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Nature of Cultural Variability in Fusarium oxysporum f. sp. apii Race 2. R. T. Awuah, Graduate student, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853; J. W. Lorbeer, Professor, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853. Phytopathology 78:385-389. Accepted for publication 29 July 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-385.

Fusarium oxysporum f. sp. apii race 2 was characterized by abundant aerial mycelium (mycelial cultures) when isolated from diseased celery plants and cultured on chloramphenicol-amended potato-dextrose agar (CPDA). Two-week-old mycelial isolates cultured on CPDA at 2427 C (12-hr photoperiod: fluorescent cool-white light; 3.875 lx) produced abundant microconidia but few macroconidia. When single microconidia were repeatedly transferred from mycelial cultures and grown on CPDA as above, cultures lacking aerial mycelium (pionnotal cultures) frequently were produced. These either originated directly from mycelial cultures or indirectly from intermediate cultures possessing both mycelial and pionnotal patches. Mycelial cultures were more virulent on celery than pionnotal cultures. Twenty-nine percent of plants infected with mycelial cultures developed vascular dicoloration and 3% died after 8 wk. None of the plants infected with pionnotal cultures developed any of these symptoms. In a sterile soil extract solution, mycelial cultures significantly produced more chlamydospores than pionnotal cultures. Mycelial cultures were less stable than pionnotal cultures and produced fewer macroconidia on CPDA. Radial growth and dimensions of conidia were similar for both culture types. Mycelial and pionnotal cultures maintained their morphological identity when passed through the host and reisolated on CPDA. It appears that only the mycelial type occurs in nature and that the intermediate and pionnotal forms are only of cultural significance.