Natural Resources Advisor, UC Cooperatiove Extension

With a finding in the KwaZulu National Botanical Gardens reported in November 2017, the invasive shot hole borer (ISHB) (polyphagous and Kuroshio borers)/ fusarium dieback pest disease complex is now known to be found on three continents. Native to parts of Asia – polyphagous SHB comes from Vietnam, while Kuroshio SHB is native to Taiwan – the problems has spread to Israel, the US, Mexico, and now South Africa. Since first being discovered, PSHB has been identified in urban areas in parts of the country that are several hundred miles apart – from Durban on the east coast to the large metropolitan area of Johannesburg further north, with the Botanical Garden in-between. Samples from the city of George, in a region on the southern coast known as the Garden Route, are currently being analyzed. South Africans are very concerned about the potential to damage their extensive urban forests and are developing a national surveillance and management plan. KwaZulu National Botanical Garden is part of the International Plant Sentinel Network, where exotic species are grown to provide an early warning system for emerging pests. The US counterpart if the Sentinel Plant Network run by the American Public Gardens Association and the National Plant Diagnostic Network. Finding this pest serves as a case in point on the value of these kinds of international networks in the battle against invasive pests.

Closer to home, there is actually, potentially, maybe some good news from the Tijuana River Valley. This is the valley that has been the hardest hit by the KSHB. In 2015 the natural willow forests were tall and lush and then just a few months later they were devastated - the before-and-after photos of this event have become iconic. John Boland, a researcher working in the Tijuana River Valley reports that in areas that were severely damaged by the pest, the willows are now vigorously recovering. They are growing back from their stumps via long re-sprouts, and this new growth is not showing signs of attack. Surveys in 2017/18 found a 6% median infestation rate, down from a 97% infestation rate observed in 2015/16. The initially observed high infestation rates in the wet forest areas may have been influenced by extremely high sewage pollution into the Tijuana River, and these high infestation rates are unlikely to occur elsewhere. Boland's “soft tree hypothesis” states that willows growing in nutrient enriched waters grow quickly, laying down wood with low density and high water content, i.e., they have ‘soft' wood. This kind of growth promotes beetle infestation, as it's easy to create tunnels, and favorable for the fungal symbiont. Willows growing elsewhere would have ‘hard' wood and suffer fewer beetle attacks. His initial data analysis supports this hypothesis. If true, this would mean that riparian areas with higher quality water could see much less severe infestation. It will be interesting to observe variations in infestation ecology around the region. Damage to avocado groves in California appears to be less severe than initially anticipated – I hope the same is true for natural areas.

Boland, J.M. 2018. The Kuroshio Shot Hole Borer in the Tijuana River Valleyin 2017-18 (Year Three):
Infestation Rates, Forest Recovery, and a New Model. Final Report for US Navy, US Fish and Wildlife Service and Southwest Wetlands Interpretive Association. 74 pages.

Photo: A typical resprouting arroyo willow. The original trunk is large, broken and riddled with KSHB holes. At the time of the photo, the four vertical resprouts were growing strongly and had no KSHB holes. (John Boland)

Ambrosia beetles comprise a group of over 6,000 species in the Scolytinae subfamily. Most of these beetles typically attack decomposing and dead trees. The Polyphagous/Kuroshio Shot Borers have been reports on over 300 landscape and wildland living tree species, including avocado. Decline and death of trees has been noted in California since 2012, and the full economic extent is still unclear. The beetles feeds on a fungal symbiont that is introduced into the tree, and it is the fungus that spreads throughout the tree and causes the tree decline and death.

What was once thought to be another species of beetle (Tea Shot Hole Borer) and then identified as a new species - Polyphagous Shot Hole Borer- and now expanded to include another species of borer – Kuroshio Shot Hole – is showing that its fungal partners can be quite diverse. A recent publication indicates the increasing tangled association of the shot hole borer/disease complex that is affecting avocado and other tree species.

Two Novel Fungal Symbionts Fusarium kuroshium sp. nov. and Graphium kuroshium sp. nov. of Kuroshio Shot Hole Borer (Euwallacea sp. nr. fornicatus) Cause Fusarium Dieback on Woody Host Species in California

Francis Na, Joseph D. Carrillo, Joey S. Mayorquin, Cedric Ndinga-Muniania, and Jason E. Stajich, Department of Plant Pathology and Microbiology, University of California, Riverside, 92521; Richard Stouthamer, Department of Entomology, University of California, Riverside, 92521; Yin-Tse Huang, Department of Plant Pathology, National Chung Hsing University, Taichung 402, Taiwan, ROC, and School of Forest Resources and Conservation, Institute of Food and Agricultural Sciences, University of Florida, Gainesville; Yu-Ting Lin and Chi-Yu Chen, Department of Plant Pathology, National Chung Hsing University, Taichung 402, Taiwan, ROC; and Akif Eskalen,^† Department of Plant Pathology and Microbiology, University of California, Riverside, 92521

https://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-07-17-1042-RE

Shot hole borer (SHB)-Fusarium dieback (FD) is a new pest-disease complex affecting numerous tree species in California and is vectored by two distinct, but related ambrosia beetles (Euwallacea sp. nr. fornicatus) called polyphagous shot hole borer (PSHB) and Kuroshio shot hole borer (KSHB). These pest-disease complexes cause branch dieback and tree mortality on numerous wildland and landscape tree species, as well as agricultural tree species, primarily avocado. The recent discovery of KSHB in California initiated an investigation of fungal symbionts associated with the KSHB vector. Ten isolates of Fusarium sp. and Graphium sp., respectively, were recovered from the mycangia of adult KSHB females captured in three different locations within San Diego County and compared with the known symbiotic fungi of PSHB. Multigene phylogenetic analyses of the internal transcribed spacer region (ITS), translation elongation factor-1 alpha (TEF1-α), and RNA polymerase II subunit (RPB1, RPB2) regions as well as morphological comparisons revealed that two novel fungal associates Fusarium kuroshium sp. nov. and Graphium kuroshium sp. nov. obtained from KSHB were related to, but distinct from the fungal symbionts F. euwallaceae and G. euwallaceae associated with PSHB in California. Pathogenicity tests on healthy, young avocado plants revealed F. kuroshium and G. kuroshium to be pathogenic. Lesion lengths from inoculation of F. kuroshium were found to be significantly shorter compared with those caused by F. euwallaceae, while no difference in symptom severity was detected between Graphium spp. associated with KSHB and PSHB. These findings highlight the pest disease complexes of KSHB-FD and PSHB-FD as distinct, but collective threats adversely impacting woody hosts throughout California.