First report of Fusarium proliferatum causing rot of garlic bulbs in North America

In September 2001, bulbs of Allium sativum L. (cultivar 'Italian Red') rotted in the drying shed at the Plant Introduction farm in Pullman, Washington. Softened cloves displayed water-soaked tan lesions, often with white mycelium near the bulb axis. Similar symptoms were noted in the same cultivar commercially grown in Idaho, 10 km from Pullman, and purchased at a retail outlet in July 2002. In each case, surface-disinfested tissue plated on agar produced a Fusarium with catenate microconidia borne on polyphialides.

Pathogenicity tests were conducted: eight garlic plants (W6-672) were grown in the greenhouse. Three plants and a control were used for each of two replications (reps). For each treated plant, six injections of 25 µl of isolate AsatF1 (10⁶ conidia per ml in nutrient yeast extract broth [NBY]) were delivered to developing cloves. Controls received sterile NBY. At 55 days, tissue was dissected from cloves in each rep, disinfested (15 sec in 70% ethanol, 3 min in 0.5% NaOCl) and plated on antibiotic malt extract agar for recovery of fungi. The experiment was repeated once.

Subsequent experiments employed cured garlic (cultivar 'Italian Red' grown in California): Five garlic bulbs in each of two reps were inoculated with AsatF1; another five served as controls in each rep. Inoculations were repeated in two additional reps of five bulbs, but substituting AsatF8 for AsatF1. Inoculum (10⁶ conidia per ml in sterile H2O) was injected into each bulb at three sites while sterile H2O was injected into controls. After 11 weeks, bulb tissue was disinfested as above, and plated to antibiotic malt extract agar or Komada's medium.

Every bulb injected with AsatF1 in greenhouse experiments displayed rot corresponding to material rotted in the drying shed. No controls displayed symptoms. Tissue excised from symptomatic cloves and plated on agar produced Fusarium isolates indistinguishable from AsatF1. The fungus did not grow from controls.

All cured garlic bulbs injected with AsatF1 or AsatF8 developed symptoms. Internal tan rot progressed from injection sites toward the clove apex, with occasional white mycelium in rot cavities. Fusarium isolates indistinguishable from AsatF1 or AsatF8 were isolated from all treatment bulbs. No controls developed symptoms, or produced Fusarium when tissue was plated to agar.

Isolates were identified as Fusarium proliferatum (T. Matsushima) Nirenberg (Nelson et al. 1983; Nirenberg and O'Donnell 1998). F. proliferatum has been reported on onion in the northwestern USA (Mohan et al. 1997). Our isolates were also pathogenic to onion (data not shown). To our knowledge this is the first report of a Fusarium in section Liseola attacking garlic in North America. Seefelder et al. (2002) have reported F. proliferatum and fumonisins in garlic bulbs in Germany.