Fungi from Galleries of the Emerald Ash Borer Produce Cankers in Ash Trees

Nickolas N Rajtar,Benjamin W Held,Robert A Blanchette

doi:10.3390/f12111509

Abstract

The emerald ash borer (EAB, Agrilus planipennis) is a devastating invasive pest that has killed millions of ash trees in the United States and Canada. EAB was discovered in the US in 2002 and first reported in Minnesota in 2009. It attacks ash trees that are native to the United States, including Fraxinus americana (white ash), F. nigra (black ash) and F. pennsylvanica (green ash). It also attacks Chionanthus virginicus (white fringe tree). Seven species of fungi isolated and identified only from EAB-infested trees in a previous study as having the potential to cause cankers were used to test their pathogenicity in F. americana (white ash). The fungi used were Cytospora pruinosa, Diplodia mutila, Diplodia seriata, Paraconiothyrium brasiliense, Phaeoacremonium minimum, Phaeoacremonium scolyti, and Thyronectria aurigera. Two field experiments that used F. americana used two inoculation methods: woodchip and agar plug inoculations. Results indicated that all of the fungi tested caused cankers in varying amounts, as compared to the controls. The largest cankers were caused by D. mutila (270 mm2), C. pruinosa (169 mm2), and D. seriata (69 mm2). All fungi except for T. aurigera were re-isolated and sequenced to confirm Kochs’ postulates. Canker-causing fungi found in association with EAB galleries have the potential to contribute to tree dieback and mortality.

Highlights

The emerald ash borer (EAB) (Agrilus planipennis) is a destructive invasive pest that has killed hundreds of millions of ash trees (Fraxinus spp.), causing great economic and ecological damage
The ash species F. chinensis, F. mandshurica, and F. rhychophylla co-evolved with the beetle and likely developed defense mechanisms, while ash trees native to North America, F. americana, F. nigra, and F. pennsylvanica did not, making these trees highly susceptible to attack by the beetle [7]
Seven fungal isolates were obtained from a previous study by the authors: Cytospora pruinosa, Diplodia mutila (EAB 42-6, MZ452622), Diplodia seriata (EAB 64-12, MZ452623), Paraconiothyrium brasiliense (EAB 58-13, MZ452626), Phaeoacremonium minimum (EAB 66-10, MZ452624), Phaeoacremonium scolyti (EAB 64-22, MZ452625), and Thyronectria aurigera (EAB 45-20, MZ452628) were used for inoculations in field trials [10] (Figure 1)

Summary

Introduction

The emerald ash borer (EAB) (Agrilus planipennis) is a destructive invasive pest that has killed hundreds of millions of ash trees (Fraxinus spp.), causing great economic and ecological damage. The larvae of the borer feed in the phloem of the tree and repeated attacks can girdle the stem [1,2] This buprestid, with characteristic metallic-green coloration, is native to China and other Eastern Asian countries [1]. The ash species F. chinensis, F. mandshurica, and F. rhychophylla co-evolved with the beetle and likely developed defense mechanisms, while ash trees native to North America, F. americana (white ash), F. nigra (black ash), and F. pennsylvanica (green ash) did not, making these trees highly susceptible to attack by the beetle [7]. In addition to these ash species, Chionanthus virginicus (white fringe tree) is susceptible [8]. The larvae bore serpentine-shaped galleries into the tree [9]

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