Forest pathology in New Zealand No. 23
MA Dick 2007
Causal organism Nectria fuckeliana C. Booth
Fig. 1 – Canker, or “flute”, associated with a pruned
branch stub on the stem of Pinus radiata
Stem cankers or ‘flutes’ range in size from minor depressions to flattened areas of the trunk to deep incisions. Flutes may extend up to several metres in length.
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Fig. 2 – Fruit bodies of N. fuckeliana formed
on a branch stub |
Fig. 3 – Fruit bodies of N. fuckeliana on a stem canker |
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Pinus radiata
Widespread and common in Dunedin, Southland and South Canterbury. Scattered locations in Otago Lakes, Central Otago, and Mid Canterbury (Fig 4).
Fig. 4 – Distribution of N. fuckeliana in June 2007
To view map online click here.
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Nectria fuckeliana is a Northern Hemisphere fungus commonly recorded there as a saprophyte or weak pathogen of species of Picea and Abies. In contrast to the disease in P. radiata in New Zealand stem cankers on these species are uncommon. Nectria fuckeliana has been recognised in New Zealand since the 1990s but could have entered the country in the late 1980s.
Nectria fuckeliana does not invade the stem through intact bark and cankers are generally associated with wounds such as the stubs of pruned branches. Bark cracks in the crotches of branches are also considered a likely entry point. Death of the cambium, particularly on the stem above the infection point results in the characteristic depressed cankers which may extend to over a metre in length. Lateral extension is limited and girdling of the stem with consequent crown death is extremely rare. Sapwood colonised and killed by Nectria becomes a grey-blue to pale brown colour (Fig 5) but the structural integrity of the wood is not affected.
However once dead the sapwood may be further invaded by a suite of other organisms including decay fungi which do cause cellular breakdown (Fig 6). Extensive huhu (Prionoplus reticularis) workings have also been found in the decayed segments of living trees. Typically even trees that have severe stem cankers retain a healthy green crown, though stem breakage at an infected whorl may occur.
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| Fig. 5 - Sapwood stained by N. fuckeliana |
Fig. 6 – Dead sapwood may be further colonised by decay fungi |
Nectria fuckeliana forms three different types of spores but ascospores provide the primary inoculum in nature. Ascospores are produced in red/orange fruiting bodies which form in tight clusters on the branch stubs, around the stub collar and on the bark of cankered parts of the stem. Individual fruit bodies are approximately half a mm in diameter and clusters may reach several cm in diameter. With age the fruit bodies become a dull beige to dark brown colour, making them difficult to see. Fruit bodies do not begin to appear until at least nine months after initial infection has taken place. Clusters may continue to enlarge for several years as successive layers of new fruit bodies are produced around and on top of the existing group. Fruitbodies are not produced on all infected trees.
Nectria ascospores ooze out of the fruiting bodies in droplets when adequate moisture is present and are primarily dispersed by water-splash or in wind-borne water droplets. Free water is also necessary for the ascospores to germinate. Germination can occur at a broad range of temperatures, from 5 to 28 ºC but at either end of the scale it is very slow or abnormal. The optimum temperature is between 18 and 25 ºC.
Tree response to infection is variable. Cankers may continue to extend and elongate for several years after the initial infection has occurred resulting in an extremely malformed stem. Many trees appear able to contain the infection and over several growing seasons the depressions around the pruned whorl will slowly occlude.
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Nectria fuckeliana is currently confined to about 140,000 ha of P. radiata plantations in the southern half of the South Island. Forest companies in the affected region estimate that up to 15% of trees develop stem cankers post-pruning. The development of new cankers after, but not associated with, silvicultural operations is thought to be rare. Although the actual volume loss of wood per tree may be low Nectria affects the most valuable part of the tree and stems with visible cankers at harvest will generally be culled. On some trees cankers become occluded with time and consequently older stems may show no signs of the earlier infection. As affected stands have not yet reached harvestable age the extent of internal degrade in logs that appear harvestable is currently unknown. A research project to quantify the level of degrade in harvested logs started in 2007.
Trials established to determine the effect of silviculture, stub treatment and environment on disease development indicate that pruning in winter results in more infection than summer pruning. The reason for this result is not yet clear but avoiding winter pruning should ensure that disease levels are reduced. Incidence is also related to stub size with stubs less than 30 mm diameter rarely associated with fluting. A number of methods of exploiting this result have been trialled. These include pruning earlier while branch diameter is smaller, the painting of larger stubs with a barrier/fungicide compound such as Garrison and increasing stocking to limit branch growth. None of these methods have provided complete control.
There is preliminary evidence from field trials that some P. radiata genotypes do not develop severe flute canker disease. The identification of parents that may provide full or partial resistance is in the early stages but it is hoped that genotype selection will be a tool for the management of Nectria flute canker in the future.
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