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.
Highlights
IntroductionA leaf spot and blight disease, caused by the filamentous fungus Alternaria alternata (abbreviated as Alternaria leaf spots and blights (ALSB)), is among the most common diseases of numerous economic crops, including crucifers (such as cabbage,(2021) 3:15 field
A leaf spot and blight disease, caused by the filamentous fungus Alternaria alternata, is among the most common diseases of numerous economic crops, including crucifers (such as cabbage,(2021) 3:15 field
The objectives of this study were to (i) investigate whether procymidoneresistant (ProR) populations of A. alternata has developed in commercial fields of cabbage and broccoli that were repetitively exposed to procymidone; (ii) compare biochemical characteristics between ProS and ProHR isolates, and between ProHR isolates from broccoli and cabbage; (iii) reveal possible molecular resistant mechanisms of A. alternata against procymidone
Summary
A leaf spot and blight disease, caused by the filamentous fungus Alternaria alternata (abbreviated as ALSB), is among the most common diseases of numerous economic crops, including crucifers (such as cabbage,(2021) 3:15 field. DCFs and phenylpyrroles are reported to target the same enzyme, a fungal two-component histidine kinase (HK), which belongs to group III hybrid histidine kinases involved in an osmotic-regulatory signal transduction cascade of mitogen-activated protein kinase (MAPK) signaling pathway, i.e., HOG-MAPK (Avenot et al 2005; Lin & Chung 2010; Ren et al 2016). This cascade perceives extracellular stimuli, and maintains intracellular osmotic homeostasis. The typical structure of fungal group III HKs from major phytopathogenic fungi is composed of three domains: Histidine kinases-Adenylate cyclasesMethyl accepting proteins and Phosphatases (HAMPs, functionally named as “sensor domain”) at a highly variable N-terminus, a central transmitter domain composed of histidine kinase A (HisKA) and cognate histidine kinase-like ATPase catalytic subdomains (HATPase_c), and a C-terminus receiver domain harboring a three amino-acid signature (DDK) (Herivaux et al 2016)
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