Sensitivity of Alternaria alternata from Citrus to Boscalid and Polymorphism in Iron-Sulfur and in Anchored Membrane Subunits of Succinate Dehydrogenase

Abstract

Vega, B., and Dewdney, M. M. 2015. Sensitivity of Alternaria alternata from citrus to boscalid and polymorphism in iron-sulfur and in anchored membrane subunits of succinate dehydrogenase. Plant Dis. 99:231-239.Boscalid, a succinate dehydrogenase inhibitor (SDHI), was registered in 2011 to control Alternaria brown spot (ABS) of citrus, caused by Alternaria alternata. In this study, the effect of boscalid on mycelial growth, conidial germination, and resazurin reduction was established in a subset of 16 sensitive isolates using three different media. Conidial germination and mycelial growth inhibition were not suppressed even at higher concentrations of boscalid, although effective concentration to inhibit 50% growth (EC 50 ) values were established with each method. Resazurin reduction produced the lowest EC 50 values and was selected for further sensitivity tests. In total, 419 isolates, never exposed to boscalid and collected from Florida tangerine orchards between 1996 to 2012, were tested for boscalid sensitivity. The sensitivity distribution was a unimodal curve with a mean EC 50 value of 0.60 µg/ml and a range of 0.07 to 5.84 µg/ml. The molecular characterization of the succinate dehydrogenase (SDH) genes were also determined in a subset of 15 isolates, exhibiting great variability in boscalid sensitivity, by cloning and sequencing the sdhB, sdhC, and sdhD genes. Sequence comparisons of the SDH complex revealed the presence of mutations in 14 of 15 isolates. In total, 21 mutations were identified. Double and multiple mutations were observed in SDHC and SDHD, respectively. In SDHB, 4 mutations were observed while, in SDHC and SDHD, 5 and 12 mutations were detected, respectively. No mutations were found in the highly conserved histidine residues at positions 277 in SDHB, 134 in SDHC, and 133 in SDHD, typically observed in SDHI-resistant isolates. Our findings suggest that A. alternata populations from Florida are sensitive to boscalid and it could be used in ABS spray programs. Boscalid resistance is currently not a problem, although further monitoring for resistance is advisable.Alternaria brown spot (ABS), caused by Alternaria alternata (Fr.) Keissl., is the most important foliar fungal disease of tangerine plants (Citrus reticulata Blanco) and their hybrids (34,35). The disease affects the leaves, twigs, and young fruit. The first symptoms observed on young leaves is the presence of small brown to black spots surrounded by a yellow halo. Lesions expand rapidly, due to the production of the host-specific ACT toxin (1,46,47), and form irregular necrotic areas. On mature fruit, blemishes that reduce marketability can vary from small necrotic spots to large and depressed dark brown areas (12,51). Although cultural practices such as disease-free nursery stock, wide tree spacing, avoidance of excessive vegetative growth, and the elimination of overhead irrigation (15,47) can help to reduce disease severity, multiple fungicide applications are essential to achieve optimal disease control.Over the last decade, qui...

Double Mutations in Succinate Dehydrogenase Are Involved in SDHI Resistance in Corynespora cassiicola

“…SDHI-resistance has become a serious problem in agriculture, leading to a decrease of control efficacy [20,26,27]. Several studies have shown that double mutation in SDH gene further reduces susceptibility to SDHI than single point mutation, leading to a reduction of control efficacy of SDHI fungicides [6,20,21].…”

Efficacy of SDHI fungicides, including benzovindiflupyr, against Colletotrichum species

“…The polymorphism in sdhB , sdhC and sdhD gene sequences found in our study resembled the polymorphism found in A. alternata . In A. alternata isolates resistant to boscalid, 21 mutations were identified in the genes encoding fungicide‐targeted subunit proteins of SDH; however, no mutations were found in the highly conserved H277 in SDHB, H134 in SDHC and H133 in SDHD, typically observed in boscalid‐resistant isolates …”

“…In A. alternata isolates resistant to boscalid, 21 mutations were identified in the genes encoding fungicide-targeted subunit proteins of SDH; however, no mutations were found in the highly conserved H277 in SDHB, H134 in SDHC and H133 in SDHD, typically observed in boscalid-resistant isolates. 43 In laboratory-induced SDHI-resistant mutants of Mycosphaerella graminicola (Fuckel) J. Schröt. in Cohn, the causal agent of Septoria leaf blotch on wheat, 27 amino acid substitutions occurred at different positions in the three subunits SDHB, SDHC and SDHD, and it was indicated that resistance might be conditioned by sequence variation in multiple subunits of the enzyme.…”

Suppression of Ripe Rot on ‘Zesy002’ Kiwifruit with Commercial Agrochemicals

“…There have been previous reports that germination rates and hyphal development are decreased by this fungicide. Both germination rate and hyphal growth of Alternaria alternata and Penicillium digitatum were inhibited by boscalid treatment (Vega and Dewdney, 2015;Xu et al, 2020). Also, it was reported that the germination rate of Puccinia striiformis causing stripe rust in wheat (Kang et al, 2019) or Colletotrichum acutatum causing anthracnose in chili (Gao et al, 2017), was decreased by pyraclostrobin.…”