Identification of Fusarium Basal Rot Pathogens of Onion and Evaluation of Fungicides against the Pathogens

Abstract

Onion (Allium cepa L.) is an economically important vegetable crop worldwide. However, various fungal diseases, including Fusarium basal rot (FBR), neck rot, and white rot, reduce onion production or bulb storage life. FBR caused by Fusarium species is among the most destructive onion diseases. In this study, we identified Fusarium species associated with FBR in Jeolla and Gyeongsang Provinces in South Korea and evaluated fungicides against the pathogens. Our morphological and molecular analyses showed that FBR in onions is associated with Fusarium commune, Fusarium oxysporum, and Fusarium proliferatum. We selected seven fungicides (fludioxonil, hexaconazole, mandestrobin, penthiopyrad, prochloraz-manganese, pydiflumetofen, and tebuconazole) and evaluated their inhibitory effects on mycelial growth of the pathogens at three different concentrations (0.01, 0.1, and 1 mg/mL). We found that prochloraz-manganese was highly effective, inhibiting 100% of the mycelial growth of the pathogens at all concentrations, followed by tebuconazole. Fludioxonil showed < 50% inhibition at 1 mg/mL for the tested isolates.

Keywords: Chemical control; Fusarium basal rot; fungicide; onion; pathogenicity.

Figure 1.

Typical symptoms of Fusarium basal rot (FBR) in onion. (A, B) FBR symptoms include leaf curling, yellowing, and rotting of bulbs, with whitish mycelium on the bulb surface. (C) Bulb tissue appears dark brown when cut open.

Figure 2.

A phylogenetic tree of Fusarium isolates generated through maximum-likelihood analysis of TEF1 sequences. Node numbers indicate bootstrap values from 500 replicates. The scale bar indicates 0.05 nucleotide substitutions per site.

Figure 3.

Morphological characteristics of Fusarium commune, Fusarium oxysporum, and Fusarium proliferatum. (A) Monophialides, (B, C) polyphialides, (D) micro- and macroconidia, and (E) chlamydospores in the middle of hyphae of F. commune. (F) Microconidia in false heads from monophialides, (G) micro- and macroconidia, and (H) chlamydospores in the middle of hyphae of F. oxysporum. (I) Mono- and polyphialides, (J) a chain of microconidia, and (K) micro- and macroconidia of F. proliferatum. (L) Chlamydospore formation was absent in the culture of F. proliferatum. Scale bars = 10 µm.

Figure 4.

Pathogenicity tests of Fusarium. commune 16-560, Fusarium. oxysporum 19-385, and Fusarium. proliferatum 17-073. Onion seeds (Allium cepa L. cv. Hwangryongball) were soaked in the conidial suspension (8 × 10⁵ conidia/mL) of each isolate for 90 min and planted in pots. Pots without the tested pathogens were used as controls. At 12 days after inoculation, photographs were taken (A) and seedling survival rate was measured (B). different letters on bars indicate significant differences according to Tukey’s test (p < 0.05). (C) Mycelial agar plugs from PDA cultures of each isolate were inoculated on the vertically cut basal plates of onion bulbs. Agar plugs without mycelia were used as controls. Photographs were taken 8 days after inoculation.

Figure 5.

Onion seeds and seedlings infected with Fusarium basal rot pathogens under pathogenicity test.