Poster Presentation Science Protecting Plant Health 2017

Spread and Severity of Fusarium Dieback – Polyphagous Shot Hole Borer Throughout Urban Forests in Los Angeles County (#154)

Shannon C Lynch 1 2 , A Eskalen 2 , G Gilbert 1
  1. Department of Environmental Studies, University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064
  2. Department of Plant Pathology and Microbiology, University of California, Riverside, 3401 Watkins Ave., Riverside, CA 92521

Trees in California’s urban forests remove the CO2 equivalent of 120,000 cars and provide USD 1.0 billion annually for ecosystem services. These benefits are under threat by the polyphagous shot hole borer (PSHB; Euwallacea sp.) and its symbiotic fungal pathogen Fusarium euwallaceae, which together cause Fusarium dieback (FD) on 51 tree species that support reproduction of the beetle. The scope of the problem is limited by insufficient data on FD distribution and impacts. The objectives of this study were to: 1) determine FD occurrence in Los Angeles; 2) identify indicators for disease spread. We conducted a rapid assessment at the urban-rural interface of the San Gabriel and Santa Monica Mountains (89 km), marking the location of confirmed positive and negative hosts every 2 km with a GPS. In the San Fernando valley, we counted and marked the location of every infested and non-infested hosts within 45 randomly selected grids (65 ha each). FD was detected on 550 trees across 30 new locations along the urban-rural interface, and two new reproductive hosts (Aesculus californica and Quercus chrysolepis). Platanus racemosa is the most common reproductive host, suggesting it is important for FD spread. Most individuals of each rare host species were infested, suggesting potential losses in biodiversity in the landscape. FD was detected on 413 of the 5,684 trees surveyed in the San Fernando Valley. Liquidambar styraciflua was the most common host, but a larger proportion of sycamore trees were infested (28% versus 5%). The likelihood of PSHB arrival was a function of sycamore abundance, but grids differed in the magnitude of disease associated with sycamore and other reproductive host densities. Preliminary results suggest the beetle is using locations with sycamore for movement through landscape, but local dynamics are idiosyncratic. Monitoring sites over time will better determine local impacts.