Pathogenic Strains Of Fusarium Oxysporum Research Articles

Fusarium oxysporum f. sp. passiflorae infecting passionfruit in New Zealand in a changing taxonomic landscape

Fusarium oxysporum f. sp. passiflorae (FOP) is reported for the first time in Northland, New Zealand. The identity of this host-specific pathogen was confirmed by pathogenicity testing, morphological characters, and DNA sequencing. Pathogenic strains of Fusarium oxysporum secrete unique proteins or effectors, ‘secreted in xylem’ (SIX), which are likely to contribute to host-specific virulence. Sequence analysis of the EF-1a gene, β-tubulin and the effector genes SIX6 and SIX9 confirmed that New Zealand isolates belong to FOP. This study confirmed that the three New Zealand EF-1α haplotypes of FOP had identical SIX6 and SIX9 sequences, indicating that the same homolog of each gene, SIX6a and SIX9a, is shared by both haplotypes of FOP. SIX genes are rarely detected in non-pathogenic strains of Fusarium oxysporum species complex (FOSC) and pathogenicity tests are necessary to confirm its pathogenicity status.

Soil management under tomato-wheat rotation increases the suppressive response against Fusarium wilt and tomato shoot growth by changing the microbial composition and chemical parameters

Crop rotation is a common practice aimed at enhancing both plant disease suppression and crop productivity under open field condition. The objective of the present study was to evaluate the effects of tomato-wheat rotation on Fusarium wilt suppression and shoot growth on tomato in comparison to tomato monoculture. The following factors were determined and integrated through a transdisciplinary approach: (i) soil chemical parameters; (ii) abundance, composition, richness and diversity of soil fungal and bacterial communities; (iii) soil biochemical and enzymatic indicators; (iv) abundance of Fusarium oxysporum isolates in rhizosphere under tomato cultivation and evaluation of the pathogenicity of the isolates; (v) suppressive response of the bulk soil samples against a selected pathogenic strain of Fusarium oxysporum f. sp. lycopersici on tomato; and (vi) tomato shoot growth. The integrated approach included soil amplicon sequencing by metabarcoding analyses carried out by Illumina MiSeq platform in association with the standard procedure for soil analyses. In addition, the pathogenicity of F. oxysporum from the tomato rhizosphere and the suppressive response of the soil bulk to Fusarium wilt were performed by bioassays under greenhouse condition. The results showed that, if durum wheat was rotated with cherry tomato for at least four consecutive years, it had the following beneficial effects: (i) increased suppressive response of the bulk soil samples against F. oxysporum f. sp. lycopersici, (ii) decreased abundance of the F. oxysporum isolates in tomato rhizosphere, (iii) decreased pathogenicity of the F. oxysporum isolates, and (iv) increased tomato shoot growth. The increased abundance of certain microbial groups belonging to potential biocontrol agents against fusaria-related wilt including Bacillales (Bacillus and Paenibacillus), Pseudomonadales (Pseudomonas), Streptomycetales (Streptomyces), Eurotiales (Aspergillus and Penicillium) and Mortierellales (Mortierella), as well as the highest tomato shoot growth, coincided with the end of the second wheat rotation cycle. There were significant Spearman cross-correlations and two-way ANOVA interactions among the microbiome variables with the (i) soil chemical parameters, (ii) abundance and pathogenicity of the F. oxysporum isolates in tomato rhizosphere, (iii) soil suppression to Fusarium wilt, and (iv) tomato shoot growth. The results indicate that a soil managed under tomato-wheat rotation displays increased suppressive response against Fusarium wilt and improved shoot growth in tomato plants by changing the microbial composition and chemical parameters of the soil.

Population structure and linkage disequilibrium in a large collection of Fusarium oxysporum strains analysed through iPBS markers

AbstractIn the current study, 160 pathogenic strains of Fusarium oxysporum collected from tomato, eggplant and pepper were studied. Eighteen inter‐primer binding site (iPBS)‐retrotransposon primers were used, and these primers generated 205 scorable polymorphic bands. The number of polymorphic bands per primer varied between 9 and 19, with a mean of 11 bands per primer. The highest polymorphism information content (PIC) value was determined as 0.27, and the lowest was 0.05. The unweighted pair‐group method with arithmetic averages (UPGMA) dendrogram including a heat map revealed that the 160 pathogenic strains of F. oxysporum were divided into two main clusters. The first cluster mainly included F. oxysporum f. sp. capsici (FOC) and F. oxysporum f. sp. melongenae (FOMG) isolates. The second cluster mainly comprised F. oxysporum f. sp. lycopersici (FOL) and F. oxysporum f. sp. radicis lycopersici (FORL) isolates. The highest percentage of loci in significant linkage disequilibrium (LD) was detected for FOL, whereas the lowest level of LD was found for FOC, and 95.2%, 99.4%, 99.1% and 97.4% of the relative kinship estimates were less than 0.4 for FOL, FOMG, FORL and FOC, respectively. LD differences were detected among formae speciales, and LD was higher in FOL as compare to FOC species. The findings of this study confirm that iPBS‐retrotransposon markers are highly polymorphic at the intraspecific level in Fusarium spp.

Host-Induced Silencing of Pathogenicity Genes Enhances Resistance to Fusarium oxysporum Wilt in Tomato.

This study presents a novel approach of controlling vascular wilt in tomato by RNAi expression directed to pathogenicity genes of Fusarium oxysporum f. sp. lycopersici. Vascular wilt of tomato caused by Fusarium oxysporum f. sp. lycopersici leads to qualitative and quantitative loss of the crop. Limitation in the existing control measures necessitates the development of alternative strategies to increase resistance in the plants against pathogens. Recent findings paved way to RNAi, as a promising method for silencing of pathogenicity genes in fungus and provided effective resistance against fungal pathogens. Here, two important pathogenicity genes FOW2, a Zn(II)2Cys6 family putative transcription regulator, and chsV, a putative myosin motor and a chitin synthase domain, were used for host-induced gene silencing through hairpinRNA cassettes of these genes against Fusarium oxysporum f. sp. lycopersici. HairpinRNAs were assembled in appropriate binary vectors and transformed into tomato plant targeting FOW2 and chsV genes, for two highly pathogenic strains of Fusarium oxysporum viz. TOFOL-IHBT and TOFOL-IVRI. Transgenic tomatoes were analyzed for possible attainment of resistance in transgenic lines against fungal infection. Eight transgenic lines expressing hairpinRNA cassettes showed trivial disease symptoms after 6-8weeks of infection. Hence, the host-induced posttranscriptional gene silencing of pathogenicity genes in transgenic tomato plants has enhanced their resistance to vascular wilt disease caused by Fusarium oxysporum.

Evaluation of the significance of cell wall polymers in flax infected with a pathogenic strain of Fusarium oxysporum.

BackgroundFusarium oxysporum infection leads to Fusarium-derived wilt, which is responsible for the greatest losses in flax (Linum usitatissimum) crop yield. Plants infected by Fusarium oxysporum show severe symptoms of dehydration due to the growth of the fungus in vascular tissues. As the disease develops, vascular browning and leaf yellowing can be observed. In the case of more virulent strains, plants die. The pathogen’s attack starts with secretion of enzymes degrading the host cell wall. The main aim of the study was to evaluate the role of the cell wall polymers in the flax plant response to the infection in order to better understand the process of resistance and develop new ways to protect plants against infection. For this purpose, the expression of genes involved in cell wall polymer metabolism and corresponding polymer levels were investigated in flax seedlings after incubation with Fusarium oxysporum.ResultsThis analysis was facilitated by selecting two groups of genes responding differently to the infection. The first group comprised genes strongly affected by the infection and activated later (phenylalanine ammonia lyase and glucosyltransferase). The second group comprised genes which are slightly affected (up to five times) and their expression vary as the infection progresses. Fusarium oxysporum infection did not affect the contents of cell wall polymers, but changed their structure.ConclusionThe results suggest that the role of the cell wall polymers in the plant response to Fusarium oxysporum infection is manifested through changes in expression of their genes and rearrangement of the cell wall polymers. Our studies provided new information about the role of cellulose and hemicelluloses in the infection process, the change of their structure and the expression of genes participating in their metabolism during the pathogen infection. We also confirmed the role of pectin and lignin in this process, indicating the major changes at the mRNA level of lignin metabolism genes and the loosening of the pectin structure.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0762-z) contains supplementary material, which is available to authorized users.

Determination of the <i>In vitro</i> Effect of <i>Trichoderma harzianum</i> on Phytopathogenic Strains of <i>Fusarium oxysporum</i>

Fusarium oxysporum is a well-known soil-borne fungi and it is difficult to control their pathogenic strains by conventional strategies. The cultures of two strains of Trichoderma harzianum (T16 and T23) were examined in laboratory conditions and with pot experiments for the control of pathogenic strains of Fusarium oxysporum f. sp. melongenae (Fomg), Fusarium oxysporum f. sp. lycopersici (Fol), Fusarium oxysporum f. sp. niveum (Fon) and F. oxysporum f. sp. melonis (Fom). The T16 and T23 strains showed significant inhibition of mycelial growth in the pathogenic strains of F. oxysporum and the maximum inhibition were recorded when the T. harzianum strain T16 was used (72.69%). Both T. harzianum strains produced volatile and non-volatile metabolites that inhibited growth of F. oxysporum strains on PDA medium. In vitro colonization study demonstrated the root-colonizing ability of these antagonists. The interaction between T. harzianum isolates (T16 and T23) and pathogenic F. oxysporum hyphae showed no overgrowth, hyphal coiling, cell wall degradation or any hyphal penetration around any of the tested F. oxysporum hyphae. Pre-treatment of soil with T16 significantly reduced the severity of Fusarium wilt disease. The disease severity in control plants reached to 90-95% whereas those of the T16-Fomg and T16-Fol treated seedlings of eggplants were 37.74% and 47.12%, respectively, on the 21st day. In this study, while both T. harzianum isolates had a considerable antagonistic effect on the tested pathogens, T16 was found to be more successful than T23. The strong repressive effect of T. harzianum (T16) towards pathogenic Fusarium oxsporum can be applied in biological control of these pathogens.

Determination of the <i>In vitro</i> Effect of <i>Trichoderma harzianum</i> on Phytopathogenic Strains of <i>Fusarium oxysporum</i>

Fusarium oxysporum is a well-known soil-borne fungi and it is difficult to control their pathogenic strains by conventional strategies. The cultures of two strains of Trichoderma harzianum (T16 and T23) were examined in laboratory conditions and with pot experiments for the control of pathogenic strains of Fusarium oxysporum f. sp. melongenae (Fomg), Fusarium oxysporum f. sp. lycopersici (Fol), Fusarium oxysporum f. sp. niveum (Fon) and F. oxysporum f. sp. melonis (Fom). The T16 and T23 strains showed significant inhibition of mycelial growth in the pathogenic strains of F. oxysporum and the maximum inhibition were recorded when the T. harzianum strain T16 was used (72.69%). Both T. harzianum strains produced volatile and non-volatile metabolites that inhibited growth of F. oxysporum strains on PDA medium. In vitro colonization study demonstrated the root-colonizing ability of these antagonists. The interaction between T. harzianum isolates (T16 and T23) and pathogenic F. oxysporum hyphae showed no overgrowth, hyphal coiling, cell wall degradation or any hyphal penetration around any of the tested F. oxysporum hyphae. Pre-treatment of soil with T16 significantly reduced the severity of Fusarium wilt disease. The disease severity in control plants reached to 90-95% whereas those of the T16-Fomg and T16-Fol treated seedlings of eggplants were 37.74% and 47.12%, respectively, on the 21st day. In this study, while both T. harzianum isolates had a considerable antagonistic effect on the tested pathogens, T16 was found to be more successful than T23. The strong repressive effect of T. harzianum (T16) towards pathogenic Fusarium oxsporum can be applied in biological control of these pathogens.

Induction of Systemic Resistance of Benzothiadiazole and Humic Acid in Soybean Plants Against Fusarium Wilt Disease

The ability of benzothiadiazole (BTH) and/or humic acid (HA) used as seed soaking to induce systemic resistance against a pathogenic strain of Fusarium oxysporum was examined in four soybean cultivars under greenhouse conditions. Alone and in combination the inducers were able to protect soybean plants against damping-off and wilt diseases compared with check treatment. These results were confirmed under field conditions in two different locations (Minia and New Valley governorates). The tested treatments significantly reduced damping-off and wilt diseases and increased growth parameters, except the number of branches per plant and also increased seed yield. Application of BTH (0.25 g/L) + HA (4 g/L) was the most potent in this respect. Soybean seed soaking in BTH + HA produced the highest activities of the testes of oxidative enzymes followed by BTH in the four soybean cultivars. HA treatment resulted in the lowest increases of these oxidative enzymes. Similar results were obtained with total phenol but HA increased total phenol more than did BTH in all tested cultivars.

Induction of systemic resistance in soybean plants against Fusarium wilt disease by seed treatment with benzothiadiazole and humic acid

The ability of benzothiadiazole (BTH), humic acid (HA) and their combination when used in seed soaking to induce systemic resistance against a pathogenic strain of Fusarium oxysporum was examined in four soybean cultivars under greenhouse conditions. Both inducers and their combination were able to protect soybean plants against damping-off and wilt diseases compared with check treatment. These results were confirmed under field conditions in two different locations that is, Minia and New Valley governorates, Egypt. The tested treatments significantly reduced damping-off and wilt diseases and increased growth parameters, except number of branches plant-1, and seed yield. Application of BTH (0.25) + HA (4 g/L) was the most potent in respect to this treatment. Soybean seed soaking in BTH+HA recorded the highest activities of oxidative enzymes followed by BTH in four soybean cultivars. Whereas, HA treatment recorded the lowest increase of these oxidative enzymes. Also, similar results were obtained in case of total phenol but HA increased the total phenol more than BTH in all tested cultivars. Key words: Induce systemic resistance, soybean, wilt disease, benzothiadiazole (BTH), humic acid (HA).

Induction of Systematic Resistance in Soybean Plants against Fusarium Wilt Disease by Seed Treatment with Benzothiadiazole and Humic Acid

The ability of benzothiadiazole (BTH), humic acid (HA) and their combination when used as seed soaking to induce systemic resistance against a pathogenic strain of Fusarium oxysporum was examined in four soybean cultivars under greenhouse conditions. Both inducers and their combination were able to protect soybean plants against damping-off and wilt diseases compared with check treatment. These results were confirmed under field conditions in two different locations i. e Minia and New Valley governorates. The tested treatments significantly reduced damping-off and wilt diseases and increased growth parameters, except number of branches plant-1, and seed yield. Application of BTH (0.25) + HA (4 g/l) was the most potent in this respect treatment. Soybean seed soaking in BTH+ HA recorded the highest activities of the testes of oxidative enzymes followed by BTH in the four soybean cultivars. Whereas, HA treatment was recorded the lowest increased of these oxidative enzymes. Also, similar results were obtained in case of total phenol but HA increased the total phenol more than BTH in all tested cultivars.

Glomus fasciculatum in defense responses to fusarial wilt of Coleus forskohlii

The activities of enzymes responsible for disease resistance in Coleus forskohlii, pre-inoculation with an arbuscular mycorrhizal fungus, Glomus fasciculatum, and/or infection with a pathogenic strain of Fusarium oxysporum, and the biological control effect of G. fasciculatum on fusarial wilt in Coleus forskohlii were investigated. The experiment was carried out with four treatments, viz. plants pre-inoculated with G. fasciculatum, plants pre-inoculated with G. fasciculatum followed by infection with F. oxysporum, plants infected with F. oxysporum, and plants without any of the two microorganisms. Mycorrhization (%) was reduced in pathogen-challenged plants. In G. fasciculatum- and F. oxysporum- treated plants, an increase in phenolic content was observed on the 20th day while the maximum increase in phenylalanine ammonia-lyase activity was observed on the 15th day after challenge inoculation with F. oxysporum compared with that of other treatments. In G. fasciculatum- and F. oxysporum-treated plants, an increase in peroxidase and polyphenol oxidase activity was observed on the 15th day after pathogen challenge compared with that of other treatments. The observation revealed that the arbuscular mycorrhizal fungus Glomus fasciculatum systemically induced resistance against wilt disease of Coleus forskohlii via production of phenolics, the defence enzymes phenylalanine ammonia-lyase and polyphenol oxidase, and pathogenesis-related proteins, and peroxidase in response to Fusarium oxysporum infection. G. fasciculatum reduced the disease severity and promoted plant growth significantly (P<0.05) in single-treated and in combination with F. oxysporum-treated plants compared with control plants. The results suggested that G. fasciculatum is a potentially effective protection agent against F. oxysporum.

Lack of biocontrol capacity in a non-pathogenic mutant of Fusarium oxysporum f. sp. melonis

The aim of this study was to assess the biocontrol capacity of rev157, a non-pathogenic mutant of a pathogenic strain of Fusarium oxysporum f. sp. melonis (Fom24). Inoculated in association with the virulent parental strain, the mutant rev157 did not protect the host plant (muskmelon) against infection by Fom24. Applied on flax, a non-host plant, the mutant rev157 was not able to protect it against its specific pathogen F. oxysporum f. sp. lini. On the contrary the parental strain Fom24 did protect flax as well as a soil-borne biocontrol strain (Fo47). Since the mutant rev157 was affected neither in its growth in vitro nor in its capacity to penetrate into the roots, it can be speculated that the mutation has affected traits responsible for interactions within the plant. In F. oxysporum the pair of strains Fom24/rev157 is a good candidate to identify genes involved in the biocontrol capacity of F. oxysporum and to test the hypothesis of a link between capacity to induce the disease and capacity to induce resistance in the plant.

(268) Nonspecific Fusarium Wilt Resistance in Watermelon Using Copper Inducible IPT Gene

`Crimson Sweet' watermelon was transformed with a copper inducible ipt gene. Clonally propagated transformed and non-transformed plants were sprayed with three different concentrations (0, 50, 100 mm) of CuSO4 at the 2–3 leaf stage twice in a 24-hour period prior to their inoculation with fusarium wilt organisms. Plants were inoculated via root dip with two different isolates of Fusariumoxysporum sp. niveum Race 2. The pathogenic strains of Fusariumoxysporum sp. niveum Race 2, Fl 99-1, and Calg 15-19, were cultured on PDA solid medium and then transferred to a sterile flask filled with 50 mL of potato dextrose broth (PDB) liquid medium. These flasks were placed on a shaker at 100 rpm for 4–5 days before the inoculation date and the final concentration of 106 spores/mL was adjusted for inoculation. Roots of the experimental units were gently washed and were then infected by dipping them in to the beaker containing the isolates for 1 min. Inoculated plants were transferred to planting trays and maintained under growth chamber conditions. Plants then were watered as needed. Fusarium wilt symptoms initially appeared approximately 7–10 days after the infection. The Cu-ipt transformants exhibited a clear and significant resistance over non-transformed plants. The severity of disease development was relatively higher in the control Cu-ipt and non-transformed plants when compared to the plants treated with CuSO4. The control Cu-ipt plants were comparatively healthier than the control non-transformed plants. Infected plants were removed from soil 2 weeks after inoculation, washed and the stems were cut vertically for rating of browning of the vascular system.

Fusarium control by Fusarium

To control Fusarium wilt disease in an environmentally friendly way, beneficial strains of Fusarium are being used to control plant pathogenic strains of Fusarium oxysporum. Deborah R. Fravel (Agriculture Research Service, Beltsville, MD, USA) and George Lazarovits (Agriculture and Agri-Food Canada, Ontario, Canada) found one strain, CS-20, which reduced wilt by 49.6%. Mixing beneficial strains of a fungus (Trichoderma virens strain G1-3) and a bacterium (Burkholderia vietnamiensis strain Bc-F) also reduced wilt incidence by 41.6%. The mechanism of biocontrol might rely on induced systemic resistance. http://www.ars.usda.gov/is/pr/2000/000714.htm

Process of tomato root colonization by a pathogenic strain of Fusarium oxysporum f. sp. lycopersici in comparison with a non‐pathogenic strain

A pathogenic strain of Fusarium oxysporum f. sp. lycopersici transformed with the glucuronidase (GUS) reporter gene was used to study the colonization process of tomato (Lycopersicon esculentum) roots in hydroponic culture. The plants were treated exactly as those used in a different study with a non‐pathogenic strain of F. oxysporum in order to compare the two types of colonization process. The pathogenic strain rapidly colonized the root surface, forming a dense network of hyphae – shown by the GUS staining assay – as early as 48 h after inoculation. The first images of the pathogen penetrating into the epidermis of the root were observed 24 h after inoculation. The plant showed defence reactions mainly in the hypodermis, but also in the cortex. Generally these barriers failed to prevent the centripetal growth of the pathogen towards the stele. The GUS activity moved with the actively growing hyphae into the tissues; the stele appeared intensely stained 7 d after inoculation, whereas by this time the hyphae at the root surface did not show any more staining. The hyphae formed a dense network at the apex of the root, but direct contact between hyphae and living cells was prevented by several layers of sloughed cap cells. Although rarely observed, penetration of the pathogen into the apex was possible and led to a rapid destruction of apical cells. The only differences between this pattern of root colonization and that observed for a non‐pathogenic strain of F. oxysporum appear to concern the frequency of dead apices and the intensity of fungal colonization in the cortex. When the pathogen passed around the barrier formed in the hypodermis it always reached the xylem, although this barrier and other defence reactions induced at different levels in the cortex always prevented the non‐pathogenic strain from reaching the stele. These observations suggest that the main differences between the two types of interaction – between the plant and either the pathogenic or the non‐pathogenic strain – are quantitative rather than qualitative.

Extracellular hydrolytic enzyme production by pathogenic strains of Fusarium oxysporum f.sp. elaeidis

Extracellular pectin esterase, polygalacturonase, C x cellulase and C 1 cellulase enzyme activities were detected in culture filtrates of F. oxysporum f.sp. elaeidis strains pathogenic to oil palm. The ten strains examined secreted varying amounts of these enzymes. Generally, enzyme production and pH of the culture medium increased with incubation period and attained maximum values between the 5th and 7th day. Optimum production of pectin methylesterase and polygalacturonase occurred under alkaline conditions (pH 7·1–8·4), whereas a slightly acidic condition (pH 5·7–6·0) favoured C x cellulase and C 1 cellulase production. Polygalacturonase and C x cellulase were constitutively secreted although there was increased synthesis in presence of their specific substrates.

The Use of Nitrate-Nonutilizing Mutants and a Selective Medium for Studies of Pathogenic Strains of Fusarium oxysporum

A chlorate-containing selective medium was developed for tracing chlorate-resistant, nitrate-nonutilizing (nit) mutants of pathogenic strains of Fusarium oxysporum. Neither wild-type strains of Fusarium spp. nor most of the fungi from six field soils could grow on this medium, whereas most nit mutants of six formae speciales grew on it. The nit mutants retained their pathogenicity. Survival in soil of nit mutants belonging to three formae speciales was comparable to that of their wild-type parents

Genetic relationships between pathogenic strains of Fusarium oxysporum, Fusarium solani and an isolate of Nectria haematococca

The Fusarium state of an isolate of N. haematococca Berk. & Br. that occurred in a mutant culture of F. oxysporum readily formed stable heterokaryons with isolates of F. o. f. pisi , f. lycopersici , and f. cubense and F. solani f. pisi and f. phaseoli . This indicates a close genetical relationship between these Fusaria and between them and N. haematococca which, like F. oxysporum f. pisi , causes wilt of Onward pea. Heterokaryons also formed between different formae speciales of pathogenic F. oxysporum . In addition, genetic crossing was achieved in culture between the Nectria and four strains of F. o. f. pisi and with two of F. o. f. lycopersici (cause of tomato wilt). Nutritional deficiencies induced by ultraviolet irradiation, wild-type differences in perithecial production and differences in pathogenicity for pea or tomato were used as genetic markers. In 931 random single-ascospore cultures from single perithecia resulting from crosses, recombinations were demonstrated between the Nectria , the isolates of F. oxysporum and those of F. solani . All three types of markers, including specific pathogenicity, were interchangeable between the parentals of each cross. Single-ascospore cultures from perithecia arising on colonies from spore mixtures of F. oxysporum and conidial Nectria produced perithecia in 48–94% of the colonies, depending on which F. oxysporum strain was used as parent, whereas single-ascospore colonies from the homothallic Nectria were always perithecial. When Nectria was crossed with a mildly virulent mutant of F. o . f. pisi , the resulting single-ascospore cultures had a decreased ability to cause wilt. Crossing Nectria with F. s. f. pisi indicated that, although several cultural characters were recombined, no recombination occurred between the ability to cause pea wilt ( Fusarium state of Nectria ) and the ability to cause foot rot ( F. solani ). This may mean that these abilities are either genetically allelic or closely linked ; hence, a distinction that can be made between the two Fusaria by means of pathogenicity tests is likely to be more reliable than any comparisons of cultures. These results do not necessarily imply that N. haematococca is the wild-type perfect state of F. oxysporum or of F. solani . The relationships established experimentally between these fungi are discussed with reference to their pathogenicity and cultural morphology.

Absence of pectolytic enzymes in a pathogenic strain of Fusarium oxysporum f. lycopersici.

THE pectic enzymes produced by Fusarium oxysporum Schl. emend. Sny. and Han. f. lycopersici (Sacc.) Sny. and Han. have been considered to play an important part in the production of symptoms and of wilting in tomato plants infected with Fusarium 1. It has been suggested that pectic enzymes are not responsible for the full range of symptoms2,3 and that other metabolites may be involved. The pectic enzymes have also been considered important for the invasion of a wide range of plants by fungal parasites2.

Heterokaryosis and parasexual recombination in pathogenic strains of Fusarium oxysporum.

SUMMARY: Mutants of Fusarium oxysporum f. pisi (cause of pea wilt), distinguished from the parent types by their different morphology, pathogenicity and nutritional requirements, were obtained by ultraviolet irradiation. Pairs of mutants with different nutritional requirements (auxotrophs) formed balanced heterokaryons on non-supplemented medium. Most of the auxotrophs were less pathogenic than the wild-types, whereas heterokaryons between the mutants were not. Single conidia of a heterokaryon between an auxotroph from race 1 and one from race 2 of Fusarium oxysporum gave a small proportion (3 in 108) of colonies which were able to grow on non-supplemented medium. These three prototrophs were presumably diploid because vegetatively they gave new strains with various combinations of colour, nutritional requirements, actinomycete tolerance and pathogenicity. The results indicate that, in fusaria, heterokaryosis plays a part in variation of virulence and that Fusarium oxysporum, which has no known sexual stage, has a system similar to the parasexual cycle described by Pontecorvo for certain other Fungi Imperfecti, that permits the segregation and recombination of genetic factors outside the sexual stage.