Abstract
Powdery mildew is caused by Podosphaera xanthii, and is one of the most important diseases that attacks Spanish cucurbit crops. Fungicide application is the primary control tool; however, its effectiveness is hampered by the rapid development of resistance to these compounds. In this study, the EC50 values of 26 isolates were determined in response to the succinate dehydrogenase inhibitor (SDHI) fungicides boscalid and fluopyram. From these data, the discriminatory doses were deduced and used for SDHI resistance monitoring during the 2018 and 2019 growing seasons. Of the 298 isolates analysed, 37.9% showed resistance to boscalid and 44% to fluopyram. Although different phenotypes were observed in leaf disc assays, the resistant isolates showed the same phenotype in plant assays. Compared to sensitive isolates, two amino acid changes were found in the SdhC subunit, A86V and G151R, which are associated mostly with resistance patterns to fluopyram and boscalid, respectively. Furthermore, no significant differences were observed in terms of fitness cost between the selected sensitive and resistant isolates analysed here. Lastly, a loop-mediated isothermal amplification (LAMP) assay was developed to detect A86V and G151R mutations using conidia obtained directly from infected material. Our results show that growers could continue to use boscalid and fluopyram, but resistance management practices must be implemented.
Highlights
Cucurbits are very important crops in Spanish agriculture
The results showed that P. xanthii isolates carrying only A86V were present in all samples tested, indicating resistance to succinate dehydrogenase inhibitor (SDHI) fungicides
The purpose of this work was to study, for the first time in Spain, the fungicide resistance situation of P. xanthii populations to SDHI fungicides, one of the families with relatively more active ingredients registered in recent years
Summary
Cucurbits are very important crops in Spanish agriculture. Within the vegetable sector, annual cucurbit production reached 3 million tons and yielded revenues of more than €1.9 billion in 2018 [1]. One of the most destructive diseases that affect these crops is powdery mildew, which is an important limiting factor for cucurbit production in. Spain [2,3]. Despite the substantial efforts that have been invested in plant breeding programs to combat powdery mildew disease, chemical control continues to be the principal practice for managing most cucurbit crops; it has been hampered by the emergence of resistant populations in the field soon after the introduction of certain classes of site-specific fungicides. In southern Spain, resistance to the most popular anti-powdery mildew fungicides, such as quinone outside inhibitors (QoIs), demethylation inhibitors (DMIs) and methyl benzimidazole carbamate (MBC) fungicides, has been reported [7,8,9,10,11]. Multiresistant isolates have been found in several areas of more intense cropping [9]
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