In 2010 in Yuma, AZ, field-grown fennel (Foeniculum vulgare, Apiaceae) exhibited previously undescribed disease symptoms. The lower stems in contact with soil developed a brown decay and leaves on these stems became chlorotic. White mycelium and orange sporodochia were observed on affected tissues near the soil line. Diseased stems later wilted, died, and resulted in reduced quality of the fennel; these plants were not harvested. Disease distribution was patchy and prevalence was approximately 5%. Symptomatic tissues were surface sterilized in a dilute (1%) bleach solution for 3 min and tissues from the margins of the decay were placed into petri plates containing acidified corn meal agar (2 ml of 25% lactic acid/liter). Isolations consistently resulted in the recovery of a presumptive Fusarium species. Isolates were transferred to carnation leaf agar and incubated at 22°C under fluorescent lights for 10 days. Morphologies of all isolates were identical, with macroconidia being long and slender, slightly curved, with elongated, bent apical cells and notched basal cells. Conidia were borne on monophialides. Microconidia were sparse and chlamydospores were not observed. For two isolates, a portion of the translation elongation factor 1-alpha gene (TEF) was amplified with primers ef1 and ef2 (3). Based on a comparison of 668 base pairs, both isolates had the same sequence, which differed by one base pair from an accession (GQ915502.1) of Fusarium avenaceum in GenBank. The same single base pair also separated the two fennel isolates from an isolate of F. avenaceum (GL 13) previously recovered from Eustoma grandiflorum (=Lisianthus russellianus) (2). Thus, both morphological and molecular criteria support identification of the recovered fungus as F. avenaceum (Fries) Saccardo. Partial TEF sequences were deposited in GenBank (Accession Nos. JN254784, JN254785, and JN254786 for the two fennel isolates and GL 13, respectively). All isolates are archived in the Department of Plant Pathology at University of California, Davis. Pathogenicity was tested by cutting shallow slits into fennel stems, inserting one colonized agar plug into each cut, and wrapping the stems with Parafilm. Five isolates from fennel were tested on 10 stems each. Control plants were inoculated with uncolonized agar plugs. Plants were maintained at 24 to 26°C in a greenhouse. After 6 to 8 days, a brown decay developed on 70 to 90% of Fusarium-inoculated stems at the points of inoculation. Foliage later became chlorotic and F. avenaceum was recovered from all symptomatic stems. Control plants were symptomless. The experiment was completed two times and results were the same. In addition, F. avenaceum isolate GL13 from E. grandiflorum (2) was inoculated onto fennel plants with the same method. However, these inoculated plants remained symptomless. To our knowledge, this is the first report of a stem and crown rot disease of fennel caused by F. avenaceum. Apparently, the only other published account of a Fusarium disease of fennel is root rot caused by F. solani (1). The inability of the Eustoma isolate of F. avenaceum to cause disease in fennel suggests that these two crown rot pathogens may have restricted host ranges.
References: (1) J. H. Gupta and V. P. Srivastava. Indian J. Mycol. Plant Pathol. 8:206, 1979. (2) S. T. Koike et al. Plant Dis. 80:1429, 1996. (3) K. O'Donnell et al. Proc. Nat. Acad. Sci. U.S.A. 95:2044, 1998.