Fusarium Yellows Research Articles

Cross Pathogenicity and Vegetative Compatibility of Fusarium oxysporum Isolated from Sugar Beet.

Fusarium oxysporum f. sp. betae causes Fusarium yellows in sugar beet (Beta vulgaris). The F. oxysporum population from sugar beet can be highly variable in virulence and morphology and many isolates are nonpathogenic. Rapid and reliable methods to identify pathogenic isolates from nonpathogenic F. oxysporum generally are unavailable. Little is known about nonpathogenic isolates, including the role they may play in population diversity or virulence to sugar beet. Sugar beet is often grown in rotation with other crops, including dry edible bean (Phaseolus vulgaris) and onion (Allium cepa), with F. oxysporum able to cause disease on all three crops. Thirty-eight F. oxysporum isolates were collected from symptomatic sugar beet throughout the United States to investigate diversity of the F. oxysporum population and the influence of crop rotation on pathogenic variation. These isolates were characterized for pathogenicity to sugar beet, dry edible bean, and onion, as well as vegetative compatibility. Pathogenicity testing indicated that some F. oxysporum isolates from sugar beet may cause disease on onion and dry edible bean. Furthermore, vegetative compatibility testing supported previous reports that F. oxysporum f. sp. betae is polyphyletic and that pathogenic isolates cannot be differentiated from nonpathogenic F. oxysporum using vegetative compatibility.

Comparative Pathogenicity and Virulence of Fusarium Species on Sugar Beet.

In all, 98 isolates of three Fusarium spp. (18 Fusarium oxysporum, 30 F. graminearum, and 50 Fusarium sp. nov.) obtained from sugar beet in Minnesota were characterized for pathogenicity and virulence on sugar beet in the greenhouse by a bare-root inoculation method. Among the 98 isolates tested, 80% of isolates were pathogenic: 83% of the F. oxysporum isolates, 57% of the F. graminearum isolates, and 92% of the Fusarium sp. nov. isolates. Symptoms varied from slight to moderate wilting of the foliage, interveinal chlorosis and necrosis, and vascular discoloration of the taproot without any external root symptoms. Among the pathogenic isolates, 14% were highly virulent and 12% were moderately virulent. Most of the highly virulent isolates (91%) and moderately virulent isolates (89%) were Fusarium sp. nov. All pathogenic isolates of F. graminearum and most pathogenic isolates (87%) of F. oxysporum were less virulent. In general, more-virulent isolates induced first foliar symptoms earlier compared with less-virulent isolates. This study indicates that both F. oxysporum and Fusarium sp. nov. should be used in greenhouse and be present in field studies used for screening and developing sugar beet cultivars resistant to Fusarium yellows complex for Minnesota and North Dakota.

Organic inputs, tillage and rotation practices influence soil health and suppressiveness to soilborne pests and pathogens of ginger

A field experiment was established in which an amendment of poultry manure and sawdust (200 t/ha) was incorporated into some plots but not others and then a permanent pasture or a sequence of biomass-producing crops was grown with and without tillage, with all biomass being returned to the soil. After 4 years, soil C levels were highest in amended plots, particularly those that had been cropped using minimum tillage, and lowest in non-amended and fallowed plots, regardless of how they had been tilled. When ginger was planted, symphylans caused severe damage to all treatments, indicating that cropping, tillage and organic matter management practices commonly used to improve soil health are not necessarily effective for all crops or soils. During the rotational phase of the experiment, the development of suppressiveness to three key pathogens of ginger was monitored using bioassays. Results for root-knot nematode (Meloidogyne javanica) indicated that for the first 2 years, amended soil was more suppressive than non-amended soil from the same cropping and tillage treatment, whereas under pasture, the amendment only enhanced suppressiveness in the first year. Suppressiveness was generally associated with higher C levels and enhanced biological activity (as measured by the rate of fluorescein diacetate (FDA) hydrolysis and numbers of free-living nematodes). Reduced tillage also enhanced suppressiveness, as gall ratings and egg counts in the second and third years were usually significantly lower in cropped soils under minimum rather than conventional tillage. Additionally, soil that was not disturbed during the process of setting up bioassays was more suppressive than soil which had been gently mixed by hand. Results of bioassays with Fusarium oxysporum f. sp. zingiberi were too inconsistent to draw firm conclusions, but the severity of fusarium yellows was generally higher in fumigated fallow soil than in other treatments, with soil management practices having little impact on disease severity. With regard to Pythium myriotylum, biological factors capable of reducing rhizome rot were present, but were not effective enough to suppress the disease under environmental conditions that were ideal for disease development.

Molecular and Pathogenic Characterization of Fusarium redolens, a New Causal Agent of Fusarium Yellows in Chickpea.

The association of Fusarium redolens with wilting-like symptoms in chickpea in Lebanon, Morocco, Pakistan, and Spain is reported for the first time, together with the molecular and pathogenic characterization of isolates of the pathogen from chickpea of diverse geographic origin. Maximum parsimony analysis of sequences of the translation elongation factor 1α (TEF-1α) gene grouped all F. redolens isolates from chickpea in the same main clade. Pathogenicity assays using three chickpea cultivars and isolates from different geographic origins indicated that F. redolens is mildly virulent on chickpea. Moreover, infection of chickpea by F. redolens induces a disease syndrome similar to that caused by the yellowing pathotype of F. oxysporum f. sp. ciceris, including leaf yellowing and necrosis that develop upward from the stem base, and premature senescence of the plant. In contrast, F. redolens does not cause discoloration of the vascular tissues in chickpea but does cause brown necrotic lesions in the tap root and necrosis of lateral roots. F. redolens is not easily differentiated from F. oxysporum f. sp. ciceris using morphology-based diagnosis, and the two species cause similar symptoms on chickpea; therefore, the use of molecular protocols should help to avoid misdiagnoses of Fusarium yellows in chickpea.

Fusarium Yellows Affects Postharvest Respiration Rate, Sucrose Concentration, and Invert Sugar in Sugarbeet

Fusarium Yellows Affects Postharvest Respiration Rate, Sucrose Concentration, and Invert Sugar in Sugarbeet

Genetic variability among isolates of Fusarium oxysporum from sugar beet

Fusarium yellows, caused by the soil‐borne fungus Fusarium oxysporum f. sp. betae (Fob), can lead to significant yield losses in sugar beet. This fungus is variable in pathogenicity, morphology, host range and symptom production, and is not a well characterized pathogen on sugar beet. From 1998 to 2003, 86 isolates of F. oxysporum and 20 other Fusarium species from sugar beet, along with four F. oxysporum isolates from dry bean and five from spinach, were obtained from diseased plants and characterized for pathogenicity to sugar beet. A group of sugar beet Fusarium isolates from different geographic areas (including nonpathogenic and pathogenic F. oxysporum, F. solani, F. proliferatum and F. avenaceum), F. oxysporum from dry bean and spinach, and Fusarium DNA from Europe were chosen for phylogenetic analysis. Sequence data from β‐ tubulin, EF1α and ITS DNA were used to examine whether Fusarium diversity is related to geographic origin and pathogenicity. Parsimony and Bayesian MCMC analyses of individual and combined datasets revealed no clades based on geographic origin and a single clade consisting exclusively of pathogens. The presence of FOB and nonpathogenic isolates in clades predominately made up of Fusarium species from sugar beet and other hosts indicates that F. oxysporum f. sp. betae is not monophyletic.

Phytotoxic effect of selected crude plant extracts on soil-borne fungi of common bean

Common bean (Phaseolus vulgaris L.) is an important food and cash crop particularly in Eastern, Southern and Great lake region. The efficacy of water based crude plant extracts of Neem (Azadirachta indica), Mexican marigold (Tagetes minuta), Tobacco (Nicotiana tobacum) and Peri-winkle (Vinca rosea) in controlling soil-borne fungi (Fusarium oxysporum Schl. f. sp. phaseoli) of common bean (Phaseolus vulgaris L.) was studied in the western province of Kenya. Thirty farms were used in the study carried out in August-September 2005 and March-June 2006. Broth microdilution method was used to determine minimum inhibitory concentration (MIC). All crude plant extracts controlled Fusarium yellows with varying efficacy. Neem extract was the most effective, while peri-winkle was the least. Wilt incidence was not affected by dosage regime. Neem is inhibitory to Fusarium growth at lower dosage than tobacco, Mexican marigold and peri-winkle with MIC ranging between 2.5 and 10.0 mg ml-1

Baseline Sensitivity of Fusarium Species Associated with Fusarium Diseases to Metconazole, Triticonazole, and Thiabendazole Fungicides

Baseline Sensitivity of Fusarium Species Associated with Fusarium Diseases to Metconazole, Triticonazole, and Thiabendazole Fungicides

Fungitoxic properties of four crude plant extacts on fusarium oxysporum schl. F. sp phaseoli.

Fusarium yellows is a disease of common beans (Phaseolus vulgaris, L.) caused by Fusarium oxysporum Schl. F. sp. phaseoli, it has been found to be important in Busia district of Western province, Kenya. The study on fungitoxic properties of four locally available crude plant extracts was aimed at evaluating their efficacy in controlling Fusarium yellows infestation under the field conditions. Crude plant extracts from Azadirachta indica, Tagetes minuta, Nicotiana tobacum and Vinca rosea were tested against Fusarium oxysporum Schl. F. sp. phaseoli. Participatory On-Farm Trials (POFT) in six (6) divisions were carried out in August-September 2005 and March- June 2006; a total of thirty (30) farms were randomly surveyed. Minimum Inhibitory Concentration (MIC) of crude plant extracts against Fusarium was determined by broth microdilution method. Analysis of variance (ANOVA) was performed on the data, using Genstat 8th edition statistical program (Release 8.11, Lawes Agricultural Trust, Rothamsted Experimental Station, Harpenden, UK). Means were separated using LSD. Crude plant extracts exhibited fungitoxic activity against Fusarium oxysporum Schl. F. sp. phaseoli, with varying degree of efficacy. Nicotiana tobacum and Vinca rosea were not effective, Azadirachta indica and Tagetes minuta exhibited significant control over Fusarium. Azadirachta indica performed better amongst all the plant extracts. Common bean treatment with Benomyl 1 significantly reduced (P≤0.05) wilt incidence and increased growth in comparison to negative (–ve) control. Azadirachta indica formulation gave a significant reduction in wilt incidence compared to the other three crude plant extracts formulations. It reduced the wilt incidence by 17.24% in comparison to Tagetes minuta, Nicotiana tobacum and Vinca rosea whose wilt incidence reduction ranged from 5.84-9.8%. Azadirachta indica inhibited Fusarium growth at lower dosage than Tagetes minuta, Nicotiana tobacum and Vinca rosea. Extracts from Azadirachta indica and Tagetes minuta are effective, cheap and ecofriendly promising methods for protecting common bean against Fusarium oxysporum Schl. F. sp. phaseoli.

Biofumigation with Brassica plants and its effect on the inoculum potential of Fusarium yellows of Brassica crops

The use of Brassica crops as green manure in the so-called biofumigation treatment has been successfully exploited for the management of soilborne pathogens and is gaining interest particularly in the case of less intensive agricultural systems. A study was undertaken to investigate possible negative side-effects of biofumigation in order to prevent possible damage caused by wilt pathogens able to attack both plants used for biofumigation as well as agricultural crops. To do so, firstly the response of different Brassicas, including some used in biofumigation, to the formae speciales of Fusarium oxysporum known for being pathogenic on Brassica crops was evaluated. Secondly, the effect of green manure treatments on yield, quality of crops, and inoculum densities, infection and survival of Fusarium oxysporum f. sp. conglutinans and F. oxysporum f. sp. raphani was evaluated. In the second part of the work, four Brassica crops, selected for their response (susceptibility or resistance) to F. oxysporum f. sp. conglutinans and to F. oxysporum f. sp. raphani were evaluated in order to determine their response to the two pathogens during subsequent crops grown in soil where plants were incorporated as green manure into the soil at the end of each cycle. Moreover, the dynamics of the populations of F. oxysporum f. sp. conglutinans and F. oxysporum f. sp. raphani in the soil after several biofumigation cycles was studied. Many of the Brassica crops used for biofumigation tested were susceptible to F. oxysporum f. sp. conglutinans and or to F. oxysporum f. sp. raphani. Green manure treatment, carried out by growing nine cycles of biocidal plants, with a short crop cycle of 30–35 days, did not reduce Fusarium wilts on susceptible Brassica hosts. The population of the pathogen was partially increased as a result of the incorporation of tissues of the susceptible plants. When Brassica crops grown were resistant to the two F. oxysporum pathogens used for soil infestation, green manure simulation did inhibit both pathogens, thus confirming its biocidal activity. The results obtained under our experimental conditions show that biofumigation treatment is not applicable for soil disinfestation on crops susceptible to the same formae speciales of F. oxysporum affecting Brassica species used for biofumigation. Brassica crops resistant to Fusarium yellows should be grown where biofumigation is applied. Moreover, alternation of crops used for biofumigation should be encouraged.

Response of Sugarbeet Lines to Isolates of Fusarium oxysporum f. sp. Betae from the United States

Nine isolates of Fusarium oxysporum Schlechtend.Fr. f. sp. betae (Stewart) Snyd & Hans, that cause Fusarium yel lows of sugarbeet ( Beta vulgaris L.), were tested for their interaction with different sugarbeet lines. two isolates were tested in the presence or absence of the sugarbeet cyst nematode, Heterodera schachtii (Schmidt). In the absence of the cyst nematode, differences in disease severity were detected in sugarbeet lines depending upon the isolates. A small number of lines showed reduced disease response to several isolates, but for the some lines, responses varied, depending upon the pathogen isolate. In the presence of H. schachtii, two isolates of F. oxysporum f. sp. betae increased disease severity in some lines and decreased it in others. this variability in host response may explain some of the variable results growers report after planting sugarbeet lines with resistance to Fusarium yellows.

First Report of a Novel Fusarium Species Causing Yellowing Decline of Sugar Beet in Minnesota.

In the United States, yellows disease of sugar beet (Beta vulgaris), which causes wilt, early death, and yield reduction, is caused primarily by Fusarium oxysporum f. sp. betae (3,4), but F. graminearum (2) has also been implicated. During the past 3 years, a similar disease causing yellowing and severe decline appeared in some sugar beet fields of central and southwest Minnesota planted with cultivars resistant to yellows. The disease has become a concern to the local sugar beet industry, which produces 56% of sugar beets in the United States. From 2005 to 2007, isolations were made from sugar beets collected in commercial fields and from a Fusarium screening nursery showing symptoms of yellowing, interveinal chlorosis, scorching, stunting, vascular discoloration of the taproot, and early death of plants. Of 96 Fusarium isolates recovered and used in root-dip inoculation trials in the greenhouse, 58 were pathogenic to sugar beets. On the basis of morphology, 12 were identified as F. oxysporum, 6 as F. graminearum, and 40 as a novel Fusarium species. The remaining 38 isolates were nonpathogenic. All three pathogenic Fusarium species were isolated from taproots, but only the novel Fusarium was isolated from petioles. In culture, the novel Fusarium exhibited a bright orange color on the underside of potato dextrose agar medium and produced micro- and macroconidia sparsely. Hyphal tip isolates of all novel Fusarium isolates were pathogenic, causing typical yellowing symptoms and plant death to the Fusarium yellows susceptible sugar beet cv. VDH46177 in replicated greenhouse trials. Isolates were successfully reisolated from the symptomatic plants, fulfilling Koch's postulates. Restriction fragment length polymorphism (RFLP) endonuclease digestion patterns (Alu1, Fnu4HI, HaeIII, and HhaI) of the internal transcribed spacer (ITS) region of 40 pathogenic novel isolates showed a distinct pattern compared with known Fusarium species. Thin layer chromatography analysis of 13 novel isolates detected the type A trichothecenes neosolaniol and 4,15-diacetoxyscirpenol. Partial sequences of the translation elongation factor 1-α (TEF) from 12 single-spored novel Fusarium isolates were generated. BLAST analysis of the TEF sequence against the FUSARIUM-ID (1) and GenBank databases did not match any known Fusarium species. On the basis of pathogenicity, morphology, RFLP patterns, mycotoxin production, and TEF sequence analysis it appears that this is a new species of Fusarium, but additional multilocus phylogenetic analyses are warranted. The natural occurrence of this novel Fusarium pathogen in sugar beet may have implications in breeding for resistance to Fusarium yellows, since yellow decline has been observed in purportedly Fusarium-tolerant cultivars in the Minnesota and North Dakota production regions.

Fusarium yellows of celery caused by fusarium oxysporum f. sp. APII in Argentina

SUMMARY In Argentina, celery is grown in the outskirts of the principal cities in the provinces of Buenos Aires, Santa Fe, and Mendoza. Since the late 1990s, plants growing in La Plata (Buenos Aires) have shown rapid decline in commercial fields. Symptoms are delayed growth, yellowed leaves, discoloured vascular tissue and reduced root system. The objectives of this work were to determine the causal agent of the disease and to analyze the effects of inoculum levels on the disease. Fusarium oxysporum (Fo) was consistently isolated on potato-dextrose-agar (PDA). The “forma specialis” was identified by pathogenicity tests where each of seven selected strains of Fo on colonized rice that was dried and ground were mixed into soil. Initial Fusarium wilt symptoms were recorded 50 days after inoculation, and Fo was reisolated from affected roots, stems, crowns, and petioles suggesting that the pathogen is Fo f. sp. apii (Foa). In another assay different concentrations of inoculum (10 1 to 10 4 cfu per g of soil) were tested to determine the effects of Foa on the plants (cv. Golden Boy). Wilt severity, chlorosis, plant height, and dry weight were assessed weekly. Typical symptoms developed at all levels of inoculum. Disease severity increased when the inoculum density was increased. To our knowledge, this is the first report of Foa in Argentina. The possibility that the pathogen was introduced on imported seed during the last few years is discussed.

Isolation and molecular analysis of R-gene in resistant Zingiber officinale (ginger) varieties against Fusarium oxysporum f.sp. zingiberi

Marker assisted selection (MAS) of resistant varieties is a reliable and faster method of selecting the right varieties for cultivation. The aim of the present study is to find the genes responsible for resistance in highly resistant varieties. In the present work we report the presence of a Resistance (R) gene of CC–NBS–LRR class of plant resistance genes. Both direct PCR amplification from genomic DNA as well as cDNAs, yielded a 0.6 kb DNA sequence indicating the absence of an intron. Sequence analysis of the PCR amplicon obtained from the genomic DNA showed very high homology to R-genes. An interesting observation from the present study is the presence of the R-gene in only resistant varieties. Neither the partially resistant or susceptible varieties showed the presence of this gene sequence. This in turn raises interesting questions on the evolution of these ginger varieties. The cloned R-genes provide a new resource of molecular markers for rapid identification of fusarium yellows resistant ginger varieties.

Fungal cell wall degrading enzyme-producing bacterium enhances the biocontrol efficacy of antibiotic-producing bacterium against cabbage yellows

The antagonistic rhizobacterium, Pseudomonas fluorescens strain LRB3W1, controls cabbage Fusarium yellows caused by Fusarium oxysporum f. sp. conglutinans and inhibits its growth in vitro. Conversely, Serratia marcescens strain B2, known as a biocontrol bacterium, did not inhibit growth of F. oxysporum f. sp. conglutinans and the suppressive effect of strain B2 against cabbage yellows was less than that of strain LRB3W1. However, combined inoculation of strain LRB3W1 with strain B2 was more effective than treatment with strain LRB3W1 alone. Colonization of each bacterium in the cabbage rhizosphere was not influenced by the presence of the other bacterium. When bud-cells of the pathogen were treated with 2,4-diacetylphloroglucinol from strain LRB3W1 and fungal cell wall degrading enzyme from strain B2 simultaneously, an additive inhibitory effect was observed. According to the results the fungal cell wall degrading enzyme-producing bacterium enhanced the biocontrol effect of the antibiotic-producing bacterium against cabbage Fusarium yellows.

First Report of Fusarium Yellows of Sugar Beet Caused by Fusarium oxysporum in Michigan

Fusarium yellows of sugar beet (Beta vulgaris L.), caused by Fusarium oxysporum Schlechtend.Fr. f. sp. betae (Stewart) Snyd & Hans., has been a long-term problem in the western United States (3) and recently was reported in Minnesota and North Dakota (4). This disease is typified by interveinal yellowing and wilting of the foliage. Roots have no external symptoms but show internal vascular discoloration. In 2005, 12 sugar beet roots from Michigan with yellows-type symptoms were received by the author. Isolations were made from the cortical and vascular tissue of the crown and tap root. Fusarium spp. isolates were obtained from 10 of the beets, and 16 isolates were identified as Fusarium oxysporum on the basis of morphology and pigmentation on potato dextrose agar and spores and phialides on carnation leaf agar (2). F. oxysporum isolates were tested for pathogenicity by dipping roots of 5-week-old susceptible sugar beet plants (FC716) in a suspension of 104 spores per ml for 8 min, 10 plants per isolate. Two known pathogenic isolates of F. oxysporum f. sp. betae, Fob13 and Fob216c (4), were used for comparison. For a negative control, plants were dipped in sterile water. Beets were planted in Cone-tainers (3.8 cm diameter × 21 cm) containing pasteurized potting mix. Plants were placed in a greenhouse at 24 to 27°C and fertilized with 15-30-15 fertilizer every 2 weeks to avoid chlorosis from nutrient deficiency. Plants were rated weekly for foliar symptoms for 6 weeks using a Fusarium yellows rating scale of 0 to 4 in which 0 = no disease and 4 = complete plant death (1). After the final rating, plants were removed from soil and the tap root examined for root symptoms. Root segments were surface disinfested with 0.5% sodium hypochlorite and cultured on potato dextrose agar to confirm presence of the pathogen. The experiment was done twice. Seven F. oxysporum isolates tested caused typical Fusarium yellows symptoms including interveinal yellowing, stunting, and wilting of inoculated plants. Pathogenic isolates were obtained from 7 of the 10 beets that yielded F. oxysporum. Symptoms were indistinguishable from those caused by Fob13 (average ratings ranged from 1.8 to 2.4) and milder than those caused by Fob216c (average rating 3.1). No interveinal chlorosis or wilting was observed on the control plants. Isolations from inoculated plants provided F. oxysporum cultures morphologically similar to those used in inoculation by the methods of Nelson et al. (2). No F. oxysporum was isolated from control plants. To my knowledge, this is the first report of F. oxysporum causing Fusarium yellows on beet in Michigan.

Fusarium Yellowing of Sugar Beet Caused by Fusarium graminearum from Minnesota and Wyoming.

In 2004, we received beet samples from seven fields from Minnesota and Wyoming that had foliar interveinal yellowing symptoms and vascular discoloration frequently associated with Fusarium yellows. Isolations were made from the vascular and cortical tissue. Hyphal tip isolates of Fusarium were obtained from beets, including eight isolates of Fusarium graminearum. F. graminearum was isolated from beets from three fields in Minnesota and one field in northern Wyoming. F. graminearum isolates were tested for pathogenicity by dipping roots of 5-week-old sugar beet plants (FC716) in a suspension of 104 spores per ml for 8 min, 10 plants per isolate. Spore suspensions were shaken periodically to aid mixing. A known moderately virulent isolate of F. oxysporum f. sp. betae (Fob13) (3), the causal agent of Fusarium yellows of sugar beet, was used as a positive control. For a negative control, plants were dipped in sterile water. Dipped plants were planted in Cone-tainers (3.8 cm diameter × 21 cm; Stuewe and Sons, Inc., Corvallis, OR) containing pasteurized potting mix and placed in a greenhouse at 24 to 27°C. Plants were fertilized with 15-30-15 fertilizer biweekly. After 2 weeks, plants were rated weekly for 5 weeks using a 0 to 4 scale in which 0 = no disease and 4 = complete plant death (2). After the final rating, plants were removed from soil and the tap root was examined for symptoms. Root segments were surface disinfested with 0.5% sodium hypochlorite and plated on potato dextrose agar to confirm presence of the pathogen. The experiment was done twice. Three of the eight isolates of F. graminearum caused mild to moderate foliar symptoms (rating 2 to 3), including interveinal yellowing, wilting, and stunting of inoculated plants, and mild vascular discoloration was observed in some root sections. Pathogenic isolates were originally from different beets. Foliar symptoms were similar to those caused by Fob13, but the F. oxysporum f. sp. betae caused more vascular discoloration than did the F. graminearum isolates. No interveinal yellowing or wilting was observed on foliage of the control plants, and no vascular discoloration was observed in a cross section of the root. Cultures of F. graminearum or F. oxysporum recovered from inoculated plants were morphologically similar to isolates used for the inoculations. No Fusarium was isolated from the roots of plants soaked in sterile water. An interesting note is that no isolates of F. graminearum were recovered among more than 100 Fusarium isolates collected from sugar beet roots from Colorado over a 4-year period. F. graminearum was recovered in one sample from Wyoming in 2004. However, in the 2004 samples from Minnesota, this species was isolated at the same frequency as F. oxysporum. While F. graminearum has been isolated from beets in the Red River Valley (1), it has not previously been reported to cause symptoms on growing sugar beet.

Beet Root-Rot Inducing Isolates of Fusarium oxysporum from Colorado and Montana.

A root-tip rot of sugar beet (Beta vulgaris L.) caused by Fusarium oxysporum Schlecht. emend. Snyd. & Hans. has been reported from Texas (2). This disease is typified by yellowing of the foliage, vascular discoloration, and a rot of the root tip. During 2002 and 2003, sugar beet samples from several fields in Colorado and Montana, some with tip rot symptoms, were received by the authors. Isolations were made from the root vascular tissue and tissue adjacent to the rot in Colorado and from the rot tissue in Montana. Isolates of Fusarium were obtained and identified as Fusarium oxysporum. At the ARS laboratory in Colorado, F. oxysporum isolates were tested for pathogenicity by dipping roots of 5-week-old sugar beet plants (FC716) in a suspension of 104 spores per ml for 8 min, 10 plants per isolate. One known isolate of F. oxysporum f. sp. betae that causes Fusarium yellows, Fob13, was used for comparison. For a negative control, plants were dipped in sterile water. Beets were planted in Cone-Tainers (3.8 cm in diameter × 21 cm) containing pasteurized potting mix. Plants were placed in a greenhouse at 24 to 27°C and fertilized with 15-30-15 fertilizer every 2 weeks to avoid chlorosis from nutrient deficiency. Plants were rated weekly for foliar symptoms for 6 weeks with a Fusarium yellows rating scale of 0 to 4, in which 0 = no disease and 4 = complete plant death (1). After the final rating, plants were removed from the soil and the taproot was examined for rot symptoms. Root segments were surface disinfested with 0.5% sodium hypochlorite and cultured on potato dextrose agar to confirm the presence of the pathogen. The experiment was done twice. Six of ten F. oxysporum isolates tested caused root vascular discoloration and foliar symptoms, including interveinal yellowing and wilting, of inoculated plants. A rot of the root tip was observed on the roots of plants inoculated with three of the six pathogenic isolates. Isolate Fob13 caused only vascular discoloration and foliar symptoms with no rot. Similar experiments were done in Montana with the exception that 3-week-old plants (cv. Monohikari) were used and planted in 10-cm plastic pots with five seedlings per pot. Inoculum levels were 105 spores per ml of F. oxysporum f. sp. betae (isolate 216C) or tip rot isolates (3 isolates), and the experiments were terminated 4 weeks after planting. The root rot isolates caused foliar symptoms, vascular discoloration, and root rot similar to that seen in the field, whereas isolate 216C caused only foliar wilt symptoms and vascular discoloration. Isolations from inoculated plants in Colorado and Montana resulted in F. oxysporum cultures similar to those used in inoculation. To our knowledge, this is the first report of F. oxysporum causing root rot of sugar beet outside of Texas.

Provide a Means to Develop Collard Cultivars

Collard (Brassica oleracea L. Acephala Group) is a leafy green vegetable adapted to the southeastern United States. The number of commercially available collard cultivars is limited, and the most popular cultivars are susceptible to fusarium yellows, a disease that most cabbage (B. oleracea Capitata group) cultivars are resistant to. We hypothesized that hybrids of cabbage and collard would look more like collard, because heading of cabbage is at least partially recessive to the nonheading growth habit of collard. We also postulated that cabbage–collard hybrids might be used directly as collard cultivars. To test these postulates, cytoplasmic male sterile cabbage inbreds were crossed to different male fertile collard inbreds and hybrid seed was produced. Resulting cabbage–collard hybrids were compared to conventional collard cultivars in three replicated field trials in South Carolina. In all trials, cabbage–collard hybrids exhibited size and weight more similar to conventional collard than cabbage, and throughout most of the growing season the collards remained nonheading. In addition, the cabbage–collard hybrids were much more uniform than open-pollinated collard cultivars. Among cabbage–collard hybrids there was significant variation with some hybrids appearing more collard-like than others. The collard inbreds designated A and B may have the greatest potential for making promising cabbage–collard hybrids. Particular hybrids (i.e., A3 or B2), derived from these inbreds and tested in this study, can perform better than certain conventional collards and may serve as possible new cultivars of this vegetable crop.

First Report of Fusarium oxysporum Causing Yellows on Sugar Beet in the Red River Valley of Minnesota and North Dakota.

In 2002, somel sugar beet (Beta vulgaris L.) fields in the Red River Valley (RRV) of Minnesota and North Dakota had symptoms characteristic of Fusarium yellows (4). In 2004, ≈5% of fields in the RRV had symptomatic plants. Interveinal yellowing of older leaves typically began in mid-July and as the disease progressed, younger leaves turned yellow. Sometimes, one side of the leaf was yellow or necrotic while the other side remained green. As leaves died, they remained attached to the crown. Transverse sections of roots revealed a light gray-brown discoloration of the vascular tissue but no external rotting of roots. Isolations from 35 symptomatic roots collected in eight fields yielded 25 isolates identified as F. oxysporum (from single conidia grown on homemade potato dextrose agar and carnation leaf agar) (3). Pathogenicity was determined by dipping roots of 5-week-old sugar beet plants (cv. ACH 9363) in a suspension of 104 conidia per ml for 8 min (12 isolates, 10 to 12 plants per isolate). Plants were planted in Cone-tainers (3.8 cm diameter × 21 cm; Stuewe and Sons, Inc. Corvallis, OR) containing sterile soil. Three known cultures of F. oxysporum Schlecht. emend. Snyd. & Hans. f. sp. betae Stewart (= F. conglutinans var. betae Stewart [4]) also were included (13 and 216c from L. Hanson, USDA-ARS, Fort Collins, CO; 0-1122 from The Pennyslvania State University Fusarium Research Center). The control was sterile water. Plants were placed in a greenhouse at 24 to 27°C with natural light supplemented with illumination from high-pressure sodium-vapor lamps for 16 h daily and lightly fertilized biweekly to avoid chlorosis from nutrient deficiency. After 6 to 7 weeks, plants were rated for disease on a 0 to 4 scale: 0 = no disease; 1 = slight to extreme plant stunting, leaves may be wilted; 2 = chlorotic leaves, some with necrosis at margins; 3 = tap root dried and brown to black in color, leaves dying; and 4 = plant dead (1). The experiment was repeated. Disease severity differed between trials, but all isolates of F. oxysporum and F. oxysporum f. sp. betae resulted in disease ratings statistically (P < 0.05) greater than that of the water control. In Trial 1, isolates of F. oxysporum averaged a rating of 2.1 (range of 1.8 to 3.3) and F. oxysporum f. sp. betae averaged 2.1 (range of 2.0 to 2.2) compared with 0.1 for the water control. One isolate of F. oxysporum had a statistically higher rating than did the cultures of F. oxysporum f. sp. betae. In Trial 2, isolates of F. oxysporum averaged a rating of 3.3 (range of 2.7 to 3.7) and F. oxysporum f. sp. betae averaged 3.1 (range of 2.7 to 3.4) compared with 0.2 for the water control. Cultures of F. oxysporum (8 of 12) resulted in ratings statistically higher than that of the least pathogenic culture of F. oxysporum f. sp. betae. Cultures of F. oxysporum and F. oxysporum f. sp. betae recovered from inoculated plants were identical to those used to inoculate plants. To our knowledge, this is the first report of F. oxysporum f. sp. betae on sugar beet in the Red River Valley of Minnesota and North Dakota. The disease has been reported in California, Colorado, Montana, Nebraska, Oregon, Texas, and Wyoming (1,2).