In vitro inhibitory activity of xylem exudates from cucurbits towardsFusarium oxysporum microconidia

Experiments were conducted to determine if xylem exudates from three cucurbitaceous plants as well as non-cucurbit species were inhibitory to the growth of microconidia ofFusarium oxysporum f. sp.niveum, causal agent of Fusarium wilt of watermelon. Inhibitory effects of xylem exudates from watermelon(Citrullus lanatus) cultivars differentially susceptible toF.o. f.sp.niveum as well as from watermelon plants previously inoculated with virulent or avirulent races ofF. oxysporum, were compared. Results indicated that xylem fluid contained an inhibitory component that was independent of the resistance status of the cultivar or previous inoculation with virulent or avirulentF.o. f.sp.niveum races. Xylem exudates from cucumber(Cucumis sativis) also inhibited microconidia growth and colony formation while muskmelon(Cucumis melo) and tomato(Lycopersicon esculentum) xylem exudates had little inhibitory activity. Incubation with proteinase K, lysozyme, or boiled xylem fluid suggested that the active component had proteinaceous properties with an oligosaccharide moiety.     

Induced resistance in tomato plants against Fusarium wilt invoked by Fusarium oxysporum f.sp. dianthi

Tomato plants, susceptible toFusarium oxysporum f. sp.lycopersici, were inoculated by immersing the roots in a conidial suspension ofF. oxysporum f. sp.lycopersici race 1,F. oxysporum f. sp.dianthi race 2 or a mixture of both fungi. Plants inoculated withF. oxysporum f. sp.lycopersici showed disease symptoms after 2 weeks, whereas plants inoculated withF. oxysporum f. sp.dianthi or a mixture of both fungi remained symptomless for over 7 weeks, the duration of the experiment. In another experiment root systems of plants were split and each half was separately inoculated. One half was firstly inoculated withF. oxysporum f. sp.dianthi or treated with water, followed after a week by a second inoculation of the other half withF. oxysporum f. sp.lycopersici or by a water treatment. The disease symptoms in the half firstly inoculated withF. oxysporum f. sp.dianthi were significantly delayed, compared to plants of which that half had been treated with water. BecauseF. oxysporum f. sp.dianthi reduced disease symptoms caused byF. oxysporum f. sp.lycopersici without any direct interaction with this pathogen, it is concluded thatF. oxysporum f. sp.dianthi is able to induce resistance againstF. oxysporum f. sp.lycopersici in tomato plants.     

Occurrence of Fusarium oxysporum f. sp. melonis Race 1 in Japan

In 1994, Fusarium wilt of melon cultivars which are resistant to races 0 and 2 of Fusarium oxysporum f. sp. melonis was observed in southern area of the Lake Biwa region, Shiga prefecture. In commercial fields, mature plants of cv. Amus which were grafted onto cv. Enken Daigi 2, and of cv. FR Amus showed yellowing, wilting and finally death before harvesting of fruits. Diseased plants had vascular and root discolorations, and their stem sections yielded typical colonies of F. oxysporum. When the Shiga strains were tested for their pathogenicity to 12 species of cucurbits, they caused wilts only on melon. Using race differential cultivars of melon, the Shiga strains were classified as race 1 of F. oxysporum f. sp. melonis, which has not been reported in Japan. To further characterize their pathogenicity, the strains were used to inoculate 46 additional cultivars of melon, oriental melon and oriental pickling melon. All the race 1 strains were pathogenic to the cultivars tested, and their host range was apparently different from those of strains belonging to other races (races 0, 2 and 1,2y). DNA fingerprinting with a repetitive DNA sequence, FOLR3, differentiated race 1 strains from strains of races 0 and 2, but not from race 1,2y strains. Received 2 July 1999/ Accepted in revised form 30 September 1999     

Downy mildew-, powdery mildew- and fusarium wilt-resistant muskmelon breeding line PI-124111F

A muskmelon (Cucumis melo L.) breeding line, PI-124111F, is a seventh-generation selection derived from PI-124111, which is resistant to downy mildew (Pseudoperonospora cubenis pathotype 3), powdery mildew (races 1 and 2 ofSphaerotheca fuliginea) and Fusarium wilt (Fusarium oxysporum f. sp.melonis, races 0, 1 and 2). This is the only breeding line known to carry multiple-race resistance to these diseases. PI-12411IF is a monoecious muskmelon with poor fruit characteristics.     

Occurrence ofFusarium oxysporum F.SP.Melonis race 1,2 on muskmelon in Israel

Fusarium oxysporum f.sp.melonis race 1,2 was isolated from wilted muskmelon plants from two sites in Israel. The isolates caused disease in all the Israeli and French differential cultivars tested, regardless of whether they were susceptible or carrying resistance genesFom 1 orFom 2. This is the first record ofF.o. f.sp.melonis race 1,2 in Israel.     

The basis of host recognition in Fusarium oxysporum f. sp. lycopersici

Filtrates from shake-cultures of Fusarium oxysporum f. sp. lycopersici race 1, concentrated to 20% of the original volume, caused cell death in tomato leaf protoplasts from near-isogenic lines corresponding to the compatible cultivar/race reactions of whole plants. Maximum activity was found in late log phase cultures on Czapek-Dox supplemented with 2% casamino acids. Selective toxicity was associated only with the protein fraction of the culture filtrate. LD₅₀ values for susceptible Ace and Moneycross to F. oxysporum f. sp. lycopersici race 1 culture filtrates were 1·92 and 0·36 μg protein ml⁻¹. Corresponding values for cvs Royal Ace and MM161, each containing the I-gene conferring resistance to race 1, were >350. Culture filtrates from F. oxysporum f. sp. lycopersici race 2 gave LD₅₀ values of 2·34 and 2·08 μg protein ml⁻¹ on cvs Ace and Royal Ace, both susceptible to race 2. The LD⁵⁰ of cv. Ace to a non-pathogenic isolate of F. xysporum f. sp. lycopersici was > 350. Culture filtrates from non-host formae of F. oxysporum were 9–149-fold less toxic on cv. Ace. Protoplasts from Pisum sativum, Lactuca sativa, Zea mays, Gossypium barbadense and Solanum melongena, all non-hosts of F. oxysporum f. sp. lycopersici, were 6–175 times less sensitive to F. oxysporum f. sp. lycopersici filtrates than susceptible tomato. The putative toxins lycomarasmin and fusaric acid showed no differential toxicity to I⁺ and I⁻ tomato protoplasts. The results are discussed in the wider context of host-pathogen interaction in which specificity is considered as the recognition of susceptibility by a proteinaceous toxic metabolite of the pathogen. This hypothesis is further extended to include the specificity of F. oxysporum formae and races.     

Comparison of resistance-inducing abilities of virulent and avirulent races of Erysiphe graminis f.sp. hordei and a race of Erysiphe graminis f.sp. tritici in barley

Resistance to powdery mildew was induced in barley by preinoculation with virulent and avirulent races of barley powdery mildew ( Erysiphe graminis f.sp. hordei ), and with a race of wheat powdery mildew ( E. graminis f.sp. tritici ). Four inducer densities were tested in 13 different induction periods between 1 and 24 h. Generally, the resistance induced by barley powdery mildew increased up to 10-12 h of induction and was maintained in longer induction periods. The inducing abilities of virulent and avirulent races could not be distinguished up to 10-12 h of induction, after which the inducing ability of avirulent races increased significantly in relation to virulent races. Wheat powdery mildew was able to induce more resistance than barley powdery mildew in induction periods up to 8 h. In a single inoculation procedure the number of haustoria developing from virulent barley powdery mildew decreased as inoculum density increased. The effect was ascribed to induction of resistance. This reduction of infection efficiency in the compatible interaction was compared to induced resistance. However, the inoculum density needed for 50% resistance induction in the double inoculation procedure was approximately 40 times higher than the inoculum density needed for 50% reduction in infection efficiency in the single inoculation procedure.     

A new race of Fusarium oxysporum f. sp. lactucae of lettuce

Fusarium oxysporum f. sp. lactucae, the causal agent of fusarium wilt of lettuce (Lactuca sativa), occurs in most countries in which lettuce is grown and causes serious economic losses. Three races (1, 2 and 3) of the pathogen have previously been identified on the basis of their ability to cause disease on differential lettuce cultivars, as well as by means of molecular tools developed to characterize different races of this pathogen. Only race 1 has been detected in Europe so far. In this study, two isolates of F. oxysporum, obtained from lettuce plants grown in the Netherlands showing symptoms of wilt, have been characterized by combining the study of pathogenicity with differential cultivars of lettuce and molecular assays to determine whether the isolates are different from the known races of F. oxysporum f. sp. lactucae. This study reports the presence of F. oxysporum f. sp. lactucae for the first time in the Netherlands. The causal pathogen has been identified, using the IRAP‐SCAR technique, as a new race of F. oxysporum f. sp. lactucae. Specific primers have been designed to identify this new race.     

Sequence variation in the putative effector gene SIX8 facilitates molecular differentiation of Fusarium oxysporum f. sp. cubense

Fusarium oxysporum f. sp. cubense (Foc), causal agent of fusarium wilt of banana, is among the most destructive pathogens of banana and plantain. The development of a molecular diagnostic capable of reliably distinguishing between the various races of the pathogen is of key importance to disease management. However, attempts to distinguish isolates using the standard molecular loci typically used for fungal phylogenetics have been complicated by a poor correlation between phylogeny and pathogenicity. Among the available alternative loci are several putative effector genes, known as SIX genes, which have been successfully used to differentiate the three races of F. oxysporum f. sp. lycopersici. In this study, an international collection of Foc isolates was screened for the presence of the putative effector SIX8. Using a PCR and sequencing approach, variation in Foc‐SIX8 was identified which allowed race 4 to be differentiated from race 1 and 2 isolates, and tropical and subtropical race 4 isolates to be distinguished from one another.     

Pathogenic variability in Ethiopian isolates of Fusarium oxysporum f. sp. ciceris and reaction of chickpea improved varieties to the isolates

Twenty-four isolates of Fusarium oxysporum f. sp. ciceris were isolated from wilted chickpea plants obtained from different districts and ‘wilt sickplots’ of central Ethiopia to assess variability in pathogenecity of the populations. Each isolate was tested on 10 different chickpea lines and eight improved chickpea varieties. Isolates showed highly significant variation in wilt severity on the differential lines and improved varieties. Based on the reaction types induced on differential lines, isolates were grouped into four corresponding races. Of the 24 isolates, F13, F20 and F22 were the most virulent. Isolates of race 3 were found in all of the districts and ‘wilt sickplots’ studied. Improved chickpea varieties also showed differential reactions to the isolates. All varieties were resistant to isolates of race 3, while varieties Arerti and DZ-10-4 were resistant to all isolates tested, showing the lowest mean wilt severity. Varieties DZ-10-11 and Maryie were susceptible to isolates F13, F20 and F22 and showed the highest mean wilt severity. Identification of races can be useful in breeding chickpea varieties resistant to wilt. The differential reactions of the improved varieties against different races might be important in managing chickpea wilt through gene deployment.     

Genetic diversity in Fusarium oxysporum f. sp. melonis*

Fusarium oxysporum f. sp. melonis is an important vascular wilt pathogen of melon. Races 1, 2 and 1–2 of this fungus have been identified in Portugal by pathogenicity tests with appropriate hosts. The aim of this research was to examine the relationships between different races of F. o. melonis of Portuguese and French origin through analysis of random amplified polymorphic DNAs (RAPDs). DNA fingerprint profiles were developed for all the accessions. Each isolate showed 5–10 DNA bands with each of the 16 primers employed. A total of 126 bands was obtained. The size of amplified DNA fragments generated with these primers ranged from 0.5 to 3.2 kb. A phenogram based on the Jaccard coefficient of similarity was computed by the unweighed pair group method using arithmetic averages (UPGMA). It was found that Portuguese race 2 is very similar to French race 1, while French race 2 is the most dissimilar being clearly separated from all other races. The genetic diversity of these isolates is also being studied for vegetative compatibility by using the nit mutant system.     

Physiological races of Fusarium oxysporum f.sp. melonis in Tunisia

Fusarium wilt of melon, caused by Fusarium oxysporum f.sp. melonis (Fom), is an important disease; races of the pathogen were identified by inoculating differential standard host cultivars. A total of ten isolates that were obtained from 23 fields located in four different geographical regions were identified as pathogenic. Results indicate that all four known Fom races, namely, 0, 1, 2 and 1.2, were found in north and middle Tunisia. Race 1.2 was the most prevalent.     

Screening of cultivated and wild adzuki bean for resistance to race 3 of <Emphasis Type="Italic">Cadophora gregata</Emphasis> f. sp. <Emphasis Type="Italic">adzukicola</Emphasis>, cause of brown stem rot

Two diseases of adzuki bean, brown stem rot (BSR, caused by Cadophora gregata f. sp. adzukicola) and adzuki bean Fusarium wilt (AFW, caused by Fusarium oxysporum f. sp. adzukicola), are serious problems in Hokkaido and have been controlled using cultivars with multiple resistance. However, because a new race of BSR, designated race 3, was identified, sources of parental adzuki bean for resistance to race 3 were needed. Therefore, we examined 67 cultivars and lines of cultivated and wild adzuki bean maintained at the Tokachi Agricultural Experiment Station using a root-dip inoculation method. Consequently, nine adzuki bean cultivars, one wild adzuki bean accession and 30 lines (including two lines resistant to all the three races of BSR and AFW) were confirmed to be resistant or tolerant to race 3 of BSR, and we found a cultivar Akamame as well as a wild adzuki bean Acc2515 to be a new source for a resistance gene to the race 3. This cultivar also holds promise as a source of resistance against other races of BSR and AFW.     

Pathogenic races of the melon-wilt Fusarium in Israel

Two races ofFusarium oxysporum f. sp.melonis were identified among 54 isolates of this pathogen, tested on three differential lines of melon. Of the two races, race 0 was the more prevalent, comprising 92.5% of the total number of isolates tested. Race 1 has been encountered so far in one growing area only. The identity of the local races is compared with those defined in France and in two areas of melon-growing in California.     

Pathogenicity, vegetative compatibility and RAPD analysis of Fusarium oxysporum isolates from tobacco fields in Extremadura

Fusarium wilt of tobacco could be caused by Fusarium oxysporum f. sp. batatas or f. sp. vasinfectum since f. sp. nicotianae was rejected because there was no evidence of isolates specific to tobacco. Forty isolates of F. oxysporum from soil and plants from tobacco fields in Extremadura (south-western Spain) were characterized by pathogenicity on burley and flue-cured tobacco, for vegetative compatibility group (VCG), and by random amplified polymorphic DNA (RAPD). Isolates from burley were identified as race 1 of F. oxysporum f. sp. batatas based on pathogenicity on tobacco, sweet potato and cotton, and those from flue-cured as race 2. Most isolates from soil were heterokaryon self-incompatible (HSI) and the remaining isolates from soil and tobacco were grouped into four VCGs: VCG 1 (5 isolates from burley), VCG 2 (17 isolates from flue-cured and 4 from soil), VCG 3 (2 isolates from flue-cured) and VCG 4 (2 isolates from soil). This is the first report of the two races and VCGs of F. oxysporum f. sp. batatas in Spain. Analysis of RAPD revealed two clusters (C-I and C-II) related to race and VCGs. C-I included race 1 (VCG 1) isolates from burley and nonpathogenic (VCG 4 or HSI) isolates from soils. C-II included nonpathogenic (VCG 2) and race 2 (VCG 2 or VCG 3) isolates from flue-cured. VCG and RAPD markers were effective in distinguishing race 2 from race 1, suggesting that there are two genetically differentiated groups of F. oxysporum f. sp. batatas on tobacco in Extremadura.     

Fusarium oxysporum f. sp. lycopersici races and their genetic discrimination by molecular markers in West Mediterranean region of Turkey

Fusarium oxysporum f. sp. lycopersici (FOL) races and F. oxysporum f. sp. radicis lycopersici (FORL), the causal agents of root rot and crown rot diseases, respectively, cause serious economic losses in tomato greenhouses where production is intensive in the West Mediterranean region of Turkey. The isolates were collected from West Mediterranean region of Turkey and were characterized by specific primers based on three races (r1, r2, r3), besides pathogenicity tests in in vivo conditions Additionally, a scheme was developed using newly tested ISSR and SRAP markers to a genotyping database and to determine the possible origin of these pathogens. The present study provided new information on these pathogens based on their races and their dominant existence in this region that has not been reported before. Genetic diversity detected in the same races of the pathogen may be associated with difficulties in controlling the pathogen and a possible resistance formation effort exerted by the pathogen to chemicals used in plant protection in tomato greenhouses. Molecular analyses indicated genetic diversity in pathogen isolates identified as r3, r2 and FORL, which may be associated with abiotic stress to which the pathogens were exposed.     

Factors affecting cross protection of fusarium wilt of pigeon pea by soilborne nonpathogenic fungi

Preinoculation of pigeon pea (Cajanus cajan (L.) Millsp.) seedlings with soilborne fungi nonpathogenic to pigeon pea, namely,Fusarium oxysporum f.sp.niveum; F. oxysporum f.sp.ciceris; F. solani f.sp.pisi; andCephalosporium sacchari, before challenge inoculation with the pathogenFusarium udum, or simultaneous inoculation of the seedlings with nonpathogen and pathogen was effective in controlling wilt of pigeon pea to a great extent. Inoculation with the nonpathogens before the challenge inoculation was more effective than simultaneous inoculation and gave up to 81.6% protection. The higher the spore concentration of the nonpathogenic fungi, the better was the degree of protection. The shorter the period was (in the range of 2–7 days) between nonpathogen inoculation and challenge inoculation, the greater was the degree of protection. Injury to the roots resulted in a decrease of the effect. Maximum disease reduction was obtained when the seedlings were kept at 20-30°C before inoculation; at 5°C and also at 40°C, protection was much less.     

Development of a Fusarium oxysporum f. sp. melonis functional GFP fluorescence tool to assist melon resistance breeding programmes

Fusarium wilt, caused by Fusarium oxysporum f. sp. melonis (Fom), is one of the most widespread and devastating melon diseases. This vascular disease is caused by the colonization of melon xylem vessels by any of the four Fom races reported (r0, r1, r2 and r1,2, subdivided into r1,2w and r1,2y). The macroscopic evaluation of disease symptoms (disease rating, DR) at several days post‐inoculation (dpi) with Fom spores has been the traditional method to determine the resistance of melon accessions to this fungal pathogen. In this study, one isolate from each Fom race was transformed by Agrobacterium tumefaciens to constitutively express the green fluorescent protein (GFP). Fom‐GFP transformants, as virulent as the corresponding wildtype races, were selected to develop an inoculation assay based on the non‐invasive evaluation of the fluorescence emitted by Fom‐GFP. It was determined that melon root neck was the appropriate area to follow Fom‐GFP and a fluorescence signal rating (FSR) was established in parallel to DR determination. This method allowed the evaluation of GFP signal in the root neck of inoculated melon seedlings at 11–15 dpi. The GFP signal was scored in 62 melon accessions/breeding lines inoculated with different Fom‐GFP, followed by evaluation of the macroscopic DR in the aerial part of melon seedlings at 20–28 dpi. Correlation analysis demonstrated a direct and significant relationship between FSR and DR. This method has shown to be an effective and reliable tool that can assist Fom resistance breeding programmes in melon.     

Impaired purine biosynthesis affects pathogenicity of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">melonis</Emphasis>

The vascular wilt pathogen Fusarium oxysporum f. sp. melonis causes worldwide yield losses of muskmelon. In this study, we characterized a UV-induced non-pathogenic mutant (strain 4/4) of F. oxysporum f. sp. melonis, previously identified as a potential biological control agent. During comparative analysis of vegetative growth parameters using different carbon sources, mutant strain 4/4 showed a delay in development and secretion of extracellular enzymes, compared to the wild type strain. Amendments of the growth medium with yeast extract, adenine or hypoxanthine, but not guanine, complemented the growth defect of strain 4/4, as well as secretion and partial activity of cellulases and endopolygalacturonases, indicating that the strain is an adenine auxotroph. Incubation of strain 4/4 conidia in adenine solution, prior to inoculation of muskmelon plants, partially restored pathogenicity to the mutant strain.