The N-acyl homoserine-lactone depleted Rhizobium radiobacter mutant RrF4NM13 shows reduced growth-promoting and resistance-inducing activities in mono- and dicotyledonous plants

Journal of Plant Diseases and Protection - The Alphaproteobacterium Rhizobium radiobacter (syn. Agrobacterium tumefaciens, “A. fa brum”) can live in close association with the...     

Ectopic expression of barley constitutively activated ROPs supports susceptibility to powdery mildew and bacterial wildfire in tobacco

ROPs (also called RACs) are RHO-like monomeric G-proteins of plants, well-known as molecular switches in plant signal transduction processes, which are involved in plant development and a variety of biotic and abiotic stress responses. The barley (Hordeum vulgare) ROPs RACB, RAC1 and RAC3 have been shown to be involved in cellular growth, polarity and in susceptibility to the biotrophic barley powdery mildew fungus Blumeria graminis f.sp. hordei. We produced transgenic tobacco (Nicotiana tabacum) plants expressing constitutively activated (CA) mutants of the barley ROPs RACB and RAC3 to monitor the impact of heterologous ROP expression on cell polarity and disease susceptibility of tobacco. CA HvROPs influenced leaf texture, disturbed root hair polarity and induced cell expansion in tobacco. Both barley ROPs induced super-susceptibility to the compatible powdery mildew fungus Golovinomyces cichoracearum but only CA HvRAC3 induced super-susceptibility to the bacterial leaf pathogen Pseudomonas syringae pv. tabaci. Data suggest involvements of ROPs in tobacco cell expansion, polar growth and in response to bacterial and fungal leaf pathogens.     

Metrafenone: studies on the mode of action of a novel cereal powdery mildew fungicide

Powdery mildew fungi are among the major pathogens causing diseases of cereals in the world. The mode of action of a novel systemic benzophenone fungicide, metrafenone, which is based on a precursor that is discussed in the preceding paper, has been analysed on the powdery mildew fungi of barley (Blumeria graminis Speer f. sp. hordei Marchal) and wheat (Blumeria graminis Speer f. sp. tritici Marchal). Preventive treatments reduced germination and blocked development beyond formation of appressoria, which penetrated less often. Moreover, metrafenone turned out to be an efficient curative fungicide, which rapidly affected fungal survival at low concentrations. The fungicide induced swelling, bursting and collapse of hyphal tips, resulting in the release of globules of cytoplasm. Bifurcation of hyphal tips, secondary appressoria and hyperbranching were also frequently observed. A histochemical analysis showed that metrafenone caused disruption of the apical actin cap and apical vesicle transport as well as weakening of the cell wall at hyphal tips. Finally, metrafenone strongly reduced sporulation. Reduced sporulation was associated with malformation of conidiophores that showed irregular septation, multinucleate cells and delocalisation of actin. Microtubules appeared to be only secondarily affected in metrafenone-treated B. graminis. The results suggest that the mode of action of metrafenone interferes with hyphal morphogenesis, polarised hyphal growth and the establishment and maintenance of cell polarity. Metrafenone likely disturbs a pathway regulating organisation of the actin cytoskeleton.     

Variation in performance of western flower thrips populations on a susceptible and a partially resistant chrysanthemum cultivar

Variation in host plant performance among populations of a phytophagous insect pest is a potential threat to the durability of host plant resistance. Aggressive biotypes may overcome the protective properties of formerly resistant cultivars. Therefore, it is of interest to study such variation in breeding programs for host plant resistance to insects. In the present study, the performance of ten populations of Frankliniella occidentalis, the western flower thrips, was determined on a susceptible and a partially resistant cultivar of chrysanthemum, Dendranthema grandiflora. Damage, reproduction, and adult survival were determined using an excised leaf assay. Significant differences between the two cultivars and among the ten populations were found for all three characteristics. In general, damage, reproduction and adult survival were reduced on the resistant cultivar when compared to the susceptible cultivar. Some populations showed, in comparison to the reference population from the Netherlands, much higher damage and reproduction on one or both chrysanthemum cultivars. But also in these populations performance on the resistant cultivar was poor compared to the susceptible cultivar. This revised version was published online in August 2006 with corrections to the Cover Date.     

A list of methods to detect arthropod quarantine pests in Europe*

A total of 177 species of quarantine arthropods in Europe have been analysed for detection methods that are used in surveillance. This paper provides a link to a list where the methods most frequently used, either alone or in combination, are given for each species. Inspection remains the most common method of detection (108 species). Volatile compounds produced by either insects or host plants, or those released from food attractants are used for 85 species, while light trapping accounts for 28 species. Semiochemicals are known for 73 species, but are commercially available for only 43 species.     

Piriformospora indica protects barley from root rot caused by Fusarium graminearum

Journal of Plant Diseases and Protection - The beneficial endophytic fungus Piriformospora indica colonizes barley (Hordeum vulgare L.) roots, which results in protection against diseases and...     

Benzothiadiazole Activates Resistance in Sunflower (Helianthus annuus) to the Root-Parasitic Weed Orobanche cuman

ABSTRACT The study was conducted to evaluate the potential of induced resistance to infestation of sunflower (Helianthus annuus L.) by the parasitic weed Orobanche cumana Wallr. Treatment of sunflower seeds with 40 ppm of benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (BTH) for 36 h completely prevented infection in root chambers. In pot studies using 2.86 x 10(-4) g of Orobanche seeds per gram of soil as inoculum, the total number of O. cumana shoots was reduced by 84 and 95% in the 60-ppm BTH treatment in the first and second trial, respectively. Evaluation of the disease incidences revealed that attachment of O. cumana at the sunflower root and the stage of early penetration was reduced in the BTH-treated plants. Chemical analysis of root extracts revealed synthesis of the phytoalexin scopoletin and of hydrogen peroxide in the BTH-treated sunflower roots, but no increase in lignification. Western blot analysis demonstrated accumulation of the pathogenesis-related protein chitinase in roots and stems of induced resistant plants. These results show that the phenomenon of induced resistance is not restricted to viral, bacterial, and fungal disease and demonstrate the great potential of this protection strategy as an effective component of future plant production systems.     

Model Wheat Genotypes as Tools to Uncover Effective Defense Mechanisms Against the Hemibiotrophic Fungus Bipolaris sorokiniana

ABSTRACT We investigated the interaction of several differentially resistant wheatwith the hemibiotrophic phytopathogenic fungus Bipolaris sorokiniana (teleomorph Cochliobolus sativus). Wheat genotypes Yangmai, M 3 (W7976), Shanghai 4, and Chirya 7 showed higher levels of resistancewith cv. Sonalika, used as a susceptible control. In amicroscopic inspection, we found that fungal penetration intoepidermal layer failed mostly through a cell wall-associated defense. In cases where the fungus successfully overcame epidermal, its spread within the mesophyll tissue (necrotrophic phase) wasin the more resistant genotypes. Epidermal cell wall-associated, spreading as well as the extent of electrolyte leakage of infected, correlated well with field resistance. We propose that cellular hostsuch as formation of cell wall appositions as well as the degreeearly mesophyll spreading of fungal hyphae are indicative of thepotential of the respective host genotype and, therefore, could befor the characterization of new spot blotch resistance traits in cereals.     

Seasonal variation in resistance of chrysanthemum cultivars to Frankliniella occidentalis (Thysanoptera: Thripidae)

Seasonal variation in the level of host plant resistance can have important consequences for the repeatability of tests to measure host plant resistance to insect pests. In the present study, the levels of resistance to Frankliniella occidentalis of a susceptible and a partially resistant cultivar of chrysanthemum, Dendranthema grandiflora, were determined throughout the year. Thrips damage, reproduction and adult female survival were determined on excised leaves in Petri dishes under uniform conditions. Strong seasonal fluctuations were observed in these three characteristics. On leaves from plants grown in winter, damage, reproduction, and survival were higher than on leaves from plants grown in summer. Clear differences in resistance were observed between the susceptible and the partially resistant cultivar on leaves from plants grown in winter, while differences disappeared in summer. Damage on both cultivars and survival on the susceptible cultivar were negatively correlated with mean daily solar radiation during plant growth, suggesting that the level of resistance depends on light intensity during plant growth. This was confirmed in an experiment carried out in summertime with shaded and unshaded plants. Leaves from control plants, grown under high light intensity had a higher level of resistance than leaves from shaded plants grown under reduced light intensity. There were clear differences in resistance between the cultivars under shaded conditions (low light intensity), but not under unshaded control conditions (high light intensity). This revised version was published online in July 2006 with corrections to the Cover Date.