First Report of Ilyonectria macrodidyma Causing Root Rot of Olive Trees (Olea europaea) in California
Authors: Úrbez-Torres J. R. , Peduto F. , Gubler W. D. 
Published: May 2012

The California olive industry produces 99% of the U.S. olive crop, which represented a value of over $113 million in 2010. During the 2008 and 2009 growing seasons, decline of young super-high-density olive cvs. Arbequina, Arbosana, and Koroneiki trees (<4 years old) was observed in orchards throughout Glenn, Yolo, and San Joaquin Counties. Symptomatic trees showed stunted growth and chlorotic leaves with roots having black, sunken, necrotic lesions, which frequently prolonged into the base and crown of the tree. Twenty-five trees were collected from different orchards and necrotic roots as well as infected trunk tissue were plated onto potato dextrose agar amended with 0.01% tetracycline hydrochloride. Cultures were incubated at room temperature (23 ± 2°C) until fungal colonies were observed. In 17 out of 25 trees collected (68%), light yellow fungal colonies were observed from the symptomatic tissue after 7 to 10 days. Colonies turned dark yellow to orange with age and showed an orange-dark brown reverse. Both microconidia (hyaline, ellipsoidal to ovoidal and aseptate (n = 60) (6.5) 11.5 to 13.5 (17.1) × (3) 3.4 to 4.5 (5.6) μm) and macroconidia (hyaline, cylindrical, straight and/or slightly curved with one, two or three septa (n = 60) (12.5) 26.5 to 38.5 (44.1) × (4) 5.5 to 7.5 (8.5) μm) were observed. Culture and conidial morphology were in concordance with previous published description of Ilyonectria macrodidyma (Halleen, Schroers & Crous) P. Chaverri & C. Salgado (1,3,4). Identification to species level was confirmed by sequence comparison of four Californian isolates (UCCE958, UCCE959, UCCE960, and UCCE961) with sequences available in GenBank using the internal transcribed spacer region (ITS1-5.8S-ITS2) of the rDNA (primers ITS1/ITS4), a portion of the β-tubulin gene (BT1a/BT1b), and a partial sequence of the mitochondrial small subunit rDNA (NMS1/NMS2) (4). Fungal sequences of isolates from olive from California (GenBank JQ868543 to JQ868554) showed 99 to 100% homology with previously identified and deposited I. macrodidyma isolates in Genbank for all three genes. Pathogenicity of I. macrodidyma in olive cvs. Arbequina, Arbosan, and Koroneiki was investigated using two fungal isolates (UCCE958 and UCCE960) as reported by Petit and Gubler (4). The roots of 10 1-year-old trees per fungal isolate for each olive cultivar were individually inoculated with 25 ml of a 106 conidia/ml spore suspension and placed in a lath house at the UC Davis field station. Additionally, 10 trees per cultivar were inoculated with sterile water as controls. Six months after inoculation, most of the inoculated olive plants showed chlorotic leaves similar to those observed in commercial orchards. Root necrosis for each cv. was expressed as the percentage of root length having lesions (2). No significant difference was observed between isolates and average root necrosis was 29.4, 35.6, and 38.3% in Koroniki, Arbosana, and Arbequina, respectiveley. I. macrodidyma was recovered from symptomatic roots in each of the cvs. and identified based on morphology. No root rot symptoms were observed in the controls. To our knowledge, this is the first report of I. macrodidyma causing root rot of olive trees not only in California but anywhere in the world.

References: (1) P. Chaverri et al. Stud. Mycol. 68:57, 2011. (2) M. Giovanetti and B. Mosse. New Phytol. 84:489, 1980. (3) F. Halleen et al. Stud. Mycol. 50:421, 2004. (4) E. Petit and W. D. Gubler. Plant Dis. 89:1051, 2005.