Metalaxyl Resistance of Phytophthora palmivora Causing Durian Diseases in Thailand

Thailand is the leading producer and exporter of durians worldwide. Serious diseases in durians include root rot, stem rot, and fruit rot, which are caused by Phytophthora palmivora, P. nicotianae, and Pythium cucurbitacearum, respectively. Thai farmers have applied fungicides for more than 20 years to control rot, but it remains difficult to control. Thus, the monitoring of fungicide-resistance development in pathogens is important for disease management. Pathogens were isolated from naturally infected durians between 2016 and 2017 in southern Thailand. The sequences of the internal transcribed spacer (ITS) and 5.8S regions of rDNA were used for the identification of their species. Seventeen out of twenty isolates were confirmed to be P. palmivora. All the isolates were tested for mycelium-growth sensitivity to metalaxyl, azoxystrobin, and dimethomorph. The results showed that nine isolates were resistant to metalaxyl with the 50% effective concentration (EC50) higher than 100 mg L−1. By contrast, all the isolates were sensitive to both azoxystrobin and dimethomorph, with EC50 < 1 mg L−1. Metalaxyl-resistant isolates were not controlled (−25.6% to 22.2%) by the treatment of the detached leaves of ‘Monthong’ durian with 100 mg L−1 metalaxyl prior to inoculation, but all the metalaxyl-sensitive and moderately metalaxyl-resistant isolates were better controlled (33.0% to 62.6%). These results clearly indicate that metalaxyl-resistant strains are present in the populations of P. palmivora in Thailand.

Chickpea seed rot and damping-off caused by metalaxyl-resistant Pythium ultimum and its management with ethaboxam

Metalaxyl and its isomer mefenoxam have been the primary fungicides used as seed treatments in managing Pythium seed rot and damping-off of chickpea. However, recent outbreaks of seed rot and damping-off of metalaxyl-treated chickpea seeds were found in the dryland agriculture regions of southeastern Washington and northern Idaho. Pythium spp. isolated from rotten seeds and associated soils showed high levels of resistance to metalaxyl. Large proportions (31 to 91%) of Pythium isolates resistant to metalaxyl were detected in areas where severe chickpea damping-off occurred and were observed in commercial chickpea fields over several years. All metalaxyl-resistant isolates were identified as P. ultimum var. ultimum. The metalaxyl resistance trait measured by EC50 values was stable over 10 generations in the absence of metalaxyl, and no observable fitness costs were associated with metalaxyl resistance. Under controlled conditions, metalaxyl treatments failed to protect chickpea seeds from seed rot and damping-off following inoculation with metalaxyl-resistant Pythium isolates. In culture, ethaboxam inhibited mycelial growth of metalaxyl-resistant, as well as metalaxyl-sensitive isolates. Greenhouse and field tests showed that ethaboxam is effective in managing metalaxyl-resistant Pythium. Ethaboxam in combination with metalaxyl is now commonly applied as seed treatments in commercial chickpea production.

Population Structure of Phytophthora infestans from a Single Location in Poland Over a Long Period of Time in Context of Weather Conditions

Phytophthora infestans (Mont.) de Bary is a destructive potato pathogen. Changing weather conditions are among the factors that influence the pathogen population structure. In this study, 237 P. infestans isolates were collected from a single unprotected experimental field in an area with high late-blight pressure located in Boguchwała in the southeastern part of Poland during 15 growing seasons (2000–2014). The isolates were assessed for mating type, mitochondrial haplotype, resistance to metalaxyl, virulence, and polymorphism of 14 single-sequence repeat markers (SSRs). The results revealed 89 unique genotypes among the 237 P. infestans isolates. Eighty-seven isolates belonged to genotype 34_A1, which was detected in all the years of research except 2012. Isolates of P. infestans from individual years were very similar to each other, as shown by Nei’s genetic identity based on 14 SSR markers. The obtained results on isolate characteristics were analyzed in terms of meteorological data (air temperature and precipitation) and indicated that frost, long winters, and hot, dry summers did not directly affect the P. infestans population structure. We described the variability in metalaxyl resistance and virulence among isolates of the P. infestans genotype 34_A1.

Differential Potential of Phytophthora capsici Resistance Mechanisms to the Fungicide Metalaxyl in Peppers

Metalaxyl is one of the main fungicides used to control pepper blight caused by Phytophthora capsici. Metalaxyl resistance of P. capsici, caused by the long-term intense use of this fungicide, has become one of the most serious challenges facing pest management. To reveal the potential resistance mechanism of P. capsici to fungicide metalaxyl, a metalaxyl-resistant mutant strain SD1-9 was obtained under laboratory conditions. The pathogenicity test showed that mutant strain SD1-9 had different pathogenicity to different host plants with or without the treatment of metalaxyl compared with that of the wild type SD1. Comparative transcriptome sequencing of mutant strain SD1-9 and wild type SD1 led to the identification of 3845 differentially expressed genes, among them, 517 genes were upregulated, while 3328 genes were down-regulated in SD1-9 compared to that in the SD1. The expression levels of 10 genes were further verified by real-time RT-PCR. KEGG analysis showed that the differentially expressed genes were enriched in the peroxisome, endocytosis, alanine and tyrosine metabolism. The expression of the candidate gene XLOC_020226 during 10 life history stages was further studied, the results showed that expression level reached a maximum at the zoospores stage and basically showed a gradually increasing trend with increasing infection time in pepper leaves in SD1-9 strain, while its expression gradually increased in the SD1 strain throughout the 10 stages, indicated that XLOC_020226 may be related to the growth and pathogenicity of P. capsici. In summary, transcriptome analysis of plant pathogen P. capsici strains with different metalaxyl resistance not only provided database of the genes involved in the metalaxyl resistance of P. capsici, but also allowed us to gain novel insights into the potential resistance mechanism of P. capsici to metalaxyl in peppers.

Metalaxyl Resistance in Phytophthora infestans: Assessing Role of RPA190 Gene and Diversity Within Clonal Lineages

Prior work has shown that the inheritance of resistance to metalaxyl, an oomycete-specific fungicide, is complex and may involve multiple genes. Recent research indicated that a single nucleotide polymorphism (SNP) in the gene encoding RPA190, the largest subunit of RNA polymerase I, confers resistance to metalaxyl (or mefenoxam) in some isolates of the potato late blight pathogen Phytophthora infestans. Using both DNA sequencing and high resolution melt assays for distinguishing RPA190 alleles, we show here that the SNP is absent from certain resistant isolates of P. infestans from North America, Europe, and Mexico. The SNP is present in some members of the US-23 and US-24 clonal lineages, but these tend to be fairly sensitive to the fungicide based on artificial media and field test data. Diversity in the level of sensitivity, RPA190 genotype, and RPA190 copy number was observed in these lineages but were uncorrelated. Controlled laboratory crosses demonstrated that RPA190 did not cosegregate with metalaxyl resistance from a Mexican and British isolate. We conclude that while metalaxyl may be used to control many contemporary strains of P. infestans, an assay based on RPA190 will not be sufficient to diagnose the sensitivity levels of isolates.

Genetically Diverse Long-Lived Clonal Lineages of Phytophthora capsici from Pepper in Gansu, China

Phytophthora capsici causes significant loss to pepper production in China, and our objective was to investigate the population structure in Gansu province. Between 2007 and 2011, 279 isolates were collected from pepper at 24 locations. Isolates (or subsets) were assessed for simple sequence repeat (SSR) genotype, metalaxyl resistance, mating type, and physiological race using cultivars from the World Vegetable Center (AVRDC) and New Mexico recombinant inbred lines (NMRILs). The A1 and A2 mating types were recovered from nine locations and metalaxyl-resistant isolates from three locations. A total of 104 isolates tested on the AVRDC panel resolved five physiological races. None of 42 isolates tested on the NMRIL panel caused visible infection. SSR genotyping of 127 isolates revealed 59 unique genotypes, with 42 present as singletons and 17 having 2 to 13 isolates. Isolates with identical genotypes were recovered from multiple sites across multiple years and, in many cases, had different race types or metalaxyl sensitivities. Isolates clustered into three groups with each group having almost exclusively the A1 or A2 mating type. Overall it appears long-lived genetically diverse clonal lineages are dispersed across Gansu, outcrossing is rare, and functionally important variation exists within a clonal framework.