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First Report of Neofabraea alba Causing Fruit Spot on Olive in North America

Olive (Olea europaea) is a widely planted evergreen tree primarily grown for its oil, fruit for pickling, and landscape appeal in Mediterranean and temperate climates. California produces most of the olives grown in the United States; its industry was valued at $53 million in 2011 (4). In 2005 and 2008, fruit spotting occurred on coratina and picholine cultivars in two commercial orchards in Sonoma County. The spots were scattered, slightly sunken and brown, and surrounded by a green halo. Many of the spots were associated with lenticels. A slow to moderate growing, cream to rose-colored fungus was isolated from the spots onto potato dextrose agar (PDA) amended with 0.01% tetracycline hydrochloride. Sporulation was observed in vitro on PDA after 40 days under near-UV light. Macroconidia, produced from conidiomata, were hyaline, aseptate, cylindrical to fusiform-allantoid, slightly curved, and 17 to 27 × 2.5 to 3.5 μm (average 21.1 × 2.9 μm). Microconidia were aseptate, strongly curved, hyaline, and 14 to 18 × 0.75 to 1 μm (average 16.1 × 0.9 μm). rDNA sequences of the internal transcribed spacer (ITS) region of the isolate (GenBank KC751540), amplified using primers ITS1 and ITS4, were 99.8% identical to Neofabraea alba (E.J. Guthrie) Verkley (anamorph Phlyctema vagabunda) (=Gloeosporium olivae) (AF141190). Pathogenicity was tested on detached, green fruit (cv. frantoio). Olives were surface sterilized in 10% sodium hypochlorite for 5 min and air dried. Five olives were wounded with a needle and 10 μl spore suspension (10⁵ spores/ml) was placed on each wound. An equal amount of spore suspension was placed on five unwounded olives. Water was also placed on wounded and unwounded olives to serve as a control. The olives were placed on racks in 22.5 × 30 cm crispers lined with wet paper towels and incubated at 23°C. After 21 days, the olives began to turn red. Olives wounded and inoculated with N. alba had a distinct green ring around the inoculation point where maturity was inhibited. Control olives uniformly turned red. After 35 days, wound-inoculated olives began to form a sunken, brown lesion at the inoculation point where aerial mycelium was visible. After 51 days, lesions were visibly sunken and immature conidiomata began to form in concentric rings giving a bull's eye-like appearance. Unwounded fruit exhibited uneven maturity and green spots associated with the lenticels throughout the experiment but did not develop sunken lesions. Control fruit showed no symptoms and ripened normally. After 56 days, fruit was surface sterilized in 10% sodium hypochlorite for 5 min and plated onto PDA. N. alba was isolated from the sunken and green areas of all of the wounded and unwounded fruit. No fungi grew from the control fruit. The experiment was repeated once with similar results. N. alba has been reported to cause an anthracnose disease on fruit and leaves of olives in Spain and Italy (1,2). In North America, N. alba causes a bull's eye rot on fruit of Malus and Pyrus spp. in the Pacific Northwest and coin canker of Fraxinus spp. in Michigan and Canada (3). To our knowledge, this is the first report of N. alba causing disease on olive in North America.

References: (1) J. Del Maral de la Vega et al. Bol. San Veg. Plagas. 12:9. 1986. (2) S. Foschi. Annali. Sper. Agr., n.s. 9:911. 1955. (3) T. D. Gariepy et al. Can. J. Plant Pathol. 27:118. 2005. (4) United States Department of Agriculture, National Agricultural Statistics Service, California Field Office, California Agriculture Statistics, Crop Year 2011.