![]() ![]() An English walnut in San Joaquin County shows the telltale canopy yellowing and thinning and branch dieback associated with thousand cankers disease (photo by M. The UC Davis studies on TCD are funded by a grant from the USDA’s Western Sustainable Agriculture Research and Education program administered by Montana State University. ![]() So, we think it’s a risk factor, and there may be other factors that we hope to be looking at.” “It’s very likely that crown gall got there first, and then the beetles have come in and in some way were attracted to these trees. “Based on survey work we did this past year, the numbers look very strong. “Anecdotally, we had found that crown gall seemed to be associated with TCD,” he said. But he was quick to point out that their studies do not confirm cause and effect but merely show a strong association between the two. One of the more recent research findings has been a correlation between trees infected with crown gall caused by Agrobacterium tumefacians and the incidence of thousand cankers disease (TCD), Bostock said. A dark stain on an English walnut tree could be caused by a number of pests, including thousand cankers disease or shallow bark canker. ![]() But many other questions remain unanswered. Since the pest complex was first found affecting commercial English walnuts in 2009 in California, researchers have slowly unraveled some of the questions surrounding it, such as seasonal beetle activity and relationship to tree stress. “But growers have been affected by it, there’s no question.” “It seems it hasn’t been that big a deal for most growers,” said Rick Bostock, a distinguished professor emeritus in the UC Davis Department of Plant Pathology. But the pest complex should still garner respect from producers because of its potential to reduce production and kill trees under the worst case. The walnut twig beetle and the thousand cankers disease it vectors don’t appear to be the devastating pests of English walnuts they were thought when first found in commercial orchards. ![]()
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