First Report of the Cucurbit Powdery Mildew Fungus (Podosphaera xanthii) Resistant to Strobilurin Fungicides in the United States

Resistance to strobilurin fungicides was documented in isolates collected from three fungicide efficacy experiments conducted in research fields in Georgia (GA), North Carolina (NC), and New York (NY). In these fields in 2002, strobilurins (fungicide group 11, quinone outside inhibitors [QoI]) when used alone on a 7-day schedule (use pattern not labeled) did not effectively control cucurbit powdery mildew. Strobilurin efficacy declined dramatically after the second application in New York (3). Efficacy also was reduced in commercial fields in Kentucky and research fields in Arizona, California, Kentucky, Illinois, Michigan, and Virginia in 2002 where strobilurins were used predominantly or exclusively. Isolates were collected on 22 July and 8 and 17 October after the last of four, five, and five applications of strobilurin (trifloxystrobin formulated as Flint or azoxystrobin formulated as Quadris) in experiments conducted by J. D. Moore in Chula, GA, M. McGrath in Riverhead, NY, and G. J. Holmes in Clayton, NC, respectively. A leaf-disk bioassay was used to determine fungicide sensitivity (2). Strobilurin sensitivity was determined using trifloxystrobin at 0, 0.5, 5, 50, and 100 μg/ml. Four of nine NY isolates, 19 of 21 GA isolates, and 13 of 15 NC isolates were resistant to strobilurins (grew well on disks treated with trifloxystrobin at 100 μg/ml). The geometric mean of the azoxystrobin baseline was 0.258 μg/ml for Podosphaera xanthii isolates collected in 1998 and 1999 in North America (4). Poor control with strobilurins under field conditions was associated with reduced sensitivity in vitro. Strobilurin sensitivity appeared to be qualitative as reported elsewhere (1). Two sensitive and three resistant isolates responded similarly when tested in another laboratory using kresoxim-methyl and pyraclostrobin (H. Ypema, personal communication). These findings and experiences elsewhere with QoI-resistant P. xanthii indicate that cross-resistance probably extends among multiple QoI's (1). Strobilurins have been available for commercial use in the United States since 1998, when azoxystrobin received Section 18 registration in some states. Federal registration was granted in March 1999. Strobilurin resistance was detected after 2 years of commercial use elsewhere in the world (1). All isolates tested in the current study were from research fields where selection pressure for resistance could have been higher than in commercial fields where strobilurins are used with demethylation inhibitors (DMIs; fungicide group 3) and contact fungicides in alternation or tank mixtures to prevent or delay resistance development. Resistance in commercial fields will reduce the utility of strobilurins, including those not yet registered, and eliminate an important tool for managing DMI resistance. Strobilurins and DMIs are the only systemic fungicides registered for cucurbit powdery mildew in the United States. Managing DMI resistance may be challenged by multiresistant strains. Strobilurin-resistant isolates also exhibited reduced sensitivity to DMIs, tolerating triadimefon at 50 to 100 μg/ml (2). One suggestion to improve resistance management is to apply a contact fungicide with strobilurins as well as DMIs. References: (1) H. Ishii et al. Phytopathology 91:1166, 2001. (2) M. T. McGrath et al. Plant Dis. 80:697, 1996. (3) M. T. McGrath and N. Shishkoff. Fungic. Nematic. Tests. (In press). (4) G. Olaya et al. Phytopathology (Abstr.) 90 (suppl):S57, 2000.

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First Detection of Boscalid Resistance in Podosphaera xanthii in the United States Associated With Failure to Control Cucurbit Powdery Mildew in New York and New Jersey in 2009

Plant Health Progress ◽

10.1094/php-03-17-0014-br ◽

2017 ◽

Vol 18 (2) ◽

pp. 93-93 ◽

Cited By ~ 2

Author(s):

M. T. McGrath ◽

C. A. Wyenandt

Keyword(s):

United States ◽

New York ◽

Powdery Mildew ◽

Resistance Management ◽

The United States ◽

Management Tool ◽

Podosphaera Xanthii ◽

Resistance Risk ◽

Causal Pathogen ◽

Succinate Dehydrogenase Inhibitor

The most effective management tool for powdery mildew on cucurbit crops is mobile, targeted fungicides having a moderate or high resistance risk and the causal pathogen has proven to be a high-risk organism. Information about occurrence of resistance to a new chemical group and impact on resistance management is important for guiding fungicide programs and contributing to scientific knowledge about resistance development. This brief reports the first known occurrence of resistance to a succinate dehydrogenase inhibitor fungicide in Podosphaera xanthii in the United States.

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Genotyping-by-Sequencing for Analysis of the Genetic Variation of Podosphaera xanthii, Incitant of Cucurbit Powdery Mildew

Plant Disease ◽

10.1094/pdis-03-19-0513-re ◽

2020 ◽

Vol 104 (3) ◽

pp. 951-957 ◽

Cited By ~ 2

Author(s):

Y. Xiang ◽

A. N. Miller ◽

M. McGrath ◽

M. Babadoost

Keyword(s):

United States ◽

New York ◽

Powdery Mildew ◽

Cucumis Melo ◽

Cucurbita Pepo ◽

Genotyping By Sequencing ◽

The United States ◽

Podosphaera Xanthii ◽

Internal Transcribed Spacer Region ◽

Significant Difference

This research was conducted to identify species causing powdery mildew on cucurbits and to determine genetic variations among isolates of the pathogen. We collected 109 isolates from six cucurbit species hosts (Cucumis melo, Cucumis sativus, Cucurbita maxima, Cucurbita moschata, Cucurbita pepo, and Lagenaria siceraria) in California, Illinois, Indiana, Michigan, New York, Texas, Washington, and Wisconsin in the United States and in Italy. By sequencing the internal transcribed spacer region of the nuclear rDNA of these 109 isolates, Podosphaera xanthii was found as the only species causing powdery mildew on cucurbits in the United States. Genotyping-by-sequencing was applied to these 109 isolates to investigate their genetic diversity, which showed a trend of isolates clustering from New York and Italy. In addition, the virulence of 36 isolates was compared and a significant difference (P < 0.0001) was found among them. Furthermore, results of the virulence tests of 28 isolates from Illinois showed significant effects of collection years, hosts, and locations on the virulence of the isolates.

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Sawadaea tulasnei Powdery Mildew of Norway Maple (Acer platanoides) in North America

Plant Disease ◽

10.1094/pd-90-0830c ◽

2006 ◽

Vol 90 (6) ◽

pp. 830-830

Author(s):

J. Weiland ◽

G. Stanosz

Keyword(s):

United States ◽

New York ◽

North America ◽

Powdery Mildew ◽

Acer Saccharum ◽

The United States ◽

Rdna Its ◽

Maple Leaves ◽

Norway Maple ◽

Genus 2

Norway maple leaves bearing powdery mildew were collected from one location in the fall of 2003 and four locations (as much as 1.5 km apart) in the fall of 2005 in Buffalo, NY. No powdery mildew was observed on leaves collected from sugar maples (Acer saccharum) that were present in the vicinity of affected Norway maples at two locations. Trees were located along streets and in yards. Diseased leaves were present throughout tree crowns but lower leaves were more commonly affected. White mycelium was present in irregular, discrete, scattered spots only on the upper surface of leaves and on both sides of wings of samaras. Typically, <10% of the upper leaf area bore visible mycelium. Cleistothecia were present singly or in groups on the mycelium. Morphology of cleistothecia on leaves collected each year, including simple and bifid appendages with uncinate to circinate apices, was sufficient to identify the pathogen to the genus Sawadaea (1). Other characteristics were not sufficiently distinct to make an identification of S. bicornis or S. tulasnei (1), each a European species found on Acer spp. However, a sample from 2003 was supplied by the authors for use in a study of phylogeny of the genus (2) that served as a first report of the species in the United States. Analysis of nuclear rDNA ITS sequence of this specimen (GenBank Accession No. AB193390) placed the sample in a clade with S. tulasnei specimens from Europe. In the same study, powdery mildew samples from Acer spp. in Ohio and Montreal, Canada also were placed in this clade. Thus, occurrence of S. tulasnei in North America is confirmed. S. bicornis was recently identified (based on morphology) on Norway maple in the western United States (3). Specimens from Buffalo, NY have been deposited in the U.S. National Fungus Collections (BPI 871210). References: (1) U. Braun. The Powdery Mildews (Erysiphales) of Europe. Gustav Fischer Verlag, Jena-Stuttgart-New York, 1995. (2) S. Hirose et al. Mycol. Res. 109:912, 2005. (3) C. Nischwitz and G. Newcombe. Plant Dis. 87:451, 2003.

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Outbreak of Cucurbit Powdery Mildew on Watermelon Fruit Caused by Podosphaera xanthii in Southwest Florida

Plant Disease ◽

10.1094/pdis-06-11-0521 ◽

2011 ◽

Vol 95 (12) ◽

pp. 1586-1586 ◽

Cited By ~ 11

Author(s):

C. S. Kousik ◽

R. S. Donahoo ◽

C. G. Webster ◽

W. W. Turechek ◽

S. T. Adkins ◽

...

Keyword(s):

Powdery Mildew ◽

Morphological Characteristics ◽

The United States ◽

Tween 20 ◽

Conidial Suspension ◽

Podosphaera Xanthii ◽

Its Sequence Analysis ◽

Fruit Samples ◽

Cucurbit Powdery Mildew ◽

Fruit Surface

Cucurbit powdery mildew caused by the obligate parasite Podosphaera xanthii occurs commonly on foliage, petioles, and stems of most cucurbit crops grown in the United States. (3). However, in the field, fruit infection on cucurbits including watermelon (Citrullus lanatus), is rarely, if ever, observed (2). Consequently, it was atypical when severe powdery mildew-like symptoms were observed on seedless and seeded watermelon fruit on several commercial farms in southwestern Florida during November and December 2010. Severe powdery mildew was also observed on ‘Tri-X 313’ and ‘Mickey Lee’ fruit grown at SWFREC, Immokalee, FL. Infected fruit developed poorly and were not marketable. Powdery mildew symptoms were mainly observed on young immature fruit, but not on mature older fruit. Abundant powdery mildew conidia occurred on fruit surface, but not on the leaves. Conidia were produced in chains and averaged 35 × 21 μm. Observation of conidia in 3% KOH indicated the presence of fibrosin bodies commonly found in the cucurbit powdery mildew genus Podosphaera (3). Orange-to-dark brown chasmothecia (formerly cleisthothecia) containing a single ascus were detected on the surface of some fruit samples. Conidial DNA was subjected to PCR using specific primers designed to amplify the internal transcribed spacer (ITS) region of Podosphaera (4). The resulting amplicons were sequenced and found to be 100% identical to the ITS sequences of P. xanthii in the NCBI database (D84387, EU367960, AY450961, AB040322, AB040315). Sequences from the watermelon fruit isolate were also identical to several P. fusca (synonym P. xanthii), P. phaseoli (GQ927253), and P. balsaminae (AB462803) sequences. On the basis of morphological characteristics and ITS sequence analysis, the pathogen infecting watermelon fruit can be considered as P. xanthii (1,3,4). The powdery mildew isolate from watermelon fruit was maintained on cotyledons of squash (Cucurbita pepo, ‘Early Prolific Straight Neck’). Cotyledons and leaves of five plants each of various cucurbits and beans were inoculated with 10 μl of a conidial suspension (105conidia/ml) in water (0.02% Tween 20). Two weeks after inoculation, abundant conidia were observed on cucumber (Cucumis sativus, ‘SMR-58’) and melon (Cucumis melo) powdery mildew race differentials ‘Iran H’ and ‘Vedrantais’. However, no growth was observed on melon differentials ‘PI 414723’, ‘Edisto 47’, ‘PMR 5’, ‘PMR 45’, ‘MR 1’, and ‘WMR 29’ (2,3). The powdery mildew isolate from watermelon fruit behaved as melon race 1 (3). Mycelium and conidia were also observed on fruit surface of watermelon ‘Sugar Baby’ and a susceptible U.S. plant introduction (PI 538888) 3 weeks after inoculation. However, the disease was not as severe as what was observed in the fields in fall 2010. The pathogen did not grow on plants of Impatiens balsamina or on select bean (Phaseolus vulgaris) cultivars (‘Red Kidney’, ‘Kentucky Blue’, and ‘Derby Bush’), but did grow and produce abundant conidia on ‘Pinto bush bean’. Powdery mildew on watermelon fruit in production fields can be considered as a potentially new and serious threat requiring further studies to develop management strategies. References: (1) U. Braun and S. Takamatsu. Schlechtendalia 4:1, 2000. (2) A. R. Davis et al. J. Am. Soc. Hortic. Sci. 132:790, 2007. (3) M. T. McGrath and C. E. Thomas. In: Compendium of Cucurbit Diseases. American Phytopathological Society, St. Paul, MN, 1996. (4) S. Takamatsu and Y. Kano. Mycoscience 42:135, 2001.

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First Report of Powdery Mildew Caused by Podosphaera (Sphaerotheca) fusca on Euryops pectinatus in North America

Plant Disease ◽

10.1094/pdis.2000.84.9.1048a ◽

2000 ◽

Vol 84 (9) ◽

pp. 1048-1048 ◽

Cited By ~ 1

Author(s):

G. S. Saenz ◽

S. T. Koike ◽

N. Shishkoff

Keyword(s):

United States ◽

New York ◽

North America ◽

Powdery Mildew ◽

Fungal Growth ◽

The United States ◽

Voucher Specimen ◽

First Report ◽

Humidity Chamber ◽

Powdery Mildew Pathogen

Gray-leaved Euryops (Euryops pectinatus Cass., Asteraceae) is an evergreen shrub that is widely planted in landscapes in the United States. In the fall of 1999, powdery mildew was observed on E. pectinatus planted in landscapes in Redlands (San Bernardino County), CA. Symptoms consisted only of slight cupping of leaves. Fungal growth was observed on stems, leaves, petioles, and pedicels and was ectophytic and amphigenous. The white mycelium was patchy to effuse. Hyphal appressoria were indistinct (1). Conidiophore foot cells were cylindric and sometimes were tapered toward or constricted at the base. Foot cells measured 30 to 50 by 10 to 12 μm and were followed by one to two shorter cells. Conidia were cylindric to slightly doliform, borne in chains of two to three, and measured 26 to 38 by 14 to 18 μm. Conidial length to width ratios ranged from 1.7 to 2.4. Catenate conidia had crenate edge lines (3). Conidia possessed conspicuous fibrosin bodies and from their sides produced short germ tubes without appressoria. Cleistothecia were not observed. Based on these characters, the fungus was identified as Podosphaera fusca (Fr.) U. Braun & N. Shishkoff (Podosphaera sect. Sphaerotheca) (1,2). Pathogenicity was confirmed by gently pressing diseased leaves onto leaves of healthy E. pectinatus plants. Plants were incubated in a humidity chamber at 22 to 24°C and after 12 to 14 days powdery mildew colonies developed. E. pectinatus cv. Viridis, a cultivar that lacks the extensive pubescence of E. pectinatus, also developed disease when inoculated. This appears to be the first report of powdery mildew on E. pectinatus in North America. A voucher specimen has been deposited into the University of California Herbarium (accession # UC1738635). P. fusca was also observed on cv. Viridis in a nursery in New York in 1999. It is unclear where this pathogen originated. P. fusca parasitizes a large number of asteraceous species including dandelion (Taraxacum officinalis) and sowthistle (Sonchus spp.) weeds, which occur in the area and sometimes are infected with powdery mildew. The Euryops powdery mildew pathogen may be a race that is different than those found on other composites in the United States. The fungus was observed on plants in shaded areas but not on plants in full sun. References: (1) U. Braun. Nova Hedwigia 89:1, 1987. (2) U. Braun and S. Takamatsu. Schlechtendalia 4:1, 2000. (3) H. D. Shin and Y. J. La. Mycotaxon 46:445, 1993.

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Sensitivities of Cucurbit Powdery Mildew Fungus (Podosphaera xanthii) to Fungicides

Plant Health Progress ◽

10.1094/php-04-20-0031-rs ◽

2020 ◽

Vol 21 (4) ◽

pp. 272-277

Author(s):

Mohammad Babadoost ◽

Salisu Sulley ◽

Yiwen Xiang

Keyword(s):

United States ◽

Powdery Mildew ◽

Succinate Dehydrogenase ◽

Podosphaera Xanthii ◽

Powdery Mildew Fungus ◽

Midwestern United States ◽

Quinone Outside Inhibitors ◽

Cucurbit Powdery Mildew ◽

Succinate Dehydrogenase Inhibitors

This study was conducted to evaluate the sensitivity of cucurbit powdery mildew fungus (Podosphaera xanthii) to major fungicides used for managing this pathogen in the Midwestern United States. Fungicides azoxystrobin from the quinone outside inhibitors group, cyflufenamid from the phenylacetamide group, penthiopyrad from the succinate dehydrogenase inhibitors group, quinoxyfen from the quinolines group, and triflumizole from the demethylation inhibitors group were tested for their effectiveness for preventing infection of cucurbits by P. xanthii. In 2015 and 2016, 37 isolates of P. xanthii were evaluated for their sensitivity to azoxystrobin (Quadris 2.08SC), cyflufenamid (Torino 0.85SC), penthiopyrad (Fontelis 1.67SC), and triflumizole (Procure 480SC) on cucumber ‘Bush Crop’ cotyledon leaves. The number of isolates sensitive to tested concentrations of Quadris 2.08SC, Torino 0.85SC, Fontelis 1.67SC, and Procure 480SC was 8 (22%), 21 (57%), 20 (54%), and 23 (62%), respectively. During 2015 to 2018, Quadris 2.08SC, Torino 0.85SC, Fontelis 1.67SC, quinoxyfen (Quintec 250SC), and Procure 480SC were tested for their effectiveness for managing powdery mildew on pumpkin ‘Howden’ in the field. The results showed that powdery mildew was effectively managed in the plots treated with Procure 480SC and Quintec 250SC. However, management of the disease was less successful in the plots treated with Quadris 2.08SC, Torino 0.85SC, and Fontelis 1.67SC.

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Poor Control of Cucurbit Powdery Mildew Associated with First Detection of Resistance to Cyflufenamid in the Causal Agent, Podosphaera xanthii, in the United States

Plant Health Progress ◽

10.1094/php-06-18-0029-br ◽

2018 ◽

Vol 19 (3) ◽

pp. 222-223 ◽

Cited By ~ 1

Author(s):

Margaret T. McGrath ◽

Zachary F. Sexton

Keyword(s):

United States ◽

Powdery Mildew ◽

High Resistance ◽

The United States ◽

Management Tool ◽

Common Disease ◽

Podosphaera Xanthii ◽

Resistance Risk ◽

Action Committee ◽

Group Knowledge

This report is about the first known occurrence of resistance to cyflufenamid, a Fungicide Resistance Action Committee (FRAC) code U6 fungicide, in Podosphaera xanthii in the United States. This fungus causes powdery mildew in cucurbit crops, the most common disease of this important crop group. Knowledge about resistance is important for guiding fungicide programs. Mobile, targeted fungicides, which have moderate or high resistance risk, are the most effective management tool for this disease. The pathogen is a documented high resistance risk organism having already developed resistance to fungicides in FRAC groups 1, 3, 7, 11, and 13.

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First Report of Resistance to Quinoxyfen in Podosphaera xanthii, Causal Agent of Cucurbit Powdery Mildew, in the United States

Plant Health Progress ◽

10.1094/php-03-17-0018-br ◽

2017 ◽

Vol 18 (2) ◽

pp. 94-94 ◽

Cited By ~ 3

Author(s):

M. T. McGrath

Keyword(s):

United States ◽

Powdery Mildew ◽

High Resistance ◽

The United States ◽

Management Tool ◽

Common Disease ◽

Podosphaera Xanthii ◽

Effective Management ◽

Group 13 ◽

Resistance Risk

This brief reports the first known occurrence of resistance to quinoxyfen, a quinoline (FRAC Group 13) fungicide, in Podosphaera xanthii in the United States. This fungus causes powdery mildew in cucurbit crops, the most common disease of this important crop group. Mobile, targeted fungicides, which have moderate or high resistance risk, are the most effective management tool for this disease. The pathogen is a documented high resistance-risk organism, having already developed resistance to fungicides in FRAC Groups 1, 3, 7, and 11.

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Resistance to Triadimefon and Benomyl: Dynamics and Impact on Managing Cucurbit Powdery Mildew

Plant Disease ◽

10.1094/pdis.2001.85.2.147 ◽

2001 ◽

Vol 85 (2) ◽

pp. 147-154 ◽

Cited By ~ 22

Author(s):

Margaret Tuttle McGrath ◽

Nina Shishkoff

Keyword(s):

New York ◽

Powdery Mildew ◽

Podosphaera Xanthii ◽

Pathogen Population ◽

Action Threshold ◽

Leaf Surfaces ◽

Dmi Fungicides ◽

Resistant Strains ◽

Control Loss ◽

Cucurbit Powdery Mildew

Frequency of fungicide-resistant strains of Podosphaera xanthii on pumpkins in New York before treatment varied from 3 to 80% for the demethylation inhibiting (DMI) fungicide triadimefon and from 0 to 48% for the benzimidazole fungicide benomyl between 1993 and 1996. When the initial frequency of triadimefon-resistant strains was less than 55%, one application of triadimefon plus chlorothalonil was effective. This application was made after reaching the action threshold of one leaf with powdery mildew symptoms per 50 old leaves (defined as the oldest third of the foliage). The frequency of triadimefon-resistant strains increased from 3 to 71% by 20 days after the first fungicide application in 1993. Triadimefon in the second application did not contribute to control. Loss of efficacy was due to resistance because, compared with triadimefon-treated pumpkins, pumpkins treated with other systemic fungicides were less severely infected by powdery mildew on abaxial leaf surfaces where the companion multi-site contact fungicide contributes little to control. Triadimefon was not effective in 1995 when 80% of the pathogen population was resistant before treatment. Benomyl was effective in 1995, but not in 1996 when 48% of the isolates tested were resistant to both benomyl and triadimefon before treatment. An in-field seedling assay was developed to determine local occurrence of resistant strains before the first treatment was needed. Although sensitivity of the pathogen population to the DMI fungicides myclobutanil and propiconazole also decreased after they were applied, these fungicides were more effective than triadimefon.

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Powdery Mildew Resistant Rootstocks that Impart Tolerance to Grafted Susceptible Watermelon Scion Seedlings

Plant Disease ◽

10.1094/pdis-09-17-1384-re ◽

2018 ◽

Vol 102 (7) ◽

pp. 1290-1298 ◽

Cited By ~ 7

Author(s):

Chandrasekar S. Kousik ◽

Mihir Mandal ◽

Richard Hassell

Keyword(s):

United States ◽

Powdery Mildew ◽

Disease Severity ◽

Citrullus Lanatus ◽

The United States ◽

Bottle Gourd ◽

Economic Losses ◽

Podosphaera Xanthii ◽

Lagenaria Siceraria ◽

True Leaf

Powdery mildew (PM) is a major foliar disease causing serious economic losses of cucurbit crops grown in the United States. The pathogen Podosphaera xanthii, which causes PM, is known to infect seedlings, stems, foliage, petioles, and fruit of cucurbit crops. In recent years, grafting watermelon on resistant rootstocks for managing soilborne diseases has been gaining popularity in the U.S.A. However, grafting for managing foliar diseases has not yet received adequate attention. Three greenhouse experiments were conducted during the summer months of 2012, 2013, and 2014 to determine if PM-resistant rootstocks could impart resistance to a susceptible watermelon scion. Susceptible watermelon scion ‘Mickey Lee’ seedlings were grafted onto 25 watermelon (Citrullus lanatus, C. amarus, C. mucosospermus) and four bottle gourd (Lagenaria siceraria) rootstocks. Grafted plants were inoculated with a 2 × 104 conidia ml−1 suspension of P. xanthii conidia and disease severity was rated 14 days after inoculation. Mickey Lee grafted on six PM-resistant watermelon rootstocks had significantly (P ≤ 0.05) lower PM severity on cotyledons, 2nd true leaf, and upper leaves (rating for 3rd to 7th or 8th true leaf) compared with Mickey Lee grafted on susceptible watermelon USVL677-PMS or bottle gourd USVL848-PMS rootstocks. However, some of the resistant watermelon rootstocks did not impart significant resistance to the scion. The resistant bottle gourd rootstocks USVL482-PMR and USVL351-PMR provided significantly greater levels of resistance, compared with many of the resistant watermelon rootstocks. Grafting watermelon on resistant rootstocks may help mitigate the effects of PM on susceptible scion seedlings.

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