Early blight, caused by Alternaria solani, along with brown spot, caused by A. alternata, have the potential to reduce quality and yield in potato production globally. Prior to this study, the incidence, disease impact, and fungicide resistance attributes of A. alternata in Wisconsin were poorly understood. Potato pathogens were isolated from foliar lesions at three commercial locations in Wisconsin in 2012 and 2017 and were initially morphologically identified as A. solani (n = 33) and A. alternata (n = 40). Identifications were further corroborated with the phylogenetic analysis of the internal transcribed spacer (ITS), translation elongation factor 1 (TEF1), gapdh, Alt a 1, and OPA10-2. A multigene phylogeny of ITS, TEF1, gapdh, and Alt a 1 showed five genotypes of A. alternata and one single genotype of A. solani. We demonstrated that the A. alternata isolates were virulent on potato cultivars Russet Burbank (P < 0.013) and Atlantic (P < 0.0073), though they caused less disease than A. solani (P < 0.0001 and P < 0.0001, respectively). A. alternata caused little disease on the breeding line 24-24-12 (P = 0.9929), and A. solani caused fewer disease symptoms on 24-24-12 than on Russet Burbank (P < 0.0001) or Atlantic (P < 0.0001). Breeding line 24-24-12 may be a promising source of potential resistance for the two diseases. There was no significant difference in virulence of different A. alternata genotypes, and no significant difference in virulence or genotype clustering among isolates from the three locations. Isolates of A. alternata that induced chlorosis caused larger lesion areas than isolates that did not in Russet Burbank (P < 0.0001), Atlantic (P < 0.0001), and 24-24-12 (P = 0.0365). There was no significant difference in virulence between quinone outside inhibitor (QoI)-sensitive and QoI-resistant isolates of A. alternata. This study enhanced our understanding of potato early blight and brown spot in Wisconsin, and suggested that A. alternata in addition to A. solani should be carefully monitored and possibly uniquely managed in order to achieve overall disease control.

Sclerotinia sclerotiorum population variability directly affects Sclerotinia stem rot (SSR) resistance breeding programs. In the north-central United States, however, soybean germplasm selection has often involved only a single isolate. Forty-four S. sclerotiorum isolates from Illinois, Michigan, Minnesota, Nebraska, Wisconsin, Poland, and across 11 different host species were evaluated for variation in isolate in vitro growth, in vitro oxalate production, and in planta aggressiveness on the susceptible soybean ‘Williams 82’. Significant differences (P < 0.0001) were detected in isolate in planta aggressiveness, in vitro growth, and in vitro oxalate production. Furthermore, diverse isolate characteristics were observed within all hosts and locations of collection. Aggressiveness was not correlated to colony growth and was only weakly correlated (r = 0.26, P < 0.0001) to isolate oxalate production. In addition, the host or location of collection did not explain isolate aggressiveness. Isolate oxalic acid production, however, may be partially explained by the host (P < 0.05) and location (P < 0.01) of collection. Using a representative subset of nine S. sclerotiorum isolates and soybean genotypes exhibiting susceptible or resistant responses (determined using a single isolate), a significant interaction (P = 0.04) was detected between isolates and genotypes when SSR severity was evaluated. Our findings suggest that screening of S. sclerotiorum-resistant soybean germplasm should be performed with multiple isolates to account for the overall diversity of S. sclerotiorum isolates found throughout the soybean-growing regions of the United States.

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Potato early blight caused by Alternaria solani and potato brown spot caused by A. alternata are two common potato foliar diseases. Fungicide applications in Wisconsin target early blight with the expectation of managing brown spot simultaneously. However, distributions of the two pathogens over time and space have not been previously reported in Wisconsin. In this study, six potato fields in Wisconsin were systematically sampled during 2014 to 2017 to investigate the incidences of both pathogens; incidences of the pathogens were compared by location, timing, and year. Incidence of A. alternata had been recorded starting in late June in every field every year, whereas incidence of A. solani started later in some of the fields. At the end of each growing season throughout the four years, the highest incidence (100% most of the years) of A. solani was observed in a field in Plover, located in a concentrated potato production area of Central Wisconsin, and a field in Arena, located in an isolated potato production area in southern Wisconsin. The highest incidence (100% every year) of A. alternata was observed in a field in Madison, an isolated potato field that was managed organically. Such distribution patterns could be a result of the different host range of the pathogens and abundance of inoculum in different geographic locations. In 2016, disease severity was correlated with the incidence of both pathogens (P < 0.0001 – P = 0.0299), but in 2017, disease severity was correlated with the incidence of A. solani in most of the fields (P < 0.0001 – P = 0.0799). These findings suggest that both Alternaria spp. are important for considering disease management strategies in central and southern Wisconsin.

Wood-decay fungi are one of the major threats to the old and valuable trees in Hong Kong and constitute a main conservation and management challenge because they inhabit dead wood as well as living trees. The diversity, abundance, and distribution of wood-decay fungi associated with standing trees and stumps in four different parks of Hong Kong, including Hong Kong Park, Hong Kong Zoological and Botanical Garden, Kowloon Park, and Hong Kong Observatory Grounds, were investigated. Around 4430 trees were examined, and 52 fungal samples were obtained from 44 trees. Twenty-eight species were identified from the samples and grouped into twelve families and eight orders. Phellinus noxius, Ganoderma gibbosum, and Auricularia polytricha were the most abundant species and occurred in three of the four parks. Most of the species were detected on old trees, indicating that older trees were more susceptible to wood-decay fungi than younger ones. More wood-decay fungal species were observed on Ficus microcarpa trees than on other tree species. These findings expanded the knowledge of wood-decay fungi in urban environments in Hong Kong and provided useful information for the conservation of old trees and the protection of human life and property from the danger of falling trees.

Succinate dehydrogenase inhibitors (SDHIs) are fungicides used in control of numerous fungal plant pathogens, including of Erysiphe necator, the causal agent of grapevine powdery mildew (GPM). Here, the sdhb, sdhc, and sdhd genes of E. necator were screened for mutations that may associate with SDHI resistance. GPM samples were collected from 2017 to 2020 from the U.S. states of California, Oregon, Washington and Michigan, and the Canadian province of British Columbia. Forty-five polymorphisms were identified in the three sdh genes, 17 of which caused missense mutations. Of these, the SDHC-p.I244V substitution was shown in this study to reduce sensitivity of E. necator to boscalid and fluopyram, whereas the SDHC-p.G25R substitution did not affect SDHI sensitivity. Of the other 15 missense mutations, the SDHC-p.H242R substitution was shown in previous studies to reduce sensitivity of E. necator towards boscalid, whereas the equivalents of the SDHB-p.H242L, SDHC-p.A83V, and SDHD-p.I71F substitutions were shown to reduce sensitivity to SDHIs in other fungi. Generally, only a single amino acid substitution was present in the SDHB, SDHC, or SDHD subunit of E. necator isolates but missense mutations putatively associated with SDHI resistance were widely distributed in the sampled areas and increased in frequency over time. Finally, isolates that had decreased sensitivity to boscalid or fluopyram were identified but with no or only the SDHC-p.G25R amino acid substitution present in SDHB, SDHC, and SDHD subunits. This suggests that target-site mutations is likely not the only mechanism conferring resistance to SDHIs in E. necator.

Early blight caused by Alternaria solani and brown spot caused by A. alternata are two common foliar diseases of potato, with early blight typically predominating in incidence and severity. Renewed interest in these two diseases has arisen as a result of notable differences in fungicide resistance profiles of the pathogens and inconsistent outcomes of disease management tactics. The pathogens share similar disease cycles, but they differ in the shape and size of their conidia. A. solani has a host range that includes just the Solanaceae, whereas A. alternata has a broad host range spanning numerous plant families. Such differences may result in differences in dispersal of the pathogens and subsequently influence epidemiology and management outcomes. In the commercial potato fields investigated in this study, the aerial conidial concentrations of A. solani and A. alternata differed significantly, with those of A. alternata conidia being higher in number and more variable than those of A. solani. The aerial conidial concentration of A. solani almost always significantly decreased with height (0 to 3 m above the canopy), whereas such a decrease was only observed for 4 of 12 days for A. alternata. The atmospheric concentrations of A. alternata were higher than those of A. solani at both upwind and downwind edges of the field (P < 0.0001), indicating more potential for long-distance dispersal. A higher aerial concentration of conidia at the downwind than the upwind location was observed for A. solani (P < 0.05), whereas overall no such effect was observed for A. alternata. This indicated that the potato fields investigated were likely the source of A. solani conidia, but they may not be the sole source of A. alternata. Results are consistent with inoculum of A. solani coming primarily from within the potato crop, whereas that of A. alternata may be generated from diverse plant sources across the landscape.

Quinone outside inhibitor (QoI) fungicides have been an important class in managing potato early blight caused by Alternaria solani and brown spot caused by A. alternata. Because of the single-site mode of action character of QoI fungicides, which are relied on for management of diseases in Wisconsin, and the abundant asexual conidia production of the Alternaria species, pathogen isolates with QoI resistance have been detected after just a few years of QoI fungicide usage in commercial production fields. Resistance to QoIs has been attributed to amino acid substitutions F129L and G143A in cytochrome b of A. solani and A. alternata, respectively, as a result of point mutations. The aim of this study was to assess Alternaria populations in Wisconsin for QoI resistance before and after fungicide applications in order to evaluate resistance stability. A TaqMan single nucleotide polymorphism genotyping assay was designed based on the sequences of the cytochrome b gene from Alternaria isolates collected in Wisconsin to profile QoI resistance in Alternaria populations as well as to explore factors that may influence frequency of QoI resistance in the pathogen populations. This assay successfully identified the mutations conferring QoI resistance in isolates collected from four locations each year from 2015 to 2017. During the course of this study, the frequency of A. solani isolates with the F129L mutation was consistently high and showed primarily the TTA mutation type. The frequency of A. alternata isolates with the G143A mutation started relatively low and increased at the end of the production season in each year (P = 0.0109, P = 0.2083, and P = 0.0159). A potato field managed without use of QoI fungicides showed a significantly lower (P < 0.05) frequency of A. alternata isolates carrying G143A than conventionally managed potato fields. The overall frequency of A. alternata isolates carrying G143A in the four locations was similar over the 3 years (P = 0.2971). The QoI resistance characteristics of the isolates were stable even when QoI selection pressure was removed for at least five subculture transfers, and the mutation types of codons 129 and 143 in the cytochrome b gene in A. solani and A. alternata, respectively, remained the same. This indicated that the application of QoIs in the field is not the sole factor responsible for the variation of the frequency of QoI resistance in the pathogen populations.