Manchurian Rootstocks Influence EAB Resistance when Grafted to North American Ash
Emerald ash borer (EAB; Agrilus planipennis Fairmaire Coleoptera: Buprestidae) is an invasive phloem-boring pest from Asia that has killed tens of millions of North American ash trees (Fraxinus spp.). In its native range, EAB only attacks highly stressed ash trees, functioning as a secondary pest. Because Asian ash shares a co-evolutionary history with EAB, they possess a suite of secondary defensive compounds that prevent EAB from successfully colonizing healthy trees. However, all North American ash trees, regardless of health, are susceptible to EAB, and typically die within four years of infestation. Propagating ash resistant to EAB is crucial to maintaining the North American ash resource. Because many secondary metabolites utilized in plant defense are synthesized in the roots, interactions between the roots and shoots of the tree are critical in host plant resistance. Grafting utilizes this root-shoot connectivity to confer desirable traits, such as resistance to a pest or pathogen, from the rootstock of a plant to its scion. In chapter 1, I review the literature on EAB and its life history, mechanisms of host plant resistance, the practice of horticultural grafting, and root-shoot interactions in grafted plants. In chapter 2, I determine the extent to which resistant Manchurian ash rootstocks confer EAB resistance to susceptible green ash scions. The resistance capability of the parent stock for grafted trees was validated by assessing woodpecker damage and canopy dieback. Adult EAB survivorship, total leaf area consumption, and fecundity, as well as morphological tree characteristics, were measured in the field on grafted and buffer green and Manchurian ash in Indiana in 2017. Grafting trees with resistant rootstocks holds promise for propagating EAB-resistant ash trees. Although total leaf area consumed was not affected by graft combination, beetles caged on conspecific green ash lived longer and laid more eggs than beetles caged on trees with a Manchurian ash scion or rootstock. Beetle survival did not differ between conspecific Manchurian ash and heterospecific green and Manchurian trees. Beetles caged on trees with Manchurian scions and green rootstocks laid more eggs than beetles on caged on trees with green scions and Manchurian rootstocks, and no beetles caged on conspecific Manchurian ash laid eggs. This result demonstrates that, although any grafted tree with a Manchurian rootstock or scion will be more resistant than a conspecific green tree, rootstock has a greater effect than the scion on traits such as fecundity. My findings demonstrate that EAB resistance can be conferred from rootstock to scion, and that grafting could be used to develop EAB-resistant ash.
Ginzel, Purdue University.
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