Host-Parasite Relationship in a Susceptible and a Resistant Rose Cultivar Inoculated with Sphaerotheca pannosa: I. Fungal Growth, Mechanical Barriers and Hypersensitive Reaction

Conidial germination and development of Sphaerotheca pannosa were studied on a susceptible and a resistant cultivar (rose Cardinal and rose Queen Elisabeth, respectively) in relation to the morphology of the leaf surface which was examined by scanning and transmission electron microscopy. Conidial germination was completed within 24 h in both cultivars and seemed to be higher in the resistant cultivar; fungal development, however, was stopped when the haustoria containing cells became necrotic. The only morphological difference between the leaves of the two cultivars is a thinner cuticle in rose Cardinal. Wax deposits on the leaf surface are similar; a layer of substances reacting to rhutenium red, most likely glycoproteins or protein-polysaccharide complexes, also are similar.     

Biochemical and bioassay analysis of resistance of potato (Solanum tuberosum L.) cultivars to attack by the slug Deroceras reticulatum (Muller)

A method is described for using young field slugs Deroceras reticulatum (Muller) in a bioassay study of biochemical resistance of potato (Solanum tuberosum L.) cultivars to slugs. Tuber parts or an artificial diet were provided as food sources. Comparisons were made of feeding, survival and weight gain between the susceptible cultivar Maris Piper and the resistant cultivar Pentland Dell. Biochemical analyses were made of these two cultivars and the resistant cultivars Stormont Enterprise and Majestic. Comparisons of tuber sections and peelings as food sources indicated factors affecting growth were located in the surface layers of the tubers. Phenolics and glycoalkaloids were concentrated in the surface layers but the amounts were similar in the susceptible and resistant cultivars and the bioassays indicated that neither acting alone could explain resistance. The amounts and distribution of free amino acids also did not correlate with resistance although when supplied in the artificial diet they partly inhibited feeding. Proteinaceous inhibitors of slug gut proteolytic enzymes were present throughout the tubers but were not concentrated in the surface layers and the amounts were similar in the different cultivars thus they too did not explain the difference in susceptibility between the cultivars. Bioassays using acetone extracts (low molecular weight substances) and acetone powders (high molecular weight substances) either alone or in combination indicated that the resistant cultivar Pentland Dell contained a high molecular weight substance which together with a low molecular weight substance from either the same cultivar or the susceptible Maris Piper could confer resistance. Bioassays using protein extracts supplied in the presence or absence of chlorogenic acid indicated that this mechanism could comprise enzymic oxidation of phenolics. Assays of phenolase confirmed this since activity was highest in the outer layers of the tubers and was highest in the three resistant cultivars. Thus the chief resistance factor identified was high phenolase activity acting rapidly on phenolics when the slug first bites the tuber surface. The quantity of phenolics per se did not control the resistance. Thus while phenolics must be available, resistance is compatible with low blackening on cutting the tuber.     

Changes in total phenolic contents of pearl millet callus resistant and susceptible to downy mildew

Tissue cultures were used as the host-parasite systems to study the total phenolics in relation to downy mildew infection in pearl millet. Total phenolics increased due to infection bySclerospora graminicola. As the age of the callus tissue increased, an increase in total phenolics was observed in both susceptible and resistant cultivars. The possibility of using a dual culture to study the biochemical changes occurring in the host-parasite interactions of an obligate parasite is discussed.     

Cell Wall-Bound Phenolic Acid and Lignin Contents in Date Palm as Related to its Resistance to Fusarium Oxysporum

The root cell walls of the resistant cultivars of the date palm were more resistant to the action of the cell wall-degrading enzymes (CWDE) of Fusarium oxysporum f. sp. albedinis than those of the susceptible cultivars. Date palm roots contain four cell wall-bound phenolics identified as p-hydroxybenzoic acid, p-coumaric acid, ferulic acid and sinapic acid. The contents of p-coumaric acid and ferulic acid in the resistant cultivars (IKL, SLY, BSTN) were about 2 times higher than those in the susceptible cultivars (BFG, JHL, BSK). The contents of p-hydroxybenzoic acid and sinapic acid in the resistant cultivars were 8.4 and 4.5 times, respectively, higher than those in the susceptible cultivars. The lignin contents in roots of the resistant cultivars were 1.8 times higher than those of the susceptible cultivars. The cell wall-bound phenols accumulated particularly in resistant cultivars reduced strongly the mycelial growth and the CWDE production in vitro.     

Histochemical Detection and Role of Phenolic Compounds in the Defense Response of Lactuca spp. to Lettuce Downy Mildew (Bremia lactucae)

The occurrence of phenolic compounds (PC) in the defence reaction of Lactuca spp. was detected by histochemical methods. Four staining methods, including three for light and one for fluorescence microscopy, were used for imaging the location of PC in cells of genotypes with different resistance mechanisms after infection by Bremia lactucae , race NL16. The results showed that the major role of phenolic compounds in studied Lactuca spp. is connected with their overexpression and localized accumulation during a hypersensitive response (HR). In incompatible interactions a slight accumulation of phenols near the cell wall of infected cells was detected. The negative reaction to staining with aniline sulphate verified the absence of lignin creation. In both compatible and incompatible interactions structural modifications in the host cells occurred as a callose deposition. Frequently, these deposits were widespread in susceptible genotypes. Intensive and rapid accumulation of autofluorescent phenolics was linked with the onset of HR, the main cytological feature of resistance to lettuce downy mildew in Lactuca spp.     

Characterisation of powdery mildew resistance in a segregating diploid rose population

Powdery mildew (Podosphaera pannoso) is one of the most serious fungal diseases on both greenhouse and field grown roses. Improvement of disease resistance is a major selection aim for garden rose breeders. For rose cultivars, being mostly tetraptoid, it is complicated to develop molecular markers for resistance. Hence, a segregating diploid population was established from a cross between 'Yesterday', a commercial available rose variety susceptible to powdery mildew, and R. wichurana, a rose species with resistance to certain isolates of powdery mildew. A progeny of 94 seedlings was planted in the field. The segregation of powdery mildew resistance was studied in this population by means of a bioassay with two different monoconidial isolates of powdery mildew. Based on the response to these inoculations different groups were selected: a first group of genotypes was susceptible to both isolates, other groups were susceptible to one of both isolates and a last group was resistant to both tested isolates. The disease resistance inherits for both isolates in a quantitative way. A genetic map based on AFLP and SSR markers was established and will be used for QTL analysis of powdery mildew resistance.     

New Races of Sunflower Downy Mildew (Plasmopara Halstedii) in Germany

Plasmopara halstedii, the causal agent of downy mildew of cultivated sunflower (Helianthus annuus), was documented in Germany for the first time in commercial fields. The pathogen was first observed in the Württemberg area, where races 1 and 4 were identified using a set of differential lines. Later, commericial fields near Baden were found to be infected by race 5, which is the first occurrence of that race outside of North America. Withthe discovery of race 5, there are now eight races of the sunflower downy mildew fungus that have been found in Europe. The sunflower cultivars most frequently grown in Germany were investigated for resistance to race 1, 4 and 5; while all were resistant to race 1, none were resistant to either race, 4 or 5.     

New Races of Sunflower Downy Mildew (Plasmopara Halstedii) in Germany

Plasmopara halstedii, the causal agent of downy mildew of cultivated sunflower (Helianthus annuus), was documented in Germany for the first time in commercial fields. The pathogen was first observed in the Württemberg area, where races 1 and 4 were identified using a set of differential lines. Later, commercial fields near Baden were found to be infected by race 5, which is the first occurrence of that race outside of North America. With the discovery of race 5, there are now eight races of the sunflower downy mildew fungus that have been found in Europe. The sunflower cultivars most frequently grown in Germany were investigated for resistance to race 1, 4 and 5; while all were resistant to race 1, none were resistant to either race 4 or 5.     

Cell wall alterations in the leaves of fusariosis-resistant and susceptible pineapple cultivars

Fusariosis, caused by the fungus Fusarium subglutinans f. sp. ananas (Syn. F. guttiforme), is one of the main phytosanitary threats to pineapple (Ananas comosus var. comosus). Identification of plant cell responses to pathogens is important in understanding the plant–pathogen relationship and establishing strategies to improve and select resistant cultivars. Studies of the structural properties and phenolic content of cell walls in resistant (Vitoria) and susceptible (Perola) pineapple cultivars, related to resistance to the fungus, were performed. The non-chlorophyll base of physiologically mature leaves was inoculated with a conidia suspension. Analyses were performed post-inoculation by light, atomic force, scanning and transmission electron microscopy, and measurement of cell wall-bound phenolic compounds. Non-inoculated leaves were used as controls to define the constitutive tissue characteristics. Analyses indicated that morphological differences, such as cell wall thickness, cicatrization process and lignification, were related to resistance to the pathogen. Atomic force microscopy indicated a considerable difference in the mechanical properties of the resistant and susceptible cultivars, with more structural integrity, associated with higher levels of cell wall-bound phenolics, found in the resistant cultivar. p-Coumaric and ferulic acids were shown to be the major phenolics bound to the cell walls and were found in higher amounts in the resistant cultivar. Leaves of the resistant cultivar had reduced fungal penetration and a faster and more effective cicatrization response compared to the susceptible cultivar.     

Evaluation of methods of screening strawberry cultivars for resistance to crown rot caused by Phytophthora cactorum

Four methods were evaluated in measuring resistance of strawberry cultivars to crown rot caused by Phytophthora cactorum. Meristem propagated plants grown in vitro were inoculated with mycelial discs. Four to five days after inoculation, it was possible to distinguish between cultivars with large differences in susceptibility to the disease. Ten days later, all plants were totally necrotic making it impossible to distinguish between cultivars. When detached leaves were inoculated by inoculating a plug of mycelium into the petiole, disease symptoms developed more slowly in resistant cultivars, but leaf age greatly affected the rate of symptom development. When plug plants (not cold stored) were lightly wounded in the rhizome with a scalpel and inoculated with either zoospores or mycelium, differences in disease development between cultivars were mainly as would be expected from previous information on susceptibility, but both age and size of plants influenced the rate of disease development. Unwounded, inoculated plants did not develop symptoms. When cold‐stored plug plants were either unwounded or lightly wounded with a scalpel in the rhizome and inoculated with zoospores, the relative rates of disease development consistently reflected the susceptibility to crown rot. At the time of final assessment, disease was much more severe in wounded plants, but the relative susceptibility of cultivars was not affected by the wounding.     

The Interaction of Either an Effective or a Defeated Major Gene with Non-specific Resistance on Mildew Infection (Erysiphe graminis f. sp. hordei) and Yield in Mixtures of Barley

Three-component mixtures of two barley cultivars expressing partial resistance to mildew ( Erysiphe graminis f. sp. hordei ) with the third component either expressing complete resistance or a defeated major gene gave more yield than the mean of their components in monoculture in a series of field trials. However, there was no reduction in mildew infection in the mixtures and even a significant increase in one trial. The potential for using partially resistant cultivars in mixtures is discussed.     

Association of the Production of Phenylpropanoid Compounds at the Infection Sites of Corynespora cassiicola with Soybean Resistance against Target Spot

This study investigated, at the microscopic level, whether the differential defence responses of soybean cultivars that are resistant (Fundacep 59) and susceptible (TMG 132) to target spot, caused by Corynespora cassiicola, could be associated with an increase in the production of phenolics, flavonoids and lignin at the infection sites. Many larger necrotic lesions with yellow halos were noticed on the leaves of plants from cultivar TMG 132, in contrast to the leaves of plants from cultivar Fundacep 59. Necrotic lesions also developed on the petioles of leaves of plants from cultivar TMG 132, while on the petioles and veins of leaves of plants from cultivar Fundacep 59, the lesions were of purple colour. The growth of fungal hyphae was reduced on the leaves of plants from cultivar Fundacep 59, and an apparently high density of trichomes was found in comparison with the leaves of plants from cultivar TMG 132. An appressorium‐like structure was produced at one or both extremities of the conidium of C. cassiicola, preferentially at the major and minor veins on the adaxial leaf surface of plants from both cultivars. Most cells on the leaves of plants from cultivar Fundacep 59 reacted against C. cassiicola infection by accumulating phenolic‐like compounds, which contributed to the death of many fungal hyphae and a greater maintenance of cell integrity. In contrast, fungal hyphae grew without any impedance in the leaf cells of plants from cultivar TMG 132, which was associated with signs of intense leaf tissue disorganization. Stronger autofluorescence and deposition of lignin and flavonoids were found in the cells of leaves of plants from cultivar Fundacep 59, in contrast to cultivar TMG 132. It can be concluded that soybean resistance to target spot is probably dependent on the activation of the phenylpropanoid pathway.     

On the Enhanced Callose Deposition in Barley with ml-o Powdery Mildew Resistance Genes

Carborundum treatment of barley leaves induced a callose deposition which was detected as diffuse blotches in the epidermal cells of susceptible barleys and as deeply stained tracks along the scratches in barleys with the ml-o powdery mildew resistance gene. Subsequent inoculation with powdery mildew resulted in appositions that enlarged inversely to their size in the respective varieties when inoculated without carborundum treatment. Aphids sucking the leaves resulted in rows of callose containing spots along the anticlinal cell walls. The spots were larger in the ml-o mutant than in the mother variety. Callose was deposited in connection with the pleiotropic necrotic spotting in barleys with the ml-o gene. Modification of the necrotic spotting by crossing the ml-o gene into other gene backgrounds did not result in any change in the size of appositions upon inoculation with powdery mildew. Callose deposition was never observed in cells subsidiary to the guard cells, and the absence of callose in such cells is suggested to be the reason for the sporadic occurrence of powdery mildew colonies on barleys with the ml-o gene.     

Resistance to powdery mildew in rose

Resistance and susceptibility of rose species and varieties to the powdery mildew fungus Sphaerotheca pannosa were tested on detached leaflets in Petri plates. Strains of the pathogen from greenhouse plants were maintained under aseptic conditions on rose shoots growing in agar medium in glass vials. Biological specialization in S. pannosa was confirmed; the strain on Rosa virginiana did not infect common rose varieties. Variation in susceptibility of commonly cultivated rose varieties to another strain of mildew was demonstrated, and resistance increased with age of leaf. Germination and prepenetration development of the fungus were not related to disease resistance. Fewer fungus haustoria developed normally in epidermal cells of resistant than of susceptible rose varieties. The relative importance of morphological barriers and internal resistance factors is discussed. Attempts failed to infect freshly isolated or long established callus tissue cultures of several rose varieties.     

The roles of plant phenolics in defence and communication during Agrobacterium and Rhizobium infection

Phenolics are aromatic benzene ring compounds with one or more hydroxyl groups produced by plants mainly for protection against stress. The functions of phenolic compounds in plant physiology and interactions with biotic and abiotic environments are difficult to overestimate. Phenolics play important roles in plant development, particularly in lignin and pigment biosynthesis. They also provide structural integrity and scaffolding support to plants. Importantly, phenolic phytoalexins, secreted by wounded or otherwise perturbed plants, repel or kill many microorganisms, and some pathogens can counteract or nullify these defences or even subvert them to their own advantage. In this review, we discuss the roles of phenolics in the interactions of plants with Agrobacterium and Rhizobium.     

Resistance to oat powdery mildew in Britain and Europe: a review

The evolution of virulence in UK oat powdery mildew (Blumeria graminis f.sp. avenae) populations is presented along with comparative information on the deployment of resistant cultivars. Virulence frequencies have followed classical gene‐for‐gene principles, and there are no effective resistance genes currently deployed in cultivars grown in the UK. The incidence of powdery mildew in continental Europe and pathogen variation is reviewed as well as other strategies for the control of this disease. New resistant sources have been identified and are being used in breeding programmes throughout Europe.     

Quantitative Differences in Powdery Mildew Resistance Among Spring Barley Cultivars

Quantitative powdery mildew resistance in compatible host-pathogen-combinations was measured by the number of pastules/cm² leaf area. Spring barley cultivar ‘Proctor’ was significantly less infected than ‘Golden Promise”. Using these two cultivars (having no effective major resistance gene) as controls, MO- and AR-resistant cultivars were inoculated with virulent mildew isolates. ‘Mona”, ‘Grit’ and ‘Nudinka’ had a higher or, at least, the same level of quantitative resistance as ‘Proctor”. None of the remaining cultivars showed the high susceptibility expressed by ‘Golden Promise”. Ranking of host genotypes was nearly constant while that of mildew isolates varied considerably. Only a small portion of the observed variance was due to interaction between host cultivars and pathogen isolates. ‘Triesdorfer Diva’ gave a resistant infection type after inoculation with different AR-virulent isolates, indicating that this cultivar has major resistance other than that conditioned by gene Ml-a12.     

Modification of competence for in vitro response to Fusarium oxysporum in tomato cells. I. Selection from a susceptible cultivar for high and low polysaccharide content

Plant cell walls play a major role in the outcome of host-parasite interactions. Wall fragments released from the plant, and/or the fungal pathogen, can act respectively as endogenous and exogenous elicitors of the defence response, and other wall components, such as callose, lignin, or hydroxyproline-rich glycoproteins, can inhibit pathogen penetration and/or spreading. We have previously demonstrated that calli from tomato cultivars resistant in vivo to Fusarium oxysporum f.sp. lycopersici show a high amount of polysaccharides in vitro. The aim of the present work was to assess the possible role of polysaccharide content and/or synthetic capacity in determining the competence of plant cells for active defence. For this purpose, tomato cell clones with increased and decreased polysaccharide (FL⁺, FL^-) and callose (A⁺, A^-) content have been selected by means of specific stains as visual markers and tested for the effect of these changes on the extent of response to Fusarium. The analysis of several parameters known to be indicative of active defence (cell browning after elicitor treatment, peroxidase and -glucanase induction and inhibition of fungal growth in dual culture) clearly shows that FL⁺ and A⁺ clones have acquired an increased competence for the activation of defence response. The results are thoroughly discussed in terms of an evaluation of the relative importance of constitutive and/or inducible polysaccharide synthetic capacity for plant response to pathogens, and their possible regulation by plant physiological backgrounds.     

Patterns of resistance of Israeli wild emmer wheat to pathogens I. Predictive method by ecology and allozyme genotypes for powdery mildew and leaf rust

The association of ecological factors and allozyme markers with genotypes of tetraploid wild emmer wheat,Triticum dicoccoides, varying in resistance to four cultures of the pathogenErysiphe graminis tritici, and to one culture ofPuccinia recondita tritici, which incite the diseases powdery mildew and leaf rust respectively, were explored theoretically and practically. The study involved 233 accessions comprising 10 populations representing the ecological range ofT. diococcoides in Israel. Our results indicate that genetic polymorphism for resistance to both pathogens is structured geographically, and is predictable by climatic as well as allozymmc markers. Three variable combinations of water factors and temperature differentials significantly explain 0.27 and 0.14 of the spatial variance for resistance to powdery mildew and leaf rust, respectively, suggesting the involvement of natural selection. Several allozyme genotypes, singly, or in combination, are significantly associated with disease resistance. We conclude thatT. dicoccoides populations in Israel, which grow in the center of diversity of the species, contain large amounts of unexploited disease resistant genotypes. The populations could be effectively screened and utilized for producing resistant cultivars by means of ecological factors and allozyme markers as predictive guidelines.