Lethal Cankers Caused by Phytophthora spp. in Almond Scions: Specific Etiology and Potential Inoculum Sources

Etiology of a new lethal canker syndrome of almond trees was investigated in the San Joaquin Valley of California. Phytophthora citricola was isolated most frequently from cankers limited to the aboveground scion portions of trees; whereas P. cactorum usually was isolated from cankers originating at or below the soil surface. Repeated observations and isolations indicated that some of the cankers associated with each species were perennial. In pathogenicity tests, isolates of P. cactorum and P. citricola caused bark cankers in excised segments of almond shoots and branches, as well as root and crown rot on potted almond seedlings. Only P. citricola caused significant disease in root and crown tissues of peach seedlings. When pear fruits and almond seedlings were used as bait, P. cactorum and P. citricola were isolated from orchard soil, debris collected in natural depressions where scaffold branches and the tree trunk joined at a common point, and debris deposited on tree surfaces during nut harvest. Control strategies for Phytophthora diseases of almond should consider aboveground as well as belowground modes of attack by P. citricola and P. cactorum. Debris infested with these pathogens and deposited on trees during harvest may play a role in the disease epidemiology.

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First Report of Phytophthora citrophthora on Penstemon barbatus in Italy

Plant Disease ◽

10.1094/pd-90-1260a ◽

2006 ◽

Vol 90 (9) ◽

pp. 1260-1260 ◽

Cited By ~ 1

Author(s):

A. Garibaldi ◽

D. Bertetti ◽

D. Minerdi ◽

M. L. Gullino

Keyword(s):

Its Region ◽

The United States ◽

Phytophthora Species ◽

Crown Rot ◽

Pathogenicity Test ◽

Phytophthora Citrophthora ◽

First Report ◽

Blastn Analysis ◽

Root And Crown Rot ◽

Pathogenicity Tests

Penstemon barbatus (Cav.) Roth (synonym Chelone barbata), used in parks and gardens and sometimes grown in pots, is a plant belonging to the Scrophulariaceae family. During the summers of 2004 and 2005, symptoms of a root rot were observed in some private gardens located in Biella Province (northern Italy). The first symptoms resulted in stunting, leaf discoloration followed by wilt, root and crown rot, and eventually, plant death. The diseased tissue was disinfested for 1 min in 1% NaOCl and plated on a semiselective medium for Oomycetes (4). The microorganism consistently isolated from infected tissues, grown on V8 agar at 22°C, produced hyphae with a diameter ranging from 4.7 to 5.2 μm. Sporangia were papillate, hyaline, measuring 43.3 to 54.4 × 26.7 to 27.7 μm (average 47.8 × 27.4 μm). The papilla measured from 8.8 to 10.9 μm. These characteristics were indicative of a Phytophthora species. The ITS region (internal transcribed spacer) of rDNA was amplified using primers ITS4/ITS6 (3) and sequenced. BLASTn analysis (1) of the 800 bp obtained showed a 100% homology with Phytophthora citrophthora (R. & E. Sm.) Leonian. The nucleotide sequence has been assigned GenBank Accession No. DQ384611. For pathogenicity tests, the inoculum of P. citrophthora was prepared by growing the pathogen on autoclaved wheat and hemp kernels (2:1) at 25°C for 20 days. Healthy plants of P. barbatus cv. Nano Rondo, 6 months old, were grown in 3-liter pots (one plant per pot) using a steam disinfested substrate (peat/pomix/pine bark/clay 5:2:2:1) in which 200 g of kernels per liter of substrate were mixed. Noninoculated plants served as control treatments. Three replicates were used. Plants were maintained at 15 to 20°C in a glasshouse. The first symptoms, similar to those observed in the gardens, developed 21 days after inoculation, and P. citrophthora was consistently reisolated from infected plants. Noninoculated plants remained healthy. The pathogenicity test was carried out twice with similar results. A nonspecified root and crown rot of Penstemon spp. has been reported in the United States. (2). To our knowledge, this is the first report of P. citrophthora on P. barbatus in Italy as well as in Europe. References: (1) S. F. Altschul et al. Nucleic Acids Res. 25:3389, 1997 (2) F. E. Brooks and D. M. Ferrin. Plant Dis. 79:212, 1995. (3) D. E. L. Cooke and J. M. Duncan. Mycol. Res. 101:667, 1997. (4) H. Masago et al. Phytopathology 67:425, 1977.

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Characterization of Oomycete Species Associated With Root and Crown Rot of English Walnut in Chile

Plant Disease ◽

10.1094/pdis-07-18-1160-re ◽

2019 ◽

Vol 103 (4) ◽

pp. 691-696 ◽

Cited By ~ 2

Author(s):

Jeannette Guajardo ◽

Sebastián Saa ◽

Natalia Riquelme ◽

Gregory Browne ◽

Cristian Youlton ◽

...

Keyword(s):

Phytophthora Cinnamomi ◽

Juglans Regia ◽

Pythium Ultimum ◽

Crown Rot ◽

Root Rots ◽

Root And Crown Rot ◽

Pathogenicity Tests ◽

English Walnut ◽

Crown And Root Rot ◽

Root Tissues

English (Persian) walnut (Juglans regia) trees affected by root and crown rot were surveyed in five regions of central Chile between 2015 and 2017. In each region, nine orchards, ranging from 1 to 21 years old, were randomly selected and inspected for incidence and severity of tree decline associated with crown and root rot. Soil and symptomatic crown and root tissues were collected and cultured in P5ARP semiselective medium to isolate potential oomycete pathogens, which were identified through morphology and molecularly using ITS sequences in the rDNA gene and beta tubulin gene. The most frequently isolated species was Phytophthora cinnamomi. Pathogenicity tests were conducted with representative oomycete isolates. P. cinnamomi, P. citrophthora, and Pythium ultimum were all pathogenic in J. regia. Nevertheless, only P. cinnamomi and P. citrophthora were pathogenic to English walnut. Py. ultimum caused limited levels of root damage to English walnut seedlings. Our research indicates that as the Chilean walnut industry has expanded, so have walnut crown and root rots induced by oomycetes.

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First report of Fusarium commune causing root and crown rot on maize in Italy

Plant Disease ◽

10.1094/pdis-01-21-0075-pdn ◽

2021 ◽

Author(s):

Monica Mezzalama ◽

Vladimiro Guarnaccia ◽

Ilaria Martino ◽

Giulia Tabome ◽

Maria Lodovica GULLINO

Keyword(s):

Plant Disease ◽

Tap Water ◽

Morphological Characteristics ◽

Plant Diseases ◽

Crown Rot ◽

Conidial Suspension ◽

Fusarium Spp ◽

First Report ◽

Root And Crown Rot ◽

Pathogenicity Tests

Maize (Zea mays L.) is a cereal crop of great economic importance in Italy; production is currently of 62,587,469 t, with an area that covers 628,801 ha, concentrated in northern Italy (ISTAT 2020). Fusarium species are associated with root and crown rot causing failures in crop establishment under high soil moisture. In 2019 maize seedlings collected in a farm located in San Zenone degli Ezzelini (VI, Italy) showed root and crown rot symptoms with browning of the stem tissues, wilting of the seedling, and collapsing due to the rotting tissues at the base of the stem. The incidence of diseased plants was approximately 15%. Seedlings were cleaned thoroughly from soil residues under tap water. Portions (about 3-5 mm) of tissue from roots and crowns of the diseased plants were cut and surface disinfected with a water solution of NaClO at 0.5% for 2 minutes and rinsed in sterile H20. The tissue fragments were plated on Potato Dextrose Agar (PDA) amended with 50 mg/l of streptomycin sulfate and incubated for 48-72 hours at 25oC. Over the 80 tissue fragments plated, 5% were identified as Fusarium verticillioides, 60% as Fusarium spp., 35% developed saprophytes. Fusarium spp. isolates that showed morphological characteristics not belonging to known pathogenic species on maize were selected and used for further investigation while species belonging to F. oxysporum were discarded. Single conidia of the Fusarium spp. colonies were cultured on PDA and Carnation Leaf Agar (CLA) for pathogenicity tests, morphological and molecular identification. The colonies showed white to pink, abundant, densely floccose to fluffy aerial mycelium. Colony reverse showed light violet pigmentation, in rings on PDA. On CLA the isolates produced slightly curved macronidia with 3 septa 28.1 - 65.5 µm long and 2.8-6.3 µm wide (n=50). Microconidia were cylindrical, aseptate, 4.5 -14.0 µm long and 1.5-3.9 µm wide (n=50). Spherical clamydospores were 8.8 ± 2.5 µm size (n=30), produced singly or in pairs on the mycelium, according to the description by Skovgaard et al. (2003) for F. commune. The identity of two single-conidia strains was confirmed by sequence comparison of the translation elongation factor-1α (tef-1α), and RNA polymerase II subunit (rpb2) gene fragments (O’Donnell et al. 2010). BLASTn searches of GenBank, and Fusarium-ID database, using the partial tef-1α (MW419921, MW419922) and rpb2 (MW419923, MW419924) sequences of representative isolate DB19lug07 and DB19lug20, revealed 99% identity for tef-1α and 100% identity to F. commune NRRL 28387(AF246832, AF250560). Pathogenicity tests were carried out by suspending conidia from a 10-days old culture on PDA in sterile H2O to 5×104 CFU/ml. Fifty seeds were immersed in 50 ml of the conidial suspension of each isolate for 24 hours and in sterile water (Koch et al. 2020). The seeds were drained, dried at room temperature, and sown in trays filled with a steamed mix of white peat and perlite, 80:20 v/v, and maintained at 25°C and RH of 80-85% for 14 days with 12 hours photoperiod. Seedlings were extracted from the substrate, washed under tap water, and observed for the presence of root and crown rots like the symptoms observed on the seedlings collected in the field. Control seedlings were healthy and F. commune was reisolated from the symptomatic ones and identified by resequencing of tef-1α gene. F. commune has been already reported on maize (Xi et al. 2019) and other plant species, like soybean (Ellis et al. 2013), sugarcane (Wang et al. 2018), potato (Osawa et al. 2020), indicating that some attention must be paid in crop rotation and residue management strategies. To our knowledge this is the first report of F. commune as a pathogen of maize in Italy. References Ellis M L et al. 2013. Plant Disease, 97, doi: 10.1094/PDIS-07-12-0644-PDN. ISTAT. 2020. http://dati.istat.it/Index.aspx?QueryId=33702. Accessed December 28, 2020. Koch, E. et al. 2020. Journal of Plant Diseases and Protection. 127, 883–893 doi: 10.1007/s41348-020-00350-w O’Donnell K et al. 2010. J. Clin. Microbiol. 48:3708. https://doi.org/10.1128/JCM.00989-10 Osawa H et al. 2020. Journal of General Plant Pathology, doi.org/10.1007/s10327-020-00969-5. Skovgaard K 2003. Mycologia, 95:4, 630-636, DOI: 10.1080/15572536.2004.11833067. Wang J et al. 2018. Plant Disease, 102, doi/10.1094/PDIS-07-17-1011-PDN Xi K et al. 2019. Plant Disease, 103, doi/10.1094/PDIS-09-18-1674-PDN

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Phytophthora crown rot of almond trees

Australian Journal of Agricultural Research ◽

10.1071/ar9860277 ◽

1986 ◽

Vol 37 (3) ◽

pp. 277 ◽

Cited By ~ 6

Author(s):

T Wicks ◽

TC Lee

Keyword(s):

Mating Type ◽

Root Rot ◽

South Australia ◽

Soil Samples ◽

Crown Rot ◽

Mature Trees ◽

Phytophthora Spp ◽

Almond Trees ◽

Almond Orchards ◽

Poorly Drained Soils

Phytophthora cambivora, P. citrophthora, P. cryptogea, and P. megasperma were isolated from either crown cankers or the soil around the crown of declining almond trees in South Australia. Severe root rot and crown cankers developed on Chellaston almond seedlings grown in soil artificially infested with the A1 but not the A2 mating type of P. cambivora. Cankers on inoculated plants were similar to those on naturally infected plants. Cankers did not develop on almond seedlings grown in soil infested with either P. citrophthora, P. cryptogea or P. megasperma. Neither extensive root rotting nor crown cankers developed in apricot and peach seedlings grown in soil artificially infested with the A1 mating type of P. cambivora. Phytophthora spp. were detected in 44% of the soil samples collected near the crown of dead and declining trees from 26 commercial almond orchards. In a severely affected orchard up to 17% of mature trees were either dead, missing or in a serious state of decline. Naturally infected trees were frequently found in poorly drained soils and were often associated with dripper irrigation outlets placed close to the trunk.

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Population Structure of Pythium ultimum from Greenhouse Floral Crops in Michigan

Plant Disease ◽

10.1094/pdis-03-18-0394-re ◽

2019 ◽

Vol 103 (5) ◽

pp. 859-867 ◽

Cited By ~ 2

Author(s):

Johanna Del Castillo Múnera ◽

Lina M. Quesada-Ocampo ◽

Alejandro Rojas ◽

Martin I. Chilvers ◽

Mary K. Hausbeck

Keyword(s):

Population Dynamics ◽

Population Structure ◽

Control Strategies ◽

Ornamental Plants ◽

Infected Plant ◽

Pythium Ultimum ◽

Crown Rot ◽

Common Source ◽

Root And Crown Rot ◽

Pathogen Populations

Pythium ultimum causes seedling damping-off and root and crown rot in greenhouse ornamental plants. To understand the population dynamics and assess population structure of P. ultimum in Michigan floriculture crops, simple sequence repeats (SSRs) were developed using the previously published P. ultimum predicted transcriptome. A total of 166 isolates sampled from 2011 to 2013 from five, one, and three greenhouses in Kalamazoo, Kent, and Wayne Counties, respectively, were analyzed using six polymorphic and fluorescently labeled SSR markers. The average unbiased Simpson’s index (λu, 0.95), evenness (E5, 0.56), and recovery of 12 major clones out of the 65 multilocus genotypes obtained, suggests that P. ultimum is not a recent introduction into Michigan greenhouses. Analyses revealed a clonal population, with limited differentiation among seasons, hosts, and counties sampled. Results also indicated the presence of common genotypes among years, suggesting that sanitation measures should be enhanced to eradicate resident P. ultimum populations. Finally, the presence of common genotypes among counties suggests that there is an exchange of infected plant material among greenhouse facilities, or that there is a common source of inoculum coming to the region. Continued monitoring of pathogen populations will enhance our understanding of population dynamics of P. ultimum in Michigan and facilitate improvement of control strategies.

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Phytophthora crown rot of Florida Strawberry: Inoculum Sources and Thermotherapy of Transplants for Disease Management

Plant Disease ◽

10.1094/pdis-11-20-2476-re ◽

2021 ◽

Author(s):

Juliana Silveira Baggio ◽

Marcus Vinicius Marin ◽

Natalia A. Peres

Keyword(s):

Heat Treatment ◽

Chemical Control ◽

Pathogen Resistance ◽

Crown Rot ◽

Phytophthora Cactorum ◽

Inoculum Sources ◽

Phytophthora Spp ◽

Commercial Farms ◽

Important Disease

Phytophthora crown rot, caused mainly by Phytophthora cactorum, and also by the recently reported P. nicotianae, is an important disease in the Florida strawberry annual production system. Mefenoxam is the most effective and widely used fungicide to manage this disease. However, due to pathogen resistance, alternatives to chemical control are needed. Phytophthora spp. were rarely recovered during the summer from soil of commercial farms where the disease was observed during the season. In a more detailed survey on research plots, neither of the two species was recovered one month after the crop was terminated and water was shut off. Therefore, Phytophthora spp. does not seem to survive in the soil over summer in Florida. In a field trial, asymptomatic nursery transplants harboring quiescent infections were confirmed as the major source of inoculum for these pathogens in Florida. Heat treatment of P. cactorum zoospores at 44oC for as little as 5 min was effective in inhibiting germination and colony formation; however, oospore germination was not inhibited by any of the tested temperatures in vitro. In the field, thermotherapy treatment of inoculated plants was shown to have great potential to serve as a non-chemical approach for managing Phytophthora crown rot in production fields and reducing mefenoxam-resistant populations in nursery transplants.

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First Report of Root and Crown Rot of Almond Caused by Phytophthora spp. in Turkey

Plant Disease ◽

10.1094/pdis-06-10-0411 ◽

2010 ◽

Vol 94 (10) ◽

pp. 1261-1261 ◽

Cited By ~ 3

Author(s):

İ. Kurbetli ◽

K. Değirmenci

Keyword(s):

Tap Water ◽

Morphological Characteristics ◽

Prunus Dulcis ◽

Selective Medium ◽

Internal Transcribed Spacers ◽

Crown Rot ◽

First Report ◽

Phytophthora Spp ◽

Agar Plug ◽

Root And Crown Rot

Almond (Prunus dulcis) production is currently increasing in Turkey. Losses of approximately 1% associated with root and crown rot of almond seedlings were observed in two commercial nurseries in Ankara and Düzce provinces in 2009. Aboveground symptoms were leaf chlorosis and wilt. Feeder roots were decayed, necrosis occurred on taproots and basal stems, and plants collapsed within several weeks. Roots were washed in tap water and 9 to 10 pieces (3 to 5 mm long) of root tissue taken from the margins of canker lesions, without surface disinfection, were placed on selective medium P5ARPH-CMA (2). Plates were incubated for 3 to 5 days at 20°C in darkness and a number of Phytophthora spp. were recovered. Actively growing mycelium was transferred to carrot piece agar containing β-sitosterol (per liter: carrot piece, 40 g; agar, 20 g; β-sitosterol, 20 mg). Isolates were identified as Phytophthora cactorum and P. citrophthora on the basis of morphological characteristics (1). P. cactorum produced abundant sporangia, oogonia, and paragynous antheridia on carrot piece agar plus β-sitosterol. It had conspicuously papillate and caducous sporangia with short pedicel. Sporangia were usually ovoid but sometimes nearly spherical. P. citrophthora did not produce sexual structures in single culture. It produced papillate, noncaducous sporangia, which were usually ovoid and obpyriform, often asymmetrically shaped and rarely possessed more than one apex. P. citrophthora did not grow at 35°C but it grew well at 30°C. Isolate identities were confirmed by sequence analysis of the ribosomal DNA internal transcribed spacers 1 and 2 (GenBank Accession Nos. HM357622, HM357623, HM357624, HM357625) using primers ITS1 and ITS2 (3). One representative isolate of each species was used to inoculate eight 2-year-old almond plants with an agar plug with actively growing mycelium that was attached to exposed cambium of basal stems. Agar plugs without mycelium were used for eight control plants. All plants inoculated with Phytophthora spp. collapsed within 3 to 4 weeks. Control plants remained healthy. Phytophthora spp. were reisolated from necrotic basal stems. To our knowledge, this is the first report of P. cactorum and P. citrophthora of almond in Turkey. References: (1) M. E. Gallegly and C. Hong. Phytophthora, Identifying Species by Morphology and DNA Fingerprints. The American Phytopathological Society, St. Paul, MN, 2008. (2) S. N. Jeffers and S. B. Martin. Plant Dis. 70:1038, 1986. (3) S. G. Roy et al. J. Phytopathol. 157:666, 2009.

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Four Phytophthora Species Causing Foot and Root Rot of Apricot in Italy

Plant Disease ◽

10.1094/pdis-93-8-0844c ◽

2009 ◽

Vol 93 (8) ◽

pp. 844-844 ◽

Cited By ~ 5

Author(s):

A. Pane ◽

S. O. Cacciola ◽

S. Scibetta ◽

G. Bentivenga ◽

G. Magnano di San Lio

Keyword(s):

Root Rot ◽

Sandy Loam ◽

Phytophthora Nicotianae ◽

Crown Rot ◽

Optimum Growth ◽

Phytophthora Spp ◽

Leaf Chlorosis ◽

Breadth Ratio ◽

Root And Crown Rot ◽

Fourth Species

In the summer of 2006, 1-year-old apricot (Prunus armeniaca L.) trees with leaf chlorosis, wilting, and defoliation associated with root and crown rot were observed in a nursery in Sicily (Italy). Of 3,000 plants, ~2% was affected. Four Phytophthora spp. (45, 25, 20, and 10% of the isolations of the first, second, third, and fourth species, respectively) were isolated from decayed roots and trunk bark on BNPRAH (3). Axenic cultures were obtained by single-hypha transfers. Isolates of the first species formed petaloid colonies on potato dextrose agar (PDA) and had an optimum growth temperature of 25°C. On V8 agar (VA), they produced persistent, papillate (often bipapillate), ovoid to limoniform sporangia (length/breadth ratio 1.4:1). They did not produce gametangia when paired with A1 and A2 isolates of Phytophthora nicotianae. The second species formed arachnoid colonies, had an optimum growth of 30°C, and produced uni- and bipapillate, ellipsoid, ovoid or pyriform sporangia (length/breadth ratio 1.3:1). All isolates were A2. The third species formed rosaceous colonies on PDA, had an optimum temperature of 28 to 30°C, and produced papillate (sometime bipapillate), ellipsoid or limoniform (length/breadth ratio 2:1), caducous sporangia with a tapered base and a long pedicel (as much as 150 μm). All isolates were A1 type. The fourth species formed petaloid-like colonies on PDA and had an optimum growth of 26 to 28°C. On VA, it produced papillate (sometimes bipapillate), ovoid (length/breadth ratio 1.3:1), and decidous sporangia with a short pedicel (<4 μm). The isolates were homothallic and produced oogonia (25 to 31 μm in diameter) with paragynous antheridia and aplerotic oospores. On the basis of morphological and cultural characters, the species were identified as P. citrophthora, P. nicotianae, P. tropicalis and P. cactorum. Identification was confirmed by the electrophoretic analysis of total mycelial proteins and four isozymes (acid and alkaline phosphatases, esterase, and malate dehydrogenase) on polyacrylamide gel (1). Analysis of internal transcribed spacer (ITS) regions of rDNA using the ITS 4 and ITS 6 primers for DNA amplification (2) revealed 99 to 100% similarity between apricot isolates of each species and reference isolates from GenBank (Nos. AF266785, AB367355, DQ118649, and AF266772). The ITS sequence of a P. citrophthora isolate from apricot (IMI 396200) was deposited in GenBank (No. FJ943417). In the summer of 2008, pathogenicity of apricot isolates IMI 396200 (P. citrophthora), IMI 396203 (P. nicotianae), IMI 396201 (P. tropicalis), and IMI 396202 (P. cactorum) was tested on 3-month-old apricot seedlings (10 plants for each isolate) that were transplanted into pots filled with soil prepared by mixing steam-sterilized sandy loam soil (4% vol/vol) with inoculum produced on autoclaved kernel seeds. Ten control seedlings were grown in autoclaved soil. Seedlings were maintained in a screenhouse and watered daily to field capacity. Within 40 days of the transplant, all inoculated seedlings showed leaf chlorosis, wilting, and root rot. Control seedlings remained healthy. All four Phytophthora spp. were reisolated solely from inoculated plants. To our knowledge, this is the first report of Phytophthora root and crown rot of apricot in Italy and of P. tropicalis on this host. References: (1) S. O. Cacciola et al. Plant Dis. 90:680, 2006. (2) D. E. L. Cooke et al. Fungal Genet. Biol. 30:17, 2000. (3) H. Masago et al. Phytopathology 67:425, 1977.

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First Report of Root and Crown Rot of Wasabi (Wasabia japonica Matsum.) Caused by Phytophthora cryptogea in Michigan

Plant Disease ◽

10.1094/pdis-04-12-0345-pdn ◽

2012 ◽

Vol 96 (9) ◽

pp. 1379-1379 ◽

Cited By ~ 1

Author(s):

L. L. Granke ◽

B. R. Harlan ◽

R. P. Naegele ◽

M. K. Hausbeck

Keyword(s):

Sequence Similarity ◽

Soil Surface ◽

Pcr Primers ◽

Root Tissue ◽

Crown Rot ◽

Phytophthora Cryptogea ◽

First Report ◽

Wasabia Japonica ◽

Root And Crown Rot ◽

Crown And Root Rot

In September 2011, a Phytophthora sp. was isolated from wasabi (Wasabia japonica Matsum.) grown commercially in hydroponic culture in a large production facility in southwest Michigan. Approximately 20% of the plants were affected, resulting in serious losses for the grower. Plants exhibited severe wilting and root and crown rot, with soft water-soaked lesions on the crown and dark lesions on the roots. Small pieces of root tissue with dark lesions were excised and plated onto potato dextrose agar and unclarified V8 agar plates amended with 25 ppm of benomyl, 100 ppm of ampicillin, 30 ppm of rifampicin, and 100 ppm of pentachloronitrobenzene. Isolates of a Phytophthora sp. were recovered from root tissue. Isolates produced sporangia abundantly in culture. Sporangia averaged 48 μm long × 34 μm wide and were ellipsoid to ovoid, occasionally obpyriform, and were nonpapillate and noncaducous. Distinct hyphal swellings were noted and chlamydospores were observed rarely in culture. The isolate used for inoculations did not produce oospores alone in culture but was able to produce oospores when paired with an A1 culture of P. capsici and incubated in the dark. Oospores were not observed when the isolate was paired with an A2 culture of P. capsici. No growth was observed at 35°C, and the isolate was identified as Phytophthora cryptogea based on morphological and physiological traits. Pathogen identity was further confirmed using PCR primers specific to P. cryptogea (1). In addition, a BLAST search was conducted using the nucleotide database collection in GenBank comparing our isolate against Phytophthora spp., with 99% sequence similarity to P. cryptogea in two sequenced genes, beta tubulin and cytochrome c oxidase 1 (2). Sequences for the isolate were deposited in the GenBank database under accession numbers JX041520 and JX041521. To fulfill Koch's postulates, six small, potted wasabi seedlings were inoculated by placing 3 g of 1-month-old infested millet (100 g of millet, 72 ml of distilled water, 0.08 g of asparagine, and 10 7-mm diameter V8 agar plugs with actively growing P. cryptogea) onto the soil surface of each pot under coconut coir mulch. Plants were watered heavily after soil infestation and as needed thereafter. Three control plants were inoculated with sterile millet seed. The experiment was repeated once. Wilting was observed within 5 and 7 days, respectively, in the first and second experiment. All six inoculated plants were severely wilted within 25 and 56 days, respectively, except for a single plant in the second experiment that never wilted. Root and crown rot was observed on wilted plants and dark lesions could be observed on root tissue. P. cryptogea was recovered from five of the six plants inoculated in each experiment. None of the control plants in either experiment displayed symptoms of wilting, and the pathogen was not recovered from these plants when pieces of root tissue were excised and plated onto amended V8 agar. To our knowledge, this is the first report of P. cryptogea causing crown and root rot of wasabi. References: (1) D. Minerdi et al. Eur. J. Plant Pathol. 122:227, 2008. (2) L. M. Quesada-Ocampo et al. Phytopathology 101:1061, 2011.

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Inoculum Sources and Survival of Xanthomonas axonopodis pv. allii in Colorado

Plant Disease ◽

10.1094/pd-89-0507 ◽

2005 ◽

Vol 89 (5) ◽

pp. 507-514 ◽

Cited By ~ 15

Author(s):

David H. Gent ◽

Jillian M. Lang ◽

Michael E. Bartolo ◽

Howard F. Schwartz

Keyword(s):

Cultural Practices ◽

Management Strategies ◽

Soil Surface ◽

Field Studies ◽

Leaf Blight ◽

Weed Species ◽

Alternate Host ◽

Inoculum Sources ◽

Xanthomonas Axonopodis ◽

Disease Epidemiology

Xanthomonas leaf blight, caused by the bacterium Xanthomonas axonopodis pv. allii, is an emerging disease of onion in the western United States and worldwide, but few management strategies have been developed because little is known about disease epidemiology and pathogen survival. Therefore, we sought to identify and quantify primary inoculum sources of the pathogen in Colorado. Growth chamber and field studies evaluated survival and dissemination of X. axonopodis pv. allii in association with weed, alternate host, and volunteer onion plants, irrigation water, and crop debris. Epiphytic X. axonopodis pv. allii was recovered from the foliage of nine asymptomatic weed species and Medicago sativa, but the bacterium was not recovered from plants in locations where an epidemic of Xanthomonas leaf blight did not occur the prior year. The bacterium also was isolated from volunteer onion with characteristic Xanthomonas leaf blight symptoms. A rifampicin mutant of X. axonopodis pv. allii strain O177 was recovered consistently from the irrigation tail water of onion fields inoculated with the bacterium; populations as large as 3.02 × 104 CFU/ml were recovered. X. axonopodis pv. allii was recovered from infested onion leaves 9 months after they were placed on the soil surface or buried to a depth of 25 cm, but culturable populations of the pathogen decreased 104 to 106 more in buried leaves. Cultural practices that avoid or eliminate X. axonopodis pv. allii inoculum sources should reduce Xanthomonas leaf blight losses to onion.

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