First Report of the Occurrence of Fusarium oxysporum f. sp. niveum Race 2 in Commercial Watermelon Production Areas of Maryland and Delaware

Resistance to race 1 of Fusarium oxysporum f. sp. niveum (E.F. Sm.) W.C. Snyder & H.N. Hans) is the most widely used tool for management of Fusarium wilt of watermelon (Citrullus lanatus (Thunb.) Matsum. and Nakai). However, this resistance is ineffective against the more aggressive F. oxysporum f. sp. niveum race 2. Race 2 was first identified in Israel in 1973 (2) and was subsequently reported in Texas (1981), Oklahoma (1988), and Florida (1989) (1). From July through September 2000 in Maryland and Delaware, 123 isolates of F. oxysporum f. sp. niveum were collected from wilted plants or plants with vascular discoloration from commercial production fields with moderate to severe wilt severity. Race determination was performed in the greenhouse on five race-differential cultivars. Differential cvs. Sugar Baby, Crimson Sweet, Charleston Gray, Allsweet, and Dixielee range from susceptible to highly resistant (in the order listed) to race 1, but all are susceptible to race 2. Seeds were planted in pots containing pasteurized vermiculite and peat moss (2:3 mixture) growth medium infested with 1 × 105 conidia per g of growth medium. Conidia were harvested from 5- to 6-day-old cultures of F. oxysporum f. sp. niveum grown in a liquid mineral salts medium (2). Control pots were treated with water or diluted liquid medium. Pots were maintained in a greenhouse at 19 to 28°C. Three replicate pots containing seven plants each were used for each isolate-cultivar combination. Race 0 obtained from B. D. Bruton (USDA-ARS, Lane, OK), and races 1 and 2 obtained from the American Type Culture Collection (Manassas, VA) were included in the tests for comparative purposes. Eleven isolates from Maryland and three isolates from Delaware consistently caused 60 to 100% wilt or mortality of all cultivars, with a mean of more than 75% wilt. Percent wilt of cvs. Sugar Baby and Dixielee to races 0, 1, and 2, and the 14 isolates from Maryland and Delaware was 78, 76, 100, and 95 to 100% and 0, 14, 59, and 63 to 93%, respectively. All control plants remained healthy. In each of the three replicated tests, these 14 isolates caused an equal or greater incidence of wilt as the reference race 2 isolate, and cvs. Dixielee and Allsweet were susceptible to these isolates. Therefore, these isolates were identified as F. oxysporum f. sp. niveum race 2. This is the first report of F. oxsporum f. sp. niveum race 2 occurring in Maryland and Delaware. References: (1) R. D. Martyn and B. D. Bruton. HortScience 24:696, 1989. (2) D. Netzer. Phytoparasitica 4:131, 1976.

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First Report of Fusarium oxysporum f. sp. niveum Race 2 as Causal Agent of Fusarium Wilt of Watermelon in Indiana

Plant Disease ◽

10.1094/pd-89-0108a ◽

2005 ◽

Vol 89 (1) ◽

pp. 108-108 ◽

Cited By ~ 12

Author(s):

D. S. Egel ◽

R. Harikrishnan ◽

R. Martyn

Keyword(s):

United States ◽

Fusarium Oxysporum ◽

Fusarium Wilt ◽

Citrullus Lanatus ◽

The United States ◽

Mineral Salts ◽

First Report ◽

Race 1 ◽

Race 2 ◽

Greenhouse Assay

Fusarium oxysporum f. sp. niveum race 1 is uniformly distributed throughout watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai) growing regions, but F. oxysporum f. sp. niveum race 2 has a limited known distribution in the United States (Texas, Florida, Oklahoma, Maryland, and Delaware) (3,4). Since the spring of 2001, commercial watermelon fields in Knox and Gibson counties in southwestern Indiana have been observed with symptoms of one-sided wilt and vascular discoloration typical of Fusarium wilt. Race 2 of F. oxysporum f. sp. niveum was suspected as the casual agent since the diseased watermelon cultivars are considered resistant to races 0 and 1. Two isolates of F. oxysporum obtained from wilted watermelon plants in two different commercial fields and one isolate obtained from a wilted seedling in a transplant house were compared for pathogenicity in a greenhouse assay. Known isolates of F. oxysporum f. sp. niveum races 0, 1, and 2 were obtained from Don Hopkins (University of Florida, Apopka), Kate Everts (University of Maryland/University of Delaware, Salisbury, MD), and Ray Martyn (Purdue University, West Lafayette, IN), respectively, and were used for comparison. All isolates were grown in shake cultures in a mineral salts liquid medium. (1). After 72 hr, the predominately microconidal suspensions were filtered through cheesecloth and adjusted to 1 × 105 conidia/ml with the aid of a hemacytometer. A concentration of 1 × 105 condia/ml was shown previously to cause the desired disease reaction in the standard cultivars. Seedlings of the differential cvs, Black Diamond (universal susceptible), Charleston Gray (race 0 resistant), and Calhoun Gray (race 0 and 1 resistant) were grown in a 1:1, (v:v) sand/ vermiculite mixture to the first true-leaf stage after which the plants were uprooted and the roots carefully washed prior to root dip inoculation. Subsequent to inoculation, seedlings were planted in a sand/vermiculite/ peat mixture (4:1:1, [v:v:v]) with four seedlings to a 15-cm-diameter pot. The experimental design was a randomized complete block with five replications. Two isolates from the commercial field plants caused an average of 100% wilt on cv. Black Diamond, 95% wilt on cv. Charleston Gray, and 80% wilt on cv. Calhoun Gray, resulting in a designation of race 2. The isolate from a commercial transplant house resulted in 100, 60, and 15% wilt, respectively, on the three standard cultivars resulting in a race 1 designation. The presence of F. oxysporum f. sp. niveum race 2 in Indiana is significant because Indiana currently ranks fifth in the United States in watermelon production and there are no commercially available cultivars that possess resistance to race 2. To our knowledge, this is the first report of F. oxysporum f. sp. niveum race 2 in Indiana and the first report of race 2 from the Midwest region of the United States. Race 2, first described from the United States in 1985 (2), has now been confirmed in six states. References: (1) R. Esposito and A. Fletcher. Arch. Biochem. Biophys. 93:369, 1961. (2) R. Martyn, Plant Dis. 69:1007, 1985. (3) R. Martyn, Plant Dis. 71:233, 1987. (4) X. Zhou and K. Everts. Plant Dis. 87:692, 2003.

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First Report of Fusarium Wilt Caused by Fusarium oxysporum f. sp. niveum Race 2 in Georgia Watermelons

Plant Disease ◽

10.1094/pdis-92-6-0983b ◽

2008 ◽

Vol 92 (6) ◽

pp. 983-983 ◽

Cited By ~ 10

Author(s):

B. D. Bruton ◽

W. W. Fish ◽

D. B. Langston

Keyword(s):

Fusarium Oxysporum ◽

Fusarium Wilt ◽

Citrullus Lanatus ◽

Fusarium Species ◽

Block Design ◽

Genetic Resistance ◽

Morphological Characteristics ◽

First Report ◽

Race 1 ◽

Race 2

Watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai) is the number one specialty crop grown in Georgia, a state that ranks fourth nationally in watermelon production. In the last 5 years, Fusarium wilt caused by Fusarium oxysporum f. sp. niveum (Fon) has been the greatest yield-limiting disease of watermelon in Georgia. In 2004, a seedless-watermelon field of ‘Regency’ and ‘Tri-X 313’ in Berrien County, GA exhibited approximately 40% of wilted plants. Affected plants also exhibited strong discoloration in the crown xylem. Plant samples (cultivars unknown) from a similarly affected field were also tested from Crisp County, GA. Xylem tissue was excised from the main stem of eight diseased plants in the area between the second and third internode, surface sterilized for 1 min in 1% NaOCl, rinsed with 80% ethanol, and plated onto water agar amended with 100 μg/liter of streptomycin sulfate. Fungi with the morphological characteristics of Fusarium oxysporum (4) were consistently recovered from the diseased tissue of all eight plants. The isolates were hyphal tipped and maintained in vials of sterile artificial potting mix until ready for use (1). Isolates were grown on Esposito and Fletcher medium (2) for 10 days, filtered through cheesecloth, and adjusted to 1 × 106 spores/ml. Reference isolates of race 1 and 2 were used as comparisons for race determination of the unknowns. In each of four studies, plants at the two-leaf stage were removed from potting mix, washed gently, and their roots were uniformly trimmed to 2.5 cm. Before repotting, the seedlings were subjected to a 2-min root-dip in the respective spore-containing media. In each study, approximately 40 plants of each watermelon differential were inoculated with the respective isolates. In disease scoring, each plant was considered a rep. ‘Black Diamond’ is susceptible to races 0, 1, and 2; ‘Charleston Gray’ is resistant to race 0; ‘Calhoun Gray’ is resistant to races 0 and 1, and PI-296341-FR (3) is resistant to races 0, 1, and 2 of Fon. Four plants were planted per 15-cm plastic pot, maintained in an air-conditioned headhouse for 24 h, and then placed in the greenhouse in a randomized complete block design. After 30 days, all plants were rated as to healthy, wilted, or dead plants. From eight isolates tested, one isolate from each county was determined to be Fon race 2 on the basis of its ability to wilt/kill a high percentage of the race 1 resistant differential, i.e., ‘Calhoun Gray’. Mean disease percentages for the isolates from each of the two counties on the watermelon differentials were 95 and 100% on ‘Black Diamond’, 68 and 80% on ‘Charleston Gray’, and 70 and 86% on ‘Calhoun Gray.’ Because of apparent genetic drift within our PI-296341-FR population, we determined that these data were not useful for identifying race 2. In fact, we observed a range of 17 to 80% wilt/death in the PI-296341-FR over a total of four studies that included a known race 2 isolate (Calg 13(15); E. Vivoda). To our knowledge, this is the first report of race 2 in Georgia and it increases the number of states to seven in which race 2 has been identified. Five of the top 10 watermelon-producing states have now reported race 2 of Fon for which there is no genetic resistance within commercial cultivars. References: (1) B. D. Bruton et al. Plant Dis. 84:907, 2000. (2) R. Esposito and A. Fletcher. Arch. Biochem. Biophys. 93:369, 1961. (3) R. D. Martyn and D. Netzer. HortScience 26:429, 1991. (4) P. E. Nelson et al. Fusarium Species: An Illustrated Manual for Identification. Pennsylvania State University Press, University Park, 1983.

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Identifying Races of Fusarium oxysporum f. sp. niveum in South Carolina Recovered From Watermelon Seedlings, Plants, and Field Soil

Plant Disease ◽

10.1094/pdis-11-19-2385-re ◽

2020 ◽

Vol 104 (9) ◽

pp. 2481-2488

Author(s):

Anthony P. Keinath ◽

Virginia B. DuBose ◽

Melanie M. Katawczik ◽

W. Patrick Wechter

Keyword(s):

South Carolina ◽

Fusarium Oxysporum ◽

Management Practices ◽

Integrated Management ◽

Citrullus Lanatus ◽

Field Soil ◽

Specific Primers ◽

Race 1 ◽

Race 2 ◽

Race Identification

Fusarium wilt of watermelon (Citrullus lanatus), caused by the soilborne fungus Fusarium oxysporum f. sp. niveum, is the most serious disease of watermelon in South Carolina and other southeastern U.S. states. Isolates of F. oxysporum collected from field-grown plants, greenhouse-grown seedlings, and field soil between 1999 and 2018 were inoculated onto three differential watermelon cultivars to identify races. Of 197 isolates obtained from plants, 12% were nonpathogenic, 2% were race 0, 23% were race 1, and 63% were race 2. One collection of isolates from greenhouse seedlings was exclusively race 1 and the other was exclusively race 2. Seventeen of 81 soil isolates were pathogenic: five were race 1 and 12 were race 2. Reactions of C. amarus PI 296341-FR, Carolina Strongback, and SP-6, cultigens with resistance to race 2, did not differ significantly among five highly virulent race 2 isolates and a standard race 2 isolate, indicating a lack of a race 3 phenotype. Forma specialis-specific primers matched phenotypic race identification for 74% of the isolates. Race-specific primers based on a secreted-in-xylem elicitor present in race 0 and 1 isolates matched phenotypic race identification for 66% of the isolates. Because a majority of the F. oxysporum f. sp. niveum isolates from South Carolina were race 2, integrated management practices should be used until commercial cultivars with resistance to race 2 are available.

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Identification and Expression of Secreted In Xylem Pathogenicity Genes in Fusarium oxysporum f. sp. pisi

Frontiers in Microbiology ◽

10.3389/fmicb.2021.593140 ◽

2021 ◽

Vol 12 ◽

Author(s):

Sascha Jenkins ◽

Andrew Taylor ◽

Alison C. Jackson ◽

Andrew D. Armitage ◽

Helen J. Bates ◽

...

Keyword(s):

United Kingdom ◽

Fusarium Oxysporum ◽

Major Gene ◽

Housekeeping Gene ◽

Gene Profiling ◽

In Planta ◽

The United Kingdom ◽

Differential Cultivars ◽

Race 1 ◽

Race 2

Fusarium oxysporum is a soilborne fungal plant pathogen responsible for causing disease in many economically important crops with “special forms” (formae speciales) adapted to infect specific plant hosts. F. oxysporum f. sp. pisi (FOP) is the causal agent of Fusarium wilt disease of pea. It has been reported in every country where peas are grown commercially. Disease is generally controlled using resistant cultivars possessing single major gene resistance and therefore there is a constant risk of breakdown. The main aim of this work was to characterise F. oxysporum isolates collected from diseased peas in the United Kingdom as well as FOP isolates obtained from other researchers representing different races through sequencing of a housekeeping gene and the presence of Secreted In Xylem (SIX) genes, which have previously been associated with pathogenicity in other F. oxysporum f. spp. F. oxysporum isolates from diseased United Kingdom pea plants possessed none or just one or two known SIX genes with no consistent pattern of presence/absence, leading to the conclusion that they were foot-rot causing isolates rather than FOP. In contrast, FOP isolates had different complements of SIX genes with all those identified as race 1 containing SIX1, SIX6, SIX7, SIX9, SIX10, SIX11, SIX12, and SIX14. FOP isolates that were identified as belonging to race 2 through testing on differential pea cultivars, contained either SIX1, SIX6, SIX9, SIX13, SIX14 or SIX1, SIX6, SIX13. Significant upregulation of SIX genes was also observed in planta over the early stages of infection by different FOP races in pea roots. Race specific SIX gene profiling may therefore provide potential targets for molecular identification of FOP races but further research is needed to determine whether variation in complement of SIX genes in FOP race 2 isolates results in differences in virulence across a broader set of pea differential cultivars.

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Incidence of Fusarium oxysporum f. sp. lycopersici Races in Tomato in Sinaloa, Mexico

Plant Disease ◽

10.1094/pdis-04-10-0255 ◽

2010 ◽

Vol 94 (11) ◽

pp. 1376-1376 ◽

Cited By ~ 3

Author(s):

P. Sánchez-Peña ◽

S. O. Cauich-Pech ◽

J. Núñez-Farfán ◽

R. D. Núñez-Cebreros ◽

S. Hernández-Verdugo ◽

...

Keyword(s):

Fusarium Oxysporum ◽

Fusarium Wilt ◽

Vascular Bundles ◽

Potato Dextrose Broth ◽

Solanum Lycopersicum L ◽

Differential Cultivars ◽

Differential Cultivar ◽

Race 1 ◽

Production Area ◽

Race 2

Sinaloa State is the main producer of tomatoes (Solanum lycopersicum L) in Mexico where production attained 15,784 ha in 2008 (3). Fusarium wilt of tomato caused by Fusarium oxysporum f. sp. lycopersici (Sacc) Snyder & Hansen causes significant yield losses in Sinaloa each year (2). Three pathotypes or races of F. oxysporum f. sp. lycopersici have been described: races 1, 2, and 3 (1). The purpose of this study was to determine the races of F. oxysporum f. sp. lycopersici present in Sinaloa and distribution of these races in the region. F. oxysporum f. sp. lycopersici isolates were obtained from plants showing symptoms of yellowing and necrosis of vascular bundles. Plants were sampled from 50 fields throughout the production area in Sinaloa from November 2008 to March 2009. Four differential cultivars were used to identify the races of 26 F. oxysporum f. sp. lycopersici isolates collected across Sinaloa: Bonny Best (susceptible to all races), UC-82-L (susceptible to races 2 and 3), F1 MH-1 (susceptible to race 3), and IR-3 (resistant to all races). A microconidial suspension was prepared for each isolate (1 × 106 CFU/ml) from cultures grown in potato dextrose broth with constant agitation (110 rpm) at 28°C for 7 days. Five 25-day-old seedlings (three fully expanded true leaves) from each differential cultivar were immersed in the appropriate microconidial suspension for 10 min, then individually transplanted into a pot containing 1 kg of sterilized commercial potting mix, and grown in a growth chamber at 25 to 28°C and 60 to 75% relative humidity for 5weeks with 14-h light/10-h darkness. Control plants (root dipped for 10 min in sterile water) were grown similarly and remained asymptomatic. Susceptible cv. Bonny Best showed typical symptoms of Fusarium wilt including epinasty, yellowing, defoliation, vascular necrosis, and wilt. None of these plants survived 25 days postinoculation for any of the isolates. All UC-82-L plants inoculated with each of the 26 isolates died before 35 days, indicating that none of the isolates was of race 1. F1 MH-1 plants only survived inoculation with 3 of the 26 isolates (11.5%), indicating that the 23 isolates that killed these plants (88.5% of the 26 isolates) were of race 3, and only 3 isolates were of race 2. All IR-3 plants inoculated with the 26 isolates survived. The isolates showed variation in response to the differential cultivar UC-82L in duration from inoculation to when the plants died (variation in isolate aggressiveness). The three F. oxysporum f. sp. lycopersici race 2 isolates were restricted to the Culiacan Valley, whereas the 23 F. oxysporum f. sp. lycopersici race 3 isolates were widely distributed across Sinaloa. Koch's postulates were confirmed by reisolating the fungus from the roots and stem base of each dead, inoculated plant (4). This study provides baseline data for future surveys to monitor changes in distribution of F. oxysporum f. sp. lycopersici races in Sinaloa, Mexico. References: (1) G. Cai et al. Phytopathology 93:1014, 2003. (2) P. Sánchez-Peña. Programa de Fomento y Apoyo a Proyectos de Investigación (PROFAPI), Universidad Autónoma de Sinaloa, México, 2007. (3) Secretaría de Agricultura, Ganadería, Desarrollo Rural, Pesca y Alimentación, Servicio de Información Agroalimentaria y Pesquera, México. www.siap.gob.mx , 2009. (4) B. A. Summerell et al. Plant Dis. 87:117, 2003.

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First Report of Race 2 of Fusarium oxysporum f. sp. lycopersici, the Causal Agent of Fusarium Wilt on Tomato in Taiwan

Plant Disease ◽

10.1094/pd-90-0111c ◽

2006 ◽

Vol 90 (1) ◽

pp. 111-111 ◽

Cited By ~ 4

Author(s):

Z. M. Sheu ◽

T. C. Wang

Keyword(s):

Fusarium Oxysporum ◽

Fusarium Wilt ◽

Banding Pattern ◽

Vascular Tissue ◽

First Report ◽

Intergenic Spacer Region ◽

Stunted Growth ◽

Vascular Discoloration ◽

Race 1 ◽

Race 2

Fusarium wilt caused by Fusarium oxysporum Schlechtend.:Fr. f. sp. lycopersici (Sacc.) W.C. Snyder & N.H. Hans. is a destructive disease of tomato crops worldwide. The use of resistant varieties is the best strategy for disease control. There are three reported races of the pathogen. Recent surveys indicated that many of the commercial cultivars with resistance to F. oxysporum f. sp. lycopersici race 1 planted in Taiwan displayed Fusarium wilt symptoms. Yellowing on the older leaves was observed on one side of the stems close to fruit maturity. The yellowing gradually affected most of the foliage and was accompanied by wilting of the plants. The vascular tissue was usually dark brown and discoloration extended to the apex. The wilting became more extensive until plants collapsed and died. A total of 87 isolates obtained from typical diseased plants throughout Taiwan from 2002 to 2005 were analyzed to determine the race and distribution of this pathogen in Taiwan. Isolates were confirmed at the species level using F. oxysporum-specific primers FOF1 and FOR1 (4). Subsequently, isolates were characterized for pathogenicity, race and restriction fragment length polymorphisms of the intergenic spacer region of rDNA (IGS-RFLP) with two reference isolates, Fol 11A (race 1) and Fol 34-1 (race 2). Pathogenicity tests and race determination were conducted using root-dip inoculation (3) on 2-week-old seedlings of host differentials Bonny Best (no resistance), UC82-L (resistant to race 1), and Florida MH-1 (resistant to races 1 and 2). Thirty-six seedlings of each cultivar were arranged into three replications and inoculated with each isolate. Disease reaction was evaluated 3 weeks after inoculation. The disease severity rating (DSR) was determined on individual plants according to the following scale: 0 = plant healthy without external symptoms; 1 = slight vascular discoloration with or without stunted growth; 2 = severe vascular discoloration usually with stunted growth; and 3 = plant wilted beyond recovery or dead. The presence of severe vascular discoloration indicated a susceptible reaction. All isolates were race 2, and over 70% of the isolates showed strong virulence with a DSR >2 on cvs. Bonny Best and UC-82L. This result was different from a previous report of race 1 from Taiwan (2). Two IGS-RFLP haplotypes generated by EcoRI, RsaI, and HaeIII digestions (1) were identified. Eighty-six isolates displayed one banding pattern, and one unique isolate displayed a second banding pattern. The results demonstrated the predominance of race 2 and low diversity within the Taiwan population. To our knowledge, this is the first report regarding the predominant race and IGS-RFLP haplotype identification of F. oxysporum f. sp. lycopersici in Taiwan. Our study indicates that tomato varieties in Taiwan should possess resistance to race 2. References: (1) G. Cai et al. Phytopathology 93:1014, 2003. (2) K. S. Elias and R. W. Schneider. Phytopathology 82:1421, 1992. (3) J. W. Gerdemann and A. M. Finley. Phytopathology 41:238, 1951. (4) P. K. Mishra et al. FEMS Microbiol. Lett. 218:329, 2003.

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Races and Inoculum Density of Fusarium oxysporum f. sp. niveum in Commercial Watermelon Fields in Maryland and Delaware

Plant Disease ◽

10.1094/pdis.2003.87.6.692 ◽

2003 ◽

Vol 87 (6) ◽

pp. 692-698 ◽

Cited By ~ 27

Author(s):

X. G. Zhou ◽

K. L. Everts

Keyword(s):

Fusarium Oxysporum ◽

Fusarium Wilt ◽

Total Population ◽

Research Field ◽

Causal Agent ◽

Inoculum Density ◽

Differential Cultivars ◽

Race 1 ◽

Race 2 ◽

The Relationship

A survey was conducted to determine races and inoculum density of Fusarium oxysporum f. sp. niveum, the causal agent of Fusarium wilt of watermelon in Maryland and Delaware. Virulence on six differential cultivars was tested for each of 63 isolates of F. oxysporum f. sp. niveum, obtained from 25 commercial watermelon fields. Thirteen isolates (21%) were identified as race 0, 36 isolates (57%) as race 1, and 14 isolates (22%) as race 2. Races 0 and 1 were present in 12 (48%) and 10 (40%) of the fields, respectively. The highly aggressive race 2 was identified from five fields in two counties in Maryland and from one field in Delaware, representing 24% of the fields. Race 2 was copresent with one or two other races. Race 2 (19 isolates) predominated among the 25 isolates obtained from a research field in Maryland. Nineteen commercial fields had inoculum densities of F. oxysporum f. sp. niveum ranging from 100 to 1,200 CFU/g of soil at harvest. Within this range of inoculum densities, >20% incidence of wilt was observed when the susceptible watermelon cv. Sugar Baby was planted in samples of soil collected from these fields. The relationship (P < 0.0001) between inoculum density of F. oxysporum f. sp. niveum (X) and incidence of Fusarium wilt (Y) on Sugar Baby was best described using the monomolecular equation, Y = 1 - exp[-0.0013 (X + 166)]. The ratio of pathogenic to total population of F. oxysporum in the fields linearly increased with increasing inoculum density of F. oxysporum f. sp. niveum (R 2 = 0.4; P < 0.0009).

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First Report of Downy Mildew (Race 2) of Sunflowers in Italy

Plant Disease ◽

10.1094/pdis.1998.82.2.263d ◽

1998 ◽

Vol 82 (2) ◽

pp. 263-263 ◽

Cited By ~ 1

Author(s):

A. Zazzerini ◽

L. Tosi

Keyword(s):

Downy Mildew ◽

Disease Incidence ◽

Central Italy ◽

Systemic Fungicide ◽

Infested Soil ◽

First Report ◽

Extensive Field ◽

Differential Cultivars ◽

Race 1 ◽

Race 2

During a survey in the spring of 1997, sunflower (Helianthus annuus L.) plants with characteristic symptoms (stunting and veinal chlorosis of leaves) of downy mildew caused by Plasmopara helianthi Novot. were observed in two experimental fields in the central Italian region of Umbria. In the first field, used to evaluate 58 commercial sunflower hybrids, fewer than 1% of plants displayed typical downy mildew symptoms. In a second field, planted with the old susceptible cultivar Ala, disease incidence was 12%. The inoculum of P. helianthi (one isolate from each field) was obtained either from downy mildew-infected leaves or infested soil (from around the roots of diseased plants). All were collected according to a random distribution. From this collection, two isolates were identified as race 2 by the whole-seedling immersion technique (2) with differential cultivars: HA 89, AD 66, CM 90 RR, RHA 265, RHA 274, HIR 34, DM-2, and HA 335. All differential lines were tested twice with 30 seeds per replicate (four replicates per line). Two weeks after inoculation the percentage of susceptible seedlings (showing sporulation on cotyledons and/or first true leaves) was determined. Race 2 has long occurred in some European countries, but this is the first report of this race from Italy. Extensive field surveys carried out in the last 10 years revealed that race 1 is the only one occurring in central Italy. An isolate apparently comparable to race 2 was reported in experimental plots in 1975 (1) where only Pl1 cultivars were susceptible. Differential lines, at that time, were not available and race identification was not carried out. Italy imposes importation regulations for sunflower seed that include treatment with metalaxyl. In greenhouse trials we have not observed a reduction in sensitivity to this systemic fungicide used as seed treatment against P. helianthi race 2. Consequently, the susceptibility of some commercial hybrids, indicated as resistant to downy mildew by importers, could be attributed to irregularity in the production of seeds. Also, an insufficient seed dressing with metalaxyl does not completely control the pathogen. References: (1) M. D'Armini et al. Atti Giornate Fitopatol. 1:659, 1975. (2) T. J. Gulya et al. Helia 14:11, 1991.

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First Report of Fusarium oxysporum f. sp. lycopersici Race 2 on Tomato in Italy

Plant Disease ◽

10.1094/pdis.1999.83.10.967b ◽

1999 ◽

Vol 83 (10) ◽

pp. 967-967 ◽

Cited By ~ 1

Author(s):

V. M. Stravato ◽

R. Buonaurio ◽

C. Cappelli

Keyword(s):

Fusarium Oxysporum ◽

Fusarium Wilt ◽

Vascular Tissue ◽

Morphological Features ◽

Pathogenicity Test ◽

First Report ◽

Fungal Isolates ◽

Race 1 ◽

Race 2 ◽

I Gene

During the summer of 1997, symptoms of Fusarium wilt were observed on tomato (Lycopersicon esculentum Mill.) cvs. Monica F1 and PS 110, which bear the I gene for resistance to race 1 of Fusarium oxysporum Schlechtend.:Fr. f. sp. lycopersici (Sacc.) W.C. Snyder & H.N. Hans., in two commercial production greenhouses in Latium (Fondi) and one greenhouse in Sardinia (Oristano). Infected plants showed yellowing, stunting, vascular discoloration, and premature death. A fungus from tomato stems with discolored vascular tissue was consistently isolated on potato dextrose agar (PDA) and, based on morphological features, was identified as F. oxysporum. To verify the pathogenicity of four fungal isolates, cv. Bonny Best tomato plants, which do not carry genes for Fusarium wilt resistance, were inoculated by dipping roots of 2-week-old seedlings in a suspension of 105 microconidia per ml for 30 s. Inocula were obtained from 1-week-old fungal cultures grown on PDA. Roots of control plants were dipped in water. Seedlings were transplanted to pots containing peat and river sand (1:1, vol/vol) and placed in a greenhouse at 20 to 25°C. One month after inoculation, all fungal isolates provoked wilting of inoculated plants. No symptoms were observed on control plants. The morphological features of the fungus reisolated from diseased plants were similar to those of the original isolates. Based on the pathogenicity test, we concluded that the fungal isolates belong to F. oxysporum f. sp. lycopersici. To determine the races of the fungal isolates, differential tomato lines VFN8 (I gene for resistance to race 1), Florida MH-1 (I and I2 genes for resistance to races 1 and 2), and I3R (I, I2, and I3 genes for resistance to races 1, 2, and 3) were inoculated with the four fungal isolates, using the same procedure described for the pathogenicity test. Because disease symptoms were detected on VFN8 but not on Florida MH-1 and I3R, we deduced that the fungal isolates belong to F. oxysporum race 2. This is the first report of F. oxysporum f. sp. lycopersici race 2 in Italy. Previous research indicated that race 1 is present in Italy (1). Currently, many commercially acceptable cultivars resistant to races 1 and 2 are available to Italian greenhouse growers. Reference: (1) M. Cirulli. Phytopathol. Mediterr. 4:63, 1965.

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