Biochemical and molecular characterization of Alternaria alternata isolates highly resistant to procymidone from broccoli and cabbage

Abstract

Alternaria alternata, a causal agent of leaf blights and spots on a wide range of hosts, has a high risk of developing resistance to fungicides. Procymidone, a dicarboximide fungicide (DCF), has been widely used in controlling Alternaria leaf blights in China for decades. However, the resistance of A. alternata against DCFs has rarely been reported from crucifer plants. A total of 198 A. alternata isolates were collected from commercial fields of broccoli and cabbage during 2018–2019, and their sensitivities to procymidone were determined. Biochemical and molecular characteristics were subsequently compared between the high-level procymidone-resistant (ProHR) and procymidone-sensitive (ProS) isolates, and also between ProHR isolates from broccoli and cabbage. Compared with ProS isolates, the mycelial growth rate, sporulation capacity and virulence of most ProHR isolates were reduced; ProHR isolates displayed an increased sensitivity to osmotic stresses and a reduced sensitivity to sodium dodecyl sulfate (SDS); all ProHR isolates showed a reduced sensitivity to hydrogen peroxide (H2O2) except for the isolate B102. Correlation analysis revealed a positive cross-resistance between procymidone and iprodione, or fludioxonil. When treated with 10 μg/mL of procymidone, both mycelial intracellular glycerol accumulations (MIGAs) and relative expression of AaHK1 in ProS isolates were higher than those in ProHR isolates. Sequence alignment of AaHK1 from ten ProHR isolates demonstrated that five of them possessed a single-point mutation (P94A, V612L, E708K or Q924STOP), and four isolates had an insertion or a deletion in their coding regions. No significant difference in biochemical characteristics was observed among ProHR isolates from two different hosts, though mutations in AaHK1 of the cabbage-originated ProHR isolates were distinct from those of the broccoli-originated ProHR isolates.

The HOG‐pathway related AaOS1 leads to dicarboximide‐resistance in field strains of Alternaria alternata and contributes, together with the Aafhk1, to mycotoxin production and virulence

“…To date, five classes of fungicides have been registered to control the GLS caused by A. alternata , including dicarboximides, phenylpyrroles, sterol demethylation inhibitors (DMIs), quinone outside inhibitor (QoIs) and succinate dehydrogenase inhibitors (SDHIs). However, populations of A. alternata isolated from passionfruit, 2 radish, 5 pistachio, 13–15 dendrobium officinale, 16 cabbage and broccoli 17 are reported to be resistant to the above‐described five classes of fungicides. The modes‐of‐action (MoAs) of DMIs, 13 QoIs 18 and SDHIs 18 have been elucidated, but the targets of dicarboximide and phenylpyrroles fungicides are not.…”

Genetic Structure and Triazole Antifungal Susceptibilities of Alternaria alternata from Greenhouses in Kunming, China

“…This species is a haploid phytopathogen that can cause leaf spot and postharvest rots in >100 agricultural crops and ornamental plants ( 7 ). The susceptible host plants include crucifers, eggplants, potato, tomato, cotton, pistachio, apple, citrus, pear, and other economic crops ( 8 – 10 ). In China, diseases caused by A. alternata have been reported on a wide range of plants, including cereal crops, vegetables, and fruits ( 11 – 15 ).…”