Phytophthora blight is a destructive disease caused by the oomycete
, which affects vegetable production throughout the state of Tennessee and worldwide. Fungicides are a primary control method used ...in managing Phytophthora blight, but in some cases the efficacy of these products has been reduced or lost in the field. In 2018 and 2019, the efficacy of six fungicides was tested in vitro on 184
isolates collected in Tennessee using radial growth assays. The fungicides included in the study were mefenoxam, fluopicolide, oxathiapiprolin, dimethomorph, mandipropamid, and cyazofamid. Seven isolates were resistant to mefenoxam, 86 were resistant to fluopicolide, one was resistant to oxathiapiprolin, and 13 were resistant to cyazofamid. None were resistant to dimethomorph or mandipropamid. Of the 86 isolates resistant to fluopicolide, five were also resistant to mefenoxam. Resistance to fluopicolide and cyazofamid was widespread in Tennessee, and it was more localized for mefenoxam and oxathiapiprolin. The results of this study show that fungicide resistance is widespread in
in Tennessee, and the implications for Phytophthora blight management are discussed.
Powdery mildew is a common disease of hemp in greenhouses in Tennessee. Fungicide efficacy data can support the use or approval of new fungicides. Therefore, two greenhouse experiments were conducted ...to assess the efficacy of nine commercial fungicides against powdery mildew. 'BaOx2' or 'Sweetened' hemp cultivars, which are susceptible to powdery mildew, were inoculated with a conidial suspension 1 day before or 1 day after the first fungicide application. Two additional fungicide applications were made at 7-day intervals by thoroughly spraying the plants using a hand-held sprayer. Control plants were sprayed with water only. Weekly disease incidence and severity ratings were done three times. Disease index and the area under the disease progress curve were calculated. All the fungicides significantly reduced powdery mildew symptoms. Compared with the control plants, disease reductions ranged from 76 to 100%. Bonide sulfur, Luna Experience, and MilStop exhibited "excellent" efficacy, reducing disease symptoms by 96 to 100%. Cinnerate, Exile, Regalia, and Sil-Matrix exhibited "very good" efficacy, reducing disease symptoms by 86 to 95%. Defguard and Stargus exhibited "good" efficacy, reducing disease symptoms by 76 to 85%. Koch's postulates were performed with the PM isolate used in this study. The isolate recovered following inoculation was found to be identical to the original isolate used in the experiments. The isolate was identified as
based on ITS, IGS, and β-tubulin sequencing and phylogenetic analysis with nucleotide sequences from closely related species. These findings provide useful information for the control of hemp powdery mildew and further research.
The plant pathogen
Phytophthora capsici
can be found all throughout the USA, and the population genetics of this organism have been studied within many of these states. Until now, no work has been ...done in the state of Tennessee to investigate the population structure and genetics of
P. capsici
found there. The population structure of
P. capsici
was explored using 296 isolates collected from five counties in Tennessee in 2004, 2007, 2018, and 2019. Samples were genotyped using 39 single nucleotide polymorphism (SNP) genetic markers. Multiple analyses indicate that the population structure of
P. capsici
in Tennessee exists in isolated clusters structured by geography. Geographically separate populations were genetically distinct, suggesting there is limited or no outcrossing among populations, but there is significant sexual reproduction occurring within populations. These findings corroborate previous studies of
P. capsici
throughout the midwestern and northeastern USA, where populations are generally sexually reproducing and structured by geography. This study provides the first characterization of
P. capsici
population structure in Tennessee.
Genotyping-by-sequencing (GBS) was performed on 257 Phytophthora infestans isolates belonging to four clonal lineages to study within-lineage diversity. The four lineages used in the study were US-8 ...(n = 28), US-11 (n = 27), US-23 (n = 166), and US-24 (n = 36), with isolates originating from 23 of the United States and Ontario, Canada. The majority of isolates were collected between 2010 and 2014 (94%), with the remaining isolates collected from 1994 to 2009, and 2015. Between 3,774 and 5,070 single-nucleotide polymorphisms (SNPs) were identified within each lineage and were used to investigate relationships among individuals. K-means hierarchical clustering revealed three clusters within lineage US-23, with US-23 isolates clustering more by collection year than by geographic origin. K-means hierarchical clustering did not reveal significant clustering within the smaller US-8, US-11, and US-24 data sets. Neighbor-joining (NJ) trees were also constructed for each lineage. All four NJ trees revealed evidence for pathogen dispersal and overwintering within regions, as well as long-distance pathogen transport across regions. In the US-23 NJ tree, grouping by year was more prominent than grouping by region, which indicates the importance of long-distance pathogen transport as a source of initial late blight inoculum. Our results support previous studies that found significant genetic diversity within clonal lineages of P. infestans and show that GBS offers sufficiently high resolution to detect sub-structuring within clonal populations.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Annual epidemics of Cercospora leaf spot (CLS), caused by the fungus Cercospora beticola, can result in substantial defoliation in table beet fields in New York. High allelic and genotypic diversity ...have been described within C. beticola populations; however, information on the temporal stability of populations is lacking. C. beticola isolates were obtained from symptomatic leaves in three table beet fields in successive years. Two of the fields were organic mixed-cropping farms and the third was managed conventionally in a broad-acre cropping system. C. beticola isolates (n = 304) were genotyped using 12 microsatellite markers. Genotypic diversity (Simpson's complement index = 0.178 to 0.990), allele frequencies, and indices of differentiation between years varied. Pairwise index of differentiation values ranged from 0.02 to 0.25 for clone-corrected data, and indicated significant genetic differentiation at Farm 2. No multilocus genotype was shared between years. The shift in multilocus genotypes between years questions the role of clonally reproducing primary inoculum. Collectively, these results suggest that a dominant inoculum source for initiating annual CLS epidemics is external to the field of interest. These findings have implications for CLS disease management in conventional and organic table beet production.
Growers seeking alternatives to traditional polyethylene plastic mulch may use biodegradable plastic mulches (BDMs). However, plasticulture systems typically also use plastic drip tape underneath the ...mulch, which must be removed from the field and disposed of at the end of the season, making tilling the BDM into the soil more difficult and expensive. A potential solution to this dilemma may be to use other irrigation methods, such as overhead sprinklers, that could be more easily removed from the field and reused from year to year. At Knoxville, TN, in 2019 and 2020, we grew three cultivars of romaine lettuce ( Lactuca sativa ) on BDM with two irrigation systems (overhead sprinklers above the mulch and drip irrigation tape under the mulch) to compare water use, disease, and yield in these two irrigation systems. Water use was higher in overhead vs. drip irrigation in both years; however, the difference in water use was much smaller in 2019 due to higher rainfall amounts during the time period the lettuce was growing in the field (March to May). Disease incidence and severity were very low both years for both irrigation systems. There were no differences in marketable yield (number of heads) between irrigation treatment in 2019. In 2020, marketable yield by number was greater in the drip vs. overhead irrigation treatment. Unmarketable yield in 2019 was due to heads that were too small; in 2020, unmarketability was predominantly due to tipburn in overhead irrigated ‘Jericho’. Overall, marketable lettuce yield did not differ between irrigation treatments in 2019 and was similar for ‘Parris Island Cos’ in 2020. Although quantitative weed counts were not made, observations of weed pressure between rows showed that weed pressure was higher in overhead irrigated compared with drip irrigated subplots. This highlights the need to have a between-row weed management program in place. The results of this study suggest that with attention to cultivar and weed management, overhead irrigation could be a viable alternative to drip irrigation for lettuce production on BDM, especially for early spring lettuce when rainfall is historically more plentiful.
Cercospora leaf spot (CLS), caused by Cercospora beticola Sacc., is the dominant foliar disease affecting the profitability of table beet production in New York, USA. The disease is controlled ...primarily with fungicides, but sustainability is critically threatened by strobilurin resistance within the C. beticola population and a limited suite of alternatives. This study evaluated the temporal dynamics of strobilurin resistance within the C. beticola population over three years, and quantified sensitivity to selected demethylation inhibitors (DMIs; propiconazole and difenoconazole) and succinate dehydrogenase inhibitors (SDHIs; fluopyram, penthiopyrad, boscalid and benzovindiflupyr). Resistance to azoxystrobin (EC
50
> 0.2 µg mL
−1
) was prevalent and detected in all fields sampled in 2015 and 2016, and six of 10 fields in 2017. Within fields, the incidence of azoxystrobin-resistant isolates ranged between 4.8% and 70%. No significant differences were detected in the frequency of azoxystrobin-resistant isolates within populations from fields with conventional and organic practices in 2016 and 2017. Reduced sensitivity to propiconazole and difenoconazole was detected in less than 1% of isolates. The EC
50
values for the SDHIs were higher than the maximum concentrations (≥ 20 µg mL
−1
) evaluated. Two small-plot replicated field trials were also conducted over two years to evaluate alternatives to the commercial standards, propiconazole and azoxystrobin. Two applications of benzovindiflupyr + difenoconazole and pydiflumetofen + difenoconazole significantly reduced CLS severity and epidemic progress compared with plots that received propiconazole and non-treated controls.
Genetic variability and mefenoxam sensitivity are described for Phytophthora spp. isolates recovered from nine ornamental plant species at four locations in Sicily, Southern Italy. Fifty-four ...isolates of Phytophthora spp. were obtained from symptomatic plant tissues. Mating type analyses revealed that all isolates are heterothallic, consisting of mating type A1. Fungicide sensitivity assays determined that all isolates are sensitive to mefenoxam. Characterisation of 48 isolates was performed using molecular and phylogenetic analyses of the internal transcribed spacer of ribosomal DNA (rDNA-ITS), beta-tubulin (β-tub) and elongation factor 1 alpha (EF-1α) regions. The isolates belonged to Phytophthora nicotianae (34 isolates), P. arecae (syn. palmivora) (five) and P. niederhauserii (nine). The pathogenicity of three representative isolates was tested on seedlings or cuttings of each host, grown in a growth chamber. All isolates were pathogenic and reproduced symptoms identical to those observed in the nursery. This is the first report of disease caused by P. nicotianae, P. arecae or P. niederhauserii on several ornamental plants in Italy, and of the sensitivity of these Phytophthora species to mefenoxam.
Advances in enzyme stabilization and immobilization make the use of enzymes for industrial applications increasingly feasible. The lactoperoxidase (LPO) system is a naturally occurring enzyme system ...with known antimicrobial activity. Stabilized LPO and glucose oxidase (GOx) enzymes were combined with glucose, potassium iodide, and ammonium thiocyanate to create an anti-fungal formulation, which inhibited in-vitro growth of the plant pathogenic oomycete Pythium ultimum, and the plant pathogenic fungi Fusarium graminearum and Rhizoctonia solani. Pythium ultimum was more sensitive than F. graminearum and R. solani, and was killed at LPO and GOx concentrations of 20 nM and 26 nM, respectively. Rhizoctonia solani and F. graminearum were 70% to 80% inhibited by LPO and GOx concentrations of 242 nM and 315 nM, respectively. The enzyme system was tested for compatibility with five commercial fungicides as co-treatments. The majority of enzyme + fungicide co-treatments resulted in additive activity. Synergism ranging from 7% to 36% above the expected additive activity was observed when P. ultimum was exposed to the enzyme system combined with Daconil® (active ingredient (AI): chlorothalonil 29.6%, GardenTech, Lexington, KY, USA), tea tree oil, and mancozeb at select fungicide concentrations. Antagonism was observed when the enzyme system was combined with Tilt® (AI: propiconazole 41.8%, Syngenta, Basel, Switzerland) at one fungicide concentration, resulting in activity 24% below the expected additive activity at that concentration.
Triazole fungicides, which are sterol demethylation inhibitors, have become the primary systemic fungicides applied to cucurbits to control gummy stem blight caused by Didymella bryoniae. Isolates of ...D. bryoniae from South Carolina that were never exposed to tebuconazole or exposed for several years were tested for sensitivity to tebuconazole and difenoconazole. Colony diameters, percentage germination of ascospores and conidia, and germ tube lengths were measured when isolates were grown on agar amended with 0.10–10.0 mg/l tebuconazole and 0.01–1.0 mg/l difenoconazole. All 147 isolates tested were sensitive to tebuconazole and difenoconazole with mean EC50 values of 0.41 and 0.054 mg/l, respectively. Ascospore germination was greater than conidia germination on fungicide‐amended agar. Although the length of germ tubes arising from both spore types was reduced by both fungicides, the reduction was greater for ascospore germ tubes than for conidia germ tubes. Because many watermelon growers rotate crops among fields every two years, local populations of D. bryoniae have not been exposed repeatedly to tebuconazole. In addition, growers often apply a rotation of systemic and contact fungicides. Thus, despite exposure to tebuconazole for up to nine years, isolates of D. bryoniae from South Carolina remain sensitive to triazole fungicides.
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