Macadamia quick decline (MQD) has been a persistent problem since 1986 when it started killing productive 14- to 36-year-old macadamia trees in the Hilo, HI area. Fungi including Nectria regulosa, Xylaria arbuscula, Phellinus gilvus, and Acremonium recifei have been attributed to MQD and could kill twigs on healthy macadamia trees after artificial inoculation (3). The oomycete originally called Phytophthora capsici and later reclassified as P. tropicalis was also considered to be involved in the MQD complex (3). However, the primary causal agent has never been determined and the issue continues to perplex the industry. Between 2005 and 2006, a mature macadamia field on the Waiakea Experiment Station planted with cv. HAES 333 began to experience a high frequency of MQD. Trees exhibiting dull green, yellow, or brown leaves within the tree canopy were observed. Sap bleeding from the trunk, Ambrosia beetles, and Nectria fruiting bodies were consistently associated with MQD. Disease incidence was 22%. Of 21 infected trees, 53% died within an average period of 6.8 months. Four branch samples were collected from four trees showing browning of leaves, sap bleeding, Ambrosia beetles, and Nectria, and seven P. tropicalis isolates were recovered from diseased tissue on water agar or V8 agar media. No other microorganisms were isolated from diseased branches. On the basis of the morphological characteristics described by Aragaki and Uchida (1), the isolates were identified as P. tropicalis. The morphological identification was confirmed by molecular analysis of the 5.8S subunit and flanking internal transcribed spacers (ITS1 and ITS2) of rDNA amplified from DNA extracted from single-zoospore cultures with the ITS1/ITS4 primers (2,4) and sequenced (GenBank No. FJ849839). Pathogenicity tests were conducted on four 12-year-old macadamia trees in the field. A 4 × 104 zoospore/ml suspension of P. tropicalis isolate L1 was injected into branches of cv. HAES 344 to incite MQD signs and symptoms. Branches inoculated with P. tropicalis started showing the initial sign of MQD, excessive sap bleeding, within 36 days postinoculation (dpi). The presence of Ambrosia beetle frass and the appearance of orange fruiting bodies of Nectria were visible within 110 dpi. No symptoms were noted on the four control tree branches inoculated by the same method but with sterilized distilled water. P. tropicalis was reisolated from the symptomatic macadamia branches, fulfilling Koch's postulates. To our knowledge, this is the first report of P. tropicalis as the primary causal agent of MQD and its association with sap bleeding, Ambrosia beetles, and a saprotrophic species of Nectria. After completion of our research, Ko (3) reported that the MQD P. capsici was P. tropicalis, supporting our finding in this study. Quick decline of macadamia trees continues to be a serious problem in Hawaii. Minimizing tree loss in mature orchards is critical for maintaining the economic viability of Hawaii's macadamia industry. Understanding the biology of this pathosystem will enable the development of control and prevention strategies.
References: (1) M. Aragaki and J. Y. Uchida. Mycologia 93:137, 2001. (2) G. Caetano-Annolles et al. Curr. Genet. 39:346, 2001. (3) W.-H. Ko. Bot. Stud. 50:1, 2009. (4) T. J. White et al. PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA, 1990.
