Correction: Jiao et al. Effects of Fusarium solani and F. oxysporum Infection on the Metabolism of Ginsenosides in American Ginseng Roots. Molecules 2015, 20, 10535-10552.

A reader recently brought to the attention of the Editor-in-Chief and the Editorial Office of Molecules several errors in our paper [...].

Taxonomy of fungal complex causing red-skin root of Panax ginseng in China.

BACKGROUND: Red-skin root of Asian ginseng (Panax ginseng) significantly reduces the quality and limits the production of ginseng in China. The disease has long been thought to be a noninfectious physiological disease, except one report that proved it was an infectious disease. However, the causal agents have not been successfully determined. In the present study, we were to reveal the pathogens that cause red-skin disease. METHODS: Ginseng roots with red-skin root symptoms were collected from commercial fields in Northeast China. Fungi were isolated from the lesion and identified based on morphological characters along with multilocus sequence analyses on internal transcription spacer, ß-tubulin (tub2), histone H3 (his3), and translation elongation factor 1α (tef-1α). Pathogens were confirmed by inoculating the isolates in ginseng roots. RESULTS: A total of 230 isolates were obtained from 209 disease samples. These isolates were classified into 12 species, including Dactylonectria sp., D. hordeicola, Fusarium acuminatum, F. avenaceum, F. solani, F. torulosum, Ilyonectria mors-panacis, I. robusta, Rhexocercosporidium panacis, and three novel species I. changbaiensis, I. communis, and I. qitaiheensis. Among them, I. communis, I. robusta, and F. solani had the highest isolation frequencies, being 36.1%, 20.9%, and 23.9%, respectively. All these species isolated were pathogenic to ginseng roots and caused red-skin root disease under appropriate condition. CONCLUSION: Fungal complex is the causal agent of red-skin root in P. ginseng.

Synthesis and antifungal activity of 2-allylphenol derivatives against fungal plant pathogens.

2-Allylphenol (2-AP) is an effective fungicide against a number of plant pathogens, which can be metabolized and bio-transformed to four chemical compounds by Rhizoctonia cerealis. To determine if its degradation affects antifungal activity, two major metabolites derived from 2-AP including 2-(2-hydroxypropyl) phenol and 2-(3-hydroxypropyl) phenol were synthesized. Inhibition of mycelial growth of several plant pathogens by the metabolites was evaluated, and structures of two metabolites were determined by hydrogen nuclear magnetic resonance (1H NMR). Among these metabolites, only 2-(2-hydroxypropyl) phenol inhibited test pathogens effectively. EC50 values of 2-(2-hydroxypropyl) phenol for inhibition of mycelial growth of R. cerealis, Pythium aphanidermatum, Valsa mali and Botrytis cinerea ranged from 1.0 to 23.5µg/ml, which were lower than the parental fungicide 2-AP that ranged from 8.2 to 48.8µg/ml. Hyphae of R. cerealis and P. aphanidermatum treated with 2-(2-hydroxypropyl) phenol were twisted. Newly developed hyphae were slender, twisted and swollen on the tip, while old hyphae were hollow and ruptured. This is the first report indicating the formation of 2-(2-hydroxypropyl) phenol may have contributed to toxicity of 2-allylphenol in control of plant pathogens.

Effects of Fusarium solani and F. oxysporum Infection on the Metabolism of Ginsenosides in American Ginseng Roots.

American ginseng (Panax quinquefolius L.) is a highly valuable herb widely used for medicinal treatments. Its pharmacologically important compounds are the ginsenosides, which are secondary metabolites in American ginseng root. The concentrations of ginsenoside in roots can be changed by fungal infection, but it is unclear what specific root tissues are impacted and whether the change is systemic. In this study, American ginseng roots were inoculated with two fungal pathogens (Fusarium solani or F. oxysporum) and the levels of six ginsenosides (Rb1, Rb2, Rc, Rd, Re, and Rg1) were then measured in the phloem and xylem around the discolored lesions and adjacent healthy areas of the root. Results indicated that the growth of Fusarium spp. was strictly limited to phloem, and correspondingly the ginsenoside concentration was only altered in this infected phloem. The concentration of Rg1, Rd, and Rc significantly changed in phloem tissues where F. solani was inoculated, while only Rg1 and Rd changed significantly after F. oxysporum inoculation. However, no changes of any ginsenoside occurred in either xylem or phloem tissue adjacent to the inoculation point. In addition, when two Fusarium spp. were grown on ginsenoside-amended Czapek medium, the majority of ginsenosides were depleted. Therefore, pathogenic Fusarium spp. may reduce ginsenoside levels by consuming them.

Evaluation of fungicides enestroburin and SYP1620 on their inhibitory activities to fungi and oomycetes and systemic translocation in plants.

Enestroburin and SYP1620 are newly developed strobilurin chemicals carrying fungicidal activity and need to be fully characterized in activities of anti-oomycete or anti-fungi, disease prevention and systemic translocation in planta. Their inhibitory activities were examined by amending the chemical in agar media, on which selected plant pathogens were grown and mycelial growth were measured. Effective concentrations for 50% inhibition (EC50) of mycelial growth were calculated to determine the level of fungicide sensitivity of the pathogen. Azoxystrobin was used as control. To examine the prevention and systemic translocation in plants, the fungicides were either sprayed on wheat leaves or dipped on wheat roots, which then were detected using high performance liquid chromatography. All the three fungicides inhibited mycelial growth of Sphacelotheca reiliana, Phytophthora infestans, Peronophythora litchi, and Magnaporthe oryzae, with EC50 values ranging from 0.02 to 2.84µg/ml; EC50 of SYP1620 was significantly lower than that of azoxystrobin and enestroburin on Valsa mali, Gaeumannomyces graminis, Alternaria solani, and Colletotrichun orbiculare. The three QoI fungicides showed strong inhibitory activities on spore germination against the 13 pathogens tested and were highly effective on biotrophic pathogens tested. Enestroburin and SYP1620 penetrated and spread in wheat leaves, but the penetration and translocation levels were lower compared to azoxystrobin. The three fungicides were all rapidly taken up by wheat roots and transported upwards, with greater fungicide concentrations in roots than in stems and leaves. The results indicate that enestroburin and SYP1620 are systemic fungicides that inhibit a broad spectrum of fungi and oomycetes.

Metabolism of fungicide 2-allylphenol in Rhizoctonia cerealis.

2-Allylphenol is a biomimetic synthetic fungicide that mimics the compound ginkgol found in gingko fruit (Gingko biloba L.). This systemic fungicide can effectively suppress a wide range of plant diseases, including wheat sharp eyespot (Rhizoctonia cerealis). However, its degradation in environment after application is still unknown. To understand this fungicide degradation, major metabolites of 2-allylphenol in R. cerealis were examined. The parent and metabolites of 2-allylphenol were detected and quantified in the mycelia and liquid medium. Results showed that 2-allylphenol was metabolized and bio-transformed by R. cerealis, and four metabolites were found, including 2-(2-hydroxyphenyl) acetic acid (M1), 2-(2, 3-dihydroxypropyl) phenol (M2), 2-(2-hydroxypropyl)-phenol (M3) and 2-(3-hydroxypropyl)-phenol (M4). Based on the results, we propose that the biodegradation pathway is that 2-allylphenol is rapidly oxidized into metabolite M2 and hydrolyzed into M3 and M4, which formed M2, and carboxylation of M2 to 2-hydroxy-3-(2׳-hydroxyphenyl) propionic acid which undergo hydrolyzation and decarboxylation to form M1. 2-Allylphenol can be bio-transformed to new compounds by R. cerealis, suggesting the existence of microbe metabolic pathways for 2-allylphenol.

Fungicide Sensitivity of Pythium spp. Associated with Cavity Spot of Carrot in California and Michigan.

The identity of 172 isolates of Pythium spp. from cavity spot lesions on carrot produced in California and Michigan was determined, and their sensitivity to three fungicides was examined. Pythium violae accounted for 85% of California isolates, with P. irregulare, P. dissotocum (the first report as a carrot pathogen in the United States), P. ultimum, and P. sulcatum making the balance. P. sulcatum, P. sylvaticum, and P. intermedium were the most commonly recovered (85%) species in Michigan; others from Michigan included P. intermedium, P. irregulare, and an unclassified strain, M2-05. On fungicide-amended media, 93% of isolates were sensitive to mefenoxam (inhibition of mycelial growth was >60% at 10 µg active ingredient [a.i.]/ml); however, two of five isolates of P. irregulare from California were highly resistant (≤60% inhibition at 100 µg a.i./ml); about half of the isolates of P. intermedium and P. sylvaticum and a single isolate of P. violae were highly or intermediately resistant to mefenoxam (>60% inhibition at 100 µg a.i./ml, or ≤60% inhibition at 10 µg a.i./ml). P. dissotocum, P. irregulare, P. sulcatum, M2-05, and three of seven isolates of P. intermedium were insensitive to fluopicolide (effective concentrations for 50% growth inhibition [EC50] were >50 µg a.i./ml), while P. sylvaticum, P. ultimum, P. violae, and some isolates in P. intermedium were sensitive (EC50 < 1 µg a.i./ml). All isolates were sensitive to zoxamide (EC50 < 1 µg a.i./ml). Sensitivity baselines of P. violae to zoxamide and fluopicolide were established.

Antimicrobial Activity of Chestnut Extracts for Potential Use in Managing Soilborne Plant Pathogens.

Chestnut extracts were studied for antimicrobial activity against selected microorganisms, including plant pathogens. Chestnut extract on paper discs was applied to an agar medium to evaluate the inhibition to multiple microorganisms or the extract was added at various concentrations to a culture medium to evaluate the growth of target microorganisms. Chestnut type, tissue of plants (shell, pellicle, and leaf), extraction methods, and physical characteristics were studied to determine antimicrobial activity. Most test microorganisms were inhibited by the extracts at different effective concentrations for 50% growth inhibition (EC50). Pseudomonas fluorescens was the most sensitive (EC50 = 4.4 µg/µl), Phytophthora cambivora was one of the least inhibited (EC50 = 185 µg/µl), and Cryphonectria parasitica was not inhibited. Extracts of the Japanese × European chestnut (Castanea crenata × C. sativa) 'Colossal' showed a greater inhibition than those of wild trees of the Chinese species (C. mollissima). High temperature did not affect the inhibitory effect. Extracts from chestnut pellicle had the highest concentration of antimicrobial compound, compared with leaf and shell. The active fraction contained several substances with molecular masses consistent with one flavonol glycoside and several terpenoid substances. Pellicle and shell tissue reduced radish scab disease caused by Streptomyces scabies in the greenhouse.

Inhibitory Effects of Essential Oils for Controlling Phytophthora capsici.

Essential oils (EOs) were studied in vitro and in vivo for inhibiting Phytophthora capsici. Mycelial growth of P. capsici was examined on EO-amended media or after exposing it to EO volatiles. The efficacy of EOs was determined by estimating the effective concentration for 50% inhibition of P. capsici mycelial growth (EC50). Among 14 tested commercial products, oregano, palmarosa, and red thyme EOs had the lowest EC50 values (<0.15 µg/ml) for inhibiting the production and germination of sporangia and zoospores, and mycelial growth of P. capsici. The EOs had the same range of effect on inhibiting some mutant P. capsici isolates resistant to fluopicolide and zoxamide. P. capsici population in soil was reduced by the three EOs. Zucchini (Cucurbita pepo) fruit were protected against P. capsici infection when they were sprayed with red thyme (0.1 µg/ml) or oregano and palmarosa (0.2 µg/ml) EOs. Zucchini seedling emergence was affected by oregano, but not by red thyme. Zucchini seedlings survived in P. capsici-infested soil treated with red thyme at 0.1 µg/ml, while all of the nontreated seedlings died. These results taken together suggest that oregano, red thyme, and palmarosa EOs may be potential components for integrated management of P. capsici.

Microbial Communities Associated with Potato Common Scab-Suppressive Soil Determined by Pyrosequencing Analyses.

Potato common scab, caused by Streptomyces spp., is an annual production problem for potato growers, and not effectively controlled by current methods. A field with naturally occurring common scab suppression has been identified in Michigan, and confirmed to have a biological basis for this disease suppression. This field and an adjacent scab nursery conducive to disease were studied using pyrosequencing to compare the two microbial communities. Total DNA was extracted from both the disease-conducive and -suppressive soils. A phylogenetically taxon-informative region of the 16S rRNA gene was used to establish operational taxonomic units (OTUs) to characterize bacterial community richness and diversity. In total, 1,124 OTUs were detected and 565 OTUs (10% dissimilarity) were identified in disease-conducive soil and 859 in disease-suppressive soil, including 300 shared both between sites. Common phyla based on relative sequence abundance were Acidobacteria, Proteobacteria, and Firmicutes. Sequences of Lysobacter were found in significantly higher numbers in the disease-suppressive soil, as were sequences of group 4 and group 6 Acidobacteria. The relative abundance of sequences identified as the genus Bacillus was significantly higher by an order of magnitude in the disease-conducive soil.

Culture-Based Assessment of Microbial Communities in Soil Suppressive to Potato Common Scab.

A field in East Lansing, MI, showed a decline of potato common scab compared with an adjacent potato field. To confirm that the decline was due to biological factors, the soil was assayed. In the greenhouse, putative common-scab-suppressive soil (SS) was either treated with various temperatures or mixed with autoclaved SS at various ratios. Pathogenic Streptomyces scabies was incorporated into the treated soil at 106 CFU/cm3 of soil, followed by planting of either potato or radish. Disease severity was negatively correlated with the percentage of SS in the mixture and positively correlated with temperature above 60°C. The soil was screened for four groups of potential antagonists (general bacteria, streptomycetes, fluorescent pseudomonads, and bacilli) pairing in culture with S. scabies. The frequency of antagonistic bacteria in SS was higher than common-scab-conducive soil (CS) in all four groups but only pseudomonads and streptomycetes were significantly higher. The population of pathogenic Streptomyces spp. in the rhizosphere of CS was significantly higher than SS. Dilution plating of CS and SS samples showed no clear trends or differences in populations of total fungi, total bacteria, streptomycetes, fluorescent pseudomonads, and bacilli but terminal restriction fragment polymorphism analysis revealed two distinct microbial communities were present in SS and CS.

Baseline sensitivity of natural population and resistance of mutants in Phytophthora capsici to zoxamide.

Laboratory experiments were conducted to determine the baseline sensitivity of Phytophthora capsici and its risk for developing resistance to zoxamide. In total, 158 P. capsici isolates were collected from China. All 158 isolates were sensitive to zoxamide, with effective concentrations for 50% inhibition of mycelial growth of 0.023 to 0.383 µg/ml and a mean of 0.114 µg/ml, which showed a skewed unimodal distribution. Zoxamide-resistant mutants of P. capsici were obtained by either treating mycelial culture and zoospores with ultraviolet irradiation or adapting a culture on zoxamide-amended plates. The frequency of resistance selection averaged 1.8 × 10(-7). Resistant isolates were also derived by selfing or crossing two sexually compatible isolates, resulting in a mean selection frequency of 0.47. The resistance factor (RF) for zoxamide was 25 to 100 in P. capsici mutants. Through 10 culture transfers, the mutants maintained high levels of RF (between 14 and 134) and had almost equal fitness as their wild-type parents in mycelial growth, sporulation, and virulence. There was no cross resistance between zoxamide and either flumorph, metalaxyl, azoxystrobin, or etridiazole. Based on the results above, P. capsici can develop resistance to zoxamide, and the risk is predicted to be moderate in nature.

Wild Type Sensitivity and Mutation Analysis for Resistance Risk to Fluopicolide in Phytophthora capsici.

Crown, root, and fruit rot caused by Phytophthora capsici is an increasing problem for vegetable growers in Michigan and the United States. The newly registered fungicide fluopicolide is effective to limit crop loss but the potential for P. capsici to develop resistance is not well known. A laboratory study assessed the risk of P. capsici developing resistance to fluopicolide. Baseline sensitivity to fluopicolide was determined using 126 P. capsici Michigan isolates. Values of effective concentrations for 50% inhibition of mycelial growth ranged from 0.08 to 0.24 µg/ml and were distributed as a unimodal curve, indicating that all isolates were sensitive to fluopicolide. Mutants resistant to fluopicolide were obtained from five isolates by screening zoospores on fluopicolide-amended (5 µg/ml) media at a mutation frequency above 1.0 × 10-7. The mutant isolates were clustered with either intermediate (resistance factor [RF] = 3.53 to 77.91) or high (RF = 2481.40 to 7034.79) resistance. Resistance was stable through 10 mycelial transfers on fungicide-free medium. All resistant mutants showed similar fitness in zoospore production, cyst germination, and virulence compared with their sensitive parents, with few exceptions. No cross-resistance was detected between fluopicolide and five other fungicides. There could be a moderately high risk of field populations of P. capsici developing resistance to fluopicolide, and populations should be monitored.

Baseline sensitivity and resistance-risk assessment of Phytophthora capsici to iprovalicarb.

Iprovalicarb has been used to control Phytophthora capsici, a devastating pathogen of many economically important crops. To evaluate the risk of fungicide resistance, 158 isolates of P. capsici were examined for sensitivity to iprovalicarb by measuring mycelial growth. Values of effective concentrations for 50% mycelial growth inhibition varied from 0.2042 to 0.5540 µg/ml and averaged 0.3923 (±0.0552) µg/ml, with a unimodal distribution. This is the first report of P. capsici isolates highly resistant to iprovalicarb (resistance factor >100). Resistance of the isolates was stable through 10 transfers on iprovalicarb-free medium, and most resistant isolates had the same level of fitness (mycelial growth, zoospore production, and virulence) as their corresponding parents, indicating that iprovalicarb resistance was independent from other general growth characters. There was cross-resistance among all tested carboxylic acid amide (CAA) fungicides, including iprovalicarb, flumorph, dimethomorph, and mandipropamid, but not with non-CAA fungicides, including azoxystrobin, chlorothalonil, cymoxanil, etridiazole, metalaxyl, and zoxamide. Based on the present results, resistance risk of P. capsici to CAAs could be moderate and resistance management should be considered.

Effect of Soil Inoculum Density of Fusarium oxysporum f. sp. vasinfectum Race 4 on Disease Development in Cotton.

The effect of soil inoculum density (0 to 106 conidia/g of potting mix) of Fusarium oxysporum f. sp. vasinfectum race 4 on plant growth (weight, height, and number of nodes), Fusarium wilt symptoms, vascular discoloration, and number of CFU per gram of stem tissue was determined on five cotton cultivars grown in a greenhouse. In the susceptible cv. DP744, symptoms of wilt and reductions in plant growth occurred at inoculum levels of 103 conidia/g of potting mix and higher, whereas plant growth of the resistant Pima cv. Ph800 was not affected by any soil inoculum densities. Cvs. DP340, Ph72, and UltEF all responded similarly, with a relatively moderate negative growth response to soil inoculum densities generally beginning at 104 conidia/g of potting mix. F. oxysporum f. sp. vasinfectum race 4 was recovered from the stems of all cultivars by plating macerated stem tissue on a semiselective medium. There was a significant positive correlation between CFU and inoculum density above 104 conidia/g of potting mix for DP744 and UltEF. The fungus was recovered relatively infrequently from stems of Ph 800 plants inoculated with any inoculum density of F. oxysporum f. sp. vasinfectum. Fusarium wilt caused by F. oxysporum f. sp. vasinfectum race 4 is an inoculum density- dependent disease; the disease was generally mild and the fungus was rarely recovered from stems at inoculum levels less than 104 conidia/g of potting mix.