VARIATION IN PHOMOPSIS ISOLATES FROM ROSACEOUS FRUIT TREES IN JAPAN
S KANEMATSU1, T KOBAYASHI2, A KUDO1 and Y OHTSU1
1National Institute of Fruit Tree Science, MAFF, Tsukuba, 305-8605, Japan; 2Tokyo University of Agriculture, Setagaya, Tokyo, 156-8502, Japan
Background and objectives
Phomopsis isolates whose conidia are morphologically similar each other are isolated from rotted fruit of peach, canker of apple and Japanese pear. These isolates can divided into two groups named the W-type and the G-type. The G-type isolates develop bigger lesions on the twig of peach, apple and Japanese pear than those of W-type ones by artificial inoculation, regardless of their isolated hosts. Deduced phylogeny based on the DNA sequence of ribosomal DNA internal transcribed spacers of the W- and G-type shows that both the isolates are clearly separated. This phylogeny also indicates that the G- type isolates and Fusicoccum sp. from peach tree, whose conidia are also similar to that of Phomopsis sp., form the identical cluster. In order to clarify the biological relationships within and between these groups, we crossed the those isolates and investigated their fertility.
Materials and methods
In the test A, sexual crosses were set up between the fertile W-type selected tester mating type strains (2a or 2b) and each of the W- or G-type isolate from peach, apple and Japanese pear. In the test B, all the combinations among G-type and Fusicoccum sp. were conducted. Both the tests were performed by using the protocol of Brayford  with some modifications. Segments of twigs of peach or Japanese pear were inoculated for both the tests. Cultured twigs were examined periodically for the formation of perithecia and its maturity. When a mature perithecia were produced on the twigs, ascospores were applied for mono-spore isolation. The results of crosses were finally observed after 12 months.
Results and conclusions
In the test A, mature Diaporthe perithecia were produced in the sexual crosses involving the W-type tester strains and the W-type strain from peach, apple and Japanese pear. On the other hand, mature Diaporthe perithecia with ascospores were produced on rare occasions in the crosses among certain G-type isolates and in the crosses between G-type isolates or Fusicoccum sp. isolated from peach tree in the test B. Ascospores produced by the cross among G-type isolates were morphologically similar to those of W-type isolates. According to the system of Wehmeyer , their teleomorphs would both be identified as D. eres. Until now, ascogenous stages were not produced in the crosses between any combinations of the W- and G-type (including Fusicoccum sp.) under similar conditions. These data indicated that the groups represented two distinct mating populations i.e. biological species. This agreed with the result of ribosomal DNA internal transcribed spacers analyses.
1. Brayford D, 1990. Mycological Research 94, 691-697.
2. Wehmeyer LE, 1933. University of Michigan Studies, Scientific Series 9, 1-349.