In 1999 and 2000, greenhouse-grown leek (Allium porrum) transplants produced in coastal California (Monterey County) developed a root and basal rot. Affected roots were initially gray and water soaked in appearance and later became pink, soft, and rotted. Basal plates were also affected, becoming water soaked and rotted. Severely affected transplants grew poorly and had chlorotic older leaves; many of these plants collapsed. Disease incidence varied greatly, though some transplant plantings had more than 50% disease. Similar symptoms were found in commercial, field-planted leek crops in the same region. The problem caused significant economic loss to transplant producers because of the loss of plants and the reduction in quality of surviving infected plants. Isolations from transplant and field samples consistently recovered a Fusarium species from both root and basal plate tissues. Single-spore subcultures were grown on carnation leaf agar and incubated under fluorescent light. All isolates produced abundant macroconidia that were stout, thick walled, and had prominent septa. Foot cells were indistinct to slightly notched. Conidiophores were monophialidic. Microconidia were absent and chlamydospores were present. Colonies on potato dextrose agar produced abundant, dense, white, aerial mycelium. The undersurface of these cultures was carmine red. Based on these features, all isolates were identified as Fusarium culmorum. To confirm the identification, a partial sequence (645 bp) of the translation elongation factor (EF-1α) was obtained for one isolate using primers EF-1 and EF-2 (2). The EF-1α sequence from the leek isolate was identical to that of two F. culmorum isolates in Genbank (Accession Nos. AF212462 and AF212463). The next closest match was F. cerealis, which differed from the leek isolate at six nucleotide positions. To test pathogenicity of the leek isolates of F. culmorum, we prepare inocula of four isolates from transplants and three isolates from field plants. A conidial suspension (1 × 105 conidia/ml) of each isolate was applied to the roots of 3-month-old potted leek (cvs. Autumn Giant, Blauwgroene, and Cisco). For the control treatment, leek plants were treated with water. All plants were maintained in a greenhouse at 25°C. After 1 month, inoculated plants showed foliar and root symptoms similar to those observed on the original samples. F. culmorum was reisolated from these symptomatic plants. Control plants did not develop symptoms. Using the same procedures, the seven isolates were inoculated onto other Allium species, but did not cause any symptoms on shallot (A. cepa var. ascalonicum) or eight cultivars of onion (A. cepa). Two of the seven isolates caused slight root symptoms on garlic (A. sativum). All experiments were conducted two times and the results of both tests were similar. To our knowledge, this is the first report of a root and basal rot of leek in California caused by F. culmorum. The occurrence of this disease on transplants grown in a soilless rooting medium and on raised benches in enclosed greenhouses provides circumstantial evidence that the pathogen could possibly be seedborne. This disease was reported recently in Spain (1).
References: (1) J. Armengol et al. Plant Dis. 85:679, 2001. (2) K. O'Donnell et al. Proc. Natl. Acad. Sci. 95:2044, 1998.
