Wilting Persian violets (Exacum affine) were observed in a Connecticut retail outlet. Diseased stems developed a dark coloration at the nodes, while the foliage turned papery, whitish tan. The vascular tissue in affected stems was reddish brown and extended from the base of the stem upward in a unilateral pattern. Fusarium grew from the discolored stem tissue when placed on Komada's medium (2). Single spores were cultured on carnation leaf agar and identified as F. oxysporum (2). Koch's postulates were completed by growing 12 2-month-old seedlings of E. affine ‘Midget’ in potting mix amended with ground dried millet seed (2.0 g/liter of soil) that had been colonized for 2 weeks by the fungus. Symptoms appeared slowly after 8 weeks, and F. oxysporum was re-isolated from the vascular tissue. Plants grown in soil mix with sterile millet remained healthy. Similar tests at different times of the year produced the same results. Other tests examined host specificity with two new isolates on seedlings of Persian violet, carnation, lisianthus, and basil. Both isolates caused symptoms only on Persian violets. Although Haematonectria haematococca (synonym Nectria haematococca, anamorph F. solani) causes Nectria canker of Persian violet (1), this is the first report of F. oxysporum causing Fusarium wilt of Persian violet. We propose the formae specialis be F. oxysporum f. sp. exaci. An isolate has been deposited at the Fusarium Research Center at The Pennsylvania State University, University Park, under Accession No. O-2282.
References: (1) M. Daughtrey et al. Compendium of Flowering Potted Plant Diseases. American Phytopathological Society, St. Paul, MN, 1995. (2) Nelson et al. Fusarium species: An Illustrated Manual for Identification. The Pennsylvania State University Press, University Park, PA, 1983.