Vf scab resistance of Malus

The apple production in temperate regions with spring rains, the Scab caused by the fungus Venturia inaequalis is the most important constraint. To produce spotless apples and avoid damage that develops during storage, growers apply fungicide on a regular or weather-determined basis. All major apple cultivars are highly susceptible to this disease. To limit the need for fungicide applications, apple breeders are currently introgressing disease resistance from wild Malus accessions into commercial lines. The first attempts to do this were made 100years ago. As apples are self-incompatible, pseudo-backcrossing is used to eliminate unwanted traits from wild Malus and select new cultivars that are attractive to both producers and consumers. This process, from the first cross of a commercial cultivar with a wild, disease-resistant Malus, is extremely long due to apple's long juvenile phase, the need for more than seven backcross steps and the high heterozygosity of this genus. Therefore, most of today's scab-resistant cultivars rely on a single introduction of scab resistance from Malus floribunda 821, referred to as Vf. In this paper, we trace the history of Vf from its initial identification through its use in breeding and commercial production. We sum up the literature describing how and where Vf resistance has been overcome by new pathotypes of V. inaequalis. Finally, we describe the current knowledge of the genes behind Vf resistance, its mode of action and the use of Vf genes in gene technolog

Identification of Microsatellite Markers for Plasmopara viticola and Establishment of High throughput Method for SSR Analysis

The Oomycete Plasmopara viticola is the causal organism of downy mildew on grapevine (Vitis spp.). In order to set up the techniques for investigating downy mildew disease dynamics and genetic structure, co-dominant, neutral, highly reproducible and polymorphic microsatellite markers for P. viticola were developed. Five markers, two with a (TC)n repeat (loci BER and ISA), two with a (TC)n(AC)n repeat (loci CES and REX) and one with a (CT)n(CTAT)n repeat (locus GOB), were selected. Simple sequence repeat (SSR) markers revealed different degrees of polymorphism within 190 oil spots (disease symptoms) collected from an infected Italian vineyard. The most polymorphic SSR marker GOB showed 43 alleles (Nei's expected gene diversity He = 0.89) while CES, ISA, BER and REX showed 14 (He = 0.71), 4 (He = 0.57), 3 (He = 0.24) and 1 allele (He = 0), respectively. A high throughput DNA extraction method, that allowed molecular analysis of this obligate pathogen directly in the host without any isolation procedure, was developed. The quality and quantity of oil spots did not influence the SSR analysis. Amplified SSR loci were separated by electrophoresis on a Beckman-Coulter 2000XL sequencer and automatically analysed. The objective of this study was to develop molecular biological tools and methods that allow high throughput analysis of the downy mildew population

Weeds as Potential Inoculum Reservoir for Colletotrichum nymphaeae Causing Strawberry Anthracnose in Iran and Rep-PCR Fingerprinting as Useful Marker to Differentiate C. acutatum Complex on Strawberry

Strawberry anthracnose caused by Colletotrichum spp. is considered one of the most serious and destructive disease of strawberry worldwide. Weeds, as possible hosts of the pathogen, could have a role as potential inoculum reservoir. To prove this hypothesis, symptomless weeds were collected in strawberry fields showing anthracnose symptoms in Iran. Ten isolates with Colletotrichum-like colonies were recovered from symptomless Amaranthus viridis L., Convolvulus arvensis L., Fumaria officinalis L., Lactuca serriola L., and Sonchus oleraceus L. plants. The isolates were identified as C. nymphaeae, based on a combination of morphological and sequence data of TUB and GADPH genes. This identification was further validated using Rep-PCR fingerprinting analysis, which produces species-specific DNA fingerprints and unveils inter and intra variation of the species examined in this study. Moreover, rep-PCR marker was used to reveal accurate taxonomic position of Colletorichum spp. causing strawberry anthracnose belonging to the C. acutatum complex, including C. acutatum sensu stricto, C. fiorinae, C. godetiae, C. nymphaeae, C. salicis, and C. simmondsii. The C. nymphaeae isolates originating from symptomless weeds confirmed their pathogenicity on detached strawberry, proving that weeds in strawberry field may have a role as reservoir of inoculum. However, further studies are necessary to quantify their actual contribution to anthracnose epidemics in strawberry fields

Alternatives to copper-based treatments for the control of grapevine downy mildew (Plasmopara viticola): 5-year synthesis of trials in France and Italy

The objective of the UE-funded program REPCO (FP6) was to identify new alternatives to copper fungicides in organic farming. Two pathogens have been considered: grapevine downy mildew (Plasmopara viticola) and apple scab (Venturia inaequalis). Research on grapevine was carried out by GRAB (Avignon, France) and FEM-IASMA (San Michele all’Adige, Italy). Trials were carried out under natural conditions in order to test reduced doses of copper and copper-free alternatives. Five-year trials proved that copper is the best fungicide. However, the use of new low copper rate formulations and the association of copper-free alternatives with reduced doses of copper or with reduced number of copper applications (limited to the most susceptible grapevine growth stages) were able to control of downy mildew with a concomitant reduced impact to the environment. Among the tested products, Trichodex (Trichoderma harzianum T39), Mycosin (clay), Chitoplant (chitosan), Saponin (Yucca extract), and an experimental sage extract (Salvia officinalis) gave the best results. The level of plant protection was similar to copper (standard), but not significantly different from the untreated control. Efficacy was good under low or medium disease pressure, but decreased when the pressure increased. The trials carried out after REPCO project confirmed that blossom stage is the most risky period for downy mildew infection and that a reduction of the number of treatments, with consequent reduction of the yearly total amount of copper, is possible by applying treatments during this period. These results show that more research efforts are needed to substitute copper in organic grape growing, and at present the practical approach must focus on combining strategies (alternative products and reduced copper, understanding the most susceptible stages of the plant, applying disease forecasting models, etc.)

Co-inoculated Plasmopara viticola genotypes compete for the infection of the host independently from the aggressiveness components

During Plasmopara viticola epidemics only few genotypes produce most of the secondary lesions and dominate in the population. Selection of dominant genotypes is hypothesized to be linked to environmental conditions and can occur rapidly, particularly if there is also difference between genotypes in terms of fitness and aggressiveness. Measurements of aggressiveness components can largely determine the rate of epidemic development, although the components of aggressiveness do not take into account potential direct competition between genotypes. Differences in aggressiveness have been also reported to be greater under non-optimal conditions suggesting for genotype adaptation to different conditions. To evaluate differences in latency at non-optimal conditions, we characterized genotypes deriving from different climatic regions at three different temperatures (15, 25 and 35°C) and we found no differences. To investigate whether other factors may impact on competition between P. viticola genotypes, we evaluated polycyclic infections of P. viticola by co-inoculating three genotypes with similar aggressiveness components in two different co-inoculation experiments and an increasing prevalence of one of the two genotypes was observed. Competition was not related to the origin of the genotype and we hypothesize that competitive selection is modulated by differences in the secretion of effector molecules which can contribute to the establishment of dominant genotypes over an epidemic seaso

The perception of rhizosphere bacterial communication signals leads to transcriptome reprogramming in Lysobacter capsici AZ78, a plant beneficial bacterium

4openInternationalThe rhizosphere is a dynamic region governed by complex microbial interactions where diffusible communication signals produced by bacteria continuously shape the gene expression patterns of individual species and regulate fundamental traits for adaptation to the rhizosphere environment. Lysobacter spp. are common bacterial inhabitants of the rhizosphere and have been frequently associated with soil disease suppressiveness. However, little is known about their ecology and how diffusible communication signals might affect their behavior in the rhizosphere. To shed light on the aspects determining rhizosphere competence and functioning of Lysobacter spp., we carried out a functional and transcriptome analysis on the plant beneficial bacterium Lysobacter capsici AZ78 (AZ78) grown in the presence of the most common diffusible communication signals released by rhizosphere bacteria. Mining the genome of AZ78 and other Lysobacter spp. showed that Lysobacter spp. share genes involved in the production and perception of diffusible signal factors, indole, diffusible factors, and N-acyl-homoserine lactones. Most of the tested diffusible communication signals (i.e., indole and glyoxylic acid) influenced the ability of AZ78 to inhibit the growth of the phytopathogenic oomycete Pythium ultimum and the Gram-positive bacterium Rhodococcus fascians. Moreover, RNA-Seq analysis revealed that nearly 21% of all genes in AZ78 genome were modulated by diffusible communication signals. 13-Methyltetradecanoic acid, glyoxylic acid, and 2,3-butanedione positively influenced the expression of genes related to type IV pilus, which might enable AZ78 to rapidly colonize the rhizosphere. Moreover, glyoxylic acid and 2,3-butanedione downregulated tRNA genes, possibly as a result of the elicitation of biological stress responses. On its behalf, indole downregulated genes related to type IV pilus and the heat-stable antifungal factor, which might result in impairment of twitching motility and antibiotic production in AZ78. These results show that diffusible communication signals may affect the ecology of Lysobacter spp. in the rhizosphere and suggest that diffusible communication signals might be used to foster rhizosphere colonization and functioning of plant beneficial bacteria belonging to the genus LysobacteropenBejarano Ramos, A.; Perazzolli, M.; Pertot, I.; Puopolo, G.Bejarano Ramos, A.; Perazzolli, M.; Pertot, I.; Puopolo, G

Weeds as potential inoculum reservoir for Colletotrichum nymphaeae causing strawberry anthracnose in Iran and Rep-PCR fingerprinting as useful marker to differentiate C. acutatum complex on strawberry

3openInternationalInternational coauthor/editorStrawberry anthracnose caused by Colletotrichum spp. is considered one of the most serious and destructive disease of strawberry worldwide. Weeds, as possible hosts of the pathogen, could have a role as potential inoculum reservoir. To prove this hypothesis, symptomless weeds were collected in strawberry fields showing anthracnose symptoms in Iran. Ten isolates with Colletotrichum-like colonies were recovered from symptomless Amaranthus viridis L., Convolvulus arvensis L., Fumaria officinalis L., Lactuca serriola L., and Sonchus oleraceus L. plants. The isolates were identified as C. nymphaeae, based on a combination of morphological and sequence data of TUB and GADPH genes. This identification was further validated using Rep-PCR fingerprinting analysis, which produces species-specific DNA fingerprints and unveils inter and intra variation of the species examined in this study. Moreover, rep-PCR marker was used to reveal accurate taxonomic position of Colletorichum spp. causing strawberry anthracnose belonging to the C. acutatum complex, including C. acutatum sensu stricto, C. fiorinae, C. godetiae, C. nymphaeae, C. salicis, and C. simmondsii. The C. nymphaeae isolates originating from symptomless weeds confirmed their pathogenicity on detached strawberry, proving that weeds in strawberry field may have a role as reservoir of inoculum. However, further studies are necessary to quantify their actual contribution to anthracnose epidemics in strawberry fields.openKarimi, Kaivan; Arzanlou, Mahdi; Pertot, IlariaKarimi, K.; Arzanlou, M.; Pertot, I

Selection of a biocontrol agent based on a potential mechanism of action: degradation of nicotinic acid, a growth factor essential for Erwinia amylovora

This work describes a medium-based screening method for selecting microbial biocontrol agents against Erwinia amylovora based on the degradation of a specific growth factor. Erwinia amylovora, the causal agent of the devastating fire blight disease, requires nicotinic acid or nicotinamide as an essential growth factor. Potential biocontrol agents are either selected for antimicrobial production in plate or directly on immature pears or apple blossoms. In this work, we have attempted to streamline the selection of a new potential biocontrol agent with a lower risk of non-target effects by isolation based on the ability to degrade nicotinic acid in vitro, using therefore few plant materials. A total of 735 bacteria and 1237 yeast were isolated from apple blossoms and pre-screened for nicotinic acid-degradation. Pseudomonas rhizosphaerae strain JAN was able to degrade both nicotinic acid and nicotinamide. Mutants deficient in this ability were constructed. JAN, but not the mutants, controlled E. amylovora on pear slices. On detached apple blossoms, JAN colonized apple hypanthia and strongly suppressed E. amylovora growth. Under greenhouse conditions, JAN was more effective in controlling blossom blight than P. fluorescens A506, a commercial biocontrol agent of fire blight unable to degrade nicotinic acid and nicotinamide.[Int Microbiol 2010; 13(4):195-206

Ecology and potential functions of plant-associated microbial communities in cold environments

Complex microbial communities are associated with plants and can improve their resilience under harsh environmental conditions. In particular, plants and their associated communities have developed complex adaptation strategies against cold stress. Although changes in plant-associated microbial community structure have been analysed in different cold regions, scarce information is available on possible common taxonomic and functional features of microbial communities across cold environments. In this review, we discuss recent advances in taxonomic and functional characterization of plant-associated microbial communities in three main cold regions, such as alpine, Arctic and Antarctica environments. Culture-independent and culture-dependent approaches are analysed, in order to highlight the main factors affecting the taxonomic structure of plant-associated communities in cold environments. Moreover, biotechnological applications of plant-associated microorganisms from cold environments are proposed for agriculture, industry and medicine, according to biological functions and cold adaptation strategies of bacteria and fungi. Although further functional studies may improve our knowledge, the existing literature suggest that plants growing in cold environments harbor complex, host-specific and cold-adapted microbial communities, which may play key functional roles in plant growth and survival under cold conditions