Ascochyta blight in North Dakota field pea: the pathogen complex and its fungicide sensitivity

Worldwide, Ascochyta blight is caused by a complex of host-specific fungal pathogens, including Ascochyta pisi, Didymella pinodes, and Didymella pinodella. The application of foliar fungicides is often necessary for disease management, but a better understanding of pathogen prevalence, aggressiveness, and fungicide sensitivity is needed to optimize control. Leaf and stem samples were obtained from 56 field pea production fields in 14 counties in North Dakota from 2017 to 2020 and isolates were collected from lesions characteristic of Ascochyta blight. Based on fungal characteristics and sequencing the ITS1-5.8S-ITS2 region, 73% of isolates were confirmed to be D. pinodes (n = 177) and 27% were A. pisi (n = 65). Across pathogens, aggressiveness was similar among some isolates in greenhouse assays. The in vitro pyraclostrobin sensitivity of all D. pinodes isolates collected from 2017 to 2020 was lower than that of the three baseline isolates. Sensitivity of 91% of A. pisi isolates collected in 2019 and 2020 was lower than the sensitivity of two known sensitive isolates. Resistance factors (Rf) from mean EC50 values of pyraclostrobin baseline/known sensitive isolates to isolates collected from 2017 to 2020 ranged from 2 to 1,429 for D. pinodes and 1 to 209 for A. pisi. In vitro prothioconazole sensitivity of 91% of D. pinodes isolates collected from 2017 to 2020 was lower than the sensitivity of the baseline isolates and 98% of A. pisi isolates collected from 2019 to 2020 was lower than the sensitivity of the known sensitive isolates. Prothioconazole Rf ranged from 1 to 338 for D. pinodes and 1 to 127 for A. pisi. Based on in vitro results, 92% of D. pinodes and 98% of A. pisi isolates collected displayed reduced-sensitivity/resistance to both fungicides when compared to baseline/known sensitive isolates. Disease control under greenhouse conditions of both pathogens provided by both fungicides was significantly lower in isolates determined to be reduced-sensitive or resistant in in vitro assays when compared to sensitive. Results reported here reinforce growers desperate need of alternative fungicides and/or management tools to fight Ascochyta blight in North Dakota and neighboring regions.

Characterization of field pea (Pisum sativum) resistance against Peyronellaea pinodes and Didymella pinodella that cause ascochyta blight

Ascochyta blight is one of the most destructive diseases in field pea and is caused by either individual or combined infections by the necrotrophic pathogens Peyronellaea pinodes, Didymella pinodella, Ascochyta pisi and Ascochyta koolunga. Knowledge of disease epidemiology will help in understanding the resistance mechanisms, which, in turn, is beneficial in breeding for disease resistance. A pool of breeding lines and cultivars were inoculated with P. pinodes and D. pinodella to study the resistance responses and to characterize the underlying resistance reactions. In general, phenotypic analysis of controlled environment disease assays showed clear differential responses among genotypes against the two pathogens. The released variety PBA Wharton and the breeding line 11HP302-12HO-1 showed high levels of resistance against both pathogens whereas PBA Twilight and 10HP249-11HO-7 showed differential responses between the two pathogens, showing higher resistance against D. pinodella as compared to P. pinodes. OZP1604 had high infection levels against both pathogens. Histochemical analysis of leaves using diamino benzidine (DAB) showed the more resistant genotypes had lower accumulation of hydrogen peroxide compared to susceptible genotypes. The digital images of DAB staining were analyzed using ImageJ, an image analysis software. The image analysis results showed that quantification of leaf disease infection through image analysis is a useful tool in estimating the level of cell death in biotic stress studies. The qRT-PCR analysis of defense related genes showed that partially resistant genotypes had significantly higher expression of PsOXII and Pshmm6 in the P. pinodes treated plants, whereas expression of PsOXII, PsAPX1, PsCHS3 and PsOPR1 increased in partially resistant plants inoculated with D. pinodella. The differential timing and intensity of expression of a range of genes between resistant lines challenged with the same pathogen, or challenged with different pathogens, suggests that there are multiple pathways that restrict infection in this complex pathogen-host interaction. The combination of phenotypic, histochemical and molecular approaches provide a comprehensive picture of the infection process and resistance mechanism of pea plants against these pathogens.

Challenges With Managing Disease Complexes During Application of Different Measures Against Foliar Diseases of Field Pea

Studies were undertaken across five field locations in Western Australia to determine the relative changes in disease severity and subsequent field pea yield from up to four foliar pathogens associated with a field pea foliar disease complex (viz. genera Didymella, Phoma, Peronospora, and Septoria) across four different pea varieties sown at three different times and at three different densities. Delaying sowing of field pea significantly (P < 0.05) reduced the severity of Ascochyta blight (all five locations) and Septoria blight (one location), increased the severity of downy mildew (four locations), but had no effect on seed yield. In relation to Ascochyta blight severity at 80 days after sowing, at all locations the early time of sowing had significantly (P < 0.05) more severe Ascochyta blight than the mid and late times of sowing. Increasing actual plant density from 20 to 25 plants m-2 to 58 to 78 plants m-2 significantly (P < 0.05) increased the severity of the Ascochyta blight (four locations) and downy mildew (one location), and it increased seed yield at four locations irrespective of sowing date and three locations irrespective of variety. Compared with varieties Dundale, Wirrega, and Pennant, variety Alma showed significantly (P < 0.05) less severe Ascochyta blight, downy mildew, and Septoria blight (one location each). Grain yield was highest for the early time of sowing at three locations. Varieties Alma, Dundale, and Wirrega significantly (P < 0.05) outyielded Pennant at four locations. The percentage of isolations of individual Ascochyta blight pathogens at 80 days after the first time of sowing varied greatly, with genus Didymella ranging from 25 to 93% and genus Phoma ranging from 6 to 23% across the five field locations. This fluctuating nature of individual pathogen types and proportions within the Ascochyta blight complex, along with variation in the occurrence of pathogens Peronospora and Septoria, highlights the challenges to understand and manage the complexities of co-occurring different foliar pathogens of field pea. While the search for more effective host resistance continues, there is a need for and opportunities from further exploring and exploiting cultural management approaches focusing on crop sequence diversification, intercropping, manipulating time of sowing and stand density, and application of improved seed sanitation and residue/inoculum management practices. We discuss the constraints and opportunities toward overcoming the challenges associated with managing foliar disease complexes in field pea.

Clarification on Host Range of Didymella pinodes the Causal Agent of Pea Ascochyta Blight

Didymella pinodes is the principal causal agent of ascochyta blight, one of the most important fungal diseases of pea (Pisum sativum) worldwide. Understanding its host specificity has crucial implications in epidemiology and management; however, this has not been clearly delineated yet. In this study we attempt to clarify the host range of D. pinodes and to compare it with that of other close Didymella spp. D. pinodes was very virulent on pea accessions, although differences in virulence were identified among isolates. On the contrary, studied isolates of D. fabae, D. rabiei, and D. lentil showed a reduced ability to infect pea not causing macroscopically visible symptoms on any of the pea accessions tested. D. pinodes isolates were also infective to some extend on almost all species tested including species such as Hedysarum coronarium, Lathyrus sativus, Lupinus albus, Medicago spp., Trifolium spp., Trigonella foenum-graecum, and Vicia articulata which were not mentioned before as hosts of D. pinodes. On the contrary, D. lentil and D. rabiei were more specific, infecting only lentil and chickpea, respectively. D. fabae was intermediate, infecting mainly faba bean, but also slightly other species such as Glycine max, Phaseolus vulgaris, Trifolium spp., Vicia sativa, and V. articulata. DNA sequence analysis of the nuclear ribosomal internal transcribed spacer region (ITS) was performed to confirm identity of the isolates studies and to determine phylogenetic relationship among the Didymella species, revealing the presence of two clearly distinct clades. Clade one was represented by two supported subclusters including D. fabae isolates as well as D. rabiei with D. lentil isolates. Clade two was the largest and included all the D. pinodes isolates as well as Phoma medicaginis var. pinodella. Genetic distance between D. pinodes and the other Didymella spp. isolates was not correlated with overall differences in pathogenicity. Based on evidences presented here, D. pinodes is not specialized on pea and its host range is larger than that of D. fabae, D. lentil, and D. rabiei. This has relevant implications in epidemiology and control as these species might act as alternative hosts for D. pinodes.

Relative Host Resistance to Black Spot Disease in Field Pea (Pisum sativum) is Determined by Individual Pathogens

Black spot, also known as Ascochyta blight, is the most important disease on field pea (Pisum sativum). It is caused by a complex of pathogens, the most important of which in Australia include Didymella pinodes, Phoma pinodella, and P. koolunga. The relative proportions of these and other component pathogens of the complex fluctuate widely across time and geographic locations in Australia, limiting the ability of breeders to develop varieties with effective resistance to black spot. To address this, 40 field pea genotypes were tested under controlled environment conditions for their individual stem and leaf responses against these three pathogens. Disease severity was calculated as area under disease progress curve (AUDPC), and subsequently converted to mean rank (MR). The overall rank (OR) for each pathogen was used to compare response of genotypes under inoculation with each pathogen. The expressions of host resistance across the field pea genotypes were largely dependent upon the individual test pathogen and whether the test was on stem or leaf. Overall, P. koolunga caused most severe stem disease; significantly more severe than either D. pinodes or P. pinodella. This is the first report of the host resistance identified in field pea to P. koolunga; the five genotypes showing highest resistance on stem, viz. 05P778-BSR-701, ATC 5338, ATC 5345, Dundale, and ATC 866, had AUDPC MR values <250.4, while the AUDPC MR values of the 19 genotypes showing the best resistance on leaf was less than 296.8. Two genotypes, ATC 866 and Dundale, showed resistance against P. koolunga on both stem and leaf. Against D. pinodes, the four and 16 most resistant genotypes on stem and leaf had AUDPC MR values <111.2 and <136.6, respectively, with four genotypes showing resistance on both stem and leaf including 05P770-BSR-705, Austrian Winter Pea, 06P822-(F5)-BSR-6, and 98107-62E. Against P. pinodella, four and eight genotypes showing the best resistance on stem and leaf had AUDPC MR values <81.3 and <221.9, respectively; three genotypes, viz. 98107-62E, Dundale, and Austrian Winter Pea showed combined resistance on stem and leaf. A few genotypes identified with resistance against two major pathogens of the complex will be of particular significance to breeding programs. These findings explain why field pea varieties arising from breeding programs in Australia fail to display the level or consistency of resistance required against black spot and why there needs to be a wider focus than D. pinodes in breeding programs.

Temporal and Spatial Changes in the Pea Black Spot Disease Complex in Western Australia

Black spot (also referred to as Ascochyta blight, Ascochyta foot rot and black stem, and Ascochyta leaf and pod spot) is a devastating disease of pea (Pisum sativum) caused by one or more pathogenic fungi, including Didymella pinodes, Ascochyta pisi, and Phoma pinodella. Surveys were conducted across pea-growing regions of Western Australia in 1984, 1987, 1989, 1996, 2010, and 2012. In total, 1,872 fungal isolates were collected in association with pea black spot disease symptoms. Internal transcribed spacer regions from representative isolates, chosen based on morphology, were sequenced to aid in identification. In most years and locations, D. pinodes was the predominant pathogen in the black spot complex. From 1984 to 2012, four new pathogens associated with black spot symptoms on leaves or stems (P. koolunga, P. herbarum, Boeremia exigua var. exigua, and P. glomerata) were confirmed. This study is the first to confirm P. koolunga in association with pea black spot symptoms in field pea in Western Australia and show that, by 2012, it was widely present in new regions. In 2012, P. koolunga was more prevalent than D. pinodes in Northam and P. pinodella in Esperance. P. herbarum and B. exigua var. exigua were only recorded in 2010. Although A. pisi was reported in Western Australia in 1912 and again in 1968 and is commonly associated with pea black spot in other states of Australia and elsewhere, it was not recorded in Western Australia from 1984 to 2012. It is clear that the pathogen population associated with the pea black spot complex in Western Australia has been dynamic across time and geographic location. This poses a particular challenge to development of effective resistance against the black spot complex, because breeding programs are focused almost exclusively on resistance to D. pinodes, largely ignoring other major pathogens in the disease complex. Furthermore, development and deployment of effective host resistance or fungicides against just one or two of the pathogens in the disease complex could radically shift the make-up of the population toward pathogen species that are least challenged by the host resistance or fungicides, creating an evolving black spot complex that remains ahead of breeding and other management efforts.

Diversity of pea-associated F. proliferatum and F. verticillioides populations revealed by FUM1 sequence analysis and fumonisin biosynthesis

Fusarium proliferatum and F. verticillioides are considered as minor pathogens of pea (Pisum sativum L.). Both species can survive in seed material without visible disease symptoms, but still contaminating it with fumonisins. Two populations of pea-derived F. proliferatum and F. verticillioides strains were subjected to FUM1 sequence divergence analysis, forming a distinct group when compared to the collection strains originating from different host species. Furthermore, the mycotoxigenic abilities of those strains were evaluated on the basis of in planta and in vitro fumonisin biosynthesis. No differences were observed in fumonisin B (FB) levels measured in pea seeds (maximum level reached 1.5 μg g(-1)); however, in rice cultures, the majority of F. proliferatum genotypes produced higher amounts of FB1-FB3 than F. verticillioides strains.

Candidate genes for quantitative resistance to Mycosphaerella pinodes in pea (Pisum sativum L.)

Partial resistance to Mycosphaerella pinodes in pea is quantitatively inherited. Genomic regions involved in resistance (QTLs) have been previously identified in the pea genome, but the molecular basis of the resistance is still unknown. The objective of this study was to map resistance gene analogs (RGA) and defense-related (DR) genes in the JI296 x DP RIL population that has been used for mapping QTLs for resistance to M. pinodes, and identify co-localizations between candidate genes and QTLs. Using degenerate oligonucleotide primers designed on the conserved motifs P-loop and GLPL of cloned resistance genes, we isolated and cloned 16 NBS-LRR sequences, corresponding to five distinct classes of RGAs. Specific second-generation primers were designed for each class. RGAs from two classes were located on the linkage group (LG) VII. Another set of PCR-based markers was designed for four RGA sequences previously isolated in pea and 12 previously cloned DR gene sequences available in databases. Out of the 16 sequences studied, the two RGAs RGA-G3A and RGA2.97 were located on LG VII, PsPRP4A was located on LG II, Peachi21, PsMnSOD, DRR230-b and PsDof1 were mapped on LG III and peabetaglu and DRR49a were located on LG VI. Two co-localizations between candidate genes and QTLs for resistance to M. pinodes were observed on LG III, between the putative transcription factor PsDof1 and the QTL mpIII-1 and between the pea defensin DRR230-b gene and the QTL mpIII-4. Another co-localization was observed on LG VII between a cluster of RGAs and the QTL mpVII-1. The three co-localizations appear to be located in chromosomal regions containing other disease resistance or DR genes, suggesting an important role of these genomic regions in defense responses against pathogens in pea.

Tulbaghia violacea L. II: In vivo antifungal properties towards plant pathogens

In vitro antifungal activity of crude extracts from Tulbaghia violacea against 6 economically important plant pathogenic fungi was previously reported. The in vivo control of Mycosphaerella pinodes, causative of Ascochyta blight, by different concentrations of a crude aerial part extract of T. violacea was subsequently followed qualitatively and quantitatively in terms of lesions that developed over a 6-day period at 20°C on detached pea (Pisum sativum) leaves. Infection by M. pinodes spores was prevented when the extract was applied both before and after inoculation, confirming complete inhibition of spore germination, whereas no phytotoxic effect was observed on the leaves. Additionally, the control of sorghum covered (Sporisorium sorghi) and loose (S. cruentum) kernel smuts by seed treatment with the crude extract was tested under field conditions. Before planting, different sorghum seed lots were inoculated separately with spores from the 2 pathogens at a rate of 0.5% per kg (w/w), followed by treatment with the crude extract, at a concentration of 2.0 mg/mL, 24 h later. A standard fungicide, Thiram (65 W), was applied as a positive control at a rate of 0.25% per kg (v/w). Disease incidence was quantified during harvest and expressed as percentage infected plants. Seed treatment with the extract significantly (P < 0.05) reduced the incidence of both sorghum loose and covered smut diseases, compared favourably with the standard fungicide, and resulted in significant yield increases compared to the untreated control.

The epidemiology and control of ascochyta blight in field peas: a review

Ascochyta blight is one of the most important diseases affecting field peas. The disease occurs in almost all pea-growing regions of the world and can cause significant crop losses when conditions are favourable for an epidemic. Here we review current knowledge of the epidemiology of the disease. Details are provided of disease symptoms, the disease cycle and the taxonomy of the causal fungi, Ascochyta pisi, Mycosphaerella pinodes and Phoma pinodella. The importance of seed-, soil- and air-borne inoculum is discussed along with the factors that influence survival of the causal fungi in soil, on seed or associated with pea trash. Many studies have been reviewed to establish how the fungi responsible for the disease survives from year to year, how the disease becomes established in new crops and the conditions that favour disease development. Evidence is provided that crop rotation, destruction of infected pea trash and chemical seed treatments can significantly reduce the amount of primary inoculum. Later sowing of crops has been shown to reduce the incidence and severity of disease. Fungicides have been used successfully to control the disease, although the cost of their application can significantly reduce the profitability of the crop. The best long-term strategy for effective disease control appears to be the development of ascochyta blight resistant pea varieties. Reports of physiological specialisation in ascochyta blight fungi are also documented. Despite extensive screening of germplasm, relatively few sources of resistance to ascochyta blight fungi have been found in Pisum sativum. However, the discovery of much better sources of resistance in closely related species and the development of advanced breeding methods offer new possibilities for developing useful resistance.

Mapping of quantitative trait loci for partial resistance to Mycosphaerella pinodes in pea (Pisum sativum L.), at the seedling and adult plant stages

The inheritance of resistance to Ascochyta blight, an economically important foliar disease of field pea ( Pisum sativum L.) worldwide, was investigated. Breeding resistant pea varieties to this disease, caused by Mycosphaerella pinodes, is difficult due to the availability of only partial resistance. We mapped and characterized quantitative trait loci (QTLs) for resistance to M. pinodes in pea. A population of 135 recombinant inbred lines (RILs), derived from the cross between DP (partially resistant) and JI296 (susceptible), was genotyped with morphological, RAPD, SSR and STS markers. A genetic map was elaborated, comprising 206 markers distributed over eight linkage groups and covering 1,061 cM. The RILs were assessed under growth chamber and field conditions at the seedling and adult plant stages, respectively. Six QTLs were detected at the seedling stage, which together explained up to 74% of the variance. Ten QTLs were identified at the adult plant stage in the field, and together these explained 56.6-67.1% of the variance, depending on the resistance criteria and the organ considered. Four QTLs were detected under both growth chamber and field conditions, suggesting they were not plant-stage dependent. Three QTLs for flowering date and three QTLs for plant height were also identified in the RIL population, some of which co-located with QTLs for resistance. The relationship between QTLs for resistance to M. pinodes, plant height and flowering date is discussed.

Genetics of resistance to Mycosphaerella pinodes in Lathyrus sativus

Three Lathyrus sativus accessions were screened for their reaction to Mycosphaerella pinodes infection. Accession ATC 80878 displayed the lowest percentage stem lesion values (%SL) and was significantly more resistant to M. pinodes than ATC 80407 and ATC 80053. Accession ATC 80407 was the most susceptible, displaying the severest disease symptoms. A backcross and an F2 population were generated using accessions ATC 80878 and ATC 80407 as the resistant and susceptible parents, respectively. The backcross and F2 progeny segregated in a 3 : 1 and 7 : 9 ratio, respectively, for resistance/susceptibility, using the 99% confidence intervals for the means of the parental controls in each assay to determine the point of discontinuity. The segregation data from both populations fitted a Mendelian segregation model that suggested that resistance in the L. sativus accession ATC 80878 may be controlled by 2 independently segregating genes, operating in a complementary epistatic manner.

Virulence and Genetic Variability Among Isolates of Mycosphaerella pinodes

Fifty-eight isolates of Mycosphaerella pinodes, collected from western Canada, New Zealand, France, Australia, the United Kingdom, and Ireland, were analyzed for pathogenic and genetic variation according to their virulence on six differential cultivars of field pea (AC Tamor, Bohatyre, Danto, Majoret, Miko, and Radley) and amplified fragment length polymorphism markers. The 56 isolates were classified into 15 pathotypes. Pathotype 1 consisted of 31 isolates that were virulent on all six pea differential cultivars. Pathotypes 14 and 15 consisted of eight isolates that were avirulent on all six differential cultivars or virulent on one of six differential cultivars. The analysis of molecular variance showed that 57.2% of the total variation was caused by differences among populations, and 42.8% was due to molecular diversity within populations. Phylogenetic analysis of molecular variation of isolates showed that most of the Canadian isolates and four Australian isolates formed two clustered groups, respectively, regardless of virulence on the six differential cultivars. Isolates from New Zealand were geographically clustered into two groups. However, the isolates from France, Ireland, and the United Kingdom were clustered with the Canadian isolates.

A Search for Resistance in Peas to Mycosphaerella pinodes

In 1991, 1992, and 1994, 2,936 accessions of Pisum sativum from the USDA Plant Introduction collection were screened under field conditions for resistance to Mycosphaerella pinodes without replication due to seed supply. These trials were conducted at the Oak Park Research Station at Carlow, Ireland, where conditions are highly favorable for disease expression. In 1995, the 157 most resistant accessions were again screened at Carlow and at the Food and Crops Research Station at Gore, New Zealand, in replicated trials. At both locations, disease was severe and ranged from 20 to 100% foliar blight. Five accessions were as resistant as the commercial cultivar Radley at both locations: PI 142441, PI 142442, PI 381132, PI 404221, and PI 413691. No accessions were more resistant than the cultivar Radley.