Abstract
Alternaria porri (Ellis) Clf. causes purple blotch disease on Allium plants which results in the reduction of crop yields and quality. In this study, to efficiently find natural antifungal compounds against A. porri, we optimized the culture condition for the spore production of A. porri and the disease development condition for an in vivo antifungal assay. From tested plant materials, the methanol extracts derived from ten plant species belonging to the families Cupressaceae, Fabaceae, Dipterocarpaceae, Apocynaceae, Lauraceae, and Melastomataceae were selected as potent antifungal agents against A. porri. In particular, the methanol extract of Caryodaphnopsis baviensis (Lec.) A.-Shaw completely inhibited the growth of A. porri at a concentration of 111 μg/ml. Based on chromatographic and spectroscopic analyses, a neolignan compound magnolol was identified as the antifungal compound of the C. baviensis methanol extract. Magnolol showed a significant inhibitory activity against the spore germination and mycelial growth of A. porri with IC50 values of 4.5 and 5.4 μg/ml, respectively. Furthermore, when magnolol was sprayed onto onion plants at a concentration of 500 μg/ml, it showed more than an 80% disease control efficacy for the purple blotch diseases. In terms of the antifungal mechanism of magnolol, we explored the in vitro inhibitory activity on individual oxidative phosphorylation complexes I–V, and the results showed that magnolol acts as multiple inhibitors of complexes I–V. Taken together, our results provide new insight into the potential of magnolol as an active ingredient with antifungal inhibitory action to control purple blotch on onions.
Highlights
Onions (Allium cepa L.) are one of the major vegetable crops grown in widespread areas and consumed throughout the world [1]
When the A. porri KACC 42998 strain was grown on different media such as V8, potato dextrose agar (PDA), oatmeal agar (OA), carboxymethyl cellulose agar (CMCA), and malt extract agar (MEA), we observed that spore production was the highest on V8 medium followed by the PDA, OA CMCA, and MEA medium (Fig 1C and 1D)
Our results can be useful for in vitro and in vivo antifungal assay to find antifungal compounds for the control of onion purple blotch caused by A. porri
Summary
Onions (Allium cepa L.) are one of the major vegetable crops grown in widespread areas and consumed throughout the world [1]. Alternaria porri (Ellis) Cif. is a causal agent of purple blotch disease on Allium plants such as onion, leek, and garlic plants [2] and causes severe damage to onions, resulting in losses in seed and bulb production [3]. Synthetic chemical fungicides (e.g., mancozeb) have been widely used to control Alternaria diseases on various crops and vegetables [6]. The chemical controls using synthetic fungicides have been considered as one of the most efficient and consistent means to improve both yield and quality of crops. Biological controls using natural products (e.g., plant extracts and antagonistic microorganisms) have been considered to overcome the shortcomings of conventional chemical controls [10, 11]. Some natural compounds have been used as lead molecules to develop synthetic fungicides or directly used as biochemical fungicides [13]
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