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Phytophthora identification

Scion is the leading provider of forest-related knowledge in New Zealand
Formerly known as the Forest Research Institute, Scion has been a leader in research relating to forest health for over 50 years. The Rotorua-based Crown Research Institute continues to provide science that will protect all forests from damage caused by insect pests, pathogens and weeds. The information presented below arises from these research activities.

From Forest Health News 210, October/November 2010.

The genus Phytophthora contains some of the most destructive species of plant pathogens known to man. By the end of the 20th century there were about 60 species described. Since then the number has more than doubled and it continues to rise steadily. This is partly due to the development of molecular techniques that help to tease out subtle differences between species. Another driving factor is the increasing number of serious plant and tree diseases found to be caused by Phytophthora species. World trade has spread unrecognized species that cause little or no damage to their native plant communities due to co-evolution with their hosts. They may colonise plant tissues without causing any symptoms of disease or they may survive as propagules within the soil without causing root disease. When these species of Phytophthora are moved outside of their native areas, they can come into contact with new hosts with no co-evolved defense response, therefore resulting in serious disease outbreaks.

Phytophthora species tend to be invisible to the human eye. It is the diseases they cause that attract attention. They must therefore be isolated from plant tissue, or from the soil, into pure culture before they can be identified or studied. Many of them are slow growing and rapidly over-run or out-competed by bacterial or fungal colonisers of their substrate. Hence the need to use selective media containing a suite of antibiotics and antifungal chemicals when isolating into pure culture. Bioassays are typically used to extract Phytophthora spp. from soil and in recent times this technique has been extended to the use of leaf baits suspended in streams, rivers and other bodies of water (including rain gauges) to attract the motile spores that will infect the susceptible leaf tissue.

The Phytophthora can then be extracted from the freshly infected tissue. A number of simple genus specific devices that operate much like a standard pregnancy test and can be employed in the field to determine whether there is a Phytophthora sp. present have also become available on the market. A droplet of macerated plant tissue will give a blue line on the device for a positive test and molecular sequences that will help identify to species level can be derived from the device. When no match is found for a molecular sequence in the international gene databases the organism is grown on different media to promote the formation of spore structures so that a formal description can be made.

The power of the new methods is illustrated by the fact that the number of species described since the year 2000 is considerably more than the number described in the previous 120 years.

Margaret Dick

This information is intended for general interest only. It is not intended to be a substitute for specific specialist advice on any matter and should not be relied on for that purpose. Scion will not be liable for any direct, indirect, incidental, special, consequential or exemplary damages, loss of profits, or any other intangible losses that result from using the information provided on this site.
(Scion is the trading name of the New Zealand Forest Research Institute Limited.)


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