DISEASE FOCUS: Fusarium dieback and polyphagous shot hole borer
by Jim Downer and Akif Eskalen
Some fungi threaten plants when they form a disease complex with their vectors.The polyphagous shot hole borer (PSHB) (Euwallacea sp.) is considered an “ambrosia” beetle because it affiliates itself with the fungus Fusarium euwallaceae, a potent vascular wilt fungus in plants. Beetles carry fungal cells within specialized structures called mycangia near their mouth parts. When the beetle enters a tree, it creates brood galleries beyond the cambium, where its fungal symbiont begins to grow in the tree. Fusarium euwallaceae colonizes the wall of the entire brood gallery and becomes food for beetle larvae (Eskalen et al. 2012 and Mendel et al. 2012). In some host (tree) interactions with the beetle, the fungus continues to invade the sapwood, plugging the vessels, which results in symptoms of branch dieback or tree death (Eskalen and Stouthamer 2013).
The host range for PSHB is quite large, but the hosts preferred by the beetle and the lethal host range are restricted to fewer species. A current host list with susceptibility to the beetle and the Fusarium disease can be found in Eskalen et al. 2013 (see “References” below). In this study, Eskalen and others examined 335 host trees at two botanical gardens in Los Angeles County. Of the trees examined, 207 had symptoms of PSHB attack. There were 113 tree species in 40 plant families from which Fusarium was recovered. Families with the most members from which Fusarium was recovered were the Aceraceae, Fabaceae and Fagaceae. There were 27 trees from all these that were suitable for beetle reproduction, however the authors indicate this might be an underestimate.
This beetle/fungus complex poses risk to both ornamental and native trees in California, and the number of susceptible trees is five times higher than with Phytophthora ramorum, the pathogen causing sudden oak death (Eskalen et al. 2013). Other Fusarium diseases of trees are relatively unusual in California—the only other two notable ones are albizia wilt, caused by Fusarium oxysporum f. sp. Pemiciosum, and date palm wilt, caused by F. oxysporum canariensis. The Fusarium causing wilt disease associated with PSHB was recently named F. euwallaceae by Freeman and others (2013).
Beetle/fungus combination can cause mortality (wilt) on trees, such as Acer negundo (fig. 1), Quercus robur and Platanus racemosa, and on weeds, such as Ricinus communis. Pathogenicity is linked to host susceptibility and perhaps the ability of PSHB to find and enter a susceptible host. Unlike other ambrosia beetle vectored-diseases, PSHB seems to attack healthy trees that have no typical predispositions (age, drought, stress, etc.). F. euwallaceae causes necrosis to wood tissues inside the tree and lines the brood galleries made by the beetle. Necrosis extends outward from beetle galleries and in a susceptible host will extend into the sapwood, resulting in wilt symptoms on the tree. Wilt symptoms may be localized to a branch, but more typically involve collapse of the entire tree. Sometimes there is a partial response to infection; necrosis occurs but the pathogen does not spread sufficiently to cause wilt.
Fig. 1. Beetle galleries and wood discoloration (left) and branch dieback (right) on box elder (Acer negundo) caused by PSHB and Fusarium disease complex. Photos by A. Eskalen.
Jim Downer is Environmental Horticulture Farm Advisor, UC Cooperative Extension, Ventura County and Akif Eskalen is Subtropical Plant Pathology Specialist,Department of Plant Pathology and Microbiology, UC Riverside.
Eskalen A, Stouthamer R. 2012. A new beetle-fungus disease complex threatening avocado. California Avocado Society 2011 Yearbook 94:120-126.
Eskalen A, Stouthamer R, Lynch SC, Rugman-Jones P, Twizeyimana M, Gonzalez A,Thibault T. 2013. Host range of fusarium dieback and its ambrosia beetle (Coleoptera:Scolytinae) vector in Southern California. Plant Dis. 97:938-951.
Eskalen A, Gonzalez A, Wang DH, Twizeyimana M, Mayorquin JS, Lynch SC. 2012. First report of Fusarium sp. and its vector tea shot hole borer (Euwallacea fornicatus) causing Fusarium dieback on avocado in California. Plant Dis. 96:1070.
Mendel Z, Protosav A, Sharon M, Zveibil A, Ben Yahuda S, O’Donnell K, Rabaglia R, Wysoki M, Freeman S. 2012. An Asian ambrosia beetle Euwallacea fornicatus and its novel symbiotic fungus
Fusarium sp. pose a serious threat to Israeli avocado industry. Phytoparasitica, Online publication. doi:10.1007/s12600-012-0223-7.
Freeman S, Sharon M, Maymom M, Mendel Z, Protasov A, Aoki T, Eskalen A, O'Donnell K. 2013. Fusarium euwallaceae sp. nov.—a symbiotic fungus of Euwallacea sp., an invasive ambrosia beetle in Israel and California. Mycologia 105:1595-1606.