemical. It is truly systemic and works both directly by limiting the spread of the disease within the plant and indirectly by boosting the plant's immune system. Phosphite is applied as an injection to the trunks of trees or large shrubs, as a high or low volume foliar spray.
It is not registered for use as a soil drench. There is a minor use permit allowing aerial or ground application to native plants, but trunk injection is not specifically listed. Consult the Australian Pesticides and Veterinary Medicines Authority for current information on registration of this chemical.
Many recommendations in the literature are the result of trials on natural bushland. The treatment regimes used may not be the best for a cultivated situation where plants are being irrigated and are therefore growing most of the year. The optimum rate varies between plant species and the method of application.
Phosphite is generally considered to have low phytotoxicity. However, foliar phytotoxicity has been reported in some horticultural and ornamental species and in native plant species, especially in hot weather. Symptoms include foliar necrosis, defoliation, growth abnormalities and chlorosis.
When injecting a tree, the aim is to apply as much phosphite as possible without causing phytotoxicity. Generally, rates vary between 50 and 200g/L phosphite depending on the sensitivity of the species. For example, Banksia species can tolerate concentrations of 200g/L, while Eucalyptus marginata (jarrah) suffers extreme phytotoxicity at this concentration and 100g/L is generally used. If injecting trees of unknown sensitivity to phosphite, test for phytotoxicity on a small patch first.
Foliar applications are not as long lasting as injections. However, injections are not often practical in cutflower production. Preventative spray treatments - two sprays at four to five week intervals - are generally able to provide protection for a period of time. If plants are not being irrigated, spraying should be done when the plant can be expected to be actively exporting photosynthates from the leaves to the root system so that the chemical is transported to the roots where it is required, for example in summer (once in early summer and then four to five weeks later).
A wetting agent such as BS1000™ should be used when spraying.
Researchers with both Leucospermum and Leucadendron showed that in most cases, once infection is established, no control methods are effective. Phosphite as a preventative treatment is the best and cheapest option.
Growers should also be aware that despite foliar sprays of phosphite, sporangia and zoospores are still produced from infected plants and capable of infecting other plants. Consequently, phosphite may slow or prevent plant deaths but not necessarily prevent the spread of inoculum into uninfested areas.
Other fungicides
A number of chemicals are registered only for control of aerial Phytophthora such as P. infestans on potato, tomato and strawberry. These include dimethomorph (Acrobat®), azoxystrobin (Amistar®), copper fungicides and captan. Granular formulations (e.g. Ridomil Gold®) can be incorporated into soil or potting mix.
Metalaxyl-m, dimethomorph (Acrobat®) and Amistar are upwardly systemic only. Aliette® and phosphorus acid are the only truly systemic chemicals, that is, they move both upwards and downwards.
Furalaxyl (Fongarid®) has a label warning for use on grevilleas and banksias, particularly in lighter soils as damage has been recorded.
Conclusion
Phytophthora root rots are a common cause of deaths in cutflower production. Particularly with in-ground production, it is difficult to eradicate the disease once present. Prevention is a better option.
Once the disease has been found, a number of options exist for disease management, of which chemical treatment is only one. A good holistic approach focuses on healthy plants and soil playing a central role in this process.
Acknowledgements
We acknowledge and thank Dr Keith Bodman, Dr Elaine Davison, Dr Hossein Golzar, Dr Daryl Joyce and Peter Wood for their technical and editorial input.
Further reading
- Centre for Phytophthora Science and Management
- Dieback Working Group
- P. ramorum - www.aphis.usda.gov/publications/plant_health/content/printable_version/S...