The Ophiognomonia clavigignenti-juglandacearum is an invasive species of fungus that infects the butternut trees and kills them by penetrating the phloem and progressing into bark and wood tissue. It has caused a decline in butternut trees by 80% in Ontario.
The butternut canker (Ophiognomonia clavigignenti-juglandacearum) is a non-indigenous pathogenic fungus that is rapidly increasing its threat to the butternut tree, a North American tree species. Ecosystems can be essentially changed when abundant tree species are eliminated, this is the effect that the butternut canker is having on the butternut tree (Loo, 2008). Butternuts are highly nutritious and are eaten by a wide variety of wildlife species and humans. As well, the butternut tree provides habitat for a lot of wildlife and was a valuable hardwood species used for furniture (Parks, 2014) The fungus is an important factor in the decline of the butternut tree and has left the remaining butternut trees isolated and sparsely distributed (Parks, 2014). Unlike other trees (for example, the American Chestnut) the butternut will not sprout from the root crown when the top is killed by cankers. Therefore, when a butternut canker disease destroys a population, the gene pool is lost forever and there is no possibility of reproduction (Campbell, 2002)
The origin of the fungus is not known but it is thought likely to be non-indigenous in North America because of its sudden appearance, its aggressiveness and the lack of genetic variability (Loo, 2008). In artificial inoculation studies of various Juglans species, Japanese walnut (J. ailantifolia) and heartnut (J. ailantifolia var. cordiformis), both Asian species, showed the highest levels of resistance. This piece of evidence suggests it may have originated in Asia (Furnier, 1999).
Randomly amplified polymorphic DNA (RAPD) markers were used in an experiment to examine the genetic variation in the fungus. The observed monomorphism of the fungus in this experiment is consistent with the theory that it was introduced into North America as a single isolate (Furnier, et al. 1999). It also suggests that if resistant butternut genotypes are found, the pathogen will not likely be able to rapidly evolve genotypes that can attack them (Furnier, et al. 1999). Therefore, once resistance is found, there is a very low chance that a fungal isolate will arise that is capable of overcoming the host resistance (Furnier, et al. 1999).
The fungus usually starts on small branches and twigs in the crown, although it may also be observed on the trunk and near the root collar. The pathogen is very aggressive and has spread rapidly since it was first detected in 1967 (Loo, 2008). In the span of 15 years, the fungus has cause the decline of the butternut tree by 80% in Ontario (Loo, 2008). Because of its high mortality rate, the fungus has caused the butternut tree to be designated as an endangered species in Ontario (Canada).
Figure 1: Butternut stem with black discharge from butternut canker sites within the bark. (Ontario Ministry of Natural Resources and Environment)
Figure 2: The range of butternut in North America. (Ontario Ministry of Natural Resources and Environment)
During my trip to the Oak Valley Pioneer Park, I looked at butternut trees that had been grafted onto black walnut tree roots. I analyzed whether this process had helped the butternut trees gain resistance to the canker. First, I learned to identify the difference between the black walnut and the butternut tree. One way to do this is by looking at the bark - the bark of the black walnut is rough while the bark of the butternut is smooth (Hosie, 1979). Next, I determined and recorded which of the grafts had failed. A graft failure was observed when a black walnut lead was produced instead of the desired butternut lead. From those that had remained of the butternut species, I looked for signs of an infection from the canker, shown as a split in the wood and black discharge around this split. The hypothesis was that if there is a low amount of grafted butternut trees with a canker, then grafting them to the black walnut aided in protecting these butternut trees from the canker.
There were a total of 41 trees in the plantation, from these, there were 19 graft failures. From the 22 successful grafts, only 2 showed no visible signs of infection. From these results, the grafting process was not a successful method for finding immunity against the butternut canker, as 91% of the grafted butternut trees were still infected.
In order to gain a valuable outcome, more research on the butternut canker needs to be done. If the canker is better understood, maybe a fungicide could be produced, reducing the amount of trees that are infected and potentially stopping the spread altogether. And although the grafting with the black walnut species did not work, this does not suggest that grafting with another tree species that is immune to the canker will not work.