First Confirmation of Sphaerotheca macularis on Strawberry Plants in Southwestern Spain

In February 2001, in seven small tunnel production fields in southwestern Spain, symptoms and signs of powdery mildew were detected on strawberry (Fragaria × ananassa) cvs. Camarosa, Andana, and Carisma. On ‘Camarosa’ plants, white patches of mycelium developed on the lower leaf surface. Later, the leaf edges rolled upward, and reddish or necrotic irregular spots appeared on the lower leaf surface. Some immature fruits were abnormally hard and covered with a dense white mycelium. Symptoms were much less severe on ‘Andana’ and ‘Carisma’. Microscopic observations revealed conidia in chains, ellipsoidal to barrel-shaped, with fibrosin granules. Mean dimensions of conidia (N= 70) were 31.2 μm ± 2.8 × 20.39 μm ± 4.1. Cleistothecia were not detected. Based on these characteristics, the causal agent was identified as Sphaerotheca macularis (Wallr.:Fr.) Lind. (1). Pathogenicity was confirmed by inoculating leaves of 2-month-old potted ‘Camarosa’ plants. S. macularis obtained from field-grown plants was maintained on ‘Camarosa’ strawberry leaves in a growth chamber (20°C and 100% relative humidity). Inoculum was obtained from freshly sporulating infected leaves 11 days after inoculation. Conidia of S. macularis were gently brushed into 100-ml distilled water containing two drops of Tween 20. Ten strawberry ‘Camarosa’ plants were inoculated by misting with a conidial suspension (104 conidia per ml) (2). The plants were incubated at 100% relative humidity for 48 h in the greenhouse (15 to 25°C). Colonies of powdery mildew were observed on leaves after 18 days. To our knowledge, this is the first confirmation of S. macularis on strawberry in Spain. References: (1) K. G. Mukerji. No. 188 in: Descriptions of Pathogenic Fungi and Bacteria. CMI, Kew, UK, 1968. (2) R. Reuveni et al. Crop Prot. 19:335, 2000.

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First Report of Powdery Mildew on Euryops pectinatus in Italy

Plant Disease ◽

10.1094/pdis.2002.86.8.920b ◽

2002 ◽

Vol 86 (8) ◽

pp. 920-920

Author(s):

A. Garibaldi ◽

A. Minuto ◽

D. Bertetti ◽

M. L. Gullino

Keyword(s):

Powdery Mildew ◽

Leaf Surface ◽

Northern Italy ◽

Growth Chamber ◽

Growth Reduction ◽

First Report ◽

Similar Disease ◽

Commercial Farms ◽

White Mycelium ◽

Sphaerotheca Fusca

Euryops pectinatus is grown in Italy for landscape use in parks and gardens. In 2001, severe outbreaks of a previously unknown powdery mildew were observed in commercial farms located in Albenga (northern Italy). All green parts (leaves, stems, and petioles) became covered with a white mycelium. Infections were particularly severe on the upper leaf surface. With progress of the disease, infected leaves turned yellow and died. The presence of powdery mildew infections on leaves and stems only rarely was linked to growth reduction. Conidia were hyaline, cylindric to slightly doliform, did not show fibrosin bodies, borne in chains, and measured 24 to 41 × 12 to 20 μm. Cleistothecia were not observed. The pathogen was identified as Oidium sp. subgen. Fibroidium (1). Pathogenicity was confirmed by gently pressing diseased leaves on leaves of healthy E. pectinatus plants. Inoculated plants were maintained in a growth chamber at 20 to 24°C. After 12 to 14 days, powdery mildew symptoms developed. A similar disease of E. pectinatus was observed in 1999 in California and identified as being caused by Podosphaera (Sphaerotheca) fusca (2). It is possible that the powdery mildew observed in Italy belongs to the same species, also considering that recently the two genera, Podosphaera and Sphaerotheca, have been unified in the genus Podosphaera (1). References: (1) U. Braun and S. Takamatsu. Schlechtendalia 4:1, 2000. (2) G. S. Saenz et al. Plant Dis. 84:1048, 2000.

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First Report of Nigrospora osmanthi Causing Leaf Spot on Tartary Buckwheat in China

Plant Disease ◽

10.1094/pdis-08-20-1773-pdn ◽

2020 ◽

Author(s):

Quan Shen ◽

Xixu Peng ◽

Feng He ◽

Shaoqing Li ◽

Zuyin Xiao ◽

...

Keyword(s):

Relative Humidity ◽

Leaf Spot ◽

Plant Pathogens ◽

Hunan Province ◽

Tartary Buckwheat ◽

Tween 20 ◽

Pathogenicity Test ◽

Conidial Suspension ◽

Sterile Water ◽

First Report

Buckwheat (Fagopyrum tataricum) is a traditional short-season pseudocereal crop originating in southwest China and is cultivated around the world. Antioxidative substances in buckwheat have been shown to provide many potential cardiovascular health benefits. Between August and November in 2019, a leaf spot was found in several Tartary buckwheat cv. Pinku1 fields in Xiangxiang County, Hunan Province, China. The disease occurred throughout the growth cycle of buckwheat after leaves emerged, and disease incidence was approximately 50 to 60%. Initially infected leaves developed a few round lesions, light yellow to light brown spots. Several days later, lesions began to enlarge with reddish brown borders, and eventually withered and fell off. Thirty lesions (2×2 mm) collected from three locations with ten leaves in each location were sterilized in 70% ethanol for 10 sec, in 2% sodium hypochlorite for 30 sec, rinsed in sterile water for three times, dried on sterilized filter paper, and placed on a potato dextrose PDA with lactic acid (3 ml/L), and incubated at 28°C in the dark for 3 to 5 days. Fungal colonies were initially white and later turned black with the onset ofsporulation. Conidia were single-celled, black, smooth, spherical to subspherical, and measured 9.2 to 15.6 µm long, and 7.1 to 11.6 µm wide (n=30). Each conidium was terminal and borne on a hyaline vesicle at the tip of conidiophores. Morphologically, the fungus was identified as Nigrospora osmanthi (Wang et al. 2017). Identification was confirmed by amplifying and sequencing the ITS region, and translation elongation factor 1-alpha (TEF1-α) and partial beta-tublin (TUB2) genes using primers ITS1/ITS4 (Mills et al. 1992), EF1-728F/EF-2 (Carbone and Kohn 1999; O’Donnell et al. 1998) and Bt-2a/Bt-2b (Glass et al. 1995), respectively. BLAST searches in GenBank indicated the ITS (MT860338), TUB2 (MT882054) and TEF1-α (MT882055) sequences had 99.80%, 99% and 100% similarity to sequences KX986010.1, KY019461.1 and KY019421.1 of Nigrospora osmanthi ex-type strain CGMCC 3.18126, respectively. A neighbor-joining phylogenetic tree constructed using MEGA7.0 with 1,000 bootstraps based on the concatenated nucleotide sequences of the three genes indicated that our isolate was closely related to N. osmanthi. Pathogenicity test was performed using leaves of healthy F. tataricum plants. The conidial suspension (1 × 106 conidia/ml) collected from PDA cultures with 0.05% Tween 20 buffer was used for inoculation by spraying leaves of potted 20-day-old Tartary buckwheat cv. Pinku1. Five leaves of each plant were inoculated with spore suspensions (1 ml per leaf). An equal number of control leaves were sprayed with sterile water to serve as a control. The treated plants were kept in a greenhouse at 28°C and 80% relative humidity for 24 h, and then transferred to natural conditions with temperature ranging from 22 to 30°C and relative humidity ranging from 50 to 60%. Five days later, all N. osmanthi-inoculated leaves developed leaf spot symptoms similar to those observed in the field, whereas control leaves remained healthy. N. osmanthi was re-isolated from twelve infected leaves with frequency of 100%, fulfilling Koch’s postulates. The genus Nigrospora has been regarded by many scholars as plant pathogens (Fukushima et al. 1998) and N. osmanthi is a known leaf blight pathogen for Stenotaphrum secundatum (Mei et al. 2019) and Ficus pandurata (Liu et al. 2019) but has not been reported on F. tataricum. Nigrospora sphaerica was also detected in vegetative buds of healthy Fagopyrum esculentum Moench (Jain et al. 2012). To our knowledge, this is the first report of N. osmanthi causing leaf spot on F. tataricum in China and worldwide. Appropriate strategies should be developed to manage this disease.

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First Report of Powdery Mildew Caused by Podosphaera aphanis var. aphanis on Potentilla fruticosa in Italy

Plant Disease ◽

10.1094/pd-89-1362c ◽

2005 ◽

Vol 89 (12) ◽

pp. 1362-1362

Author(s):

A. Garibaldi ◽

D. Bertetti ◽

M. L. Gullino

Keyword(s):

United States ◽

Powdery Mildew ◽

The United States ◽

Pathogenicity Test ◽

First Report ◽

Potentilla Fruticosa ◽

Erysiphe Polygoni ◽

Voucher Specimens ◽

White Mycelium ◽

Sphaerotheca Macularis

Potentilla fruticosa L. (bush cinquefoil), belonging to the family Rosaceae, is an ornamental plant used in parks and gardens. During the spring and summer of 2005, severe outbreaks of a previously unknown powdery mildew were observed in several private gardens located near Biella (northern Italy). The adaxial and abaxial surfaces of leaves as well as the stems were covered with white mycelium. Buds and flowers also were affected. As disease progressed, infected leaves turned yellow and dehisced. Conidia formed in chains and were hyaline, ovoid, and measured 24.0 to 36.0 × 15.8 to 24.0 μm (average 30.1 × 20.0 μm). Fibrosin bodies were present. Chasmothecia were numerous, sphaerical, amber colored, and diameters ranged from 84.0 to 98.4 μm (average 90.4 μm). Each chasmothecium contained one ascus with eight ascospores. Ascospores measured 26.5 to 27.2 × 13.2 to 15.6 μm (average 26.8 × 14.0 μm). On the basis of its morphology, the causal agent was determined to be Podosphaera aphanis (Wallr.) U. Braun & S. Takamatsu var. aphanis U. Braun (1). Pathogenicity was confirmed through inoculations by gently pressing diseased leaves onto leaves of healthy P. fruticosa plants. Three plants were inoculated. Three noninoculated plants served as a control. Plants were maintained at temperatures ranging from 12 to 23°C. Ten days after inoculation, typical symptoms of powdery mildew developed on inoculated plants. Noninoculated plants did not show symptoms. The pathogenicity test was carried out twice. To our knowledge, this is the first report of powdery mildew on P. fruticosa in Italy. Erysiphe polygoni D.C. and Sphaerotheca macularis (Wallr.:Fr.) Lind were observed in the United States on P. fruticosa (2), while in Japan, the presence of S. aphanis var aphanis was reported (3). Voucher specimens are available at the AGROINNOVA Collection, University of Torino. References: (1) U. Braun and S. Takamatsu. Schlechtendalia 4:1, 2000 (2) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St Paul, MN, 1989. (3) S. Tanda et al. J. Agric. Sci. 39:258, 1995.

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First Report of Powdery Mildew Caused by an Oidium sp. on Spiraea japonica in Italy

Plant Disease ◽

10.1094/pdis.2004.88.9.1045c ◽

2004 ◽

Vol 88 (9) ◽

pp. 1045-1045

Author(s):

A. Garibaldi ◽

G. Gilardi ◽

M. L. Gullino

Keyword(s):

Powdery Mildew ◽

Ornamental Plants ◽

The United States ◽

Northern Italy ◽

Artificial Inoculation ◽

Conidial Suspension ◽

Sterile Water ◽

Growth Reduction ◽

White Mycelium ◽

The City

Spiraea japonica is an old-fashioned shrub widely grown in Italy in parks and gardens. In the summer of 2003, severe outbreaks of a previously unknown powdery mildew were observed in some parks and gardens in the city of Torino (northern Italy). Infected leaves became covered on both sides with a white mycelium. As the disease progressed, infected leaves turned reddish and eventually dropped prematurely. The presence of powdery mildew infections on leaves sometimes caused their distortion and growth reduction. Frequently, mycelium was observed also on the stem. Conidia were hyaline, ellipsoid, cylindrical, or dol iform, born in chains, measured 21.0 to 38.4 × 10.8 to 14.4 mm, and showed fibrosin bodies. Cleistothecia were not observed. The pathogen was identified as Oidium subgenus Fibroidium (1,2). Pathogenicity was confirmed by spraying leaves of healthy potted S. japonica plants with a conidial suspension (105 conidia per ml) prepared in sterile water from diseased leaves. Three plants were inoculated and three noninoculated plants served as control. The artificial inoculation was carried out twice. After artificial inoculation, plants were maintained in a growth chamber at 25°C. After 20 days, powdery mildew symptoms developed. Microsphaera alni and Podosphaera oxyacanthae were described as causal agents of powdery mildew on S. japonica in the United States (3), while Sphaerotheca spiraeae was considered the causal agent of a powdery mildew observed in Japan (4) and more recently in Poland. References: (1) U. Braun and S. Takamatsu. Schlechtendalia 4:1, 2000. (2) R. T. A. Cook et al. Mycol. Res. 101:975, 1997. (3) P. Pirone. Diseases and Pests of Ornamental Plants. John Wiley and Sons, NY, 1978. (4) K. Sawada. Rev. Appl. Mycol. 31:577, 1952.

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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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Epiphytic growth and survival of Tilletiopsis pallescens, a potential biological control agent of Sphaerotheca fuliginea, on cucumber leawes

Canadian Journal of Botany ◽

10.1139/b97-099 ◽

1997 ◽

Vol 75 (6) ◽

pp. 892-901 ◽

Cited By ~ 18

Author(s):

E. J. Urquhart ◽

Z. K. Punja

Keyword(s):

Biological Control ◽

Relative Humidity ◽

Powdery Mildew ◽

Canola Oil ◽

Leaf Surface ◽

Biological Control Agent ◽

Liquid Paraffin ◽

Control Agent ◽

Sphaerotheca Fuliginea ◽

Growth And Survival

The influence of low (70%) and high (90%) relative humidity on epiphytic growth, development, and survival of Tilletiopsis pallescens, a ballistospore-forming yeast-like fungus, on cucumber leaves was investigated. In addition, survival of the fungus in the presence or absence of powdery mildew (Sphaerotheca fuliginea) colonies was determined. Growth and development were visualized by scanning electron microscopy of the leaf surface, and survival was quantified as colony-forming units recovered on a semiselective medium. Development of T. pallescens from blastospores that were applied to healthy leaves at 70% relative humidity was limited to small colonies that grew adjacent to leaf veins 7 days after application. At 90% relative humidity, extensive hyphal networks had developed within 3 days of blastospore germination, and ballistospores were produced within 7 days. Growth and sporulation of T. pallescens were most extensive at the base and on the surface of leaf trichomas. In the presence of S. fuliginea, T. pallescens mycelium developed adjacent to hyphae and conidiophores of the pathogen within 3 days at both 70 and 90% relative humidity. However, at 90% relative humidity, growth of T. pallescens was more extensive and ballistospores were produced within 5 days, and there was visible collapse of mildew hyphae. There was no evidence of penetration of the leaf or mildew hyphae by T. pallescens. Survival of T. pallescens was significantly (P = 0.05) increased at 1 and 5 days postapplication at 70% relative humidity when blastospores were amended with 1% (v/v) canola oil – lecithin. Survival at 90% relative humidity was also significantly increased with canola oil – lecithin and by the presence of S. fuliginea. The addition of liquid paraffin – lecithin or liquid paraffin – Tween had no effect on survival when compared to the control. The results from this study indicate that growth and survival of T. pallescens are enhanced by high relative humidity and by the presence of powdery mildew, and canola oil – lecithin amendment improved survival on the leaf surface at reduced ambient humidity. Key words: antagonism, biological control, powdery mildew, yeast.

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First Report of Powdery Mildew on Tomato Caused by Oidium neolycopersici in Venezuela

Plant Disease ◽

10.1094/pdis-91-7-0910b ◽

2007 ◽

Vol 91 (7) ◽

pp. 910-910 ◽

Cited By ~ 1

Author(s):

J. O. Montilla ◽

M. S. González ◽

D. Renaud

Keyword(s):

Powdery Mildew ◽

Leaf Surface ◽

Distinct Species ◽

Oidium Neolycopersici ◽

First Report ◽

Physalis Peruviana ◽

Infected Tomato ◽

White Mycelium ◽

Foliar Tissue ◽

Plant Pathol

During 2004 and 2005, a powdery mildew was observed in tomato (Lycopersicon esculentum Mill. cv. Rio Grande) fields in several states in Venezuela. Symptoms included development of patches of dense white mycelium, predominantly on the upper leaf surface. Leaves in the middle and lower canopies were the most affected. Similar symptoms sometimes were observed on stems. The foliage of infected plants turned yellow and showed necrosis followed by desiccation and rapid defoliation. Microscopic examination revealed the presence of typical structures of the Oidium genus. Hyphae were hyaline and septate. Conidiophores were unbranched, erect, measured 51 to 108 (80) μm, and consisted of 3 or 4 cells. Conidia were hyaline, ellipsoidal to ovoidal, and measured 16 to 43 (30) × 12 to 22 (16) μm. Conidia were produced singly, without fibrosin bodies. Conidial polar germination was common. Appressoria were lobed. The sexual stage was not found. The fungus was identified as Oidium neolycopersici, recently recognized as a distinct species (1,2). Conidia from infected tomato leaves were shaken onto leaves of 10 plants of L. esculentum and L. pimpinellifolium (Jusl.) Mill. and five plants of a weed (Physalis peruviana L.). Noninoculated plants served as a control. Symptoms that developed on all inoculated plants were similar to those of plants naturally infected. Within 7 to 8 days, symptoms in L. esculentum consisted of small colonies that quickly covered large portions of foliar tissue. L. pimpinellifolium showed small and localized symptoms after 10 days and P. peruviana showed symptoms after 15 days. To our knowledge, this is the first report of O. neolycopersici in Venezuela. References: (1) H. Jones et al. Mol. Plant Pathol. 2:303, 2001. (2) L. Kiss et al. Mycol. Res. 105:684, 2001.

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Effects of Application Techniques on the Control of Powdery Mildew on Roses

Experimental Agriculture ◽

10.1017/s001447970000017x ◽

1970 ◽

Vol 6 (2) ◽

pp. 123-127

Author(s):

Hillela Vigodsky ◽

N. Zieslin

Keyword(s):

Powdery Mildew ◽

Leaf Surface ◽

High Volume ◽

Lower Leaf Surface ◽

Fluorescent Tracers ◽

Air Blast ◽

Adequate Control ◽

Application Techniques ◽

Low Volume

SUMMARYThree methods of applying four fungicides were tested for controlling powdery mildew on roses. Best results were obtained by air blast application, with high-volume sprays providing better control than low-volume sprays. The former, unassisted by air blast, failed to give adequate control. Fluorescent tracers showed that performance of various modes of application depended on their ability to provide a satisfactory fungicide deposit on the lower leaf surface, which was particularly susceptible to the disease.

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First report of Erysiphe corylacearum, agent of powdery mildew, on hazelnut (Corylus avellana) in Romania

Plant Disease ◽

10.1094/pdis-01-21-0024-pdn ◽

2021 ◽

Author(s):

Marco Rosati ◽

Marian Bogoescu ◽

Davide Spadaro

Keyword(s):

Powdery Mildew ◽

Crop Protection ◽

Its Region ◽

Corylus Avellana ◽

Morphological Identification ◽

Conidial Suspension ◽

Rdna Sequences ◽

White Mycelium ◽

Corylus Avellana L ◽

Erysiphe Corylacearum

Romania has an area dedicated to hazelnut (Corylus avellana L.), covering 890 hectares as of 2019. During October 2020, powdery mildew symptoms were observed on the upper side of leaves of hazelnut ‘Tonda di Giffoni’ in two commercial orchards in Dudeștii Vechi, Romania (Fig. 1). The disease was present on 70% of the trees in planting, with at least 5 leaves per tree having powdery mildew. Micromorphological examination revealed amphigenous, hyaline, branched, septate mycelial patches of 2.3 to 3.6 μm in diameter. Conidiophores measured 24-60 × 5-6 (average: 45 × 6) μm and consisted of erect, cylindrical to flexuous foot cells, followed by 1-2 shorter cells. Ellipsoid, ovoid to doliform conidia were produced singly and they measured 19-35 × 16-24 (average: 28 × 19) μm. Chasmothecia were spherical, 75 to 107 (average: 88) μm in diameter. Nine to thirteen straight, sometimes flexuous, appendages measured 54 to 92 (average: 66) μm in length and they had five times dichotomous branched apices with curved tips (Fig. 2). Each chasmothecium contained three to five ellipsoid, ovoid to subglobose asci measuring 41-58 × 29-55 μm (average 52 × 43) μm. The asci contained four to eight ascospores measuring 13-24 × 11-15 (average 18 × 14) μm. Morphological identification was confirmed by sequencing the ITS-region of rDNA using two isolates from leaves, stored as frozen mycelium at -20°C. PCR was performed with Erysiphales-specific primer pair PMITS1/PMITS2 (Cunnington et al. 2003). The obtained sequences were deposited in GenBank (Accession n° MW423075, MW423076). Blast analysis of both sequences had 100% identity to ITS rDNA sequences of Erysiphe corylacearum from Azerbaijan (Abasova et al. 2018; Accession n° LC270863), Turkey (Sezer et al. 2017; KY082910), Switzerland (Beenken et al. 2020; MN82272), Iran (Arzanlou et al. 2018; MH047243), Italy (Mezzalama et al. 2020; MW045425) and 99% identity from Georgia (Meparishvili et al. 2019; MK157199). The sequences had a lower percent identity (83%) to Phyllactinia guttata (Accession n° AB080558) (Fig. 3). Pathogenicity was verified on one-year-old plants of C. avellana ‘Tonda di Giffoni’, which were artificially inoculated with a conidial suspension from infected leaves (n = 25). Inoculated plants were incubated at 20 to 28°C with 70 to 80% relative humidity. White mycelium appeared on the upper surface of the leaves at 8 to 10 days after inoculation. No symptoms were found on control plants sprayed with sterile water. The fungus present on inoculated leaves was morphologically identical to the original isolates from diseased trees from the field. E. corylacearum is native to East Asia and was previously reported in Japan on wild species of Corylus (Takamatsu et al. 2015; Accession n° LC009928). The pathogen most likely spread into Europe from east to west of Europe (Heluta et al. 2019), through the Caucasus, starting from Turkey, Azerbaijan, Georgia, and Iran. P. guttata was considered the only causal agent of powdery mildew on hazelnut in most countries, including Romania (Brown 1995). Compared to P. guttata, which generally develops a mycelium on the underside of leaves, E. corylacearum grows with a white mycelium on the upper side of the leaves. Recently, E. corylacearum on C. avellana was reported also in Ukraine (Heluta et al. 2019), from which it could have moved to Romania. Crop protection strategies for hazelnut should be revised according to the new pathogen occurrence.

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Downy Mildew Caused by Peronospora valerianellae on Corn-Salad (Valerianella locusta) in California

Plant Disease ◽

10.1094/pdis-92-10-1470b ◽

2008 ◽

Vol 92 (10) ◽

pp. 1470-1470 ◽

Cited By ~ 1

Author(s):

S. T. Koike

Keyword(s):

Downy Mildew ◽

Growth Characteristic ◽

Leaf Tissue ◽

The United States ◽

Tween 20 ◽

Conidial Suspension ◽

Fresh Market ◽

Initial Symptoms ◽

Seedborne Inoculum ◽

Symptoms And Signs

Corn-salad or lamb's lettuce (Valerianella locusta) is a specialty leafy green, annual vegetable grown commercially in California as a fresh market commodity used in salads. In the spring (February through April) of 2008, fields in coastal California (Monterey County) showed symptoms and signs of a downy mildew. Initial symptoms consisted of irregularly shaped, light green patches observed on adaxial leaf surfaces; these lesions later turned yellow. As disease progressed, patches became brown and necrotic. The abaxial sides of affected leaves were heavily colonized by an extensive, purplish growth characteristic of a downy mildew pathogen. Symptomatic leaves were unmarketable and diseased portions of plantings were not harvested. Crop loss was estimated to be between 1 and 15% for any one particular planting. The purple growth consisted of hyaline, branched conidiophores that emerged from stomata and had branches ending in slender, curved branchlets that did not have swollen tips. Conidia were slightly brown, ovoid, mostly nonpapillate, and measured 23.7 to 29.4 μm long × 15.9 to 22.2 μm wide. When necrotic leaf tissue was examined microscopically, thick-walled, yellow brown oospores were abundant within leaf tissues. Oospores measured 33.2 to 36.9 μm in diameter. On the basis of disease symptoms and morphology of the organism, the pathogen was identified as Peronospora valerianellae (1,3). To prove pathogenicity on corn-salad, 24 3-week-old seedlings were sprayed until runoff, using a hand-held spray bottle, with a conidial suspension (1.0 × 104 sporangia/ml), incubated for 24 h in a dew chamber (18 to 20°C), and then maintained in a greenhouse (22 to 24°C). Inoculum was obtained from one section of an affected commercial field. After 10 to 12 days, symptoms and signs of downy mildew occurred on inoculated plants, and the pathogen morphology matched that of the pathogen originally observed. Twelve untreated control plants did not develop downy mildew. To test for seedborne inoculum, 10 g of seed of each of two corn-salad cultivars were added to 100 ml of a dilute (0.05%) Tween 20 solution. The suspension was agitated for 3 h, filtered through cheesecloth to remove seed, and centrifuged. The resulting pellet was examined microscopically and found to contain low numbers of oospores that were similar in morphology to those observed in necrotic leaf lesions. To my knowledge, this is the first report of downy mildew caused by P. valerianellae on corn-salad in California and the United States. The pathogen has been reported on corn-salad in England, France, Germany, Scotland, and the Ukraine (1–3). Seedborne oospores of downy mildew have been reported on corn-salad seed tested in France (1). References: (1) R. Champion and H. Mecheneau. Seed Sci. Technol. 7:259, 1979. (2) D. F. Farr et al. Fungal Databases. Systematic Mycology and Microbiology. Online publication. ARS, USDA, 2008. (3) G. Pietrek and V. Zinkernagel. Advances in Downy Mildew Research. Kluwer Academic Publishers, Dordrecht, the Netherlands, 2002.

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