Correlation Between Induced Resistance and Host Fluorescence in Barley Inoculated with Erysiphe graminis

Resistance was induced in barley by virulent and avirulent Erysiphe graminis f. sp. hordei and by E. graminis f. sp. tritici. The diameters of fluorescent haloes and papillae at the site of the primary germ tube of the inducers were larger until 12 hours after inoculation with E g. f. sp. tritici than in the corresponding periods after inoculation with E. g. f. sp. hordei. Fluorescence at the site of appressorial lobes of the inducers developed, gradually from 10 and 12 hours after inoculation with E. g. f. sp. hordei and E. g. f. sp. tritici, respectively. Penetration success of single infection units of the challenger was reduced in cells with inducer fluorescence, suggesting that resistance is principally localized to cells previously attacked by the inducer. But penetration success was independent of the average distance to the ten nearest inducer fluorescences, and penetration failures did also occur in cells without inducer fluorescence, suggesting that the induced resistance to some extent is translocated to other epidermal cells. The average diameter of the fluorescent papillae at the site of unsuccessful challenger infection units increased as a result of induced resistance.     

The early stages of mildew colony development on susceptible oats

The chronology of primary infection by Erysiphe graminis f. sp. avenae on detached leaves of a susceptible host, and growth patterns of the primary haustorium and secondary hyphae, proved similar to those of wheat and barley mildew found by other workers. The timing of formation and subsequent growth of the primary haustorium was not affected by the light regime, but the formation of subsequent haustoria was highly synchronous under an alternating dark/light cycle, and far less so under continuous illumination. Five days after inoculation almost twice as many secondary and tertiary haustoria were formed per colony under the dark/light treatment than under continuous light. Because of the synchronous formation of haustoria subsequent to the primary, haustoria selected at random from leaves of susceptible host cultivars showed a bimodal distribution in length, the less well developed tertiary haustoria being distinguished from earlier formed primary and secondary haustoria. There was also a significant positive correlation between length and the number of digitate processes/haustorium. The energy required to produce one secondary haustorium was calculated to be equivalent to that required to produce approximately 4–7 hyphal cells.     

Quantitative Studies of Resistance Induced By Avirulent Cultures of Erysiphe graminis f. sp. hordei in Barley

Quantification of resistance induced by avirulent cultures of Erysiphe graminis f. sp. hordei against virulent cultures in barley was attempted. Four mildew cultures and 4 barley varieties with known genes of virulence and resistance respectively were used. Pre, post and simultaneous inoculation of leaves was done with avirulent and virulent cultures. Pre-inoculation with avirulent cultures induced resistance in the host such that the pustule number and spore production by later inoculation of virulent cultures was reduced significantly. Once induced, such resistance was active up to 8 days. There was some indication of induced susceptibility if the inducing culture was characterized by medium virulence. Increase of inceulum density of the avirulent (inducer) culture increased the amount of induced resistance Further studies of the phenomenon of induced resistance are needed in relation to possible applications for disease control through inoculations. varietal mixtures and multilines.     

Evidence for Resistance-inducing Compounds in Conidia of Erysiphe graminis f.sp. hordei

Preparations of Erysiphe graminis f.sp. hordei conidia were spray-applied to the first leaf of barley plants which were subsequently challenge inoculated with virulent powdery mildew. The powdery mildew reducing effect of the preparations was assessed by scoring the outcome of the challenge inoculation. Homogenates of ungerminated conidia, germinated conidia, and methanol-water extracts of germinated conidia reduced the number of powdery mildew colonies. Cell wall fragments from ungerminated conidia, germinated conidia, and conidial germination fluid obtained from conidia germinated in aqueous suspension did not reduce the number of powdery mildew colonies. Microsconical analysis of the infection course following challenge inoculation indicated that the powdery mildew reducing effect is due partly to induced resistance.     

Further Investigation of the Specificity of Interactions Between Wild Lactuca spp. and Bremia lactucae Isolates from Lactuca serriola

Callus cultures derived from isogenic lines of the tomato cultivars Moneymaker and Craigella, resistant or susceptible to F. oxysporum f. sp. lycopersici, were inoculated with Fusarium oxysporum f. sp. lycopersici race 1. Fungal growth was restricted on callus derived from resistant plants, after inoculation with a conidial suspension, whereas callus derived from susceptible plants was totally overgrown by the fungus within 7 days. The concentration of the phytoalexin rishitin was significantly higher in the callus culture derived from a resistant tomato line compared with the callus culture from a susceptible line, 2 and 3 days after inoculation with mycelium. The results of the experiments were compared with experiments with whole plants. Rishitin production as well as growth of the fungus was comparable with responses in plant-fungus interaction. Therefore callus culture may be useful in studying the interaction between tomato plants and race 1 of F. oxysporum f. sp. lycopersici.     

Colonization of Fusarium Wilt‐Resistant and Susceptible Watermelon Roots by a Green‐Fluorescent‐Protein‐tagged Isolate of Fusarium oxysporum f.sp. niveum

Interactions between watermelon and a green fluorescent protein (GFP)‐tagged isolate of Fusarium oxysporum f.sp. niveum race 1 (Fon‐1) were studied to determine the differences in infection and colonization of watermelon roots in cultivars resistant to and susceptible to Fusarium wilt. The roots of watermelon seedlings were inoculated with a conidial suspension of the GFP‐tagged isolate, and confocal laser scanning microscopy was used to visualize colonization, infection and disease development. The initial infection stages were similar in both the resistant and susceptible cultivars, but the resistant cultivar responded differentially after the pathogen had penetrated the root. The pathogen penetrated and colonized resistant watermelon roots, but further fungal advance appeared to be halted, and the fungus did not enter the taproot, suggesting that resistance is initiated postpenetration. However, the tertiary and secondary lateral roots of resistant watermelon also were colonized, although not as extensively as susceptible roots, and the hyphae had penetrated into the central cylinder of lateral roots forming a dense hyphal mat, which was followed by a subsequent collapse of the lateral roots. The initial infection zone for both the wilt‐susceptible and wilt‐resistant watermelon roots appeared to be the epidermal cells within the root hair zone, which the fungus penetrated directly after forming appressoria. Areas where secondary roots emerged and wounded root tissue also were penetrated preferentially.     

Anatomical and biochemical aspects of interaction between roots of chickpea and Fusarium oxysporum f. sp. ciceris race 2

Roots of the susceptible “JG-62” and resistant “WR-315” chickpeas (Cicer arietinum L.) were inoculated with a conidial suspension of Fusarium oxysporum f. sp. ciceris. Anatomical and biochemical studies were carried out in a time-course manner to elucidate the infection process and plant defence reactions. Scanning electron microscope images revealed fungal colonisation in the root hair region. Early occurrence of fungal biofilms associated with the infected “JG-62” root epidermis was also visualised. After 96 h of inoculation, a gradual accumulation of polysaccharide positive deposits was observed in the xylem vessels of the infected “JG-62” roots. Fungal mycelium was observed in the vessel lumen of infected “JG-62” after 22 days of inoculation. Due to fungal invasion during this period, some of the vessels also appeared collapsed in “JG-62”, whereas vessels in “WR-315” remained intact. The host plant defence responses specifically linked to the susceptible interactions were the induction of ascorbate peroxidase, guaiacol peroxidase and superoxide dismutase in roots and shoots.     

Suppressiveness to Fusarium oxysporum f. sp. radicis lycopersici in re-used perlite and perlite–peat substrates in soilless tomatoes

This work reports the occurrence of suppressiveness to Fusarium oxysporum f. sp. radicis lycopersici (FORL) on recycled perlite and perlite–peat mix from closed and open soilless systems. Nine soilless systems were sampled from three different sites in Northern and Southern Italy and different parameters, including sampling site, growing period before sampling, electric conductivity of the nutrient solution, tomato cultivar, and irrigation system were considered. The effects on seed germination and FORL incidence on tomato seedlings were finally assessed with or without additional artificial inoculation of the pathogen and with or without autoclaving the samples prior to inoculation. Suppression of FORL was experimentally evaluated with a technique already adopted for rockwool. Results collected on perlite and perlite–peat confirmed the possibility to reduce FORL severity on recycled substrates. Only the composition of the substrate (perlite, perlite–peat mix) and the disinfestation did affect the incidence of FORL. Suppression of FORL was observed in not disinfected recycled perlite–peat substrates while a reduction of FORL incidence was also been recorded in disinfected and recycled perlite. Disease incidence decreased from an average ranging from 44.4% to 61.9% in new perlite to 2.5–36.3% in recycled one. Similarly disease incidence decreased from an average ranging from 35.9% to 75.2% in new perlite–peat mix to 0.4–26.4% in recycled perlite–peat mix. In conclusion the recorded data suggest the possibility to consider FORL suppression as a predictable phenomenon when recycled substrates (perlite, perlite–peat mix) are adopted.     

Primary Infection of Barley by Erysiphe graminis f. sp. hordei in Relation to Leaf-Age Dependent Resistance and the Roles of the Epidermis and Mesophyll in this Resistance

Abstract The infection frequency of both compatible and incompatible races of Erysiphe graminis f. sp. hordei decreased gradually with increasing leaf age on undetached primary barley leaves. The length of secondary hyphae of the compatible race was approximately the same regardless of age, but secondary hyphae were slightly longer on younger seedlings than on older seedlings in the case of the incompatible race. Both the infection frequency and length of secondary hyphae of the two races weredistinctly different. On composite sections produced by exchanging the epidermal layers of young and relatively mature primary leaves, the infection frequency of the compatible race was higher on the epidermis of young leaves than on the epidermis of older, leaves, regardless of which mesophyll was under the epidermis. The epidermis appears to play a major role in age-dependent resistance, while the mesophyll may act disparately by providing a factor promotive to mildew infection in addition to supporting the resistance function of the epidermis.     

Increased Growth and Yield of Hydroponically Grown Greenhouse Tomato Plants Inoculated with Arbuscular Mycorrhizal Fungi and Fusarium oxysporum f. sp. radicis-lycopersici

The arbuscular mycorrhizal fungi Glomus monosporum, G. vesiculiferum, G. deserticola, G. intraradices, G. mosseae, and two unidentified species were tested to determine their effect on plant growth and fruit production of tomato (Lycopersicon esculentum Mill.) cv. Trust inoculated with Fusarium oxysporum f. sp. radicis-lycopersici (FORL) under near-commercial greenhouse conditions. Inoculation with G. monosporum and G. mosseae significantly increased fruit yield and fruit number of tomato plants grown hydroponically in sawdust. Plant height and plant dry weight increased significantly when inoculated with G. monosporum and G. mosseae. Further, plants inoculated with G. monosporum and G. mosseae showed significantly lower FORL root infection than the untreated control plants.     

Development of Early Infection Structures of Puccinia recondita f. sp. tritici in Non-host Cereal Species

The development of infection structures, derived from urediospores of Puccinia recondita f.sp. trilici in nearisogenic lines of susceptible and resistant wheat, and in non-hosts (viz. maize, oat, sorghum and barley), was examined by fluorescence microscopy and scanning electron microscopy (SEM). The infection structure formation on and in five cereal species follows a similar pattern. In sorghum, fungal development is arrested at the stage of substomatal vesicle formation, while, in maize, most fungal structures collapse during the stage of primary hypha development. By contrast, in wheat, barley and oat, the fungus forms many branched infection hyphae and haustorial mother cells.     

Ribulose Bisphosphate Carboxylase Protein and Enzymes of CO2 Assimilation in Barley Infected by Powdery Mildew (Erysiphe graminis hordei)

Effects were examined of barley powdery mildew (Erysiphe graminis f. sp. hordei) on the major leaf protein, ribulose-l,5-bisphosphate carboxylase (RuBPCase), and other enzymes of CO₂ assimilation, phosphoenolpyruvate carboxylase (PEPcase) and malic enzymes, and enzymes associated with RuBPCase in the reductive pentose phosphate pathway. Activity of RuBPCase per unit fresh weight of leaf was inhibited by infection from the first sample, 3 days after inoculation, to the last sample, 24 days after inoculation, when healthy control leaves were visibly senescing. The inhibition occurred because the amount of RuBPCase protein (measured specifically by an immunological technique) was reduced from 6 days after inoculation and because activity per unit protein declined from 3 days until 21 days after inoculation, at which time there was little protein remaining. Activity of PEPcase per unit fresh weight of leaf was initially stimulated by infection but, thereafter, it was inhibited. Inhibition, like that also affecting malic enzymes (NAD) and (NADP), 3-phosphoglycerate kinase, and glyceraldehyde-3-phosphate dehydrogenases (NAD) and (NADP), was associated with a decline in amounts of total soluble minus RuBP Case protein per unit fresh weight of leaf. Reduced amounts of leaf protein may be associated with reduced nitrate uptake by roots and fungal demand for nitrogen in mildewed plants.     

Assimilatverteilung in Sommergerstensorten mit unterschiedlicher Resistenz gegenüber dem Echten Mehltau (Erysiphe graminis f. sp. hordei)

Distribution of assimilates in cultivars of spring barley with different resistance against powdery mildew (Erysiphe graminis f. sp. hordei) Transport and distribution of radioactive labelled assimilates in spring barley cultivars with different degrees of resistance to powdery mildew were studied after ¹⁴CO₂-treatment of single leaves. Plants of the cultivars ‘Amsel’ (susceptible), ‘Asse’ (adult plant resistant), and ‘Rupee’ (resistant) were analyzed at the vegetative growth stage (5. leaf unfolded) and the generative growth stage (anthesis). At the vegetative growth stage the assimilate export from the mildew inoculated 5. leaf of ‘Amsel’ and ‘Rupee’ is decreased; in ‘Asse’, there is no considerable change of assimilate distribution due to infection. At the generative growth stage the assimilate export from the infected flag leaf of ‘Amsel’ is reduced when the fungus, is sporulating. In the cultivar ‘Asse’ the assimilates are bound at the infection site until the seventh day after inoculation, then the transport of assimilates to the ear is increased. In ‘Rupee’ mildew inoculation causes an enhanced assimilate transport to the ear. The changes in assimilate distribution due to mildew inoculation are discussed with respect to the different types of host-parasite-interactions and the source-sink-activities in the different cultivars.     

Investigations on Adult Plant Resistance of Barley Against Erysiphe graminis f.sp. hordei

Spring barley cultivars and lines were tested for 3 years in field studies for adult plant resistance against Erysiphe graminis f.sp. hordei. The cultivars Osiris and Asse were selected for further detailed cytological studies and compared with the susceptible cultivar Peruvian. Under controlled greenhouse conditions, the percentage of conidia that had formed a functional haustorium and secondary hyphae (infection efficiency) was reduced in fifth leaves of the adult plant resistant cultivars. On fifth and flag leaves of adult plant resistant cultivars, papillae were formed more frequently under primary germ tubes and appressoria, and fungal penetration was prevented more often than on the susceptible cultivar Peruvian. In ultrastructural studies various types of papillae were observed, but could not be strictly correlated with penetration success or failure of the fungus.     

Production of Intraspecific Hybrids of Fusarium oxysporum f. sp. radicis-lycoperisici and Fusarium oxysporum f. sp. lycopersici by Protoplast Fusions

The fusion of protoplasts from the cycloheximide-resistant mutant FOL(C) of Fusarium oxysporum f. sp. lycopersici (FOL) and the mycostatin-resistant mutant FORL(M) of F. oxysporum f. sp. radicis-lycopersici (FORL), produced hybrids which expressed significant differences from the parents in their pathogenicity and growth and in the electrophoretic separation patterns of their proteins, enzymes and isoenzymes. The results suggest a transformed genetic basis for these altered expressions and the feasibility of using protoplast fusion technology for examining the biology of pathogenicity genes and for elucidating the disease and virulence potential for new races from within hybridisable taxa of Fusarium spp. Such information would be useful for the design and development of long-term control systems for Fusarium diseases, particularly in breeding programs for disease resistance in crops.     

In vitro efficacy of <Emphasis Type="Italic">Hyptis suaveolens</Emphasis> L. (Poit.) essential oil on growth and morphogenesis of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f.sp. <Emphasis Type="Italic">gladioli</Emphasis> (Massey) Snyder &; Hansen

Hyptis suaveolens L. (Poit.) essential oil was tested in vitro on the growth and morphogenesis of Fusarium oxysporum f.sp. gladioli (Massey) Snyder & Hansen, which causes Fusarium corm rot and yellows in various susceptible cultivars of gladiolus. The fungitoxicity of the oil was measured by percentage radial growth inhibition using the poisoned food technique (PF) and volatile activity assay (VA). The mycelial growth of the test fungus was completely inhibited at 0.998 and 0.748 μg ml⁻¹ concentration of oil in PF and VA, respectively. Essential oil was found to be fungicidal in nature at 1.247 and 0.998 μg ml⁻¹ concentration of oil in PF and VA, respectively. Determination of conidial germination in the presence of oil was also carried out and it was found that the oil exhibited 100% inhibition of conidial germination at 0.450 μg ml⁻¹ concentration. The effect of essential oil on the yield of mycelial weight was observed and it was found that at 0.873 μg ml⁻¹ concentration no mycelium was recorded and 100% inhibition was observed. The fungitoxicity of oil did not change even on exposure to 100°C temperature or to autoclaving, and the oil also retained its fungicidal nature even after storage of 24 months. The main changes observed under light microscopy after oil treatment were a decrease and loss of conidiation and anomalies in the hyphae such as a decrease in the diameter of hyphae and granulation of cytoplasm. The treatment of the oil also showed highly reduced cytoplasm in the hyphae, showing clear retraction of the cytoplasm from the hyphae and ultimately in some areas hyphae without cytoplasm were also found. GC-MS studies of the essential oil revealed that the oil consisted of 24 compounds with 1,8-cineole as major component accounting for 44.4% of the total constituents.     

Elektronenmikroskopische Untersuchungen des Gerstenmehltaus (Erysiphe graminis DC f. sp. hordei Marchal) nach Resistenzinduktion mit mikrobiellen Stoffwechselprodukten

Electron microscopical studies on Mildew of Barley (Erysiphe graminis DC f. sp. hordei Marchal) after induced Resistance with Products of Microbial Metabolism Ultrastructural studies of the infection progress of barley by Erysiphe graminis after induction of resistance with products of microbial metabolism showed that pathogen development was affected through inducer activated defence mechanisms. The formation of papillae-like structures and an accumulation of electron dense material below the host cell wall at the penetration site appeared to be associated with reduced penetration. The haustoria were also altered in the extrahaustorial membrane and electron dense material accumulated at the cell wall of haustorial body and neck, thus apparently impairing efficient functioning and allowing only limited growth of the pathogen.     

Lauric Acid Exhibits Antifungal Activity Against Plant Pathogenic Fungi

This study aimed at examining the effects of the saturated fatty acid lauric acid on mycelial growth of Rhizoctonia solani and Pythium ultimum and on infection of barley seedlings with Blumeria graminis f. sp. hordei. Mycelial growth of R. solani and P. ultimum in agar culture was significantly reduced by lauric acid at concentrations of 100 μm and above, while no fungal growth occurred in liquid culture at concentrations above 50 μm . Application of lauric acid at concentrations ranging from 250 to 1000 μm to barley leaves before or after inoculation with B. graminis f. sp. hordei led to significant reductions in infection. This study provides the first report of the activity of lauric acid against plant pathogenic fungi and indicates the need for investigation of its mechanism of action.     

The Relationship between Systemic Resistance Induced by Pruning and Accumulation of Antifungal Substances in Barley Seedlings

Infection by a compatible race of Erysiphe graminis f. sp. hordei on barley secondary leaves was significantly suppressed upon pruning of the primary leaves when E. graminis hordei was inoculated 3–12 h after the pruning, but it, was rather enhanced during 15–21 h. The accumulation of antifungal substances was detected in hot ethanol extracts of barley seedlings from 15–27 h after pruning the primary leaves. Taking the time of the infection process of a challenger (E. graminis, hordei) into consideration, timing of systemic resistance induced upon pruning coincided with the accumulation of antifungal substances.     

Race cultivar-specific differences in callose deposition in soybean roots following infection with Phytophthora megasperma f.sp. glycinea

Primary roots of soybean (Glycine max (L.), Merrill, cv. Harosoy 63) seedlings were inoculated with zoospores from either race 1 (incompatible, host resistant) or race 3 (compatible, host susceptible) of Phytophthora megasperma f.sp. glycinea and total callose was determined at various times after inoculation. From 4 h onward, total callose was significantly higher in roots showing the resistant rather than the susceptible response. Local callose deposition in relation to location of fungal hyphae was determined in microtome sections by its specific fluorescence with sirofluor and was quantified on paper prints with an image-analysis system. Callose deposition, which occurs adjacent to hyphae, was found soon after inoculation (2, 3 and 4 h post inoculation) only in roots displaying the resistant response, and was also higher at 5 and 6 h after inoculation in these resistant roots than in susceptible roots. Early callose deposition in the incompatible root-fungus reaction could be a factor in resistance of soybean against P. megasperma.Abbreviation pi post inoculation