First Report of a Fusarium sp. and Its Vector Tea Shot Hole Borer (Euwallacea fornicatus) Causing Fusarium Dieback on Avocado in California

Abstract

Per capita consumption of avocado in the United States has nearly doubled between 2000 and 2010. The California avocado industry supplies almost 40% of U.S. demand and the remaining 60% is supplied by imports from Latin America and New Zealand. The Tea Shot Hole Borer (TSHB) is an ambrosia beetle from Asia that forms a symbiosis with a new, yet undescribed Fusarium sp. and is a serious problem for the Israeli avocado industry (3). The beetle also causes severe damage on the branches of tea (Camelia sinensis) in Sri Lanka and India (1). In California, TSHB was first reported on black locust (Robinia pseudoacacia) in 2003, but there are no records of fungal damage (4). In 2012, nine backyard avocado trees (cvs. Hass, Bacon, Fuerte, and Nabal) exhibiting branch dieback were observed throughout the residential neighborhoods of South Gate, Downey, and Pico Rivera in Los Angeles County. Upon inspection, symptoms of white powdery exudate, either dry or surrounded by wet discoloration of the outer bark in association with a single beetle exit hole, were found on the trunk and main branches of the tree. Examination of the cortex and wood under the exit hole revealed brown discolored necrosis. The TSHB was also found within galleries that were 1 to 4 cm long going against the grain. Symptomatic cortex and sapwood tissues were plated onto potato dextrose agar amended with 0.01% tetracycline (PDA-tet). The TSHB was dissected and plated onto PDA-tet after surface disinfestation following methods described by Kajimura and Hijii (2). After 5 days of incubation at room temperature, regular fungal colonies with aerial mycelia and reddish brown margins were produced. Single spore isolations were used to establish pure culture of the fungus. Fifty conidia were hyaline, clavate with a rounded apex, and initially aseptate (4.1 to 12.0 × 2.4 to 4.1 μm) becoming one- to three-septate (7.6 to 15.1 × 2.8 to 4.5 μm, 9.2 to 17.2 × 3.4 to 4.8 μm, and 13.5 to 17.6 × 4.3 to 4.7 μm, respectively). Identity of the fungal isolates was determined by amplification of the rDNA genes with primers ITS4/5 and EF1/2, respectively. Sequences were deposited into GenBank under Accession Nos. JQ723753, JQ723760, JQ723756, and JQ723763. A BLASTn search revealed 100% similarity to Fusarium sp. (Accession Nos. JQ038020 and JQ038013). Detached green shoots of healthy 1-year-old avocado were wounded to a depth of 1 to 2 mm and 5-mm mycelial plugs from 5-day-old cultures (UCR 1781 and UCR 1837) were placed mycelial side down onto the freshly wounded surfaces and then wrapped with Parafilm. Control shoots were inoculated with sterile agar plugs and five replicates per treatment were used. Shoots were incubated at 25 ± 1°C in moist chambers for 3 weeks. Lesions were observed on all inoculated shoots except for the control. Mean lesion lengths were 10.7 and 12.8 cm for UCR1781 and UCR1837, respectively, and were significantly different (P ≤ 0.05) from the control. Both isolates were reisolated from 100% of symptomatic tissues of inoculated shoots to complete Koch's postulates. This experiment was conducted twice and similar results were obtained. To our knowledge, this is the first report of Fusarium sp. and its vector E. fornicatus causing Fusarium dieback on Avocado in California. References: (1) W. Danthanarayana. Tea Quarterly 39:61, 1968. (2) H. Kajimura and N. Hijii. Ecol. Res. 7:107, 1992; (3) Mendel et al., Phytoparasitica, DOI 10.1007/s12600-012-0223-7, 2012. (4) R. J. Rabaglia. Annals Entomol. Soc. Amer. 99:1034, 2006.

Destructive Tree Diseases Associated with Ambrosia and Bark Beetles: Black Swan Events in Tree Pathology?

“…These diseases have developed in forest ecosystems, urban landscapes and agricultural settings, under varied environments, and in geographically diverse locations (2,42,51,56,73,89,94,98,114,133,137,149,169). The most significant of these diseases have had huge and surprising impacts.…”

“…Recently, a branch dieback and canker disease was associated with E. fornicatus on avocado and other trees (32,42,114) (Fig. 11).…”

“…in Florida, another species of Fusarium in California and Israel, and a fourth species in Australia (42,114; Freeman et al, unpublished; Kasson et al, unpublished). Artificial inoculations of avocado in Florida, California and Israel indicate that the new species are moderately pathogenic (42,114, Ploetz et al., unpublished). In California and Israel, the great and surprising impact of Fusarium dieback on avocado and its unexplained (and expanding?)…”

“…Some of these problems, such as Dutch elm disease (DED), have a long history, have been extensively researched, and are fairly well understood (23,56,70,113,149,168,169). In contrast, other similar diseases developed recently and are poorly or partially understood (2,3,42,51,73,89,94,98,114,127,137,154,158, 164,165). Significant data gaps may exist for the ecology, epidemiology and management of the latter diseases.…”

Exotic plant disease threats to the New Zealand avocado industry and climatic suitability: a review

“…This fungus and its associated ambrosia beetle vectors have been described as causing a serious dieback disease in Israel (Mendel et al 2012;Mendel et al 2017) and California (Eskalen et al 2012). Further examination has shown that molecular phylogenetics distinguishes six morphologically identical clades of F. euwallaceae, of which at least four attack and reproduce in avocado (O'Donnell et al 2016).…”