Measures of disease intensity in powdery mildew (Erysiphe graminis) of winter wheat. 1. Errors in estimating pustule number

Assessments of pustule number and severity of powdery mildew on winter wheat in the Netherlands were made in commercial fields and in experimental plots. The sample variance (s²) of the number of pustules per leaf (m) was fairly constant over years, varieties, growth stages and leaf postitions, but depended strongly on the average pustule number: . The effect of sample size on the precision of the estimate is discussed and it is concluded that it is difficult to estimate low disease intensities accurately. Estimates are given for the detection level of pustule counts in relation to sample size.Mildew intensity on the lower surface of leaves can be estimated from the intensity on the upper surface. This method reduces the duration of the observation, but introduces an additional error. At low disease intensities and small sample sizes this method is more efficient than sampling mildew on both surfaces of leaves. The common practice of assessments of the upper surface of leaves only may not be the most efficient method.Samenvatting Aantallen puistjes meeldauw per blad werden geteld in praktijkpercelen en veldproeven met wintertarwe. De steekproefvariantie van het aantal puistjes was tamelijk constant in de jaren, rassen, gewasstadia en bladposities, maar was sterk afhankelijk van het gemiddeld aantal puistjes . Het effect van de steekproefgrootte op de nauwkeurigheid van de schatting wordt besproken en het blijkt dat het moeilijk is om lichte aantastingen nauwkeurig te schatten. Er worden schattingen gegeven van de detectiegrens in afhankelijkheid van de steekproefgrootte.Meeldauwaantastingen aan de onderkant van het blad, kunnen worden geschat uit de aantasting op de bovenkant van het blad. Deze methode levert een tijdsbesparing op, maar ook een extra onnauwkeurigheid. Alleen bij lichte aantastingen en kleine steekproeven is deze methode efficiënter dan een directe tweezijdige bemonstering. Het schatten van meeldauw op de bovenkant van bladeren is, hoewel algemeen gebruikelijk, waarschijnlijk niet de meest efficiënte methode.     

Possible Root Infection of Cercospora beticola in Sugar Beet

A potential primary infection site of the foliar pathogen Cercospora beticola in sugar beet is described. Sugar beet seedlings of the susceptible cv. Auris were grown in a standard soil for 14 days. A monoconidial culture of a C. beticola isolate was grown to produce conidia. In experiment 1, roots were immersed in a conidial suspension of isolate code IRS 00-4, or in tap water (control), for 2 days. After incubation seedlings were potted in a peat – fine river sand mixture and placed at low relative humidity (RH) (<80%) or high RH (100%). Twelve days after infection, seedlings at high RH showed more disease incidence (90%) than seedlings grown at low RH (disease incidence = 25%), whereas no disease symptoms developed in the control seedlings. Cercospora leaf spots (CLSs) developed on the cotyledons, leaves, petioles and stems of the seedlings. In experiment 2, roots were immersed in a conidial suspension of isolate code IRS 00-2 for 5 h. Thirty-four days after infection at high RH, 100% disease incidence was observed in the treated seedlings and one CLS in the control treatment. First indications of leaf spot development were observed as reddish purple discolouration of individual parenchymatic cells. Because splash dispersal and symptoms due to infested soil were excluded, we showed that it is possible to obtain CLS symptoms in sugar beet seedlings when their roots were immersed in conidial suspensions of C. beticola, thus demonstrating that roots can be a primary infection site.     

Pathogenicity and genetic variation of Phytophthora cactorum from silver birch and strawberry

Phytophthora cactorum strains isolated from necrotic stem lesions on Betula pendula seedlings or from Fragaria ananassa plants suffering from crown rot were pathogenic to their host plants. Only isolates from birch caused clear lesions on non-wounded bark of birch. P. cactorum isolates from birch were not detrimental to strawberry. Random Amplified Polymorphic DNA (RAPD) analysis revealed variation within P. cactorum, isolates from silver birch having different banding patterns than those from strawberry. UPGMA analysis clustered isolates from silver birch and strawberry plants into separate groups. The data show that the recent outbreak in Finland of P. cactorum in birch could not be caused by the import of strawberry plants affected by crown rot.     

Roles of reactive oxygen species in interactions between plants and pathogens

The production of reactive oxygen species (ROS) by the consumption of molecular oxygen during host–pathogen interactions is termed the oxidative burst. The most important ROS are singlet oxygen (¹O₂), the hydroxyperoxyl radical (HO₂·), the superoxide anion , hydrogen peroxide (H₂O₂), the hydroxyl radical (OH^-) and the closely related reactive nitrogen species, nitric oxide (NO). These ROS are highly reactive, and therefore toxic, and participate in several important processes related to defence and infection. Furthermore, ROS also play important roles in plant biology both as toxic by-products of aerobic metabolism and as key regulators of growth, development and defence pathways. In this review, we will assess the different roles of ROS in host–pathogen interactions with special emphasis on fungal and Oomycete pathogens.     

Dynamics of cercospora leaf spot disease determined by aerial spore dispersal in artificially inoculated sugar beet fields

Cercospora beticola is one of the most important fungal pathogens of sugar beet, causing cercospora leaf spot (CLS) disease. Due to the decreasing efficacy of various fungicides caused by resistance traits, the development of a sustainable disease management strategy has become more important. Therefore, detailed knowledge about the epidemiology of the pathogen is crucial. Until now, little was known about the spatiotemporal dispersal of C. beticola spores from the primary inoculum source. Rapid detection of C. beticola spores could facilitate a more precise and targeted disease control. Therefore, a TaqMan real-time PCR assay for detection and quantification of C. beticola spores caught with Rotorod spore traps was established. In 2016 and 2017, field trials were conducted to monitor C. beticola aerial spore dispersal and disease development within an inoculated field and in the adjacent noninoculated area. With the established detection method, C. beticola spores were successfully quantified and used as a measure for aerial spore dispersal intensity. The analysis of the spatiotemporal spread of C. beticola spores revealed a delay and decrease of aerial spore dispersal with increasing distance from the inoculated area. Consequently, disease incidence and severity were reduced in a similar manner. These results imply that spore dispersal occurs mainly on a small scale within a field, although long distances can be overcome by C. beticola spores. Moreover, secondary aerial spore dispersal from sporulating leaf spots seems to be the main driver for CLS disease development. These results provide an important basis for further improvement of CLS control strategies.     

Identification of resistance to Cercospora leaf spot of cowpea

Twelve selected cowpea cultivars were screened for resistance to Cercospora leaf spot (CLS) disease caused by Pseudocercospora cruenta and Cercospora apii s. lat. under artificial epiphytotic conditions in a replicated field trial, with the objective of developing a quantitative measure of disease resistance. CLS incidence, leaf spotting score, lesion density, lesion size, proportion of nodes infected, diseased leaf area, conidia number mg⁻¹ and fascicle density were assessed in 12 cowpea genotypes at crop maturity. Proportion of nodes infected and leaf spotting score were best able to quantitatively differentiate between the levels of resistance, and allow the exploitation of quantitative resistance to the disease. Both lesion density and lesion size were important in determining the final leaf spotting score but the former was epidemiologically more important than the latter, indicated by its correlation to most of the CLS symptom measures. There was differential resistance to the P. cruenta and C. apii s. lat. among the cowpea varieties screened. Among the cowpea lines screened, resistance to P. cruenta was more common than resistance to C. apii s. lat. Nevertheless, P. cruenta was considered the more aggressive and epidemiologically more important than C. apii s. lat. on the varieties tested evidenced by the strong correlation of P. cruenta incidence with acropetal spread of CLS, intensity of leaf spotting, conidia number mg⁻¹ and fascicle density. The highly susceptible varieties namely VRB7, Los Banos Bush Sitao no.1 and CB27 were susceptible to both Cercospora pathogens. The cowpea variety VRB-10 was completely resistant to both pathogens and is a useful source of resistance in CLS breeding programmes.     

Molecular analysis of <Emphasis Type="Italic">Cercospora beticola</Emphasis> isolates for strobilurin resistance from the Central High Plains,USA

Cercospora leaf spot (CLS), caused by Cercospora beticola, is the most destructive foliar disease and is a problem in sugar beet production areas, such as Central High Plains (states of Colorado, Montana, Nebraska and Wyoming) in the United States. The disease can be controlled by strobilurin fungicides, referred to as quinone outside inhibitors (QoIs), with a single target site on C. beticola. Strobilurin resistance has been reported in beet production areas from the United States, including the Central High Plains. Although strobilurin resistance is quantitatively inherited, it is considered that it has low to medium heritability in the population. Effective diagnostic tools are required for the rapid detection of C. beticola strobilurin resistance. The study obtained a partial nucleotide sequence of the C. beticola cytochrome b gene and determined to a putative protein with ~386 amino acid residues. Eighty C. beticola isolates (2004–2011) from the Central High Plains were analyzed for mutations. We found a single nucleotide polymorphic (SNP) site which led to G143A mutation and was present in 2 C. beticola QoI-resistant isolates. Partial sequences obtained from 82 C. beticola QoI-sensitive isolates showed identical cytochrome b gene. We developed a PCR-RFLP assay that involved an in vitro digestion using Fnu4HI restriction enzyme for the rapid molecular detection of G143A mutation in the C. beticola population. Results indicated the PCR-RFLP assay was reliable, sensitive, and can be used for the rapid detection of C. beticola strobilurin resistance.     

Lack of influence of host plant disease resistance on the evolution of resistance to sterol demethylation-inhibiting (DMI) fungicides in<Emphasis Type="Italic">Cercospora beticola</Emphasis>

Field experiments were conducted during 1997 and 1998 to determine the effects of sugar beet cultivar susceptibility to Cercospora leaf-spot on the sensitivity ofCercospora beticola isolates to the triazole fungicide flutriafol. Four cultivars with different levels of disease resistance were treated in experimental plots with six spray applications of flutriafol. Disease assessments were carried out at 15-day intervals. Sensitivity to flutriafol was measured on isolates collected from the plots ∼15 days after the last flutriafol application. Measurements of disease severity and calculations of AUDPC (area under disease progress curve) values showed a distinct differentiation among cultivars, reflecting their level of disease resistance. Disease severity was significantly lower in cvs. ‘Bianca’ and ‘Areth’ than in ‘Univers’ and ‘Rizor’ both in the untreated and in the flutriafol-treated plots. Fungal isolates from flutriafol-treated plots were less sensitive to the fungicide than were isolates from untreated plots. However, no differences in isolate sensitivity were observed among the cultivars, as regards their level of disease resistance. Despite the fact that the use of resistant cultivars cannot eliminate selectively the resistant strains, it can eliminate both resistant and sensitive isolates. Reducing the number of treatments with DMIs, by applying them only when environmental conditions are favorable for disease development, is a prerequisite for successful resistance management; therefore, the use of disease-resistant varieties could aid toward management of DMIs resistance inC. beticola. http://www.phytoparasitica.org posting May 6, 2003.     

基于高通量测序检测甜菜生尾孢中真菌病毒多样性

为明确甜菜生尾孢Cercospora beticola中真菌病毒的多样性,2020年自新疆维吾尔族自治区、内蒙古自治区、黑龙江省和北京市采集感染褐斑病甜菜病叶,鉴定获得78株甜菜生尾孢菌株,运用高通量的宏转录组测序技术检测甜菜生尾孢所携带的真菌病毒种类;将分离到的甜菜生尾孢按照菌落生长表型分为4组,进行宏转录组测序,将得到的序列经过拼接、聚类和基因功能注释,筛选出真菌病毒相关同源序列。序列分析结果显示,4组样品共比对到6个真菌病毒科,包括双分体病毒科Partitiviridae、双核糖核酸病毒科Birnaviridae、低毒病毒科Hypoviridae、裸露RNA病毒科Narnaviridae、泛欧尔密病毒科Botourmiaviridae和转座病毒科Metaviridae,以及未分科的负义单链RNA病毒。研究结果丰富了甜菜生尾孢真菌病毒资源库。     

Identification of the G143A mutation associated with QoI resistance in Cercospora beticola field isolates from Michigan,United States

BACKGROUND: Cercospora leaf spot (CLS), caused by the fungus Cercospora beticola, is the most serious foliar disease of sugar beet (Beta vulgaris L.) worldwide. Disease control is mainly achieved by timely fungicide applications. In 2011, CLS control failures were reported in spite of application of quinone outside inhibitor (QoI) fungicide in several counties in Michigan, United States. The purpose of this study was to confirm the resistant phenotype and identify the molecular basis for QoI resistance of Michigan C. beticola isolates. RESULTS: Isolates collected in Michigan in 1998 and 1999 that had no previous exposure to the QoI fungicides trifloxystrobin or pyraclostrobin exhibited QoI EC₅₀ values of ?0.006 µg mL^?1. In contrast, all isolates obtained in 2011 exhibited EC₅₀ values of > 0.92 µg mL^?1 to both fungicides and harbored a mutation in cytochrome b (cytb) that led to an amino acid exchange from glycine to alanine at position 143 (G143A) compared with baseline QoI‐sensitive isolates. Microsatellite analysis of the isolates suggested that QoI resistance emerged independently in multiple genotypic backgrounds at multiple locations. A real‐time PCR assay utilizing dual‐labeled fluorogenic probes was developed to detect and differentiate QoI‐resistant isolates harboring the G143A mutation from sensitive isolates. CONCLUSION: The G143A mutation in cytb is associated with QoI resistance in C. beticola. Accurate monitoring of this mutation will be essential for fungicide resistance management in this pathosystem. Copyright © 2012 Society of Chemical Industry     

Root infection of sugar beet by <Emphasis Type="Italic">Cercospora beticola</Emphasis> in a climate chamber and in the field

Sugar beet root infection by Cercospora beticola, the causal agent of Cercospora leaf spot (CLS), was studied in a climate chamber and in the field. In the climate chamber, root incubation of susceptible seedlings with a conidial suspension resulted in disease incidences that were significantly different for two sugar beet cultivars (Auris: 0.8 ± 0.14 and A00170: 0.5 ± 0.18; P < 0.05) with regard to the control treatment 35 days after root incubation in a standard potting soil-fine river sand mixture. In a field trial with susceptible cv. Savannah with soil-incorporated CLS-infested leaf material, disease developed four weeks earlier in the infested plots than in the control plots. The probability that disease develops in the field was significantly higher for the infested than for the control plots (P < 0.05). Symptomless plants from infested field plots transferred to the glasshouse to induce leaf spot symptoms showed a significantly higher probability to induce symptom development (0.4 ± 0.08), than plants from control plots (0.02 ± 0.02) (P <0.05) 14 days after transfer. This probability was significantly higher than for plants that remained in three of the infested field plots (0.2 ± 0.04; 0.2 ± 0.05 and 0.2 ± 0.04 respectively), except for one infested field plot (0.4 ± 0.05) on July 5. We conclude that C. beticola is able to infect sugar beet seedlings through their roots and that latent CLS infections in sugar beet lead to symptom development at high temperatures (> 20 °C) and high relative humidity (> 95) in our climate chamber or after canopy closure in the field. Quantification of root infection and long term survival in soil is necessary to assess its contribution to the epidemiology and life cycle of Cercospora beticola. Cultural methods such as a wider crop rotation, management of crop debris and ploughing systems may provide control strategies alternative to or reducing fungicide input.     

The use of digital image analysis and real‐time PCR fine‐tunes bioassays for quantification of Cercospora leaf spot disease in sugar beet breeding

Cercospora leaf spot, caused by the fungus Cercospora beticola, is a major fungal sugar beet disease worldwide and the cause of significant yield losses. The disease is most successfully countered by the introduction of genetic tolerance into elite sugar beet hybrids. To this end, breeding programmes require high quality biological assays allowing discrimination of minor differences between plants within a segregating population. This study describes the successful implementation of image analysis software in the bioassays for quantification of necrotic lesions at different stages of C. beticola infection, allowing selection on minor phenotypic differences during the sugar beet breeding process for C. beticola resistance. In addition, a real‐time PCR assay was developed for the quantification of C. beticola pathogen biomass in infected beet canopy. The use of both techniques, even in an early stage of infection, fine‐tunes current bioassays, allowing more accurate and efficient selection of resistant breeding material.     

Evaluation of drench treatments with phosphonate derivatives against Pseudomonas syringae pv. syringae on pear under controlled environment conditions

Several phosphonate derivatives including theoomycetic antifungal agents phosphonate andtris-o-ethylphosphonate (fosetyl), theethylene-releasing compound 2-chloroethylphosphonate(ethephon), and the antibiotic2-epoxypropylphosphonate (phosphomycin) were evaluatedfor in vitro and in planta activityagainst Pseudomonas syringae pv. syringae.Inhibition of colony growth in CYE agar byphosphonate, fosetyl and etephon was very slight(minimal inhibitory concentrations MIC= 0.31–;0.62 gHP /l). Also, survival of P. syringae pv. syringae in aqueous solutions ofphosphonate or fosetyl was high. Only phosphomycinshowed significant antibacterial activity invitro (MIC=10-20 µg HP /ml) comparedto streptomycin (1-2 µg a.i./ml). Potted pearplants irrigated with these chemicals and inoculatedwith Pseudomonas syringae pv. syringae had significantly less disease than non-treatedcontrols ( P<0.001). Phosphomycin was the mostactive compound with a median effective dose(ED₅₀) of less than 0.62 g HP /l.Activities of the other phosphonates were weak butconsistent between experiments. The ED₅₀s on wholeplants were 2.1, 3.3, and 6.9 g HP /l for ethephon, phosphonate and fosetyl, respectively. TheED₅₀of P. syringae pv. syringaeincreased from 6.5 in non-treated controls to 7.7-8.8log₁₀ cfu/ml on plants treated with phosphonatesat 1.86 g HP /l. It was concluded thatdrench treatment with fosetyl is not a practicaloption for control of P. syringae pv. syringae on pear.     

Development of a field biotest using artificial inoculation to evaluate resistance and yield effects in sugar beet cultivars against <Emphasis Type="Italic">Cercospora beticola</Emphasis>

Cercospora beticola resistance and disease yield loss relationships in sugar beet cultivars are best characterised under field conditions with heavy natural infection; this does not occur regularly under German climatic conditions. Since Cercospora resistance reduces the rate of pathogen development, high yield loss was observed in studies using artificial inoculation. Our study, therefore aimed to optimise inoculum density to obtain cultivar differentiation, which correlates to natural infection. In 2005 and 2006, field trials were carried out to determine the effect of different inoculum densities on Cercospora resistance of three sugar beet cultivars possessing variable resistance. The epidemic progress and white sugar yield loss (WSY_loss) were determined and their relationship evaluated. An optimal inoculum concentration range (between 10,000–20,000 infectious Cercospora units ml⁻¹ inoculum suspension) was determined which allowed maximum resistance parameter differentiation in terms of C. beticola disease severity (DS), area under the disease progress curve (AUDPC) and WSY_loss. The correlation between AUDPC and WSY_loss was identical for all cultivars independent of the resistance level, demonstrating that tolerant reactions of the cultivars under study were not detectable. This study provides evidence that even under optimal inoculum levels necessary to obtain maximum differentiation between cultivars, climatic conditions are important for disease management, but remain unpredictable, indicating that artificial inoculation needs to be optimised, but that single field locations are not sufficient and reliable to evaluate Cercospora resistance.     

Epidemiology of cercospora leaf spot on sugar beet: modeling disease dynamics within and between individual plants

ABSTRACT Disease dynamics of Cercospora leaf spot (CLS) of sugar beet was analyzed at two hierarchical scales: as vertical profiles within individual plants and in relation to disease on neighboring plants. The relative contribution of different leaf layers to increase in CLS was analyzed using a simple continuous-time model. The model was fitted to data from two field trials in the Netherlands: one in an area with a long history of CLS, the other in an area where CLS has only recently established; in each case these were unsprayed and twice-sprayed treatments. There were differences in the relative contribution of different leaf layers to disease increase on the target leaf layer according to the CLS history and whether the plants were sprayed or unsprayed. In both field trials, parameter estimates giving the relative contribution of the target leaf layer to disease increase at that leaf layer were higher than those for the lower leaf layer. On only a few occasions the contribution of an upper leaf layer to disease increase at the target leaf layer was significant. Thus, CLS increase at the target leaf layer was determined mainly by disease severity at that leaf layer and to a lesser extent by disease at the lower leaf layer. Our continuous-time model was also used to analyze CLS increase on an individual sugar beet plant in relation to its own and its neighbor's level of disease in field trials at five locations in the two CLS areas over two years. In all field trials, the contribution of the target plant itself to disease increase (auto-infection) was larger than that of its neighboring plants (allo-infection). The overall analysis in the two CLS areas also indicated a larger contribution of the target plant to its disease increase than of neighboring plants, and this pattern was also apparent in a pooled analysis across all sites. Thus, CLS increase on a sugar beet plant was mainly determined by the disease severity on that plant and to a lesser extent by its within-row neighboring plants.     

Interactive Effects of Host, Pathogen and Mineral Nutrition on Grey Leaf Spot Epidemics in Uganda

Grey leaf spot incited by Cercospora zeae-maydis is a new devastating foliar disease of maize in East Africa. For effective control, elucidation of the most critical elements of the grey leaf spot disease pyramid is important. This study investigated the role of mineral nutrition, pathogen variability and host resistance in the epidemic. Trials were conducted under field and controlled environments. The 28 isolates used in the controlled environment varied significantly (P 0.05) in parasitic fitness measured indirectly as disease efficiency, but no infection pattern could be attributed to known C. zeae-maydis pathotypes. Data from field trials showed that host resistance and mineral nutrition significantly (P 0.05) affected disease efficiency, with highest disease development occurring in nitrogen-augmented plots. Exclusive phosphorus application had no clear effect on grey leaf spot epidemics but combined application with nitrogen significantly (P 0.05) reduced the predisposition effects of nitrogen to the disease. Overall, treated plots had less disease than unfertilised plots. Fertiliser application had no effect on sporulation capacity, while cultivars significantly affected it. Geographic differences in amount of disease were observed, suggesting environment influences on grey leaf spot incidence. The results suggest that the current grey leaf spot epidemics in East Africa are due to favourable cultivars, poor mineral nutrition and environmental interactions.     

Precise, simple screening for resistance in potato varieties to common scab using paper pots

Resistance to common scab pathogen Streptomyces turgidiscabies of seven potato varieties was compared in the field with a newly developed paper pot method. Seedlings raised in soil in paper pots containing inocula at 1 × 10³ to 10⁷cfu/g soil were transplanted into a scab-free field and grown for 3 months. The disease severity of the seven varieties in the field trials differed in iteration and from year to year, even though their resistance levels were approximately similar at the expected levels. With the paper pot method, the seven varieties had different resistance levels, which were almost completely consistent with the results of the field trials, at more than 1 × 10⁵cfu/g soil. Significant differences in disease severity between resistant and susceptible varieties were observed (P = 0.05) for 2 years, and the resistance level of the varieties was elucidated.     

Characterization of cytochrome b from European field isolates of Cercospora beticola with quinone outside inhibitor resistance

Cercospora leaf spot (CLS), caused by the fungal pathogen Cercospora beticola, is the most important foliar disease of sugar beet worldwide. Control strategies for CLS rely heavily on quinone outside inhibitor (Q_OI) fungicides. Despite the dependence on Q_OIs for disease control for more than a decade, a comprehensive survey of Q_OI sensitivity has not occurred in the sugar beet growing regions of France or Italy. In 2010, we collected 866 C. beticola isolates from sugar beet growing regions in France and Italy and assessed their sensitivity to the Q_OI fungicide pyraclostrobin using a spore germination assay. In total, 213 isolates were identified with EC₅₀ values greater than 1.0???g?ml^?1 to pyraclostrobin, all of which originated from Italy. To gain an understanding of the molecular basis of Q_OI resistance, we cloned the full-length coding region of Cbcytb, which encodes the mitochondrial Q_OI-target enzyme cytochrome b in C. beticola. Cbcytb is a 1,162-bp intron-free gene with obvious homology to other fungal cytb genes. Sequence analysis of Cbcytb was carried out in 32 Q_OI-sensitive (<0.080???g?ml^?1) and 27 Q_OI-resistant (>1.0???g?ml^?1) isolates. All tested Q_OI-resistant isolates harboured a point mutation in Cbcytb at nucleotide position 428 that conferred an exchange from glycine to alanine at amino acid position 143 (G143A). A PCR assay developed to discriminate Q_OI-sensitive and Q_OI-resistant isolates based on the G143A mutation could detect and differentiate isolates down to approximately 25?pg of template DNA. Microsatellite analyses suggested that Q_OI resistance emerged independently in multiple genotypic backgrounds at multiple locations. Our results indicate that Q_OI resistance has developed in some C. beticola populations in Italy and monitoring the G143A mutation is essential for fungicide resistance management in this pathosystem.     

Control of bean common mosaic by deployment of the dominant gene I

The deployment of the dominant gene I for the control of bean common mosaic predisposes a bean crop to the risk of death by black root if one or more recessive genes to prevent the hypersensitive plant reaction are lacking. However, during 3 years of observation in Kenya black root occurred only exceptionally to more than 10%, and in such cases the yields from crops having the dominant gene I exceeded significantly those from mosaic-susceptible crops. The use of the dominant resistance factor in Kenya is therefore recommended, but continued caution and monitoring of the disease situation are required. Broadening the resistance basis by adding recessive genes to control black root is advisable.Samenvatting Het gebruik van het dominante gen I voor de bestrijding van de virusziekte bonerolmozaïek stelt een bonengewas bloot aan het gevaar van systemische necrose, als niet een of meer recessieve genen aanwezig zijn om deze overgevoeligheidsreactie van de planten te voorkomen. Gedurende een periode van drie jaar werden waarnemingen verricht om na te gaan hoe vaak deze necrose optrad. Slechts bij uitzondering bleek meer dan 10% van de planten te worden aangetast. In die gevallen werd de opbrengst toch aanmerkelijk verhoogd door de aanwezigheid van het dominante gen I. Het gebruik van dit gen ter bestrijding van het bonerolmozaïek wordt daarom voor Kenya aanbevolen. Voortdurend moet echter worden nagegaan in welke mate necrose-inducerende stammen van het virus voorkomen. Een verbreding van de erfelijke basis van de resistentie ter voorkoming van de necrotische reactie door introductie van recessieve resistentiegenen is raadzaam.

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Résumé L'emploi du gène dominant I pour la lutte préventive contre la mosaïque commune du haricot, en I'absence d'un ou plusiers gènes récessifs pour éviter une réaction hypersensible de la plante, entraine un danger de mort de la plante par racines noires. Néanmoins, pendant les 3 ans d'observations au Kénya, les cas de racines noires ne sont apparus qu'exceptionnellement à un taux superieur à 10%, et, dans de tels cas, le rendement du végétal ayant le gène dominant I était, de façon significative, supérieur à celui des végétaux prédisposées à la mosaïque. L'utilisation au Kénya du facteur de résistance dominant est par conséquent recommendé mais une attention et une surveillance permanentes de I'évolution de la maladie sont nécessaires. On peut conseiller I'élargissement de la base de résistance par addition de gènes récessifs pour le contrôle de la maladie des racines noires.

     

Foliar spray of a cell wall protein fraction from the biocontrol agent <Emphasis Type="Italic">Pythium oligandrum</Emphasis> induces defence-related genes and increases resistance against Cercospora leaf spot in sugar beet

After a cell wall protein fraction (CWP) of Pythium oligandrum was sprayed on sugar beet leaves, we screened leaves for induced expression of defence-related genes and for resistance against Cercospora leaf spot. In a western blot analysis, the CWP was primarily retained on the surface of leaves without degradation for at least 48 h after spraying. In northern blot analyses, four defence-related genes (β-1, 3-glucanase, acidic class III chitinase, 5-enol-pyruvylshikimate-phosphate synthase and oxalate oxidase-like germin) were expressed more rapidly in CWP-treated leaves compared to control leaves treated with distilled water (DW). When CWP was applied to a suspension of cultured cells of sugar beet, an oxidative burst was observed that did not occur after the DW treatment. In growth chamber trials after inoculation with Cercospora beticola, the severity of Cercospora leaf spot was significantly reduced in CWP-treated plants compared to the DW-treated controls. In a field experiment, CWP treatment was also effective against the disease. CWP did not reduce growth rate of the pathogen in plate tests. The results together suggest that the CWP from P. oligandrum can be retained on the leaf surface and induce expression of disease resistance genes, thereby reducing Cercospora leaf spot on sugar beet.