Global incidence of wheat rusts and powdery mildew during 1969–2010 and durability of resistance of winter wheat variety Bezostaya 1

Disease incidence and severity was studied for winter wheat variety Bezostaya 1 and susceptible checks based on data from international nurseries from 1969 to 2010 and from 51 countries across major winter wheat production regions totalling 1,047 reports. The frequency of leaf rust and stripe rust occurrence was stable over time with marked increases in severity in 2001–2010 especially in Europe and Central and West Asia. Substantial global reductions in stem rust occurrence were recorded and attributed primarily to use of resistance genes although the recent emergence of race Ug99 makes wheat more vulnerable. The occurrence of powdery mildew remained globally stable over time. It was the most important foliar disease in Western and Southern Europe, where the frequency was very high for all time periods coupled with slight increases in severity during 2001–2010. The durable resistance of variety Bezostaya 1 to all four diseases was demonstrated in the study using comparisons of disease severities of Bezostaya 1 and the most susceptible entries. The Lr34/Yr18/Pm38 pleiotrophic set possessed by Bezostaya 1 is currently an important target for selection because it is now amenable to molecular selection. Increased use of genes like Lr34 combined with strategies to minimize cultivation of extremely susceptible varieties will contribute to long term maintenance of low and non-damaging disease levels. The durable disease resistance of Bezostaya 1, combined with its adaptability and good end-use quality, was a significant reason for its huge impact in agriculture over the last 50?years.     

Occurrence of boscalid resistance in cucumber powdery mildew in Japan and molecular characterization of the iron–sulfur protein of succinate dehydrogenase of the causal fungus

A total of 74 mass isolates of cucumber powdery mildew fungus (Podosphaera xanthii) were collected from commercial greenhouses with a history of boscalid use in Ibaraki Prefecture, Japan, and tested in a leaf disk assay for their sensitivity to boscalid. The mildew development of 40 of 74 isolates and five sensitive reference isolates on the disks was completely suppressed at 5 μg boscalid/ml. The minimum inhibitory concentrations (MIC) for the remaining 34 isolates were 50 μg/ml or higher, and 21 of these isolates also grew well at 500 μg/ml. Six single-spore isolates were resistant to boscalid with MIC values higher than 500 μg/ml; four of these were moderately resistant (MR), and two were very highly resistant (VHR) isolates. The growth of MR isolates was almost completely suppressed at 500 μg/ml, whereas two VHR isolates grew vigorously at 500 μg/ml. In foliar inoculation tests of potted cucumber plants, the efficacy of boscalid (500 μg/ml) against both MR and VHR isolates was very low. Partial DNA fragment of the iron–sulphur protein subunit (SdhB) gene of succinate dehydrogenase was PCR-amplified from five sensitive and five resistant isolates and directly sequenced, revealing that VHR isolates possess a substitution from a highly conserved histidine (CAT) to tyrosine (TAT) in a third cysteine-rich center of a putative SdhB, whereas MR isolates so far have not been found to have such substitution in SdhB.     

Expression of the yeast Δ-9 desaturase gene in tomato enhances its resistance to powdery mildew

Tomato plants (Lycopersicon esculentumMill.) transformed with the yeast Δ-9 desaturase gene were found to display enhanced resistance to powdery mildew (Erysiphe polygoniDC. edmund. Salm.) and to possess higher levels of 16:1, 16:2 and 18:1 fatty acids. Lipid peroxides also increased. The improved resistance of the transgenic plants was apparently associated with the increases in 16:1 and 16:2 fatty acids.     

African wheat germplasm – a valuable resource for resistance to rust diseases

Known and unknown genes conferring seedling and adult plant resistance (APR) to leaf rust, stem rust and stripe rust were detected either singly or in combination in a set of 136 African wheat genotypes using multi-pathotype tests with characterized Australian Puccinia triticina (Pt), P. graminis f. sp. tritici (Pgt) and P. striiformis f. sp. tritici (Pst) pathotypes. Lines Beladi 132, IYN 68/9.44, Kenya Kifaru and Kenya Mbweha were postulated to carry resistance against multiple pathotypes of Pt, Pgt and Pst, whereas IAR/W/163-3, Grano Di Moggio Tipo 44 and Trigo 48 had resistance against all pathotypes tested in the current study. Field evaluation with the three rust pathogens detected low to high APR in more than 50% of lines, and while most tested positive with markers linked to known APR genes (csLV34, csLV46G22, TM10KASPAR, csGS, Cfb5006 and csSr2), many carried unidentified and useful resistance to all three rusts. Genetic analysis of F₃ mapping populations based on seven genotypes showed either monogenic or digenic inheritance of APR to leaf rust, stem rust and stripe rust. The lines postulated to carry effective uncharacterized seedling genes and APR genes are of great potential value in diversifying resistance to help achieve durable control of all three rust diseases of wheat.     

What types of powdery mildew can infect wheat-barley introgression lines?

This work is a detailed study of the infection of fungal biotrophic pathogens causing powdery mildew diseases on introgression lines originating from the intergeneric hybridisation between wheat and barley (Triticum aestivum L. × Hordeum vulgare L.). Powdery mildew fungi are among the most widespread biotrophic pathogens of plants also and infect dicot and monocot species. Most powdery mildew species are strictly host specific. They colonize only a narrow range of species or one particular host species. The intergeneric hybridisation between wheat and barley could result in expansions of host ranges of the barley powdery mildew. Our experiments covered natural infections in the field and artificial infections under greenhouse conditions. Formae speciales of powdery mildew were identified on the basis of the sequencing results of ribosomal internal transcribed spacer sequences (rDNA-ITS). We identified Blumeria graminis f.sp. tritici isolate 14 (HM484334) on the wheat parent and all wheat-barley introgression lines and B. g. f. sp. hordei isolate MUMH1723 (AB 273556) on the barley parent, respectively. The wheat-barley introgression lines were inoculated with barley powdery mildew under greenhouse conditions. According to our results the added barley chromosomes (or segments) do not cause host range expansion of barley powdery mildew.     

Variation in the nrDNA ITS sequences of some powdery mildew species: do routine molecular identification procedures hide valuable information?

During the past years, nrDNA ITS sequences have supported the identification of many powdery mildew fungi because comprehensive analyses showed that differences in these sequences have always correlated with the delimitation of different species and formae speciales of the Erysiphales. Published data, obtained using direct sequencing of the PCR products, suggested that even one to five nucleotide differences in the ITS sequences delimit different, albeit closely related, species, and/or indicate differences in host range patterns. Here we show that such differences in the ITS sequences can be detected even in a single sample of a powdery mildew fungus. We sequenced the ITS region in 17 samples, representing six powdery mildew species, both directly and after cloning the PCR products. Among these, samples of O. longipes exhibited two or three, samples of O. neolycopersici three or four, those of an Oidium sp. from Chelidonium majus up to seven, and a sample of another Oidium sp. from Passiflora caerulea two different ITS types determined after cloning. No ITS nucleotide polymorphisms were found in samples of O. lycopersici and Erysiphe aquilegiae. This suggests that some powdery mildew taxa are more variable at the ITS level than others. Thus, although the ITS sequences determined by direct sequencing represent robust data useful in delimitation and phylogenetic analysis of distinct species of the Erysiphales, these need to be used with precaution, and preferably determined after cloning, especially when dealing with closely related taxa at species and sub-species levels. With this method a hitherto undetected genetic diversity of powdery mildews can be revealed.     

Structure of populations of wheat powdery mildew (Erysiphe graminis DC f.sp. tritici Marchal) in Central Europe in 1993–1996: I. Dynamics of virulence

In 1993–1996, the virulence of regional populations of the wheat powdery mildew pathogen (Erysiphe graminis DC f. sp. tritici Marchal) from the Czech Republic, Austria, Hungary and Slovakia against 13 resistance genes was investigated. The populations differed mainly at the regional level. Populations from the Czech Republic, mainly from the western regions, showed higher values of virulence against the Pm4b gene. Lower frequency of virulence against Pm4b was found in Austria, and the lowest value was observed in Hungary. The differences in frequencies of virulence against Pm4a and Pm4b showed a similar geographic pattern across the four countries: a continuous decline from west to east and from north to south. Virulence against Pm2 decreased in all countries considered; virulence to pm5, Pm6, Pm8 and Mli was high throughout. Genes and gene combinations that can ensure a relatively effective biological protection against this pathogen across Central Europe at present are Pm3b, Pm2+Mld and Pm1+2+9. Czech and Slovak populations were the most complex: virulence complexity reached a maximum in Slovakia in 1994. A similar evolution, though less significant, was observed in the Czech Republic. Data on complexity of isolates suggest that Central European populations of wheat powdery mildew tend to reach an intermediate level representing the optimal number of virulence genes. This process is probably a consequence of stabilizing selection.     

Virulence variation of cucurbit powdery mildews in the Czech Republic – population approach

Kosman diversity models were applied to analyses of virulence (disease reaction patterns) variation of 115 isolates of two cucurbit powdery mildew (CPM) species, Golovinomyces orontii (Go) and Podosphaera xanthii (Px), collected in the Czech Republic from 2010 through 2012. Diversity within and distances between Go and Px populations and each other in a spatio-temporal context and with regard to original host plant species were analyzed based on virulence patterns of individual isolates on a set of 21 melon (Cucumis melo L.) race differential genotypes. Significant differentiation among the Go and Px pathogen populations was revealed, and the results clearly demonstrate and confirm that the set of differential C. melo genotypes is well composed because of high differentiation capacity. Differentiation of pathogens among years was significant for both species. No significant difference between Go isolates from different host plant species was established due to high variability among Go isolates, but there was significant host-specific differentiation among Px isolates. Differentiation of pathogens among regions was not detected. These results revealed high virulence variation in isolates of Go and Px, and their spatio-temporal fluctuations. High diversity in virulence of Go isolates supports the treatment of Go as a complex of different (sub)species with distinct virulence factors. Similar relationships of selected Go isolates in a UPGMA dendrogram in a previously reported multigene phylogenetic tree support the logic and suitability of the Kosman assignment based approach to population studies of organisms with asexual or mixed modes of reproduction. The approach applied in this study provides a complex view of virulence structures of powdery mildew populations, and when combined with race determination and denomination on melon, it may serve as a base to understandvirulence variation of these CPM species from a spatio-temporal viewpoint.     

Annual dynamics of Zymoseptoria tritici populations in wheat cultivar mixtures: A compromise between the efficacy and durability of a recently broken-down resistance gene?

Cultivar mixtures slow polycyclic epidemics but may also affect the evolution of pathogen populations by diversifying the selection pressures exerted by their plant hosts at field scale. We compared the dynamics of natural populations of the fungal pathogen Zymoseptoria tritici in pure stands and in three binary mixtures of wheat cultivars (one susceptible cultivar and one cultivar carrying the recently broken-down Stb16q gene) over two annual field epidemics. We combined analyses of population “size” based on disease severity, and of population “composition” based on changes in the frequency of virulence against Stb16q in seedling assays with more than 3000 strains. Disease reductions were observed in mixtures late in the epidemic, at the whole-canopy scale and on both cultivars, suggesting the existence of a reciprocal protective effect. The three cultivar proportions in the mixtures (0.25, 0.5, and 0.75) modulated the decrease in (a) the size of the pathogen population relative to the two pure stands, (b) the size of the virulent subpopulation, and (c) the frequency of virulence relative to the pure stand of the cultivar carrying Stb16q. Our findings suggest that optimal proportions may differ slightly between the three indicators considered. We argue potential trade-offs that should be taken into account when deploying a resistance gene in cultivar mixtures: between the dual objectives “efficacy” and “durability,” and between the “size” and “frequency” of the virulent subpopulation. Based on current knowledge, it remains unclear whether virulent subpopulation size or frequency has the largest influence on interepidemic virulence transmission.     

Powdery mildew of Arabidopsis thaliana: a pathosystem for exploring the role of silicon in plant–microbe interactions

Silicon (Si) has been used in agriculture to protect plants against disease for hundreds of years and its prophylactic effects in monocots and dicots are well documented. The mechanisms by which Si exerts its protective effects in planta, however, are uncertain and presently the subject of debate. In this study, we sought to determine if Arabidopsis thaliana could be used to clarify the role of Si in plant–pathogen interactions. Accordingly, X-ray microanalysis mapping, light microscopy, scanning and transmission electron microscope techniques were used to examine the leaves of Si− fed A. thaliana plants inoculated with the powdery mildew fungus, Erysiphe cichoracearum. The results of this study demonstrate for the first time, that A. thaliana is a species that absorbs Si and that the incidence of powdery mildew disease for Si− fed plants is significantly lower compared to control plants. In particular, treatment with Si appeared to induce the production of an electron-dense, fungitoxic substance that accumulated within and around the collapsed fungal haustoria of infected epidermal cells within the leaves of disease-resistant plants. These results with Arabidopsis corroborate recent observations in other non-related species and support the emerging theory that the mechanisms by which Si imparts resistance to plants are complex and are not entirely explained by the traditionally proposed role of Si as a reinforcer of mechanical resistance. Collectively, the findings of the present study have established the Arabidopsis thaliana-Erysiphe cichoracearum pathosystem as a valid model to investigate the role of Si in plant–microbe interactions.     

Comment on ‘In Turkish wheat cultivars the resistance allele of LR34 is ineffective against leaf rust’

Journal of Plant Diseases and Protection -     

The dynamics of Phytophthora infestans populations in the major potato-growing regions of Asia – A review

Asia is now the largest potato-producing region of the world and late blight, caused by Phytophthora infestans, is the most important pathogen limiting production. This review documents, in both the historical and the current context, the population structure of P. infestans in the major areas of potato production in Asia. Information from diverse sources regarding the stated or inferred clonal pathogen lineages present, population changes, and possible migration routes of the pathogen into the countries of this region have been reviewed to aid researchers and those involved in managing late blight in Asia. The single most important factor for population change and resultant epidemics in this region has been found to be migration of pathogen genotypes from Europe and the Americas. Reducing the impact of such migration in the future will necessitate putting in place improved phytosanitary measures. To achieve this, data sharing using global networks such as AsiaBlight and EuroBlight is imperative.     

Mechanism and molecular markers associated with rust resistance in a chickpea interspecific cross (Cicer arietinum × Cicer reticulatum)

A gene that controls resistance to chickpea rust (Uromyces ciceris-arietini) has been identified in a recombinant inbred line (RIL) population derived from an interspecific cross between Cicer arietinum (ILC72) × Cicer reticulatum (Cr5-10), susceptible and resistant to rust, respectively. Both parental lines and all RILs displayed a compatible interaction but differed in the level of infection measured as Disease Severity (DS) and Area Under the Disease Progress Curve (AUDPC). Histological studies of the seedlings of resistant parental Cr5-10 line revealed a reduction in spore germination, appressorium formation, number of haustoria per colony and colony size, with little host cell necrosis, fitting the definition of partial resistance. A Quantitative Trait Locus (QTL) explaining 31% of the total phenotypic variation for DS in seedlings and 81% of the AUDPC in adult plants in the field was located on linkage group 7 of the chickpea genetic map. The AUDPC displayed a bimodal distribution with high frequency of susceptible lines and both the AUDPC and markers showed the same distorted segregation. Consequently, it was hypothesised that a single dominant gene (proposed as Uca1/uca1) controlled resistance to rust in adult plants. This allowed us to locate the gene on the genetic linkage map. Two Sequence Tagged Microsatellite Sites (STMS) markers, TA18 and TA180 (3.9 cM apart) were identified that flank the resistance gene. These findings could be the starting point for a Marker-Assisted Selection (MAS) programme for rust resistance in chickpea.     

Factors contributing to enhanced pink snow mould resistance of winter rye (Secale cereale L.) – Pivotal role of crowns

Frost tolerance, resistance to Microdochium nivale and associated biochemical changes were investigated in leaves and crowns of six winter rye inbred lines. Investigated lines differed in their susceptibility to snow mould but not in frost tolerance. It was shown that winter rye resistance is linked to defence response in crowns expressed by higher activity of antioxidant enzymes – CAT, SOD and total peroxidases – on the 1st day after inoculation and β-d-glucosidase activation during the course of pathogenesis. It could be postulated that changes in leaves are attributed to reaction to cold while changes in crowns are attributed to pathogen reaction.     

Note: Proline — An inducer of resistance against pearl millet downy mildew disease caused by<Emphasis Type="Italic">Sclerospora graminicola</Emphasis>

In an attempt to find a suitable alternative to the otherwise perilous chemical control strategy of disease management, the amino acid proline was evaluated for its efficiency to elicit resistance in pearl millet (Pennisetum glaucum (L.) R. Br.) against downy mildew disease caused bySclerospora graminicola (Sacc.) Schroet both under greenhouse and field conditions. Proline treatment to seeds at 50 mM concentration for 3 h, significantly enhanced the seed germination and seedling vigor of pearl millet in comparison with the control. The same concentration and duration of seed treatment protected the pearl millet plants from downy mildew by offering 58% protection under greenhouse and 67% protection under field conditions. Studies revealed that 3 days were required for proline-treated plants to develop resistance, which was systemic and was sustained throughout the life of the plants. Apart from disease protection, proline was also found effective in enhancing vegetative and reproductive growth of the plants, as evidenced by the increase in height, fresh weight, leaf area, tillering capacity, 1000-seed weight and grain yield in comparison with the control plants. http://www.phytoparasitica.org posting Oct. 3, 2004.     

Translocation of apple proliferation phytoplasma via natural root grafts – a case study

Apple proliferation (AP), a phytoplasma-induced disease of apple trees, was proven to be transmitted through infected grafting material and sap-sucking insects. To date there are little firm data on disease propagation in the field via natural root grafts. This question was thus addressed in the present case study by investigating trees of a 24-year old commercial apple orchard (‘Red Chief’ on MM 111), where the existence of root connections was discovered accidentally. After having displayed specific AP symptoms, nine trees were cut down and the stubs were infiltrated or brushed with glyphosate. Herbicide injury, however, remained not only restricted to the treated stubs, but also spread to approximately 50 neighbouring trees. Surprisingly, none of the pollinators (‘Granny Smith’ on M 9) growing interjacently and alternating between herbicide-damaged main crop trees was affected. Respective to the position of the nine AP-infected and glyphosate-treated cut stumps, four sections in the orchard were defined, from which a total of 122 trees was sampled and analysed using qualitative real-time PCR for detection of AP phytoplasma. The pathogen was found in 71.4% of ‘Red Chief’ trees with severe herbicide damage and 18.8% of trees with partial herbicide damage. None of the 31 investigated pollinators was AP-infected. Our data indicate that root connections seem to play a role for the spread of AP phytoplasma at least in older orchards and between trees on vigorous rootstocks.     

Insecticide resistance management of Bemisia tabaci (Hemiptera: Aleyrodidae) in Australian cotton – pyriproxyfen,spirotetramat and buprofezin

BACKGROUND

Bemisia tabaci is a globally significant agricultural pest including in Australia, where it exhibits resistance to numerous insecticides. With a recent label change, buprofezin (group 16), is now used for whitefly management in Australia. This study investigated resistance to pyriproxyfen (group 7C), spirotetramat (group 23) and buprofezin using bioassays and available molecular markers.

RESULTS

Bioassay and selection testing of B. tabaci populations detected resistance to pyriproxyfen with resistance ratios ranging from 4.1 to 56. Resistance to spirotetramat was detected using bioassay, selection testing and sequencing techniques. In populations collected from cotton, the A2083V mutation was detected in three populations of 85 tested, at frequencies ≤4.1%, whereas in limited surveillance of populations from an intensive horticultural region the frequency was ≥75.8%. The baseline susceptibility of B. tabaci to buprofezin was determined from populations tested from 2019 to 2020, in which LC₅₀ values ranged from 0.61 to 10.75 mg L⁻¹. From the bioassay data, a discriminating dose of 200 mg L⁻¹ was developed. Recent surveillance of 16 populations detected no evidence of resistance with 100% mortality recorded at doses ≤32 mg L⁻¹. A cross-resistance study found no conclusive evidence of resistance to buprofezin in populations with high resistance to pyriproxyfen or spirotetramat.

CONCLUSIONS

In Australian cotton, B. tabaci pest management is challenged by ongoing resistance to pyriproxyfen, while resistance to spirotetramat is an emerging issue. The addition of buprofezin provides a new mode-of-action for whitefly pest management, which will strengthen the existing insecticide resistance management strategy. © 2023 Commonwealth of Australia. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.     

Precision trunk injection technology for treatment of huanglongbing (HLB)-affected citrus trees—a review

Journal of Plant Diseases and Protection - The outbreak and spread of huanglongbing (HLB) disease have brought about tremendous negative effects and economical losses on citrus production all over...     

Comparing the effectiveness of R genes in a 2-year canola–wheat rotation against Leptosphaeria maculans,the causal agent of blackleg disease in Brassica species

European Journal of Plant Pathology - Blackleg, which is caused by the ascomycete fungus Leptosphaeria maculans, is a major disease of canola in Canada and worldwide. In Canada, canola is usually...