A. Nachmias

Biological and immunochemical characterization of a low molecular weight phytotoxin isolated from a protein—lipopolysaccharide complex produced by a potato isolate of Verticillium dahliae Kleb

Abstract A low molecular weight (less than 3000) phytotoxic polypeptide fraction was partially purified from the protein-lipopolysaccharide (PLP) complexes produced in culture by a strain of Verticillium dahliae pathogenic on potato. The toxic fraction was host-specific and antigenically identical to the high molecular weight PLP. Immunodiffusion and immunofluorescence assays showed that a substance is present in stems and tubers of infected potato plants which was antigenically related to the low molecular weight toxin. When used as a tool for screening new potato varieties for tolerance to Verticillium wilt disease, the low molecular weight toxin was more reliable than the PLP because non-specific sensitivity to macromolecules was not observed.

Comparison of protein-lipopolysaccharide complexes produced by pathogenic and non-pathogenic strains of Verticillium dahliae Kleb. from potato☆

Abstract Phytotoxic protein-lipopolysaccharide (PLP) complexes were isolated from dialyzed culture fluids of a pathogenic strain of Verticillium dahliae; PLP complexes which lack phytotoxic activity were isolated from a mutant non-pathogenic strain of the fungus. A comparison of profiles of the wild-type and mutant PLP eluting from an Agarose A-5 m column revealed quantitative differences between the 2 major protein peaks of each strain. The corresponding peaks of the 2 strains had similar molecular weights and in gel immunodiffusion were antigenically indistinguishable, but differences between them were seen in their chemical composition when analysed for protein, lipid and carbohydrate content. Antiserum prepared against the components of peak 1 from the pathogenic strain reacted with an antigen in extracts of Verticillium-infected potato plant tissue which was apparently identical to a moiety produced by the pathogen in culture.

Comparison of Verticillium dahliae-produced phytotoxic peptides purified from culture fluids and infected potato stems

Abstract Phytotoxins produced by a strain of Verticillium dahliae pathogenic on potato have been purified from culture fluids and xylem extracts of potato stems infected by the fungus. The toxins were very similar to each other in molecular mass (approximately 1000 D), HPLC profile, amino acid composition, biological activity and antigenic cross-reactivity. Visible Verticillium wilt symptoms were observed when either peptide, in amounts as low as 20 ng (2 × 10 −6 mol), was injected into detached potato leaves. The differential activity of the peptides on hosts and non-hosts of the fungus, as well as on susceptible and tolerant cultivars, indicated host specific properties. An immunofluorescence assay detected the toxin in potato stems before extensive colonization occurred, suggesting that the toxin may be translocated in advance of the growing hyphae.

SIGNIFICANCE OF THE ROOT SYSTEM IN VERTICILLIUM WILT TOLERANCE IN POTATO AND RESISTANCE IN TOMATO

Reciprocal (susceptible/resistant) grafting experiments were used to determine whether the root is the major site of resistance to Verticillium wilt disease in tomato and tolerance in potato. Increased resistance/tolerance was seen in both species when the resistant cultivar served as rootstock. In potato, however, stem-dipping experiments indicated that tolerance was independent of the root. In experiments designed to determine whether a major gene plays a role in Verticillium wilt tolerance in potato, tomato races 1 and 2 of V. dahliae did not show differential pathogenicity on tolerant potato cultivars inoculated by the root-dip method. The results indicate that the root system plays an important role in the defense mechanisms to Verticillium wilt both in tomato and potato.

VERTICILLIUM WILT OF POTATO IN ISRAEL

Potato early dying, incited byVerticillium dahliae, was found to be the major factor in reducing the yield potential of potato grown under our conditions. Immediate attempts at control involved the screening of commercial cultivars and lines bred in both the USA and Europe, and various soil fumigation treatments. This work led to the selection of cultivars with reasonably high tolerance and the elucidation of the role that the nematodePratylenchus thornei plays in the disease syndrome. Elimination of the nematode by sprinkler applied metham-sodium in conjunction with some of the above cultivars now produces yields of 40–60 metric T/ha, depending on the cultivar and season. More recently, our research has been directed towards the use of serological methods to detect the pathogen in locally produced seed tubers and the utilization of an extracellular toxin produced by the pathogen in determining resistance to the fungus. With regard to the former, we have been able to produce an antibody that reacts solely toV. dahliae and can be used in several serological tests to determine the presence of the pathogen. Our work has also shown that utilization of the toxin offers a highly reliable technique in determining at least certain forms of resistance.ResumenSe descubrió que la muerte prematura de la papa causada porVerticillium dahliae era el factor más importante en la reducción del potencial de rendimiento de la papa cultivada bajo nuestras condiciones. Los intentos inmediatos de control consistieron en el tamizado de líneas y cultivares comerciales mejorados tanto en los EE.UU. como en Europa, y en varios tratamientos de fumigación del suelo. Este trabajo llevó a la selectión de cultivares con una tolerancia razonablemente alta y a la elucidación del papel que juega en el síndrome de la enfermedad el nematodoPratylenchus thornei. La eliminación del nematodo por medio de Metam-Sodium aplicado en aspersión, en combinación con algunos de los cultivares mencionados, tiene ahora como resultado rendimientos de 40 a 60 toneladas métricas por hectárea según el cultivar y la temporada. Ultimamente, nuestra investigación ha sido dirigida hacia la aplicación de métodos serológicos para detectar el patógeno en los tubérculos-semillas producidos localmente y hacia la utilización de una toxina extracelular producida por el patógeno para determinar la resistencia al hongo. Respecto a esto, hemos podido producir un anticuerpo que sólo reacciona conV. dahliae y puede ser utilizado en varias pruebas serológicas para determinar la presencia del patógeno. Nuestro trabajo también ha demostrado que la utilización de la toxina nos ofrece una técnica altamente confiable para determinar por lo menos ciertas formas de resistencia.

Erwinia carotovora subsp.atroseptica infection promotes verticillium wilt development in potato in Israel

SummaryThe interaction betweenErwinia carotovora subsp.atroseptica (Eca) andVerticillium dahliae and its effect on symptom development in potato cultivars showing different degrees of resistance to them was examined over two seasons in irrigated fields in a hot, dry climate. Four cultivars were used: Cara, highly resistant to blackleg and tolerant to Verticillium wilt; Pentland Crown also resistant to blackleg but susceptible to Verticillium wilt; Désirée, moderately susceptible to blackleg but tolerant to Verticillium wilt; and Maris Bard, susceptible to both diseases.Seed tubers were inoculated with Eca immediately after harvest in Scotland, to simulate natural infection, and then planted in Israel in soil naturally infested byV. dahliae. In Verticillium tolerant cultivars, Eca infection increased the rate of colonization byV. dahliae and significantly enhanced symptom development, as assessed by suppression of plant height and wilt symptoms.

Effect of light duration and growth season on verticillium wilt in potato

The tolerance response to Verticillium in potato is affected by external abiotic and biotic factors. Short-day photoperiod enhanced disease level in the susceptible cvs. ‘Nicola’ and ‘Maris Bard’, as well as in the tolerant cvs. ‘Cara’ and ‘Désirée‘. This was observed under field conditions, where plant reaction in the spring was compared with that in autumn. In some cases the disease was more pronounced in autumn, with its shorter days, than in spring, when the days are longer. The effect was measured by disease symptom severity, fungus colonization in the plants, plant height and final yield. This disease enhancement was more pronounced under controlled growth room conditions, where the only different parameter was illumination. A short photoperiod of 8 h increased the disease level relative to 16 h of light.

Assessing progenies of potato for resistance to early blight

SummaryProgenies from crosses between cultivars varying widely in resistance to early blight (Alternaria solani Sor.), were assessed for resistance as true seedlings in a glasshouse in Scotland. The resistance of a representative sample of surviving genotypes from each progeny was compared with samples of the same progenies not previously exposed to the fungus, both in the glasshouse in Scotland and in the field in Israel. The exposed population was more resistant. Resistance was identified more effectively in adult plants from tubers in the glasshouse than in true seedlings and agreement between glasshouse and field assessment was better when progenies were compared rather than individual genotypes. The mid parent and progeny mean scores of the unexposed population were correlated at both sites, thus confirming that the resistance is heritable. Selecting resistant individuals at the seedling stage is suggested as a useful tool for resistance breeding, having first chosen the best parents for crossing.

A 9-year monitoring study of diseases on potato seed tubers imported to Israel

Potato seed tubers are imported annually to Israel from northern Europe. Although the seed is registered as certified, a survey carried out over a 9-year period indicated that most lots were affected by latent or active bacterial and fungal infections. Latent infection byErwinia carotovora subsp.atroseptica, the causal agent of blackleg, at a level of 103 cells/g peel, was present in 30% of the lots in most years. Black scurf caused byRhizoctonia solani was present in 20–70% of the imported lots, with a moderate to high level of infection in all years except 1985. In contrast, although many lots were affected by powdery scab, common scab, and Fusarium dry rot in most years, disease incidence within lots was generally low. The gangrene pathogen (Phoma exigua) was rarely detected. The survey findings are of marked importance, due to the extensive use of soil fumigation in Israeli agriculture.

Diagnosis ofVerticillium dahliae in potato seed tubers with immunoassay techniques

SummaryThree immunoassay techniques were compared with an isolation technique for detectingVerticillium dahliae in seed potato tubers. Immunofluorescent labelling was the most sensitive technique and the percentage infection levels determined for eight field-grown tuber samples were similar to those obtained by a traditional isolation method. Modified Mancini and Ouchterlony assays were less sensitive and gave the lowest values.