A Blight Disease of Dill in California Caused by Itersonilia perplexans

In 2000, a foliar disease was observed in commercial field plantings of dill (Anethum graveolens) in coastal California. Initial symptoms consisted of a gray-green discoloration and wilting of the tips of dill leaves. As disease developed, many of the leaves discolored and collapsed, which gave the foliage a blighted appearance and made the leaves unsuitable for harvest. A fungus was consistently isolated from symptomatic leaves. Slow growing colonies were cream-colored, velutinous, and flat with minimal aerial mycelium. Mycelium was hyaline and had clamp connections. Ovoid to subglobose sporogenous cells were abundant and measured 11 to 14 × 12 to 16 μm. Spores were ballistospores that were bilaterally symmetrical, lunate, 16 to 18 × 12 to 13 μm, and germinated with hyphae or secondary ballistospores. The fungus was identified as Itersonilia perplexans Derx (1,2). Pathogenicity was tested by preparing spore suspensions (1 × 10⁴) and spray-inoculating potted dill plants. Plants were placed in a dew chamber for 24 h and then maintained in a greenhouse (23 to 25°C). After 10 days, leaves showed symptoms similar to those observed in the field and the same fungus was reisolated. Control plants sprayed with water did not develop symptoms. Inoculations were repeated and the results were the same. In another experiment, sets of dill plants and flowers of five cultivars of potted florist's chrysanthemum (Dendranthema × grandiflorum [= Chrysanthemum × morifolium]) and one cultivar of Leucanthemum paludosum (= C. paludosum) were spray-inoculated with several dill isolates and incubated as described previously. Dill plants developed symptoms and the fungus was reisolated. However, flowers of the six chrysanthemum cultivars did not develop any petal blight symptoms. Inoculations were repeated and the results were the same. These findings are consistent with previous studies that indicate I. perplexans consists of pathogenic forms that are host-specific to either chrysanthemum and closely related Compositae or to Apiaceae plants such as dill, carrot (Daucus carota subsp. sativus), parsley (Petroselinum crispum), and parsnip (Pastinaca sativa) (1,2,3). To our knowledge, this is the first report of I. perplexans on dill in California, and also appears to be the first record of this disease in North America. References: (1) T. Boekhout. Mycol. Res. 95:135, 1991. (2) T. Boekhout et al. Can. J. Microbiol. 37:188, 1991. (3) A. G. Channon. Ann. Appl. Biol. 51:1, 1963.