Tolerance of septoria leaf blotch in winter wheat

For individual varieties, tolerance of septoria leaf blotch was quantified by the slope of the relationship between disease and yield. Variation in disease severity and the associated yield responses were provided across two sites and three seasons of field experiments. Slopes were fitted by residual maximum likelihood for two contrasting models: (i) a fixed-effects model, where no prior assumptions were made about the form of the variety slopes; and (ii) a random-effects model, where deviations in individual variety slopes away from the mean variety slope formed a normal random population with unknown variance. The analyses gave broadly similar results, but with some significant differences. The random model was considered more reliable for predicting variety performance. The effects of disease were quantified as symptom area and green canopy duration. Models of the relationship between symptom area and yield were site-specific. When site effects were not taken into account, these models had poor predictive precision. Models based on the canopy green area gave robust predictions of yield and were not site-specific. Differences in disease tolerance were detected in a comparison of 25 commercial winter wheat varieties. Tolerance was not detected directly through symptom measurements, but instead through measurements of canopy green area, which provides a measurement of the effects of disease that accounts for differences in canopy size across sites and seasons. The varieties showing greatest tolerance tended to have lower attainable yield than the intolerant varieties. Presence of the 1BL/1RS chromosome translocation, which has been reported to increase radiation use efficiency, appeared to be associated with intolerance.     

Infection routes for Sclerotinia sclerotiorum in apetalous and fully petalled winter oilseed rape

Near‐isogenic lines (NILs) of apetalous (AP) and fully petalled (FP) winter oilseed rape were used to investigate infection by Sclerotinia sclerotiorum, which occurs mainly via infected petals adhering to leaves in FP oilseed rape. AP1 flowers had an average of 1·4 and 0·8 petals per flower in field and polytunnel experiments, respectively. In field experiments there were no significant differences between counts of FP1 petals, FP1 stamens and AP1 stamens adhered to leaves during flowering. At any one sample time, significantly more stamens tested positive for S. sclerotiorum on AP1 than FP1 NILs, e.g. in 2004, at early flowering 37·5% and 24·2% of stamens tested positive on AP1 and FP1 NILs, respectively. In polytunnel experiments, there were significantly more sclerotinia lesions per plant in the FP1 than in the AP1 NIL. The AP1 NIL did not avoid infection completely, probably because it produced some petals, and lesions were initiated from adhered stamens as well as petals. However, while 8·5% and 16·3% of petals initiated lesions in FP1 and AP1 NILs, respectively, only 2·5% and 1·0% of stamens initiated lesions in FP1 and AP1 NILs, which suggests stamens may be less infective than petals. In field experiments the AP1 NIL had significantly less incidence of sclerotinia stem rot than the FP1 NIL in 2004 (4·9% and 7·0%, respectively). However, there was no significant difference in stem rot incidence between AP and FP lines in 2005 (3·6% and 4·3%, respectively) or 2006 (5·5% and 3·9%, respectively).     

Pathogenic Variability of Pyricularia grisea from the High- and Mid-Elevation Zones of Bhutan

ABSTRACT Thirty isolates of P. griseacollected from rice during a blast epidemic in 1995 in the high (1,800 to 2,600 m) and middle (1,200 to 1,800 m) elevations of Bhutan and 80 isolates collected from one rice cultivar from two high- and two mid-elevation sites in 1996 were analyzed for virulence. Differential varieties were indica CO39, with five near-isogenic lines (NILs) for resistance genes in the genetic background of CO39, and japonica Lijiangxintuanheigu (LTH), with five NILs for LTH. Twelve selected Bhutanese landraces also were studied. In addition, 10 blast nurseries consisting of the NIL sets, important local landraces, and representatives of international differential groups were established in the 1996 and 1997 growing seasons in the mid- and high-elevation agroecological zones. The 110 isolates were differentiated into 53 pathotypes based on the 2 NIL sets. Thirteen isolates were avirulent on all of the NILs but were compatible with some landraces. Several isolates were able to attack one of the NILs of CO39 but not CO39. These results strongly suggest that both CO39 and LTH possess previously unidentified resistance. The landraces were not uniform in their reactions to the isolates. When a reaction index taking into account all individual plant reactions was used, isolates that had been assigned to the same pathotype could be further differentiated, indicating that the NIL sets could not completely discriminate virulences in Bhutanese P. grisea populations. In the trap nurseries, disease was always present in the middle elevations, but disease was very low during July 1996 in the high elevations and only present during August and September 1997. Almost all varietal groups were more frequently attacked in the middle than in the high elevations, indicating that the virulence spectrum is wider and the conduciveness of the environment is greater in the middle elevations. Landraces from the high elevations were most susceptible, followed by international differential groups 7 and 8. The results suggest that selection has yielded landraces with more complete and complex resistance in the more disease-conducive mid-elevation environment. At the same time, the pathogen population also possesses a wider virulence spectrum in that environment.     

Resistance to Scald (Rhynchosporium secalis) in Barley (Hordeum vulgare) Studied by Near-Isogenic Lines: I. Markers and Differential Isolates

ABSTRACT Near-isogenic lines (NILs) with resistance for scald in seventh generation backcross with 'Ingrid' as recurrent parent (RP) were tested with seven differential isolates of Rhynchosporium secalis in Norway and Canada. NILs of 'Turk', 'Brier', 'CI 8162', 'La Mesita', 'Hispont', 'Atlas 46', 'Modoc', 'Hudson', 'Abyssinian', 'Steudelli', and 'CI 2222' also were evaluated for field reactions. The genetic characterization of the NILs (degree of isogeneity with Ingrid) and with each other was carried out. The molecular marker pattern shows that the backcrossing program has resulted in from 86.3 to 100% RP genome in the NILs, depending on the marker system. On an average, 96% RP genome was found in the NILs. There were certain consistent (pairwise) differences between the NILs and RP on chromosomes 3H and 7H. Both chromosomes are known to contain loci conferring resistance to R. secalis, indicating successful introgression from the donors into the NILs. Approximately two-thirds of the observed RP-NIL polymorphisms were linked to the assumed resistance in the NIL. Based on the marker and phenotypic analyses of the NILs, suggestions for a more appropriate and updated terminology of genes for resistance to R. secalis in barley are made. The proposed changes in nomenclature also indicate the differentials that are available as NILs and those lacking.     

Selectivity of weed harrowing in lupin

Three field experiments were conducted in lupin in 1997, 1998 and 1999 to study two aspects of selectivity of post‐emergence weed harrowing; the ability of the crop to resist soil covering (the initial damage effect), and the ability of the crop to tolerate soil covering (the recovery effect). Each year soil covering curves and crop tolerance curves were established in three early growth stages of lupin. Soil covering curves connected weed control and crop soil cover in weedy plots, and crop tolerance curves connected crop yield and crop soil cover in weed‐free plots. The experiments showed that both resistance and tolerance were unaffected by the growth stage of lupin within the range from the cotyledon to the 7–8 leaf growth stages. Tolerance to soil covering was also unaffected by year whereas the ability of the crop to resist soil covering was highly affected by year. Lupin showed high tolerance to soil covering but a rather low ability to resist soil covering. Harrowing at multiple growth stages supported the finding that lupin is fairly tolerant to soil covering. Advantages and disadvantages of using soil covering as a measure of crop damage is discussed. In conclusion, weed harrowing in lupin showed positive prospects because of high tolerance to crop soil cover.     

稻瘟病菌与水稻CO39近等基因系互作的寄生适合度属性研究

选用以CO39为遗传背景的水稻CO39抗稻瘟病近等基因系(NILs)与广东省稻瘟病菌4个优势生理小种(ZC₁₃、ZB₁、ZB₁₃和ZB₅)构成16个亲和性和非亲和性互作组合,对各互作的4个寄生适合度属性(病斑平均扩展速率、感病型病斑比例、侵染成功率和病斑单位面积平均产孢量)进行了方差分析、多重比较和相关分析。方差分析表明,水稻NILs各品种、稻瘟病菌各小种以及品种×小种互作间的差异极显著。经多重比较,品种CO39和C104PKT之间在接菌相同小种时差异极显著,而品种C101LAC和C101A51之间虽差异不显著,但与前两品种间的差异极显著;小种ZC₁₃和ZB₄之间除单位病斑面积产孢量外另3个属性上差异不显著,但都显著高于ZB₅和ZB₁₃,而且ZB₅也显著高于ZB₁₃。相关分析表明,4个属性中每2个属性间均呈显著正相关,说明病菌小种在这个遗传背景相同的近等基因系中,可能具有相似的生存方式,病菌一旦克服寄主某方面的抗性后,所有生存适应性都一起提高,这是由于寄主的遗传同质性引起的。     

Potential contribution of selected canopy traits to the tolerance of foliar disease by spring barley

A model of canopy photosynthesis and above-ground growth rate was used to investigate the potential impact of several canopy traits on tolerance of foliar disease by barley. Disease tolerance was defined as the reduction in predicted crop dry-matter growth rate per unit of visible disease symptoms. The traits were canopy area (leaf area index, LAI ), light extinction coefficient ( k ) and the ratio of virtual to visible lesion size ( β ). The effects of altering the area of the healthy flag leaf and its light-saturated rate of photosynthesis (P_max) in response to disease elsewhere on the plant were also investigated. The model was parameterized for spring barley and run with a solar radiation and temperature regime typical of north-east Scotland. Predicted reductions in growth rate per unit increase in disease were greatest at high disease severity and when disease was distributed relatively uniformly through the canopy. Tolerance was increased by increasing LAI to >3 and k to >0·3, but the beneficial effects depended on the severity and, to a lesser extent, the distribution of disease. Tolerance was reduced by increasing β . A sensitivity analysis performed at a single disease severity and distribution showed that tolerance was most sensitive to variations in β and compensatory adjustments in area and P_max of the flag leaf, and least sensitive to whole canopy LAI and k . Future research should quantify the genetic variation in these traits within barley germplasm to evaluate the scope for improving the disease tolerance of spring barley.     

Mechanisms of resistance in cotton to whitefly (<Emphasis Type="Italic">Bemisia tabaci</Emphasis>): antixenosis

Screenhouse experiments were conducted under free-choice conditions to determine the mechanism of resistance operating in cotton against whitefly, Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae). Twelve cotton genotypes belonging to Gossypium hirsutum and G. arboreum were assessed for oviposition preference by whitefly. The trichome density and length, distance from lower leaf surface to nearest vascular bundles, leaf lamina thickness and compactness of vascular bundles were estimated for each genotype and correlations with number of eggs laid were determined. The genotypes RS2098, CNH911, and PA183 were non-preferred for oviposition and exhibited an antixenosis mechanism of resistance. NHH44, LK861, Supriya, RS2013 and LD694 were categorized as moderately resistant while IS-376/4/1/20/72 and F846 were categorized as susceptible. Greater leaf lamina thickness and more compact vascular bundles were correlated with egg laying by whitefly.     

Tolerance to paraquat-mediated oxidative and environmental stresses in squash (Cucurbita spp.) leaves of various ages

Paraquat is labeled for row-middle application on cucurbits, but drift to crop foliage is inevitable. Experiments were conducted to determine whether differential tolerance to paraquat existed among leaves of various ages in Cucurbita spp. (squash) and other plants, and to examine whether leaves tolerant to paraquat are also tolerant to other herbicides and abiotic stresses. Physiological responses to paraquat, including antioxidant activity, were investigated in squash leaves to identify mechanisms of paraquat tolerance. Although the level of paraquat tolerance differed by leaf age, cultivar, and species, the level of paraquat injury was lower in younger leaves than in older leaves in 14 of 18 squash cultivars and 5 of 12 other species tested. Cellular leakage and lipid peroxidation were consistently lower in the youngest leaf (leaf 4) than in the older leaves. Quantum yield and relative chlorophyll content were the same in all leaves of nontreated plants. Epicuticular wax content was higher in the youngest leaf than in leaves 1, 2, and 3 of cv. ‘Joongangaehobak’ and ‘Wonbiaehobak’. However, leaf cuticle content was not consistent with leaf ages. Differential leaf response to paraquat was partially correlated with the change in catalase, peroxidase, ascorbate peroxidase (APX), and glutathione reductase activities in nontreated and treated leaves. The APX activity in the youngest leaf was generally 2 times higher than in leaves 1-3 in both nontreated and treated plants. Ascorbate antioxidant levels were also higher in the youngest leaf than those in leaves 1-3. Leaves tolerant to paraquat were also tolerant to diquat and to abiotic stresses, low temperature and drought. However, tolerance to oxyfluorfen, which has a different mode of action than paraquat and diquat, was higher in older than in younger leaves. Higher tolerance to paraquat-mediated oxidative and abiotic stresses in young leaves of most squash cultivars might contribute to the differential prevention of oxidative damage in leaves of various ages.     

Resistance of wheat to septoria tritici blotch (Mycosphaerella graminicola) and associations with plant ideotype and the 1BL−1RS translocation

Septoria tritici blotch (STB) is a major disease of wheat, reaching epidemic proportions in many parts of the world. In several studies, taller, later-maturing cultivars have had lower disease levels. This study was undertaken to investigate the genetic associations of natural field infection by STB with disease-escape mechanisms related to heading date and height components, mainly leaf spacing, in a population where height and maturity are not controlled by major genes. In field trials of a single seed-descent population of a cross between two nonsemi-dwarf cultivars, Apollo (with strong partial resistance to STB) and Thésée (susceptible), conducted over 3 years, there was a negative correlation between STB and heading date. There was no correlation between STB and distance from stem base to leaf 2; and there was an unexpected positive correlation between STB and distance from flag leaf to leaf 2, which contradicted the so-called 'ladder effect' postulated in STB epidemiology. No effect was detected of the presence of the 1BL−1RS translocation on STB levels. The largest single contributor to variation in STB levels was genetic variation between the progeny lines, and the narrow-sense heritability was 42%. These results suggest that breeders can select for STB resistance alongside optimal stature within the range of height which is adaptive in a particular environment.     

农杆菌介导的稻瘟病菌致病突变菌株的筛选

 利用1个致病能力非常弱的稻瘟病菌菌株CY2和农杆菌介导T-DNA方法,已成功地获得了5000多个转化子,并以多个转化子混合接种含25个不同抗病基因的近等基因系,获得了30个致病突变体。但突变体针对不同的无毒基因有很大差别,从含有Pi-ta²的F128-1上获得了14个突变体,从含有Pi-a的IRBL-2上获得了7个突变体,从含有Pik-s的IRBL-4上获得了2个突变体,而从其它抗病基因的品系上未发现任何突变体,故T-DNA似乎有插入热点。不同突变体在携带相同抗病基因的品系上的致病性存在很大差异,表明这些近等基因系可能还携带其它抗稻瘟病基因。CY2不能侵染普遍认为不含抗稻瘟病基因的丽江新团黑谷(LTH),而7个突变体则能成功地使其发病,表明LTH亦含有抗稻瘟病基因。     

Foliar calcium concentration of potato and its relation to genotype lateness and tolerance of cyst nematodes

In field trials during three years respectively 18, 22 and 57 potato genotypes were grown on soils moderately or heavily infested with potato cyst nematodes (Globodera pallida) after soils were fumigated or not. Nematode infection increased leaf calcium contents but genotypes that were relatively tolerant of potato cyst nematodes (Globodera pallida) had lower leaf calcium concentrations on a particular sampling date. Tolerance of potato cyst nematodes was closely linked to genotype lateness and may be related to tolerance of drought. When using the method in plant breeding and screening for genotypes with tolerance, plants may be planted in infected or in uninfected soils, without influencing the outcome. The relationship between tolerance and calcium contents was clearest when sampling took place towards the end of the growing season when the variability was at it highest with a good distinction between newly formed leaves in late maturing genotypes and aged leaves in early maturing cultivars.     

Contradictions in host plant resistance to pests: spider mite (Tetranychus urticae Koch) behaviour undermines the potential resistance of smooth-leaved cotton (Gossypium hirsutum L.)

BACKGROUND: Two‐spotted spider mites (Tetranychus urticae Koch) oviposit near leaf veins or in leaf folds on the undersides of cotton (Gossypium hirsutum L.) leaves where the humid boundary layer offers protection from desiccation. The authors predicted that the boundary layer of glabrous cotton leaves should be shallower than that of hairy leaves, providing some resistance to mites. The dynamics of mite populations, leaf damage, leaf gas exchange and crop yield on two leaf hair isolines (smooth versus hairy) in two genetic backgrounds was assessed. RESULTS: Mite colonies developed faster on the hairy leaf isolines, but leaf damage per mite was higher in smooth leaf isolines, indicating more intense damage. A 50% reduction in photosynthesis on the hairy isolines required 1.8 times more mites than smooth leaves. The yield of cotton was reduced in + mite treatments, but the magnitude of reduction was similar for hairy and smooth isolines. CONCLUSION: Paradoxically, the relative inhospitality of glabrous leaves may have induced mites to concentrate in protected leaf sections, causing more localised and more severe damage, negating the yield benefits from fewer mites. These results highlight interactions between leaf microenvironment, pest behaviour and plant productivity that may have implications for other instances of plant resistance. Copyright © 2010 Society of Chemical Industry     

Characterization of a resistance locus (Pfs-1) to the spinach downy mildew pathogen (Peronospora farinosa f. sp. spinaciae) and development of a molecular marker linked to Pfs-1

Downy mildew is a destructive disease of spinach worldwide. There have been 10 races described since 1824, six of which have been identified in the past 10 years. Race identification is based on qualitative disease reactions on a set of diverse host differentials which include open-pollinated cultivars, contemporary hybrid cultivars, and older hybrid cultivars that are no longer produced. The development of a set of near-isogenic open-pollinated spinach lines (NILs), having different resistance loci in a susceptible and otherwise common genetic background, would facilitate identification of races of the downy mildew pathogen, provide a tool to better understand the genetics of resistance, and expedite the development of molecular markers linked to these disease resistance loci. To achieve this objective, the spinach cv. Viroflay, susceptible to race 6 of Peronospora farinosa f. sp. spinaciae, was used as the recurrent susceptible parent in crosses with the hybrid spinach cv. Lion, resistant to race 6. Resistant F(1) progeny were subsequently backcrossed to Viroflay four times with selection for race 6 resistance each time. Analysis of the segregation data showed that resistance was controlled by a single dominant gene, and the resistance locus was designated Pfs-1. By bulk segregant analysis, an amplified fragment length polymorphism (AFLP) marker (E-ACT/M-CTG) linked to Pfs-1 was identified and used to develop a co-dominant Sequence characterized amplified region (SCAR) marker. This SCAR marker, designated Dm-1, was closely linked ( approximately 1.7 cM) to the Pfs-1 locus and could discriminate among spinach genotypes that were homozygous resistant (Pfs-1Pfs-1), heterozygous resistant (Pfs-1pfs-1), or homozygous susceptible (pfs-1pfs-1) to race 6 within the original mapping population. Evaluation of a wide range of commercial spinach lines outside of the mapping population indicated that Dm-1 could effectively identify Pfs-1 resistant genotypes; the Dm-1 marker correctly predicted the disease resistance phenotype in 120 out of 123 lines tested. In addition, the NIL containing the Pfs-1 locus (Pfs-1Pfs-1) was resistant to multiple races of the downy mildew pathogen indicating Pfs-1 locus may contain a cluster of resistance genes.     

小麦抗白粉病基因Pm21 的抑制基因

 小麦-簇毛麦6VS. 6AL 易位染色体含有抗白粉病基因Pm21,在我国的小麦育种中被广泛应用。近年来,一些含有Pm21 基因的小麦品种(系)开始感染白粉病。为探索含Pm21 的品种(系)感染白粉病的原因,本研究在6VS. 6AL 易位系与小麦品系(种)R14 和川农12 的杂交后代中利用分子标记CINAU17-1086 和CINAU18-723 辅助选择的遗传背景相对简单的F7 和F8 近等基因系为材料,研究了小麦抗白粉病基因Pm21 的抗病性表达。结果发现,在3 个含有6VS. 6AL 易位染色体的感病F6 植株繁殖的F7 近等基因系中发生了白粉病抗性的分离,分离比率符合13 感病︰ 3 抗病的理论值。在随机选取的F7 感病小麦单株所繁殖的F8 近等基因系中,有7 / 13 的株系一致地重感白粉病,有6 / 13 的株系发生了抗白粉病的分离,其中2 / 13 的株系分离比符合3 感病︰ 1 抗病、4 / 13 的株系分离比符合13 感病︰ 3 抗病的分离模式。这一结果指出,小麦株系中的抗白粉病基因Pm21 的抗性表达受小麦基因组中的一对显性抑制基因所控制,该基因来源于小麦品种(系)川农12或R14,建议命名为SuPm21。本研究指出,在把外源基因引入小麦的研究中,有利的外源基因与不含抑制基因的受体遗传资源同等重要。     

Host plant resistance through physico-chemical characters against major insect pests of okra occurring in the Gangetic plains of eastern India

Okra production in the tropics is threatened by attack of insect-pests, fruit and shoot borer (FSB), jassid and whitefly causing severe loss in marketable yield particularly in the Gangetic plains of eastern India. No developed varieties/hybrids are free from attack of these pests. However, improved lines of okra have potential to broaden the genetic variability may improve resistance to these pests. Therefore, the population dynamics of these pests were recorded from 24 advance breeding lines/varieties of okra during peak infestation period over two consecutive years. Morphological leaf traits and biochemical parameters were assessed as potential mediators of resistance. The population of these pests reached above economic threshold levels during the month of September. Significant genotypic differences in terms of infestation of these pests were observed. Negatively correlated physico-chemical characters might be considered as important indices for selection of genotype tolerant to these pests. As indicated by the low infestations, the tolerance of BCO-1 to FSB, whitefly and that of Arka Anamika to jassid is based on both antixenotic properties of the genotypes. The donor parents BCO-1 and Arka Anamika should, therefore, be introduced into okra breeding programs to enhance tolerance against these pests.     

Photosynthesis,transpiration and plant growth characters of different potato cultivars at various densities of Globodera pallida

Plants of four potato (Solanum tuberosum L.) cultivars were grown in pots in a greenhouse at five densities ofGlobodera pallida between 0 and 300 eggs per gram of soil. Photosynthesis and transpiration of selected leaves were measured at 30, 37, 49 and 60 days after planting. Stem length was recorded at weekly intervals. Plants were harvested 70 days afteer planting and various plant variables were determined.At 30 days after planting, when second and third stage juveniles were present in roots, both photosynthesis and transpiration rates were severely reduced byG. pallida. In the course of time these effects became less pronounced. Water use efficiency was reduced byG. pallida between 30 and 49 days, but not at 60 days after planting. The results suggest independent effects ofG. pallida on stomatal opening and on photosynthesis reactions. There were no consistent differences among cultivars in the response of leaf gas exchange rates and water use efficiency to nematode infection. Reduction of photosynthesis byG. pallida appeared additive to photosynthesis reduction due to leaf senescence.Total dry weight was reduced by 60% at the highestG. pallida density. Weights of all plant organs were about proportionally affected. Shoot/root ratio was not affected and dry matter content was reduced. Stem length and leaf area were most strongly reduced during early stages of plant-nematode interaction. The number of leaves formed was only slightly reduced byG. pallida, but flowering was delayed or inhibited. Reduction of total dry weight correlated with reduction of both leaf area and photosynthesis rate. Leaf area reduction seems the main cause of reduction of dry matter production. Tolerance differences among cultivars were evident at 100 eggs per gram of soil only, where total dry weight of the intolerant partially resistant cv. Darwina was lower than that of the tolerant partially resistant cv. Elles and of the tolerant susceptible cv. Multa. The tolerance differences were not correlated with leaf photosynthesis and transpiration. Apparently these processes are not part of tolerance of plants.     

Field Resistance to Strobilurin (Q(o)I) Fungicides in Pyricularia grisea Caused by Mutations in the Mitochondrial Cytochrome b Gene

ABSTRACT Gray leaf spot caused by Pyricularia grisea is a highly destructive disease of perennial ryegrass turf. Control of gray leaf spot is dependent on the use of preventative fungicide treatments. Strobilurin-based (Q(o)I) fungicides, which inhibit the cytochrome bc(1) respiratory complex, have proven to be very effective against gray leaf spot. However, in August 2000, disease was diagnosed in Q(o)I-treated perennial ryegrass turf on golf courses in Lexington, KY, Champaign, IL, and Bloomington, IL. To determine if resistance was due to a mutation in the fungicide target, the cytochrome b gene (CYTB) was amplified from baseline and resistant isolates. Nucleotide sequence analysis revealed an intronless coding region of 1,179 bp. Isolates that were resistant to Q(o)I fungicides possessed one of two different mutant alleles, each of which carried a single point mutation. The first mutant allele had a guanine-to-cytosine transition at nucleotide position +428, resulting in a replacement of glycine 143 by alanine (G143A). Mutant allele two exhibited a cytosine-to-adenine transversion at position +387, causing a phenylalanine-to-leucine change (F129L). Cleavable amplified polymorphic sequence analysis revealed that neither mutation was present in a collection of baseline isolates collected before Q(o)I fungicide use and indicated that suspected Q(o)I- resistant isolates found in 2001 in Indiana and Maryland possessed the F129L mutation. The Pyricularia grisea isolates possessing the G143A substitution were significantly more resistant to azoxystrobin and trifloxystrobin, in vitro, than those having F129L. DNA fingerprinting of resistant isolates revealed that the mutations occurred in just five genetic backgrounds, suggesting that field resistance to the Q(o)I fungicides in Pyricularia grisea is due to a small number of ancestral mutations.