Phytophthora diseases of trees
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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 178, October/November 2007.
Phytophthora spp. are some of the most invasive plant pathogens known world-wide. Several are of global importance as they have been widely dispersed by trade and other human-assisted mechanisms. Some species have very broad host ranges including both woody (conifers and angiosperms) and herbaceous plants, and are capable of causing multiple symptoms. As they are serious agricultural pathogens, as well as affecting native ecosystems, they are of concern to regulatory agencies, such as the New Zealand Ministry of Agriculture and Forestry (MAF).
From a forest perspective, several new species of Phytophthora that damage trees have been recognised and described, including: P. ramorum (the cause of sudden oak death), P. kernoviae (causing cankers and death of European beech in southern England), and P. alni (killing alder across the UK and Europe). In 2007, two new Phytophthora diseases of conifers were described. Phytophthora austrocedrae is a new species isolated from necrotic lesions of stem and roots of Austrocedrus chilensis in forest areas in southern Argentina and it is believed to be the cause of extensive mortality of this native cypress. Its origin is unknown but researchers report that they consider that it has been introduced. In Chile, a foliage disease of Pinus radiata caused by a new-to-science Phytophthora is typified by the relatively rapid death of needles and subsequent defoliation of trees. Newly planted seedlings and naturally regenerated plants often die in the first year of growth.
Because of the cryptic nature of many Phytophthora infections, and difficulties encountered with culturing and growing members of this genus, it has taken several years of concentrated research effort in both Argentina and Chile to isolate and identify the pathogenic agents responsible for these diseases. These two cases illustrate some of the problems associated with identifying Phytophthora spp. as the cause of disease. Although Phytophthora spp. are aggressive pathogens they are also seasonally active, delicate, ephemeral organisms that are quickly replaced in host tissues by other fungi and by bacteria. Even when successfully isolated into pure culture they may be difficult to identify to species as their spore structures (needed for identification) can be difficult to obtain and there is considerable overlap between species.
The molecular era holds the promise of substantially easing the problems of disease diagnosis of cryptic origin. Phytophthora diseases have become so important in forest ecosystems that IUFRO (International Union of Forestry Research Organisations) has allocated a research working party (7.02.09) solely to the topic of “Phytophthoras in Forests and Natural Ecosystems”. This group, which meets every 2–3 years, held its fourth meeting in August 2007 in Monterey, California.
Ensis Forest Biosecurity & Protection was represented at the meeting by Tod Ramsfield, who has expertise in the molecular identification tools that are increasingly been used to help find and identify these elusive pathogens, and Margaret Dick who studies the morphological and growth characteristics (still a vital component of the identification procedure). New Zealand had further representation at the research working party with George Gill (Senior Advisor, Incursion and Surveillance) from MAF Biosecurity NZ, and Ross Beever and Nick Waipara from Landcare Research. Tod, Margaret, and Ross co-authored two verbal presentations at the meeting, one on P. kernoviae in New Zealand, and one on a putative Phytophthora disease of kauri.
The next meeting of IUFRO Working Party 7.02.09 “Phytophthoras in Forests and Natural Ecosystems” will be held in New Zealand in 2010. The meeting will be jointly organised by Scion and Landcare Research and considerable interest has already been expressed by overseas Phytophthora researchers.
Margaret Dick and Tod Ramsfield
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(Scion is the trading name of the New Zealand Forest Research Institute Limited.)