Research Update Nematode impact gets close look


PARC scientist aims to get clear picture of how tiny roundwormcan damage cherry orchards.


By Scott Trudeau


n experiment is getting under way at the Pacific Agri-Research Centre to explore the potential impact of a particular nematode species and its effect on cherry trees. Tom Forge, a scientist at the centre, noted that few people study nematodes — microscopic roundworms inhabiting the soil.


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“Within tree fruit production, we’ve known for a long time that there are a number of species of nematodes in most producing areas of the world that are recognized to be damaging,” said Forge. “In fact, the most well-known manifestation of this is when people go to replant orchards.”’


What often occurs is that young trees planted into sites where orchards grew previously do very poorly, the result of a condition called replant syndrome or replant disease complex. Forge said several factors can contribute to depressed growth and over time it has been discovered nematodes are an important part of the entire complex. One species in particular – the root lesion nematode – is most often associated with this replant problem. If one were to ask an orchardist if they are familiar with nematodes or if their trees have nematodes, chances are it would be the root lesion species.


More recently discoveries were made in vineyards of a nematode species called the ring nematode (Mesocriconema xenoplax).


“It’s because when we look at it under a microscope, it has actually quite pretty circles on its cuticle,” said Forge. Before he came to the Okanagan, Forge did post-doctoral research in Oregon.


“It just so happens when I was there, the nematology program I was working with had started to discover that ring nematodes were in a lot of their vineyards,” he said. “They went on to do experimental work demonstrating that the nematode was quite damaging to the grapevine.”


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Recently, Forge began examining what was happening in 10- and 20-year-old cherry orchards that had been experiencing patchy growth in orchard blocks. Because he was already aware ring nematodes were found in vineyards, he wanted to see if it the same nematode was contributing to the problems in Okanagan cherries.


“I knew in the back of my head from work that had been done with peaches and also plums in California and the southeastern United States, other prunist-growing regions, that this nematode is well-known to cause problems in peach trees in particular,” said Forge. “Sure enough, we found it on about 70 per cent of the orchard blocks where we sampled.”


Samples were taken from 38 orchard blocks in the Okanagan Valley. There was no pattern to indicate the nematodes were more prevalent in one part of the valley. There was a range of densities among the sample orchards. In nine of the blocks, tree measurements were taken to see if there was a correlation between nematode populations and tree measurements of


mature trees. “We found a statistically significant negative relationship between the numbers of this nematode and leaf area, which in turn is a pretty good indicator of the overall vigour of the tree in that growing year,” said Forge. “That was consistent with what we might expect and it basically just tells us we need to do some experimental work now, where we can plant cherry trees in soil, side by side, with or without the nematodes so we can get comparisons of the effect of the nematodes that aren’t obscured by other correlating factors.” An experiment is being arranged at PARC using 36 micro plots – large concrete tubes located in the ground – that enable scientists to confine the entire root system. Half of the plots will be inoculated with the nematode and the other half will not.


“We’ll plant these cherry trees out and over the course of several years, we’ll monitor the growth of the cherry trees and compare the roots of the ones that have nematodes in them and the ones that don’t,” said Forge.


While the nematode is having a negative impact on cherry production, it’s too early to know the specifics of that impact however he said the experiment involving nematodes coincides with recent changes to fumigation restrictions on buffers around the practice. The fumigants growers once used when they replanted to eliminate nematodes are not being pulled from the market but restrictions in terms of the size of the buffers around the fields or orchard blocks are increasing, said Forge. “It doesn’t mean people can’t fumigate; it just means it’s going to be a lot harder to fumigate in the future.”


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