Differences in fungicide resistance profiles and multiple resistance to a quinone‐outside inhibitor (QoI), two succinate dehydrogenase inhibitors (SDHI), and a demethylation inhibitor (DMI) for two Stagonosporopsis species causing gummy stem blight of cucurbits

Abstract

BACKGROUND: Gummy stem blight (GSB) is a devastating disease of cucurbits that has been effectively managed with fungicide applications. However, the Stagonosporopsis spp. that cause GSB have rapidly evolved resistance to multiple classes of fungicides. To better understand the evolution and persistence of fungicide resistance in field populations, resistance profiles of unique and clonal genotypes of 113 Stagonosporopsis citrulli and 19 S. caricae isolates to four different fungicides were determined based on in vitro mycelial growth assays and molecular markers based on genes encoding fungicide targets. RESULTS: All 19 S. caricae isolates screened were resistant to tebuconazole and azoxystrobin, and sensitive to boscalid and fluopyram. All 113 S. citrulli isolates were sensitive to tebuconazole and sensitive to fluopyram, with one exception that was fluopyram-resistant. All isolates of S. citrulli except two were resistant to azoxystrobin. Phenotypic differences in response to boscalid were detected among S. citrulli isolates, but the phenotypes were not associated with multilocus genotypes (MLG) determined by 16 microsatellite loci. Additionally, isolates sharing the same MLG varied by SdhB genotype. A unique mutation of I229V in SdhB, a target of succinate dehydrogenase inhibitor fungicides, was detected for the fluopyram-resistant isolate of S. citrulli. CONCLUSION: Both the lack of association of fungicide resistance profiles with genetic similarity of isolates based on microsatellite loci and the finding that widely distributed MLG varied in fungicide resistance profiles suggest that independent evolutionary events for resistance to boscalid have likely occurred. Frequent genetic recombination within populations may be responsible for resistance to multiple fungicides. This study provides useful information for effectively managing both species of GSB fungi present in the southeastern USA and understanding the evolution of fungicide resistance within populations of plant-pathogenic fungi. EC 50 and EC 95 values of S. caricae for SDHI fungicidesThe concentrations of two SDHI fungicides, boscalid and fluopyram, required to inhibit mycelial growth by 50% (EC 50 ) and 95% (EC 95 ) for each of the 19 S. caricae isolates were determined Pest Manag Sci 2019; 75: 3093-3101

QTL associated with gummy stem blight resistance in watermelon

“…Due to the limited effectiveness of cultural practices on their own, fungicides remain critical for successful management of GSB (Stevenson et al 2004 ; Keinath 2012 ). However, recent reports of differential fungicide resistance among the three causal Stagonosporopsis species presents a significant challenge to growers since the species cannot be differentiated based on symptoms (Brewer et al 2015 ; Li et al 2016 , 2019 ; Newark et al 2019 ). In addition, fungicide applications greatly increase production costs and their repeated use may have a negative impact on the environment, particularly if residues persist in the soil.…”

Multiple mutations across the succinate dehydrogenase gene complex are associated with boscalid resistance in Didymella tanaceti in pyrethrum

“…tanaceti was SdhB-I279V. This substitution has also been identified in a single Stagonosporopsis citrulli isolate collected from watermelon [44]. The isolate was reported to be sensitive to boscalid and resistant to fluopyram.…”

Remotididymella ageratinae sp. nov. and Remotididymella anemophila sp. nov., two novel species isolated from the invasive weed Ageratina adenophora in PR China

“…However, because the phylogenetic relatives of this genus, such as Allophoma [44], Boeremia [45]. Stagonosporopsis [46, 47], Ectophoma [44], Epicoccum [48] and Similiphoma Valenz.-Lopez et al . [18] are common plant pathogens, it is necessary to determine the host range of these strains before their application for biocontrol.…”