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THE PATHOSYSTEM FUSARIUM OXYSPORUM F.SP. CALLISTEPHI/CALLISTEPHUS CHINENSIS: THERE IS NO RELATIONSHIP BETWEEN GROWTH PATTERNS OF THE PATHOGEN IN VITRO AND VIRULENCE TO SPECIFIC CHINA ASTER CULTIVARS IN VIVO

W DERCKS1, E ZEIDLER1 and A ORLICZ-LUTHARDT2

1Fachhochschule Erfurt, Fachbereich Gartenbau, 99085 Erfurt, Germany; 2Institut fur Gemuse- und Zierpfianzenbau, 99189 Erfurt-Kuhnhausen, Germany

Background and objectives
A serious limiting factor in the production of China asters (Callistephus chinensis) is a wilt disease caused by Fusarium oxysporum f.sp. callistephi [1]. The disease cannot be controlled chemically, hence the use of resistant cultivars is crucial for successful control in horticultural production [2]. The level of disease resistance is dependent not only on inherent genetic traits of any given cultivar, but also on the virulence of isolates of the pathogen present in the soil. If a correlation existed between certain growth patterns in vitro and virulence of these isolates on a cultivar in vivo, a morphological analysis of the isolates present could yield information on which cultivars should be cultivated and which avoided. This would be of considerable interest to researchers, advisors and growers. The investigations described here were carried out in order to evaluate whether or not such a correlation exists.

Materials and methods
Nine isolates from Germany and one isolate from Poland were analysed for type of spores produced (micro- and macroconidia) as well as for the growth rate of mycelium. The studies were performed in parallel on Czapek-Dox, potato dextrose, and synthetic agar. Then greenhouse experiments were conducted to assess the virulence of these isolates towards potted plants of seven China aster cultivars with different levels of resistance. Inoculation and disease rating were carried out according to [1, 2]. Rating was done at intervals of 10 days until 100 days following inoculation, when the tests were completed.

Results and conclusions
The isolates differed in mycelial growth rate. All isolates formed microconidia, but only six also formed macroconidia. Formation commenced between 1and 3 days upon inoculation of the plates and continued until 7 days after inoculation, when the experiments were stopped. There were remarkable differences in virulence of the isolates. The individual level of resistance of every cultivar was dependent on the isolate of the pathogen used. However, the cultivars could be grouped into two sets of overall resistance levels, according to the maximum amount of disease. One group comprised the more resistant cultivars Pfirsichblijte, Dunkelblau, Migelia and Nico (0-35% diseased plants), the other the more susceptible cultivars Starlight Rose, Roter Edelstein and Lachskarmin (10-100% diseased plants). At present, German growers are strongly advised to rely on cultivars from the first group, since these can be expected to suffer less damage. There was no correlation between growth rate of mycelium in vitro and virulence in vivo. The same holds true for type of spores produced in vitro and virulence in vivo. Thus it is not possible to base predictions of virulence on patterns of morphological growth. Future studies will focus on the identification of genomic markers for genotype characterization of the pathogen and their correlation with virulence in pathogen-host interactions at different levels of resistance.

References
1. Hoffmann GM, 1963. Gartenbauwissenschaft 28, 319-358.
2. Persiel F, Lein H, 1989. Zeitschrift fur Pfianzenkrankheiten und Pflanzenschutz 96, 47-59.