Genetic diversity of giant reed ( A rundo donax) in Australia

The perennial grass, Arundo donax, has shown potential as a promising biomass crop. However, it has become invasive in a number of areas and declared a noxious weed in some jurisdictions, making proposals to grow A. donax for commercial use in Australia controversial. Evidence of asexual reproduction and the presence of a single genetic clone in Australia was investigated, as such characteristics would indicate a limited risk of escape and invasion. Using amplified fragment length polymorphism markers, the genetic diversity of 218 A. donax samples from across Australia was examined. The samples were found to separate into two distinct genetic groups, or clades. There was only a small amount of genetic diversity within a clade (0.9 and 1.5%). However, there was a larger difference between the clades of 19.8%, suggesting the presence of two distinct A. donax genotypes in Australia. The low level of genetic variation in Australian A. donax that was found in this study indicates that spread is essentially by vegetative means and suggests that if grown in areas where it is separated from natural water dispersal events, A. donax poses a low risk of becoming invasive.     

Evaluation of strawberry (Fragaria L.) genetic resources for resistance to Botrytis cinerea

The grey mould disease caused by Botrytis cinerea leads to substantial economic losses in strawberry production all over the world. Control of the disease requires an extensive amount of fungicide that is applied in varying complexes because the pathogen easily develops resistance against the active compounds. Planting of resistant cultivars seems to be a promising alternative for fruit growers, but there are currently no cultivars available combining resistance to B. cinerea with attractive horticultural traits. Breeding of new cultivars requires the effective identification of resistant strawberry genotypes; therefore the current study was aimed at the evaluation of strawberry genetic resources under controlled conditions by establishing an artificial inoculation assay. The method presented in this study is an artificial inoculation of ripe fruits with a defined spore suspension under laboratory conditions. The results show that this assay is fast and simple and leads to reproducible results that correlate with field observations. Over 3 years a total of 107 strawberry genotypes of the German National Fruit Genebank at the JKI in Dresden‐Pillnitz were evaluated. Five partly resistant genotypes, cultivars Diana, Joerica and Kimberly, and Fragaria virginiana ‘Wildmare Creek’ and F. vesca subsp. bracteata, were identified with mean disease levels of <20% at 6 days post‐inoculation. The obtained results are discussed with regard to future breeding activities.     

Herbicide‐resistant weeds in the Philippines: Status and resistance mechanisms

Two major weeds in rice in the Philippines, Sphenochlea zeylanica Gaertn. and Echinochloa crus‐galli (L.) Beauv., are controlled with chemical and cultural methods. In the 1980s, after >10 years of continuous use of 2,4‐D, S. zeylanica evolved resistance to the chemical in those rice fields that had been treated with 2,4‐D once or twice every cropping season. In the 1990s, E. crus‐galli evolved resistance to butachlor and propanil in rice monocrop areas where both herbicides were used continuously for 7–9 years. Rice farmers continue to use 2,4‐D, butachlor and propanil extensively and are often unaware of herbicide resistance or the potential for cross‐resistance, its causes or its implications. In order to control herbicide‐resistant E. crus‐galli, farmers are shifting to locally available herbicides with different modes of action, such as bispyribac, an acetolactate synthase inhibitor, and cyhalofop, an acetyl coenzyme A carboxylase inhibitor. Follow‐up manual weeding or rotary weeding after herbicide spraying, a common farmers’ practice, removes the susceptible and resistant biotypes and could help to delay or prevent the evolution of resistance. Although the resistance mechanisms of both weeds are not determined yet, they could be related to enhanced degradation that is similar to the mechanisms that are shown by the resistant biotypes in other countries.     

Resistance and systemic dispersal of Xanthomonas fragariae in strawberry germplasm (Fragaria L.)

The angular leaf spot disease caused by Xanthomonas fragariae is an important plant disease with major impact for the strawberry nursery industry. Currently there is no plant protection product available for controlling the disease effectively. Planting of resistant cultivars seems to be promising, but all commercially used cultivars are susceptible and no donor with a high level of resistance has yet been found. Therefore, a total of 145 genotypes from the Fruit Genebank Dresden (Germany) were evaluated for resistance to X. fragariae by artificial inoculation. Six genotypes were classified as partly resistant, out of which only two (US4808 and US4809) are octoploid. Fragaria vesca f. alba, Fragaria nilgerrensis ‘Yunnan’, F. vesca ‘Illa Martin’ and F. moschata ‘Bauwens’ were also classified as partially resistant, but they are only of limited use for breeding because of their variable ploidy level. Fully resistant genotypes could not be detected. The systemic dispersal of the bacteria in strawberry plants was investigated after inoculation of leaves with X. fragariae strain XF3.9.C and the GFP‐tagged strain XF3.9.C_(pKAN). The systemic spread was evaluated after 3, 7, 14 and 28 days post‐inoculation (dpi) by nested PCR and fluorescence microscopy. After 3 dpi, X. fragariae could be found in all tissues tested including the inoculated leaf, its petiole, the rhizome, the heart bud up to the youngest fully expanded leaf and its petiole. The systemic spread was also detectable in partially resistant genotypes.     

Mechanisms of resistance in Brassica carinata,B. napus and B. juncea to Pseudocercosporella capsellae

Studies were undertaken to compare susceptible and resistant host responses to Pseudocercosporella capsellae in cotyledons of Brassica carinata, B. juncea and B. napus in order to define the mechanisms of resistance in these three species. On both resistant and susceptible hosts, hyphal penetration was always through stomatal openings and without infection pegs or appressoria. On resistant B. carinata ATC94129P, up to 72% of spores disintegrated and, generally, germination (<22%) and germ tube lengths (<25 μm) were comparatively low. Resistant B. napus Hyola 42 had the lowest germination (8%) and susceptible B. carinata UWA#012 had the highest (51%). On resistant B. carinata ATC94129P, germ tube extension was impeded across 24–60 h post‐inoculation (hpi) and percentage stomatal penetration lower (4%) at 60 hpi compared with susceptible B. carinata UWA#012 (26%). Stomatal densities (stomata/14 757 μm²) on resistant B. juncea Dune (2·12) and B. napus Hyola 42 (1·62) were lower than for susceptible B. juncea Vardan (2·40) and B. napus Trilogy (2·03). Resistant B. carinata ATC94129P had greater stomatal density (1·89) than susceptible B. carinata UWA#012 (1·58). Overall, B. juncea had greater stomatal density (2·26) compared with B. napus (1·83) and B. carinata (1·74). In resistant B. carinata ATC94129P, P. capsellae induced 28% stomata to close, while in susceptible B. carinata UWA#012 no such closure was induced. Epicuticular wax crystalloids were present only on resistant B. carinata ATC94129P and probably also contribute towards resistance.     

Reduced translocation is involved in resistance to glyphosate and paraquat in Conyza bonariensis and Conyza canadensis from California

Resistance to glyphosate and paraquat has evolved in some populations of Conyza spp. from California, USA. This study evaluated whether herbicide absorption and translocation were involved in the mechanism of resistance to both herbicides. Three lines of each species were used: glyphosate‐paraquat‐susceptible (GPS), glyphosate‐resistant (GR) and glyphosate‐paraquat‐resistant (GPR). Radiolabelled herbicide was applied to a fully expanded leaf, and absorption and movement out of the treated leaf were monitored for up to 24 h for paraquat and 72 h for glyphosate. Plants treated with paraquat were incubated in darkness for the first 16 h and then subjected to light conditions. More glyphosate was absorbed in C. bonariensis (52.9–58.3%) compared with C. canadensis (28.5–37.6%), but no differences in absorption were observed among lines within a species. However, in both species, the GR and GPR lines translocated less glyphosate out of the treated leaf when compared with their respective GPS lines. Paraquat absorption was similar among lines and across species (71.3–77.6%). Only a fraction of paraquat was translocated in the GPR lines (3% or less) when compared with their respective GPS or GR lines (20% or more) in both species. Taken together, these results indicate that reduced translocation is involved in the mechanism of resistance to glyphosate and paraquat in C. bonariensis and C. canadensis.     

Involvement of FgMad2 and FgBub1 in regulating fungal development and carbendazim resistance in Fusarium graminearum

Resistance to carbendazim of Fusarium graminearum is conferred by point mutation in the β₂‐tubulin gene that plays an important role in spindle assembly. The spindle assembly checkpoint is a cellular surveillance system that is critical for maintaining genomic stability. Predicted protein Mad2‐ and Bub1‐encoding genes in F. graminearum (FgMad2 and FgBub1) were isolated and characterized. There was no difference in FgMad2 and FgBub1 expression levels between carbendazim‐sensitive and ‐resistant strains; however, after carbendazim treatment FgMad2 expression increased while FgBub1 expression stayed the same. Both the FgMad2 and FgBub1 deletion mutants became more sensitive to carbendazim. The FgMad2 deletion mutants grew more slowly, produced fewer conidia and both hyphae and conidia were malformed. Conversely, deletion of FgBub1 had no effect on fungal development other than a reduction in conidia production. FgMad2 deletion mutants exhibited a severe decrease in perithecia production and pathogenicity along with a down‐regulation of trichothecene production, whereas FgBub1 deletion mutants exhibited only a slight reduction in perithecia production and was accompanied by a twofold increase in trichothecene production. Overall, the results indicate that both FgMad2 and FgBub1 are involved in carbendazim resistance and trichothecene biosynthesis, and FgMad2 plays an important role in fungal development in F. graminearum.     

Genetic analysis and molecular mapping of resistance to Puccinia striiformis f. sp. pseudo‐hordei in common wheat

This is the first genetic study reporting on the interaction and molecular mapping of resistance to the barley grass stripe rust pathogen (Puccinia striiformis f. sp. pseudo‐hordei, Psph) in common wheat. Seedlings of 638 wheat accessions were tested and it was determined that wheat is a near‐nonhost to Psph based on rare susceptibility observed in <2% of commercial cultivars and <5% of wheat landraces. As previously observed for P. striiformis f. sp. tritici (Pst), the Australian cultivar Teal was highly susceptible to Psph. In contrast, a selection of cv. Avocet carrying complementary resistance genes Yr73 and Yr74 (Avocet R; AvR) was resistant. The Teal × AvR (T/A) doubled haploid (DH) population was used to map resistance in AvR to Psph. Infection types on the T/A DH lines inoculated with Psph and Pst indicated that all DH lines carrying both Yr73 and Yr74 were also resistant to Psph; however, fewer DH lines were susceptible to Psph than expected, suggesting the resistance was more complex. QTL analysis using 9053 DArT‐Seq markers determined that resistance to Psph was polygenically inherited and mapped to chromosomes 3A, 3D, 4A and 5B. The 3DL and 5BL markers co‐located with Yr73 and Yr74, suggesting an overlap between host and non‐host resistance mechanisms.     

The ash dieback crisis: genetic variation in resistance can prove a long‐term solution

Over the last two decades, ash dieback has become a major problem in Europe, where the causative fungus has invaded the continent rapidly. The disease is caused by the invasive pathogenic fungus Hymenoscyphus pseudoalbidus (anamorph Chalara fraxinea), which causes severe symptoms and dieback in common ash, Fraxinus excelsior. It is becoming a significant threat to biodiversity in forest ecosystems and the economic and aesthetic impacts are immense. Despite the presence of the disease for at least 10 years in Scandinavia, a small fraction of F. excelsior trees have remained vigorous, and these trees exhibit no or low levels of symptoms even where neighbouring trees are very sick. This gives hope that a fraction of the ash trees will retain a sufficiently viable growth to survive. Following a period of high mortality in natural populations, selection and breeding of remaining viable ash trees could therefore provide a route for restoring the role of ash in the landscape. This paper reviews the available data on disease dissemination, and the consequences thereof in terms of symptom severity and mortality, and appraises studies that have tested the hypothesis that less‐affected trees have genetically based resistance. The implications of the results for the adaptive potential of common ash to respond to the disease through natural or assisted selection are discussed. The risks of adverse fitness effects of population fragmentation due to high mortality are considered. Finally, it is recommended that resistant trees (genotypes) should be selected to facilitate conservation of the species.     

Characterization of the first two viruses described from wild populations of hammer orchids (Drakaea spp.) in Australia

Sequences representing the genomes of two distinct virus isolates infecting wild plants of two members of the genus Drakaea (hammer orchids) in Western Australia are described. The virus isolated from Drakaea livida has a bipartite genome of 4490 nt (RNA1) and 2905 nt (RNA2) that shares closest sequence and structural similarity to members of the genus Pecluvirus, family Virgaviridae, described from legumes in the Indian subcontinent and West Africa. However, it differs from pecluviruses by lacking a P39 protein on RNA2 and having a cysteine‐rich protein gene located 3′ of the triple gene block protein genes. It is the first peclu‐like virus to be described from Australia. The name Drakaea virus A is proposed (DVA; proposed member of the family Virgaviridae, genus unassigned). The second virus isolate was identified from Drakaea elastica, a species classed as endangered under conservation legislation. The genome sequence of this virus shares closest identity with isolates of Donkey orchid symptomless virus (DOSV; proposed member of the order Tymovirales, family and genus unassigned), a species described previously from wild Caladenia and Diuris orchids in the same region. These viruses are the first to be isolated from wild Drakaea populations and are proposed to have an ancient association with their orchid hosts.     

Pseudomonas syringae pv. actinidiae: chemical control,resistance mechanisms and possible alternatives

Pseudomonas syringae pv. actinidiae (Psa) is a Gram‐negative bacterium that causes the bacterial canker of both green (Actinidia deliciosa) and yellow (Actinidia chinensis) fleshed kiwifruit. Since the emergence of an economically devastating Psa outbreak in Japan in the 1980s, the disease took a contagious turn causing severe economic loss to kiwifruit industries in Italy, South Korea, Spain, New Zealand and other countries. Research shows that the pathogenic strains isolated from different infected orchards vary in their virulence characteristics and have distinct genes coding for the production of different toxins. The global Psa outbreak has activated research around the world on developing efficient strategies to contain the pandemic and minimize loss to the kiwifruit industry. Chemical and biological control options, orchard management and breeding programmes are being employed in this global effort. Synergy between different disease control strategies has been recognized as important. Phytotoxicity, resistance development and regulatory measures in certain countries restrict the use of copper compounds and antibiotics, which are otherwise the mainstay chemicals against bacterial plant diseases. Therefore, because of the limitations of existing chemicals, it is important to develop novel chemical controls against Psa. Antimicrobial peptides, which are attractive alternatives to conventional antibiotics, have found promising applications in plant disease control and could contribute to expanding the chemical control tool box against Psa. This review summarizes all chemical compounds trialled so far against Psa and provides thoughts on the development of antimicrobial peptides as potential solutions for the future.     

Disease protection and growth promotion of potatoes (Solanum tuberosum L.) by Paenibacillus dendritiformis

One of the challenges in developing plant‐beneficial bacterial agents for agricultural application is ensuring that an effective selection and screening procedure is in place. The sporadic success of using bacterial agents in the field is usually due to the inability of added bacteria to compete with the local microorganisms. In the present study, the effectiveness of Paenibacillus dendritiformis, a unique pattern‐forming, Gram‐positive, soil bacterium, to reduce disease indices and increase yield in potato crops was examined. This bacterium was chosen as a potential agent based on genome analysis carried out in previous studies. In vitro laboratory experiments, as well as three greenhouse and one field experiment, were conducted. The results show that, in agreement with the hypothesis, P. dendritiformis significantly reduced the maceration area of tuber slices infected by Pectobacterium carotovorum subsp. carotovorum, significantly reduced disease indices in greenhouse experiments and significantly increased tuber yield of infected plants in the field. This work demonstrates the potential of preliminary screening based on genome analysis to identify effective biocontrol agents.     

Molecular,biological and physiological characterizations of resistance to phenamacril in Fusarium graminearum

Fusarium head blight (FHB), caused by Fusarium graminearum, is one of the most devastating wheat diseases in China. Phenamacril is a novel cyanoacrylate fungicide with a unique chemical structure and specific mode of action against Fusarium spp. In this study, the molecular, biological and physiological characteristics of laboratory‐induced mutants of F. graminearum with resistance to phenamacril were investigated. Compared to the wildtype strains, the phenamacril‐resistant mutants showed obvious defects in various biological and physiological characteristics, including vegetative growth, carbon source utilization, response to oxidative and osmotic stresses, sensitivity to cell wall and cell membrane integrity inhibitors, cell membrane permeability, glycerol accumulation and pathogenicity. The phenotypes of the phenamacril‐resistant mutants exhibited many variations. Sequencing indicated that the three parental strains studied were identical, and the mutants TXR1, TXR2, BMR1, BMR2, SYR1 and SYR2 each had a single point mutation in the amino acid sequence encoded by the myosin‐5 gene (FGSG_01410). These results provide new reference information for future investigations concerning the resistance mechanism of F. graminearum to phenamacril and could offer important relevant data for the management of FHB caused by F. graminearum.