1.3.19
A NEW HOST-SPECIFIC ACT-TOXIN OF THE TANGERINE PATHOTYPE OF ALTERNARIA ALTERNATA CAUSING ALTERNARIA BROWN SPOT OF MANDARINS IN AUSTRALIA

K FUJII1, H NAKAJIMA1, M KODAMA1, H OTANI2, P MAYERS3 and K KOHMOTO1

1Faculty of Agriculture, Tottori Uiversity, Tottori 680-0945, Japan; 2United Graduate School of Agricultural Sciences, Tottori University, Tottori 680-0945, Japan; 3Maroochy Horticultural Research Station, Nambour, Queensland 4560, Australia

Background and objectives
The tangerine pathotype of Alternaria alternata (Fr.:Fr) Keisler causes brown lesions on young leaves and immature fruits of limited varieties of mandarins and tangerines. The pathotype was found in Queensland, Australia in 1966 and in Florida, USA in 1976. Previously we isolated two host-specific toxins (ACT-toxin Ib and IIb) from the US isolates and elucidated their structures [1, 2]. The present objectives are to know whether the Australian isolates collected in tangerine orchards in Queensland produce the same phytotoxins, and whether they are pathogenic to potential host plants, certain Japanese pears that are known to be affected by the pathogen in the field in addition to the original citrus host.

Results and conclusions
Pathogenicity of Australian isolates was examined by spore inoculation on detached leaves. They could infect young leaves of susceptible Emperor mandarin and Japanese pear cv. Nijisseiki, but did not affect resistant rough lemon and Japanese pear cv. Chojuro. Their pathogenicity was comparable to that of US isolates of the tangerine pathotype. Two toxins were isolated from spore germination fluids of Australian isolates using HPLC. One toxin showed a peak with the same retention time of ACT-toxin Ib, and was confirmed to be the toxin Ib by UV and FAB mass-spectrum data. The other (ACT-toxin IIIb) had a peak with longer retention time. Its 1H-NMR data showed the presence of isoleucine and of an omega-epoxy-methyl-decatrienoic acid moiety with the same configuration (2E,4Z,6E) as that of AK-toxin and AF-toxin. Its FAB mass data gave an MW of 539. Amino-acid analysis indicated that the new toxin had isoleucine. All the data collected support the structure that ACT-toxin III is isoleucine -substituted ACT-toxin Ib for valine. The dilution end-point of ACT-toxin IIIb for leaf necrosis was estimated to be 10 ng/ml for susceptible Emperor mandarin and 1 g/ml for susceptible Japanese pear cv. Nijisseiki.

References
1. Kohmoto K, Itoh Y, Shimomura N et al., 1993. Phytopathology 83, 495-502.
2. Itoh Y, Kohmoto K, Shimomura Net al., 1993. Annals of the Phytopathological Society of Japan 59, 416-427.