A simple model of pea (Pisum sativum) growth affected by Mycosphaerella pinodes

The effect of epidemics of Mycosphaerella pinodes on crop growth, radiation interception efficiency (RIE) and radiation use efficiency (RUE) was studied in field conditions and the growth of diseased crop was modelled. Natural epidemics were simulated in field plots by dispersion on the ground of barley grains colonized by the fungus. Growth and leaf area index (LAI) were measured in healthy and diseased plots during the growth season. Epidemics affected crop growth mainly by decreasing RUE, with a slight decrease in RIE. This was probably due to the strong effect of M. pinodes on leaf photosynthesis and the lateness of Mycosphaerella blight epidemics on spring pea, which became serious when the canopy was already formed, thus decreasing RIE only by accelerating the senescence of leaves. The data were used to evaluate the performance of a crop growth model for diseased pea crops. The model comprised the decrease in photosynthesis rate in the leaves, the vertical gradient of disease intensity and the differences in photosynthetic function of the various layers of the canopy. This model, validated over 2 years in the field, accurately simulated the crop growth in diseased plots. Thus the decrease in RUE may be accounted for solely by photosynthesis losses in diseased leaves. This simple model may be used for disease management, defining damage thresholds for chemical application and criteria for tolerant variety selection.     

Spatio-temporal development of pycnidia and perithecia and dissemination of spores of Mycosphaerella pinodes on pea (Pisum sativum)

The pattern of development of pycnidia and perithecia of Mycosphaerella pinodes was studied in the glasshouse on pea plants (cv. Solara) sprayed with a pycnospore suspension and in field plots inoculated with barley grains colonized by the fungus. The numbers of pycnidia and perithecia were estimated on each stipule and internode of infected plants, and were related to ratings of disease severity (0–5 scale). Pycnidia were produced on both green and senescent organs, whereas perithecia only appeared on senescent organs. The development and quantity of pycnidia were related to initial inoculum concentration and the physiological stage of the plants. The formation of fruiting bodies progressed from the bottoms to the tops of plants during crop development. Spore trapping showed that both pycnospore dispersal and ascospore discharge were initiated by rainfall or dew. Pycnospores were principally trapped in the first 20 cm above the soil surface while ascospores were also trapped above the crop canopy. Pycnospores and ascospores were dispersed throughout the growing season, suggesting that ascospores also play an important role in secondary infections.     

Effects of temperature and moisture on disease and fruit body development of Mycosphaerella pinodes on pea (Pisum sativum)

The effects of temperature (5–30°C) and the duration of moisture on the development of ascochyta blight ( Mycosphaerella pinodes ) on pea seedlings, grown under controlled conditions, were investigated. The optimum temperature for monocyclic processes was 20°C. At this temperature, pycnidiospores germinated after 2 h, appressoria formed after 6 h and the germ-tube penetrated the leaf cuticle after 8 h. Disease symptoms were evident after 1 day of incubation and the first pycnidia formed after 3 days. Longer wetting periods were required for disease development and pycnidial formation at non-optimal temperatures. Disease severity and the number of pycnidia formed on leaves increased with temperature from 5 to 20°C, then decreased between 20 and 30°C. Polynomial equations were fitted to predict the stages of infection, incubation, latency and disease development as functions of temperature and duration of moisture. These equations allow comparisons of pathogen spread with plant development and could be incorporated into disease development models used for crop management programmes.     

Cultural, molecular and pathogenic variability of Mycosphaerella pinodes and Phoma medicaginis var. pinodella isolates from dried pea (Pisum sativum) in France

The characteristics of 50 isolates of Mycosphaerella pinodes and 17 isolates of Phoma medicaginis var. pinodella , originating from several regions of France where ascochyta blight is prevalent, were investigated using cultural, physiological, molecular and pathogenicity analyses. M. pinodes was distinguished from P. medicaginis var. pinodella on the basis of presence of pseudothecia, a higher proportion of larger, bicellular conidia, compared with the smaller, predominantly unicellular conidia of P. medicaginis var. pinodella , and a slower linear growth rate on agar under a 12-h light regime. RAPD analysis clearly distinguished the two species, which had low intraspecific variability. Although both species gave identical symptoms, they could be distinguished by their incubation period and aggressiveness, respectively, shorter and higher for M. pinodes . Virulence tests gave no definitive evidence for the existence of pathotypes among the M. pinodes isolates. Two unidentified isolates had similar characters to both M. pinodes and P. medicaginis var. pinodella in some features but were distinguished from them by their RAPD patterns.     

Effects of interrupted wet periods and different temperatures on the development of ascochyta blight caused by Mycosphaerella pinodes on pea (Pisum sativum) seedlings

The effect of interrupted wet periods on pycnidiospores of Mycosphaerella pinodes was studied by assessing spore viability, infection and disease development on pea seedlings. Pycnidiospores survived dry periods of up to 21 days after inoculation. Rewetting restored the infection capacity of the pycnidiospore, resulting in high levels of disease. The effects of wet–dry–wet cycles depended on when the dry period occurred during the infection process. No disease symptoms appeared when dry periods occurred during germination. A low level of disease occurred after rewetting in high relative humidity if the interruption of the wet period was long. However, a wet period resulting in leaf wetness after a dry period gave similar levels of infection to those achieved with a continuous wet period. Pycnidiospores formed appressoria but hyphae did not penetrate if a 6–12 h wet period preceded the dry period, and only a few flecks appeared during the dry period. Coalescent necrosis occurred when the dry period followed penetration. The disease was severe in each case when plants were returned to wet conditions after a period of dryness. Lesion development depended on the duration of the initial wet period, and the characteristics (temperature and duration) of both the dry period and the final wet period.     

Characterization of mechanisms of resistance against Didymella pinodes in Pisum spp.

Ascochyta blight caused by Didymella pinodes is a serious disease of pea (Pisum sativum ssp. sativum) to which little resistance has been identified so far. Only incomplete resistance is available in pea germplasm although higher levels of resistance have been reported in related Pisum species. In this study we characterized histochemically the underlying resistance mechanisms in these wild species and in the pea cv. Radley, the pea cultivar with the highest level of resistance to D. pinodes. Resistance was characterized by a reduced success of colony establishment and lesion size. Histologically this was associated with higher frequency of epidermal cell death and protein cross-linking in infected epidermal cells but not with H₂O₂ accumulation and peroxidase activity.     

Effects of Ascochyta blight (Mycosphaerella pinodes) on the photosynthesizing leaf area and the photosynthetic efficiency of the green leaf area of dried-pea (Pisum sativum)

The effects of Ascochyta blight (caused by Mycosphaerella pinodes ) on the net photosynthetic rate of glasshouse-grown dried pea were studied on foliar discs and whole plants in pots showing different disease intensities, assessed visually using a disease scale (scores 0 to 5) or by an estimation of the fraction of leaf area with necrosis using an image analyser. The photosynthetic rate of foliar discs was measured using a leaf disc electrode; for plants it was assessed by CO₂ exchange rate measurements in a closed chamber. A reduction in the net photosynthetic rate of diseased plants was correlated with an increase in disease score. A 53–56% reduction was found in foliar discs with a mean disease score of 3. On whole plants, mean scores of 3.5 and 4.0 were associated with reductions of 36% and 98% respectively. The disease also induced a decrease in the photosynthetic efficiency of the non-necrotic leaf area. The photosynthetic rate was zero when 30–40% of foliar area was necrotic on foliar discs and on whole plants. A function was derived from the foliar disc data that described the dependence of photosynthetic rate on the fraction of leaf area without necrosis. The model is discussed with reference to data obtained from entire plants and other pathosystems.     

Identification of genes expressed during spore germination of Mycosphaerella pinodes

Mycosphaerella blight, caused by Mycosphaerella pinodes, is one of the major diseases of cultivated pea (Pisum sativum L.). To isolate the genes that are up- and down-regulated during spore germination, suppression subtraction hybridization (SSH) was performed between ungerminated and germinated spores. The 232 and 128 clones from forward and reverse libraries, respectively, were collected, sequenced, and analyzed with a BLASTX homology search. About 95% of the 32 selected clones were expressed during spore germination on a paper sheet and during infection of pea leaves. We discuss the applicability of the SSH libraries for analyzing M. pinodes genes involved in the early stage of infection.     

Components of quantitative resistance to powdery mildew (Erysiphe pisi) in pea (Pisum sativum)

Components of quantitative resistance in pea ( Pisum sativum ) to Erysiphe pisi , the pathogen causing powdery mildew, were investigated. Conidium germination, infection efficiency, latent period and conidium production dynamics on cv. Quantum (quantitatively resistant) were compared with those on Pania and Bolero (susceptible). There was an additional comparison in conidium germination experiments with the resistant cv. Resal. Quantitative resistance in Quantum did not affect conidium germination, but infection efficiency of conidia on this cultivar was 34% less than on the susceptible Pania. More conidia germinated on 5-day-old leaflets than on 15-day-old leaflets but the age of the plant did not affect percentage germination or infection efficiency. The length of the latent period did not differ between cultivars. Total conidium production (AUC) per unit leaflet area on Quantum was 25% less than on Pania. The maximum conidium production per day (CMAX) per unit leaflet area on Quantum was 33% less than on Pania. The time to maximum conidium production per day (TMAX) was 10% longer on Quantum than on Pania. The cv. Bolero, reported to be susceptible, also showed some degree of quantitative resistance, but this differed from that of Quantum. Total conidium production was less on Bolero than on Quantum, but the conidia on Bolero were produced sooner, and for a shorter period, than on Quantum. The stability of these responses was tested by analysing components in three different temperature regimes and testing for interactions with temperature, and with leaflet age. Temperature affected all conidium production variables. AUC per leaflet area was nearly seven times as great and CMAX nearly 15 times greater at 23°C than at 13°C. TMAX increased by 1.5 times when temperature increased from 13°C to 18°C or 23°C. Several interactions occurred and these are described.     

Effect of pea canopy architecture on splash dispersal of Mycosphaerella pinodes conidia

To investigate the hypothesis that disrupting pathogen movement within the plant canopy could slow the development of aschochyta blight, the effect of pea canopy architecture on splash dispersal of Mycosphaerella pinodes‐conidia was studied in controlled conditions using a rainfall simulator generating rain events (2 mm) in still air. In intra‐plant dispersal experiments, a source constituted by a semi‐leafless pea plant with a single infectious lesion (108 pycnidia per cm² of lesion, 1685 conidia per pycnidium) was placed in the middle of eight healthy target plants. Spore deposition was estimated by the number of lesions that developed on each stipule of the source (auto‐deposition) and target (allo‐deposition) plants after incubation. Rates of deposition on the source and target plants were 0·53 and 0·47, respectively. On the source plant, most of the spores were splashed downwards, with few spores remaining at the infectious node and very few spores moving upwards. In inter‐plant dispersal experiments, potted plants were grouped to constitute 1‐m² canopies. A range of canopy architectures was achieved by using different plant densities and growth stages. A suspension of conidia was placed in the centre of each canopy. Resulting horizontal dispersal gradients were generally described by a negative exponential model. Canopies with a leaf area index (LAI) greater than 0·48 produced gradients with slopes that were not significantly different. A less dense canopy (LAI 0·36) produced a significantly steeper slope. Half‐distances were short and ranged between 1·6 and 6·5 cm. The barrier rate, calculated as the ratio of the mean number of lesions assessed on isolated plants to the mean number of lesions assessed on plants in canopies, increased with increasing canopy LAI.     

Breakdown of pisatin by some fungi pathogenic to Pisum sativum

Samenvatting Aan een voedingsmedium, geïnoculeerd met elk der schimmelsFusarium oxysporum f. sp.pisi ras 1,Mycosphaerella pinodes (pathogenen van de erwt),Cladosporium cucumerinum, Colletotrichum lindemuthianum enMonilinia fructigena (niet-pathogenen van de erwt) werd pisatine toegevoegd. Na incubatie gedurende een week werd het medium gecentrifugeerd. Zowel de bovenstaande vloeistof als het mycelium werd onderzocht op de aanwezigheid van pisatine. Uit de U.V.-absorptiespectra van de vloeistof en alcoholextracten van het mycelium blijkt, dat beide pathogenen het pisatine kunnen afbreken, maar de drie niet-pathogenen niet.     

Relationship between ascochyta blight on field pea (Pisum sativum) and spore release patterns of Didymella pinodes and other causal agents of ascochyta blight

Ascochyta blight of field pea, caused by Didymella pinodes, Phoma medicaginis var. pinodella, Phoma koolunga and Didymella pisi, is controlled through manipulating sowing dates to avoid ascospores of D. pinodes, and by field selection and foliar fungicides. This study investigated the relationship between number of ascospores of D. pinodes at sowing and disease intensity at crop maturity. Field pea stubble infested with ascochyta blight from one site was exposed to ambient conditions at two sites, repeated in 2 years. Three batches of stubble with varying degrees of infection were exposed at one site, repeated in 3 years. Every 2 weeks, stubble samples were retrieved, wetted and placed in a wind tunnel and up to 2500 ascospores g^?1 h^?1 were released. Secondary inoculum, monitored using seedling field peas as trap plants in canopies arising from three sowing dates and external to field pea canopies, was greatest in early sown crops. A model was developed to calculate the effective number of ascospores using predictions from G1 blackspot manager (Salam et al., 2011b; Australasian Plant Pathology, 40 , 621–31), distance from infested stubble (Salam et al., 2011a; Australasian Plant Pathology, 40 , 640–7) and winter rainfall. Maximum disease intensity was predicted based on the calculated number of effective ascospores, soilborne inoculum and spring rainfall over two seasons. Predictions were validated in the third season with data from field trials and commercial crops. A threshold amount of ascospores of D. pinodes, 294 g^?1 stubble h^?1, was identified, above which disease did not increase. Below this threshold there was a linear relationship between ascospore number and maximum disease intensity.     

Specificity of incomplete resistance to Mycosphaerella graminicola in wheat

We examined interactions between wheat (Triticum aestivum) and Mycosphaerella graminicola, causal agent of Septoria tritici blotch, to determine whether specific interactions occur between host and pathogen genotypes that could be involved in eroding resistance. The moderate resistance of the wheat cultivar Madsen has eroded significantly in the Willamette Valley of Oregon since its release in 1990. Foote is a replacement cultivar expressing moderate resistance and was released in 2000. Isolates of M. graminicola were collected from Foote and Madsen in 2004 and 2005 and tested on each cultivar in growth chamber and greenhouse experiments. There was a significant (P 相似文献   

Short-lived protection of pea plants against Mycosphaerella pinodes by prior inoculation with Pseudomonas phaseolicola

Pea plants,Pisum sativum cv. Eminent, were inoculated by spraying with a suspension (10⁸ cells/ml) ofPseudomonas phaseolicola, a bacterium not pathogenic to pea. At intervals of 0,2,4,6, or 8 days the same plants were challenged with a conidial suspension (10⁶ conidia/ml) of the peapathogenic fungusMycosphaerella pinodes. Periodically, leaf samples were examined for development of symptoms and phytoalexin content.Up to 6 days after inoculation with the non-pathogenic bacterium the plants were protected against the pathogenic fungus. Pisatin, however, seemed not to be responsible for the observed protection.Samenvatting Erwteplanten, cv. Eminent, werden bespoten met een suspensie (10⁸ bacteriën/ml) vanPseudomonas phaseolicola, een bacterie die niet pathogeen is voor erwt. Na 0,2,4,6, of 8 dagen werden dezelfde planten opnieuw geïnoculeerd, nu echter met een conidiënsuspensie (10⁶ conidiën/ml) van de voor erwt pathogene schimmelMycosphaerella pinodes. Periodiek werden bladmonsters beoordeeld naar ontwikkeling van symptomen en aanwezigheid van fytoalexinen.Tot 6 dagen na de inoculatie met de niet-pathogene bacterie bleken de planten—in afnemende mate — beschermd tegen aantasting door de pathogene schimmel. Aangezien er geen positieve correlatie bleek te bestaan tussen de pisatineconcentratie en de mate van bescherming, leek pisatine voor deze bescherming niet verantwoordelijk.     

Genetics of specific resistance in pea (Pisum sativum) cultivars to seven races of Pseudomonas syringae pv. pisi

Seven races of Pseudomonas syringae pv. pisi were distinguished using eight differential cultivars of pea (Pisum sativum). Segregation among F₂ populations of crosses between differential cultivars sequentially inoculated with races of P.s. pv. pisi provided evidence for four and possibly six putative resistance(R)/avirulence(A) gene pairs. R1, R2 and R3 are dominant resistance alleles at single loci, R4 is a dominant allele at a single locus which exhibits variable expression possibly dependent on genetic background. There is evidence that R3 and R4 are at linked loci. Homology tests provided proof of the occurrence of the alleles R2, R3 and R4 in more than one cultivar. Two other alleles, R5 and R6, were postulated to explain the observed segregation ratios in certain crosses.
It can be inferred that P.s. pv. pisi races 2, 3 and 4 each carry a different single a virulence gene, race 6 carries no apparent avirulence genes, and race 7 carries at least A2, A3 and A4. Race 1 carries Al, A3, A4 and possibly A6; race 5 carries A2, A4 and possibly A5 and A6.     

Hemibiotrophic infection of Pisum sativum by Colletotrichum truncatum

The infection of pea ( Pisum sativum ) by Colletotrichum truncatum was studied by light and electron microscopy. These investigations were facilitated by use of an Argenteum pea mutant, which has a readily detachable epidermis. Infection pegs emerging from appressoria penetrated epidermal cells directly. Large intracellular primary hyphae formed a dense stromatic mycelium confined within a single epidermal cell. Primary mycelia gave rise to thinner secondary hyphae which radiated into surrounding cells and caused extensive wall dissolution. Melanized sclerotia developed in the centre of chlorotic water-soaked lesions. Acervuli were not observed. Epidermal cells survived initial penetration by primary hyphae, as shown by their ability to plasmolyse and accumulate Neutral red, but all infected cells were dead when the secondary hyphae had formed. Six cultivars of pea were susceptible, but seven other legumes were resistant. A single isoform of polygalacturonase with a pI of 8·3 and apparent M _r of 40000 was purified from culture filtrates and the TV-terminal amino acid sequence determined. The relevance of the results to the taxonomy of C. truncatum and the relationships between infection process and host range are discussed.     

Characterization of the recessive resistance gene cyv1 of Pisum sativum against Clover yellow vein virus

Two recessive resistance genes against Clover yellow vein virus (ClYVV), cyv1 and cyv2, have been previously reported. We recently screened resistant peas from a separate set of pea lines and classified them into two groups according to their distinct modes of resistance. We later revealed that one group carries cyv2, encoding eukaryotic translation initiation factor 4E (eIF4E), in linkage group (LG) VI. We explored the possibility that the resistance gene, tentatively designated non-cyv2, that confers resistance to the other group, was actually cyv1. We found that PI 236493, which carries cyv1, had restricted cell-to-cell movement of ClYVV similar to that in non-cyv2 peas including PI 429853. PI 429853 was crossed with susceptible line PI 250438. Mapping of F₂ progeny revealed that non-cyv2 was 4?cM from the simple sequence repeat marker AB40, whose loci are close to cyv1, mo, and sbm-2 mapped in LG II, which mediates resistance to other potyviruses. Moreover, PI 429853 crossed with PI 236493 produced F₁ progeny resistant to ClYVV, raising the possibility that non-cyv2 is allelic to cyv1. Because mo was previously mapped with eIF(iso)4E in LG II, we examined the possibility that non-cyv2, cyv1, and mo encoded eIF(iso)4E. However, there was no difference in the nucleotide sequence of the eIF(iso)4E-coding region between susceptible and resistant pea lines. The eIF(iso)4E gene was equivalently expressed in both PI 429853 and PI 250438 before and after ClYVV infection. Our results suggest that these resistance genes are unlikely to encode eIF(iso)4E on LG II.     

Effect of pea plant architecture on spatiotemporal epidemic development of ascochyta blight (Mycosphaerella pinodes) in the field

The effect of pea canopy structure on epidemics of Mycosphaerella pinodes was investigated in four spring pea cultivars (Bridge, Obelisque, Solara and Athos) in two field experiments. These cultivars had similar levels of susceptibility to ascochyta blight and presented different architectural features (branching, standing ability and stem height). Stem height ranged from 77 to 95 cm in 1999 and from 72 to 92 cm in 2000, while leaf area index (LAI) ranged from 3·8 to 5·1 in 1999 and from 3·8 to 4·7 in 2000. Internode lengths varied from 4·6 to 6·0 in 1999 and from 3·8 to 4·7 in 2000. Mean distance between nodes in the canopy (MDN) ranged from 12·2 to 15·3 cm in 1999 and from 11·2 to 13·9 cm in 2000. Canopies with different architecture differed in disease progression on stipules and mainly on pods. The levels of disease on stipules and pods were strongly correlated. Moreover, if disease was considered as a function of stipule height up the stem, large differences in vertical disease distribution were observed between cultivars. Three architectural features acted on disease development: cumulative LAI above the node on which disease was assessed (LAIcum) and large MDN favouring disease development, and large internode length reducing disease severity. Modifying LAI distribution and plant organization could be one way to reduce the impact of ascochyta blight, by direct or indirect effects on environmental and dispersal conditions.     

Differential regulation of MBP kinases by a glycoproptein elicitor and a polypeptide suppressor from Mycosphaerella pinodes in pea

A polypeptide fungal suppressor from a pea pathogen Mycosphaerella pinodes plays a key role in pathogenesis by suppressing elicitor-induced defense response(s) in pea (Pisum sativum L). In this study, we show that treatment of pea tissues with the polysaccharide elicitor secreted by M. pinodes results in rapid increased activation of two myelin basic protein (MBP)-dependent kinases p44 (≈44 kDa) and p48 (≈48 kDa) within 15–30 min upon elicitation. Interestingly, the suppressor inhibited the elicitor-induced activation of only p44 kinase. While the defense-inducing signalling molecules, chitosan and salicylic acid (SA) activated the p44 and p48 kinases, methyl jasmonate (MeJA) did not. The abiotic stress signals, abscisic acid (ABA), NaCl and wounding activated the p48 kinase alone. These results demonstrate that MAPKs are differentially activated in response to pathogen invasion and abiotic stress in pea. Furthermore, specific inhibition of elicitor-induced p44 kinase activation by a MAPKK inhibitor, PD098059 and protein kinase inhibitor, K252a correlated with the suppression of elicitor-induced phenylalanine ammonia lyase (PAL) gene expression, supporting a role for p44 in the elicitor-induced defense response(s) in pea. Inhibition of p44 by the phosphoinositide (PI) turnover inhibitor, neomycin (a fungal suppressor mimic), and potentiation of p44 by the diacylglycerol (DAG) kinase inhibitor, R59022 indicated that p44 may be acting downstream of (PI) metabolism. Taken together, our results indicate that suppressor of defense elicitation from M. pinodes acts through inhibition of a MAPK (p44), possibly through a PI signaling pathway, facilitating the establishment of basic compatibility during infection of pea.     

Resistance to Peronospora viciae expressed as differential colony growth in two cultivars of Pisum sativum

Peronospora viciae sporulated frequently following localized inoculation of leaflets of Pisum sativum cv. Krupp Pelushka but rarely on cv. Early Onward (60 and < 1% of plants respectively). Similar results were obtained following inoculation of seedling shoot apices. The proportions of sporangia that germinated, formed appressoria, invaded via stomata or cuticle, and established colonies, were similar on leaflets of both cultivars. After 2 days, total hyphal lengths of most colonies were greater than 100 μm (94% in Krupp Pelushka, 64% in Early Onward), but in Krupp Pelushka there were five times as many haustoria per unit length of hypha. Subsequently, only occasional colonies in Early Onward grew to the same extent as in Krupp Pelushka, with 8% and 88% respectively attaining lengths > 800 μm after 4 days. Reduced growth of hyphae in Early Onward was associated with increased frequency of host cells stained by trypan blue.