2.2.22
WHAT VEGETATIVE COMPATIBILITY DOES AND DOES NOT REVEAL ABOUT THE POPULATION STRUCTURE OF FUSARIUM OXYSPORUM F.  SP. CUBENSE IN THAILAND

RC PLOETZ, JL HAYNES and A VAZQUEZ

University of Florida, Tropical Research and Education Center, 18905 SW 280th Street, Homestead, FL 33031-3314 USA

Background and objectives
Panama disease, caused by Fusarium oxysporum f. sp. cubense, constrains banana production worldwide. Recent work indicates that the pathogen originated in south-east Asia, coincident with its banana host [1], and that it is most diverse within this region [2]. Effective deployment of resistant, replacement cultivars will rely on prior knowledge of the pathogen's diversity in the affected areas. The objectives of this study were therefore to: (1) determine the distribution of Panama disease and the banana cultivars it affects in Thailand; (2) investigate the population structure of F. oxysporum f. sp. cubense in the country; and (3) examine external factors that might explain the observed structure.

Materials and methods
From 1990 to 1995, surveys of banana-producing regions were conducted in Thailand by several individuals. Tissue samples were recovered from plants with symptoms of Panama disease and assayed for F. oxysporum in the laboratory. Isolates were single-spored and identified as members of the species prior to storage at 4C and -80C. The genetic diversity of these isolates was characterized using vegetative compatibility and RAPDs. Pathogenic diversity of a representative subset of the isolates studied was assessed in artificial inoculation experiments with tissue-culture-derived plantlets of different banana cultivars.

Results and conclusions
With a single exception, 'Kluai nam wa ABB' (Pisang awak) was the only banana cultivar that was affected by Panama disease during the surveys. The disease was observed once on the cv. Kluai lanka AAB (Mysore) and was never seen on the race 1 susceptible cv. Kluai hom thong AAA (Gros Michel), even when it was in close proximity to diseased 'Kluai nam wa'. Thus, reports that indicate that the cv. Kluai nam wa is susceptible to race 1 may be erroneous. Alternatively, since Panama disease was also not observed on the race 2 susceptible cv. Kluai hak muk ABB (Bluggoe), there may be a new race of the pathogen present in Thailand.

Four vegetative compatibility groups (VCGs) were found during the surveys, two of which were uncommon. VCG 01218 occurred mainly in the South (Provinces of Narathiwat and Yala) and, prior to the survey, had only been collected in Java, Sumatra and peninsular Malaysia. It appears to have been introduced into southern Thailand from these last areas by Yawi-speaking Muslim populations which now reside in all of the above areas. A new VCG for this taxon, VCG 01221, was found only in northern portions of Thailand (Provinces of Chiang Rai and Nan). In contrast, VCGs 0123 and 0124-0125 were common and widespread. Two subgroups, A and B, were noted in VCG 0123. Isolates in A were compatible with only 10% of the B isolates but compatibility among isolates in the subgroups was common (84 and 60%, respectively).

The genetic diversity within and among the VCGs and the 0123 A and B subgroups, as determined by RAPDs, will be discussed. In addition, pathogenic diversity in these populations will be presented in light of these findings.

References
1. Ploetz RC, Pegg KG, 1997. Australasian Plant Pathology 26, 239-49.
2. Koenig RL, Ploetz RC, Kistler HC, 1997. Phytopathology 87, 915-23.