158 Subburayalu and Sydnor: Mortality Due to Agrilus planipennis in an Urban Forest Plantation blight resistance breeding program. Evaluation of this cultivar is especially important because black ash is the only North American ash closely related to an Asian ash under current phylogeny (Wallander 2008), and has already been shown to cross-hybridize readily with Manchurian ash. Cultivar Differences To date, the reports on the susceptibility of white ash species to EAB are somewhat mixed. Rebek et al. (2008), in a garden experiment, re- ported a survival rate of 25% for white ash cul- tivar ‘Autumn Purple’ at the end of a three-year monitoring period, and they concluded that any variation observed in the resistance was of little practical significance. Cipolloni et al. (2011), studying the phenolic chemistry of white ash cultivar ‘Autumn Purple’, concluded that this cul- tivar has very low levels of defense traits and is hence susceptible to EAB. Klooster et al. (2014), reporting on the susceptibility of white ash, high- lighted that EAB has killed 99% of the white ash in southeastern Michigan forests. More recently, however, Tanis and McCullough (2015) suggest- ed that white ash cultivar ‘Autumn Purple’ may be a less preferred host. In the current study, white ash cultivars ‘Autumn Purple’ and ‘Rose- hill’ are showing intermediate survival rates. The expression of inter-cultivar variation in host-plant resistance is not something new in for- est pest management. For example, the American elm cultivar ‘Princeton’, which was initially selected in 1922 for its vigorous growth and some degree of resistance to elm leaf beetle, was later found to be highly tolerant to the Dutch elm disease fun- gus (Santamour and Bentz 1995). Any noticeable inter-cultivar variation in the white ash species could have implications in ash tree management, given the preferential planting of the cultivars ‘Autumn Purple’ and ‘Rosehill’ as the third and fourth most popular white ash cultivars in urban areas (Nowak and Sydnor 1992). The current study was carried out with a small sample size of the white ash cultivars ‘Autumn Purple’ and ‘Rosehill’. Furthermore, for both cultivars, all but one sur- viving individual were infested at the end of the study. Hence, it is imperative that further research be carried out to confirm these implications. Similar to white ash, researchers also observed differences in survival rates of the two black ash cultivars (‘Fall Gold’ and an ‘unnamed cultivar’), suggesting that there might be some possible dif- ferences in the degree to which they are resis- tant. Black ash is generally reported as the most susceptible of all the North American species (Herms 2015; Poland et al. 2015). Klooster et al. (2014) documented 100% mortality of black ash across an extensive series of monitoring plots in southeastern Michigan forests. Even though the ‘unnamed cultivar’ in the current study showed lower mortality, further screening is needed to make any interpretations about this cultivar as well, because only four trees (out of seven) sur- vived, and all of them were infested with EAB. Blue Ash: A Less Preferred Host In 2004, in a common garden study established at Michigan State University’s Tollgate Education Center (Novi, Michigan), Herms (2015) observed that blue ash had the highest survival (65% in 2014), of the North American taxa evaluated, but lower survival and greater canopy decline than Man- churian and ‘Northern Treasure’ ash. According to Herms (2015), the decline of blue ash is advanc- ing over time, suggesting that blue ash mortality may continue to increase. From a survey of ash trees in two woodlot sites in southeastern Michi- gan in 2004–2005, Anulewicz et al. (2007) antici- pated that all blue ash would eventually die aſter the death of the preferred white ash. However, seven years later, at the same two sites, Tanis and McCullough (2012) reported a surprising survival of large numbers of blue ash trees (67% on aver- age across the two sites). The relative resistance of blue ash was also reported by Tanis and Mc- Cullough (2015) in a subsequent study. Peterson et al. (2015) found that larval EAB performance on blue ash and highly susceptible green ash did not differ, which suggests that resistance of blue ash is due to low oviposition preference. The higher survival rates of blue ash observed in the current study further affirms claims that blue ash is a less preferred host among the North American ash taxa. The finding underscores the need for evalu- ating resistance mechanisms in blue ash, includ- ing oviposition preference of EAB for blue ash. ©2018 International Society of Arboriculture
May 2018
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