Agronomic and meteorological factors affecting the severity of leaf blotch caused by Mycosphaerella graminicola in commercial wheat crops in England

Factors affecting the severity of leaf blotch on the two upper leaves of wheat plants in crops at the milky ripe growth stage (GS 73–75) were investigated using survey data from 3513 randomly selected wheat crops sampled during 1985–1996. Year‐to‐year variation in disease severity was greater than spatial variability at county level, although both showed significant differences. The presence of disease above a 5% severity threshold was modelled using random effects logistic regression (Generalised Linear Mixed Model), which enabled risk variables measured at the field level to be combined with meteorological variables estimated at county level. The final model included terms for the fixed effects of disease resistance rating, date of sowing, high risk septoria periods in May and June, number of fungicide sprays and number of days with frost (≤‐2°C) in November. The percentage of crops above the threshold decreased with later sowing, increased number of November frost days and increased number of fungicide sprays. In contrast, high risk septoria periods (rain splash events) in May and June showed a positive correlation with the percentage of crops above the threshold. There were benefits from using resistant cultivars. The model showed that a range of risk variables were of broadly equivalent importance in determining the development of leaf blotch. These risk variables should be integrated in any scheme designed to support fungicide use decisions.     

A model of the effect of fungicides on disease-induced yield loss, for use in wheat disease management decision support systems

A model of the effect of foliar-applied fungicides on disease-induced yield loss is described, parameterised and tested. The effects of fungicides on epidemics of Septoria tritici (leaf blotch), Puccinia striiformis (yellow rust), Blumeria graminis f.sp. tritici (powdery mildew) and Puccinia triticina (brown rust) on winter wheat were simulated using dose–response curve parameters. Where two or more active substances were applied together, their joint action was estimated using an additive dose model where the active substances had the same mode of action or a multiplicative survival model where the modes of action differed. By coupling the model with models of wheat canopy growth and foliar disease published previously, it was possible to estimate disease-induced yield loss for a prescribed fungicide programme. The difference in green canopy area and, hence, interception of photosynthetically active radiation between simulated undiseased and diseased (but treated) crop canopies was used to estimate yield loss. The model was tested against data from field experiments across a range of sites, seasons and wheat cultivars and was shown to predict the observed disease-induced yield loss with sufficient accuracy to support fungicide treatment decisions. A simple method of accounting for uncertainty in the predictions of yield loss is described. Fungicide product, dose and spray timing combinations selected using the coupled models responded appropriately to disease pressure and cultivar disease resistance.     

Disease Development and Buildup of Resistance to Oxadixyl in Potato Crops Inoculated with Phytophthora infestans as Affected by Oxadixyl and Oxadixyl Mixtures: Experimental and Simulation Studies

Simulator predictions for the effect of oxadixyl and oxadixyl mixtures on the control of late blight in potato crops inoculated with Phytophthora infestans and the buildup of fungal resistance to oxadixyl were compared with data obtained in field experiments. The simulator, which considered fungus, fungicides and crop variables, predicted accurately the effect of fungicide sprays (oxadixyl, oxadixyl mixed with mancozeb, and oxadixyl mixed with mancozeb and cymoxanil) on blight development. The model predicted accurately both resistance buildup and infection in non-treated crops and in crops treated with oxadixyl alone or in a mixture with mancozeb (SAN 518F), but overestimated the control efficacy of the three-way mixture Pulsan.     

Crop Growth, Disease and Yield Components of Rusted Phaseolus Beans in Ethiopia

Crop growth and disease epidemics in sprayed and non-sprayed bean plots, artificially infected with rust (Uromyces appendiculatus) 3 weeks after emergence. were assessed weekly in two cultivars, at two locations for two seasons. Disease intensity was regulated by the application of a fungicide at 5 spray frequencies. Fungicide application influenced leaf area index (LAI) and reduced rust intensity. The fungicide had no significant effect on other diseases and dead leaf area. Fungicide application increased seed yield (SY) by increased numbers of pods per plant (PP). Rust severity was strongly correlated with pustule density but the overall relationships among rust assessment parameters depended on cultivar and location. Seed yield and pods per plant were highly correlated with LAI. The relationships between LAI and seeds per pod or seed weight depended on cultivar and location. Overall rust assessment parameters (rust severity and pustule density) showed close, negative relationships with seed yield. seed weight and pods per plant but not with seeds per pod. The relationships obtained in the partially resistant line 6-R-395 were less definite than those in the susceptible line Mexican 142. The yield parameters seed yield and pods per plant, showed strong positive relationships.     

QTL mapping of multiple foliar disease and root-lesion nematode resistances in wheat

A genetic linkage map, based on a cross between the synthetic hexaploid CPI133872 and the bread wheat cultivar Janz, was established using 111 F₁-derived doubled haploid lines. The population was phenotyped in multiple years and/or locations for seven disease resistance traits, namely, Septoria tritici blotch (Mycosphaeralla graminicola), yellow leaf spot also known as tan spot (Pyrenophora tritici-repentis), stripe rust (Puccinia striiformis f. sp. tritici), leaf rust (Puccinia triticina), stem rust (Puccinia graminis f. sp. tritici) and two species of root-lesion nematode (Pratylenchyus thornei and P. neglectus). The DH population was also scored for coleoptile colour and the presence of the seedling leaf rust resistance gene Lr24. Implementation of a multiple-QTL model identified a tightly linked cluster of foliar disease resistance QTL in chromosome 3DL. Major QTL each for resistance to Septoria tritici blotch and yellow leaf spot were contributed by the synthetic hexaploid parent CPI133872 and linked in repulsion with the coincident Lr24/Sr24 locus carried by parent Janz. This is the first report of linked QTL for Septoria tritici blotch and yellow leaf spot contributed by the same parent. Additional QTL for yellow leaf spot were detected in 5AS and 5BL. Consistent QTL for stripe rust resistance were identified in chromosomes 1BL, 4BL and 7DS, with the QTL in 7DS corresponding to the Yr18/Lr34 region. Three major QTL for P. thornei resistance (2BS, 6DS, 6DL) and two for P. neglectus resistance (2BS, 6DS) were detected. The recombinants combining resistance to Septoria tritici blotch, yellow leaf spot, rust diseases and root-lesion nematodes from parents CPI133872 and Janz constitute valuable germplasm for the transfer of multiple disease resistance into new wheat cultivars.     

A foliar disease model for use in wheat disease management decision support systems

A model of winter wheat foliar disease is described, parameterised and tested for Septoria tritici (leaf blotch), Puccinia striiformis (yellow rust), Erysiphe graminis (powdery mildew) and Puccinia triticina (brown rust). The model estimates disease‐induced green area loss, and can be coupled with a wheat canopy model, in order to estimate remaining light‐intercepting green tissue and hence the capacity for resource capture. The model differs from those reported by other workers in three respects. First, variables (such as weather, host resistance and inoculum pressure) that affect disease risk are integrated in their effect on disease progress. The agronomic and meteorological data called for are restricted to those commonly available to growers by their own observations and from meteorological service networks. Second, field observations during the growing season can be used both to correct current estimates of disease severity and to modify parameters that determine predicted severity. Third, pathogen growth and symptom expression are modelled to allow the effects of fungicides to be accounted for as protectant activity (reducing infections that occur postapplication) and eradicant activity (reducing growth of presymptomatic infections). The model was tested against data from a wide range of sites and varieties and was shown to predict the expected level of disease sufficiently accurately to support fungicide treatment decisions.     

Mutations in wheat showing altered field resistance to yellow and brown rust.

Intensive screening of a small population of mutagenised wheat lines revealed a large number of lines with altered resistance to both yellow and brown rust. The parental cultivar Guardian has an intermediate level of adult plant resistance to this disease; mutants were therefore isolated that showed either enhanced resistance or enhanced susceptibility to yellow rust. Seven lines were identified that gave an altered yellow rust disease phenotype as adult plants under both field and greenhouse conditions. Simultaneous field testing for brown rust infection identified two of these lines as having increased resistance to brown rust.     

Effects of previous cropping and fungicide timing on the development of light leaf spot (Pyrenopeziza brassicae), seed yield and quality of winter oilseed rape (Brassica napus)

Light leaf spot, caused by Pyrenopeziza brassicae, was assessed regularly on double-low cultivars of winter oilseed rape during field experiments at Rothamsted in 1990-91 and 1991-92. Previous cropping and fungicide applications differed; seed yield and seed quality were measured at harvest. In each season, both the initial incidence of light leaf spot and the rate of disease increase were greater in oilseed rape crops sown after rape than those sown after cereals. The incidence of diseases caused by Phoma lingam or Alternaria spp. was also greater in second oilseed rape crops. In 1991-92 there was 42% less rainfall between September and March than in 1990-91, and much less light leaf spot developed. However, P. lingam and Alternaria spp. were more common. Only fungicide application schedules including an autumn spray decreased the incidence of light leaf spot on leaves, stems and pods, as indicated by decreased areas under the disease progress curves (AUDPC) and slower rates of disease increase. Summer sprays decreased incidence and severity of light leaf spot on pods only. In 1990-91, all fungicide treatments which included an autumn spray increased seed and oil yields of cv. Capricorn but only the treatment which included autumn, spring and summer sprays increased yields of cv. Falcon. No treatment increased the yields of cv. Capricorn or cv. Falcon in 1991-92. Fungicide applications decreased glucosinolate concentrations in the seed from a crop of cv. Cobra severely infected by P. brassicae in 1990-91, but did not increase yield.     

Surveys of diseases of spring barley in England and Wales, 1976–1980

Samples from 200–300 randomly selected spring barley crops were taken annually at growth stage 73–77 (milky ripe) from 1976 to 1980. The number of samples from each region was proportional to the area of barley grown in each region. The percentage of the area of the top two leaves affected by diseases was recorded. Mildew (Erysiphe graminis) was the most widespread and severe disease recorded. Brown rust (Puccinia hordei) and rhynchosporium (Rhyn-chosporium secalis) occurred frequently but at relatively low levels. Yellow rust {Puccinia striiformis) and septoria (Septoria nodorum) were seen on less than 50% of the samples in most years, and halo spot (Selenophoma donacis) and net blotch (Pyrenophora teres) were rarely recorded. There was an association between the severity of rhynchosporium and the number of rain days in May and June. The highest levels of brown rust occurred in the south and east and rhynchosporium was more common in Wales and the south-west than in the east, but there were no differences in the regional distribution of other diseases. Cultivar resistance, sowing date, previous cropping and fungicide usage were all found to be associated with altered disease levels. The proportion of crops treated with a foliar fungicidal spray rose from 26% in 1976 to 47% in 1980. The use of tridemorph declined but that of triadimefon increased reaching 29% of crops treated by 1980. The use of ethirimol as a seed treatment declined from 16% of crops grown from treated seed in 1976 to 7% in 1980. Estimated yield losses between 1976 and 1980 varied between 4% and 9% due to mildew, between 0.3% and 0.8% due to brown rust and between 0.2% and 0.5% due to rhynchosporium.     

A survey of foliar diseases of spring barley in England and Wales in 1967

A total of 5,250 acres of spring barley was sampled in proportion to the barley acreage in each of eight regions. The percentage leaf area ‘affected’ by each disease was recorded for the first (flag) and second leaves at a growth stage when the grain was milky ripe. The operation was automated by punching the data directly on to computer paper tape and a programme was written to analyse the results. Mildew was found to be the most important disease, causing an average loss in yield of approximately 18 %, followed in descending order by brown rust 3%, leaf blotch 1 %, yellow rust 1 %, halo spot 1 %. The total loss due to foliar diseases was estimated to be 20–25% of the national barley yield. Diseases were more severe in the southern than northern regions, e.g. mildew severity was three times higher because of the greater popularity of mildew-susceptible varieties, and the prevalence of weather more conducive to mildew development. Disease severity was not related to previous cropping, and in general the February-sown crops had more mildew than the April-sown crops.     

The integration of host resistance with fungicides in the control of oat powdery mildew

Treatment of seed with the systemic fungicides triadimenol plus fuberidazole as Baytan significantly decreased powdery mildew in field experiments involving eight oat cultivars differing widely in their resistance to the disease. This was so up to growth stage G.S. 61 (Zadoks) even in a year of high mildew incidence, although the effect diminished after flowering. Significant (P≥ 0.001) interactions between fungicide treatments and cultivars occurred in each of three years at nearly all assessment times. In years of high mildew incidence (1983 and 1984), susceptible cultivars developed similar levels of mildew in seed-treated and untreated plots by about flowering time, although cultivars with adult plant resistance (APR) had significantly less mildew when seed-treated than when left untreated. Later, as the APR was fully expressed and the fungicide effectiveness was declining, similar mildew levels were recorded on treated and untreated plots. Untreated APR cultivars generally had less mildew than treated susceptible cultivars and in a year of late and light mildew, APR alone provided good protection. Seed treatments combined with foliar sprays of tridemorph as Calixin almost completely controlled mildew except on very susceptible cultivars late in the season in high mildew years. Over all cultivars, seed treatment gave yield advantages of 5.3% (1983) and 6.6% (1984) but in 1982, a year of low mildew, the response was small. The possible influence of integrated host resistance and fungicides in stabilising the pathogen population at economically unimportant levels and the environmental benefit of using host resistance to minimise fungicide usage is discussed.     

QTL mapping of adult-plant resistance to stripe rust in a population derived from common wheat cultivars Naxos and Shanghai 3/Catbird

Stripe rust, caused by Puccinia striiformis Westend. f. sp. tritici Erikss., is a severe foliar disease of common wheat (Triticum aestivum L.) worldwide. Use of adult-plant resistance (APR) is an efficient approach to provide long-term protection of crops from the disease. The German spring wheat cultivar Naxos showed a high level of APR to stripe rust in the field. To identify the APR genes in this cultivar, a mapping population of 166 recombinant inbred lines (RILs) was developed from a cross between Naxos and Shanghai 3/Catbird (SHA3/CBRD), a moderately susceptible line developed by CIMMYT. The RILs were evaluated for maximum disease severity (MDS) in Sichuan and Gansu in the 2009-2010 and 2010-2011 cropping seasons. Composite interval mapping (CIM) identified four QTL, QYr.caas-1BL.1RS, QYr.caas-1DS, QYr.caas-5BL.3 and QYr.caas-7BL.1, conferring stable resistance to stripe rust across all environments, each explaining 1.9-27.6, 2.1-5.8, 2.5-7.8 and 3.7-9.1?% of the phenotypic variance, respectively. QYr.caas-1DS flanked by molecular markers XUgwm353-Xgdm33b was likely a new QTL for APR to stripe rust. Because the interval between flanking markers for each QTL was less than 6.5?cM, these QTL and their closely linked markers are potentially useful for improving resistance to stripe rust in wheat breeding.     

Effects of foliar sprays of azoxystrobin on take-all in wheat

Average percentages of winter wheat plants with severe take‐all were decreased by up to half by azoxystrobin applied as foliar sprays in four field experiments. Decreased take‐all in three of the experiments was associated with increased grain yield but effects on other diseases may have contributed to these responses. Standard fungicide sprays were ineffective. The effects differed, but not consistently, among different cultivars that were tested in three of the experiments. One, two or three sprays of azoxystrobin or kresoxim‐methyl, in autumn, spring or summer, were tested in the fourth experiment. Unlike azoxystrobin, kresoxim‐methyl had no consistent effects but a smaller amount was applied. Two or three sprays of azoxystrobin were more effective than a single spray but their timing was unimportant. Such control of a root disease by a foliar‐applied fungicide is unusual but may help to explain some of the unexpectedly large yield responses to azoxystrobin that have been reported. This relatively broad‐spectrum fungicide may have the potential to contribute to the practical management of take‐all but further research is needed to determine how best to exploit its effects consistently.     

The development of a STS marker linked to a yellow rust resistance derived from the wheat cultivar Moro

A sequence-tagged-site (STS) marker has been developed for a gene conferring yellow rust resistance originating from the wheat cultivar Moro. The single, dominant, seedling yellow rust resistance gene, designated YrMoro, was mapped to the group 1 chromosomes. The STS marker was developed from an AFLP band which cosegregated with the YrMoro gene. Sequence-specific primers were made which incorporated the selective bases of the AFLP primers, plus 16 and 17 additional bases extending into the AFLP band. This simple, PCR-based marker will allow wheat breeders to pyramid this resistance gene, along with other resistance genes, into a single wheat genotype.     

Methyl jasmonate stimulates polyamines biosynthesisand resistance against leaf rust in wheat plants

Abstract

Leaf rust of wheat (Triticum aestivum L.), incited by Puccinia recondita ex Desm. f. sp. tritici Eriks, is one of the most important wheat diseases in Egypt. Methyl jasmonate (MJ) is a potential plant elicitor which induces a wide range of chemical and anatomical defense reactions in conifers and might be used to increase systemic resistance against biotic damage. In the greenhouse, different concentrations of MJ (10, 20 and 30 mM) were applied as seed soaking plus foliar spray or only as foliar spray to control leaf rust and induction of secondary compound production in leaves of wheat plants. Foliar spray was applied after 30 and 50 days of sowing. Results indicated that all concentrations and treatments reduced the severity of rust disease caused by P. recondita f. sp. tritici in wheat leaves during 45 days of inoculations. Disease incidence was decreased significantly in MJ-treated plants as seed soaking plus foliar spray with 20 and 30 mM when compared to 10 mM MJ or control plants. The study revealed that, with increasing concentrations of MJ, the secondary metabolites were greatly increased. Endogenous levels of both free and conjugated putrescine, spermidine and spermine increased in response to the elicitor. Activities of polyamine biosynthetic enzymes of ornithine decarboxylase (ODC) and polyamine oxidase (PAO) displayed up to threefold increases relative to untreated control. Moreover, significant increases in activities of plant defense-related protein, enzymes as peroxidase and chitinase as well as free and conjugated phenols contents were recorded in treated plants compared with untreated and infected plants. Furthermore, MJ treatment increased the chlorophyll-a, chlorophyll-b and carotenoids pigments contents, the higher increase was obtained with combined treatment between seeds soaking plus foliar spray at 20 and 30 mM of MJ. Under field conditions, three concentrations of MJ, i.e. 10, 20 and 30 mM as combined treatment between seeds soaking plus foliar spray or only as foliar spray were applied to study their effect against rust disease. Foliar spray was applied after 30 and 80 days of sowing. Results showed that the high reduction in disease severity was obtained with combined treatments between seeds soaking plus foliar spray with MJ at 20 and 30 mM compared with other treatments and control. At the same time, all treatments increased the growth and grain yield of wheat plants. It could be suggested that combination treatment between seeds soaking plus foliar spray with methyl jasmonate might be used commercially for controlling rust disease of wheat plants under field conditions.     

Introgression of Recombinant 1RSWR.1BL Translocation and Rust Resistance Genes in Bread Wheat cv. HD2967 Through Marker-Assisted Selection

Bread wheat (Triticum aestivum L.) is one of the major cereal crops utilized worldwide for bread making. The presence of secalin locus on 1RS leads to the sticky dough and poor bread-making quality of wheat. In the present study, two donor parents, one with distal rye chromatin (1RS44:38) and another with distal wheat chromatin (Pavon MA1) without secalin, and one recipient elite wheat cultivar HD2967 were used. In 1RS44:38, the distal rye region has the Pm8 gene to which the QTL for superior root traits is linked, while in Pavon MA1 with Glu-B3/Gli-B1, the Pm8 gene was found to be absent. This distal rye region having root trait QTL was introgressed into the HD2967 derivatives using marker-assisted backcross selection. The derivatives with distal rye region introgression had higher root biomass, drought resistance, and 6–8% higher yield than the recipient parent cultivar. HD2967 is highly susceptible to yellow rust. Therefore, in the second backcross, the rust-resistant version of HD2967 (Lr57?+?Yr40) was used to introgress rust resistance in the derivatives. Background selection was done using polymorphic wheat anchored SSR markers of A, B, and D genomes of wheat which led to the selection of derivatives with?> 90% background of the recipient cultivar. The significant findings in this study include higher root biomass, improved yield, rust resistance in the derivatives, and retaining the alleles of Glu-B3/Gli-B1 along with Pm8 and the absence of secalin.

     

Possible explanations of synergism in fungicide mixtures against Phytophthom infestans

Single foliar sprays of oxadixyl, mancozeb or cymoxanil alone or the mixtures oxadixyl/mancozeb and oxadixyl/cymoxanil were applied to tomato plants (cv. Baby) one day before inoculating with a sensitive or a phenylamide-resistant strain of Phytophthora infestans. Fungicide mixtures were far more effective in controlling both sensitive and resistant strains compared to the individual components alone, representing significant levels of synergistic interactions. When analysed by the Abbott method, synergy ratios continuously decreased with increasing amounts of fungicides in the mixture. All fungicide mixtures showed higher synergy ratios against resistant strains when using the Abbott method, whereas only mixtures containing cymoxanil showed this phenomenon when using the Wadley method for interpretation of interactions. Synergism significantly decreased when the components were used as foliar split applications with intervals of 48 and 72 h. A combination of a drench application of oxadixyl and a single spray application of mancozeb or cymoxanil resulted in a high level of synergism. The explanation of synergism may be that the exposure of sporangia to sublethal concentrations of one fungicide affects them to an extent that sublethal doses of the other fungicide becomes detrimental.     

Physiological and biochemical response of potato (Solanum tuberosum L. cv. Kara) to O3 and antioxidant chemicals: possible roles of antioxidant enzymes

An Egyptian cultivar of potato (Solanum tuberosum L. cv. Kara) was grown in the field at two locations in northern Egypt: a ‘rural’ and a ‘suburban’ site, from October 2000 and November 2002. The antiozonant ethylenediurea (EDU) and the fungicide chlorothalonil (1,3‐benzenedicarbonitrile‐2,4,5,6 tetrachloroisophthalnitrile) were applied as a foliar spray to plants at both sites. It was found that foliar injury symptoms were reduced greatly in plants treated with EDU and/or chlorothalonil, and the yield of treated plants was higher than that of the untreated ones, with the EDU having a greater protection than chlorothalonil. Antiozonant (EDU) and fungicide (chlorothalonil) combination sprays were even more effective in reducing O₃ injury. Moreover, the percentage of protection was higher in the rural area than in the suburban one, and this was associated with higher levels of O₃ recorded in the rural area. The response to O₃, EDU, and chlorothalonil of the leaf antioxidant scavenger system was examined. Antiozonant‐treated plants had the highest reduced glutathione/oxidised glutathione ratio. The results suggest that EDU and chlorothalonil do not act directly as antiozonant to inhibit O₃ injury but act through maintaining some antioxidant enzymes during O₃ exposure. To the best of knowledge, this is the first report demonstrating the marked enhancement of yield and plant oxidative enzymes by fungicides as a mechanism of protecting plants against noxious oxidative stress from the environment in the developing world.     

Reduction of Winter Wheat Yield Losses Caused by Stripe Rust through Fungicide Management

Stripe rust of winter bread wheat (Triticum aestivum L.) causes substantial grain yield loss in Central Asia. This study involved two replicated field experiments undertaken in 2009–2010 and 2010–2011 winter wheat crop seasons. The first experiment was conducted to determine grain yield reductions on susceptible winter wheat cultivars using single and two sprays of fungicide at Zadoks growth stages Z61–Z69 in two farmers’ fields in Tajikistan and one farmer's field in Uzbekistan. In the second experiment, four different fungicides at two concentrations were evaluated at Zadoks growth stage Z69. These included three products from BASF – Opus (0.5 l/ha and 1.0 l/ha), Platoon (0.5 l/ha and 1.0 l/ha) and Opera (0.75 l/ha and 1.5 l/ha) – and locally used fungicide Titul 390 (0.5 l/ha and 1.0 l/ha). One and two sprays of fungicides did not differ significantly (P > 0.05) in increasing grain yield. Stripe rust reduced grain yield and 1000‐kernel weight (TKW) from 24 to 39% and from 16 to 24%, respectively. The benefits from the two concentrations of the same fungicide did not consistently resulted in significantly higher grain yield, suggesting that the lower concentrations could be more cost effective. Our study provides important information about the selection of fungicides, spray concentrations and number of spray to control stripe rust and increase grain yield. The findings could play an important role in developing stripe rust management approaches such as fungicide rotation and strategic fungicide applications in Central Asian countries.     

QTL identification and microphenotype characterisation of the developmentally regulated yellow rust resistance in the UK wheat cultivar Guardian

Yellow rust (causal agent: Puccinia striiformis f.sp. tritici) resistance in the UK wheat cultivar Guardian is developmentally regulated, resistance increasing as the plant matures. Yellow rust resistance was assessed under field conditions on plants after ear emergence to ensure maximum expression of resistance. Three quantitative trait loci (QTL) for yellow rust resistance were identified, being located on chromosomes 1B (QPst.jic-1B), 2D (QPst.jic-2D) and 4B (QPst.jic-4B). The largest resistance effect, QPst.jic-1B located to the same position on the long arm of chromosome 1B as the known durable source of yellow rust resistance, Yr29. Microscopic studies were carried out to determine what effect the resistance in Guardian had on the development of P. striiformis f.sp. tritici. While the adult plant resistance in Guardian did not prevent germinated urediniospores from establishing an effective infection site, the growth of hyphae within flag leaf tissue was significantly inhibited, slowing the development of microcolonies. 3,3-diaminabenzadine (DAB) and trypan blue staining indicated that this inhibition of hyphal growth was not associated with hydrogen peroxide accumulation or extensive plant cell death.