Molecular identification and characterization of Fusarium oxysporum f. sp. koae, the causal agent of koa wilt in Hawaii

PD: Shaobin Zhong

Acacia koa (koa) is the most important native tree in the State of Hawaii from both an economic and environmental perspective. Koa represents at least 90% of the value of the Hawaiian wood industry with annual revenue of $30 million. Providing an adequate supply of koa for future is a major concern of the forestry industry. Currently, there is extensive interest from both private landowners and public land managers to re-establish koa on many underutilized lands. However, young trees planted at mid and low elevations suffered serious wilt and dieback and 30 to 90% of the trees eventually died in a few years. Older koa plantings and natural stands are also impacted in major Hawaiian islands. A soil-borne fungus, Fusarium oxysporum f.sp. koae, has been identified as the causal agent of the disease, but its origin, distribution and etiology are still not well understood. Also, still completely unexplored is the genetic diversity of Fusarium oxysporum f.sp. koae. To manage this devastating disease, it is critical to identify and characterize the causal pathogen. Unfortunately, identification of Fusarium species and forms within a species based on traditional microscopic observation is difficult or impossible because of their similarity in morphology. Pathogenicity testing is also time consuming, labor intensive and often inaccurate. It is imperative to develop molecular tools for rapid identification of the species or forms causing the wilt disease in koa plants and for determining the genetic variation of the pathogen. Thus, the objectives of this project are to 1) develop a DNA-based system for rapid detection and identification of F. oxysporum isolates causing koa wilt disease in Hawaii and 2) determine genetic variation of F. oxysporum f. sp. koae by amplified fragment length polymorphism (AFLP) analysis and vegetative compatibility group test. Development of specific DNA primers will allow rapid and reliable identification of the presence of the particular causal organism (i.e., which species and which strain within a species) using polymerase chain reaction (PCR) technology. The new technique could be applied to both symptomatic and healthy-appearing but latently colonized tissues, or tissues in which this pathogen (and many other fungi) might co-occur. It will facilitate both more geographically extensive survey and intensive investigation of pathogen incidence in seed lots, seedlings and soils. Information of genetic variation will be useful for disease management by using appropriate resistant koa varieties.

Supported by USDA-CSREES