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.     

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.     

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.     

Detection of resistance to sterol demethylation-inhibiting (DMI) fungicides in<Emphasis Type="Italic">Cercospora beticola</Emphasis> and efficacy of control of resistant and sensitive strains with flutriafol

Single-lesion isolates ofCercospora beticola (n=150) were collected in 1998 from sugar beet fields in the area of Serres, N. Greece. In this area, sterol demethylation-inhibiting (DMI) fungicides have been used for almost 20 years to control sugar beet leaf spot. The sensitivity of these isolates to the DMI fungicides flutriafol and difenoconazole (EC₅₀ values) was determined on the basis of inhibition of mycelial growth at several fungicide concentrations. The relative growth (RG) of isolates was correlated at all tested concentrations with the respective EC₅₀ values, indicating that RG provides a reliable estimate for the sensitivity of the isolates. The highest correlation coefficients were obtained for concentrations of 1 μg ml⁻¹ flutriafol and of 0.05 μg ml⁻¹ difenoconazole, respectively. Consequently, they are proposed for monitoring of DMI sensitivity inC. beticola populations, as single discriminatory concentrations in a simplified test method. Based on the RG values at the discriminatory concentration of 1 μg ml⁻¹ flutriafol,C. beticola isolates were classified as either resistant or sensitive. The efficacy of flutriafol, applied at the commercially recommended dose, in controlling Cercospora leaf spot was examined in field experiments conducted during 1999 and 2000. Disease incidence in plots artificially inoculated with resistant isolates and treated with flutriafol was significantly higher than in similar plots inoculated with sensitive strains. These results suggest that poor disease control after application of flutriafol may be based on the presence of resistant strains within the pathogen population in northern Greece. This emphasizes the risk of the development of practical resistance if there is increased frequency of such strains within the population. http://www.phytoparasitica.org posting July 13, 2003.     

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.     

Spatial pattern of Cercospora leaf spot of sugar beet in fields in long- and recently-established areas

Spatial disease pattern of Cercospora beticola was characterised during natural epidemics of Cercospora leaf spot (CLS) in sugar beet. We applied linear regression and geostatistical analyses to characterise CLS spatial patterns in three field trials, in long-established and recently-established CLS-areas, during two consecutive years. Linear regression showed a positive influence of average disease severity of within-row neighbouring plants (0.38 < < 0.88). Semi-variograms modelled the spatial dependence of disease severity for two directions per week in both years. Disease severity displayed strong spatial dependence over time. The within-row spatial dependence was the largest, but across-row dependence was irregular and weaker. Both long- and recently established areas showed strong spatial dependence of disease severity within row, decrease in variability between years and within the second trial year and a relation between and the relative nugget. Observed differences were more field than area specific. These spatial and temporal analyses indicated that disease severities of adjacent plants were dependent; hence, we concluded that C. beticola is dispersed mainly over short distances from plant to plant.     

CERCBET 3 – a forecaster for epidemic development of Cercospora beticola*

Cercospora beticola is the most prevalent and damaging fungal disease in German sugar beet growing. Control strategies are based on action thresholds. A model has been developed which forecasts epidemic development (expressed as disease incidence) and signals when action thresholds are overridden. The plot‐specific model, CERCBET 3 uses as input meteorological parameters (temperature, relative humidity), easily accessible agronomic field characteristics and a single recording of C. beticola disease incidence. Extensive validation in 2001–03 showed that, in 80–95% of the cases, CERCBET 3 correctly forecasted the dates when thresholds were overridden. Cultivar diversity in German sugar beet growing is increasing, thus a module has been included into CERCBET 3 which reflects susceptibility to C. beticola by introducing a sporulation factor. In some cases a second or even third fungicide treatment could be necessary to control Cercospora leaf spot and so a further module which models fungicide efficacy has been elaborated. CERCBET 3 is available for sugar beet growers in an interactive form on the Internet platform ISIP, which is provided by the governmental crop protection services of Germany.     

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.     

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     

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.     

Impact of climate change on the temporal and regional occurrence of Cercospora leaf spot in Lower Saxony

Journal of Plant Diseases and Protection - The effect of climate change on the temporal and regional occurrence of Cercospora leaf spot (CLS) of sugar beet in Lower Saxony is analysed using the...     

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.     

Early detection of sugar beet pathogen Ramularia beticola in leaf and air samples using qPCR

A quantitative PCR method (qPCR) was developed for the detection and quantification of Ramularia beticola causing Ramularia leaf spot in sugar beet. R. beticola specific primers were designed based on the internal transcribed spacer region 2 (ITS2). The assay was applied on DNA extracted from spores trapped on tape from Burkard spore traps placed in an artificially inoculated sugar beet field trial and in two sugar beet fields with natural infections. R. beticola DNA was detected at variable amounts in the air samples 14 to 16 days prior to first visible symptoms. R. beticola DNA was detected in air samples from fields with natural infection at significant and increasing levels from development of the first symptoms, indicating that spore production within the crop plays a major role in the epidemic development of the disease. Sugar beet leaves sampled from the inoculated field trial were also tested with the qPCR assay. It was possible to detect the presence of R. beticola in the leaves pre-symptomatic at least 10 days before the occurrence of the visible symptoms of Ramularia leaf spot. This is the first report of a molecular assay, which allows screening for the presence of R. beticola in plant material and in air samples prior to the appearance of visible symptoms. An early detection has potential as a tool, which can be part of a warning system predicting the onset of the disease in the sugar beet crop and helping to optimise fungicide application.     

Fitness of Cercospora Beticola Field Isolates – Resistant And – Sensitive to Demethylation Inhibitor Fungicides

Isolates of Cercospora beticola resistant to fungicides that inhibit sterol demethylation (DMIs) were collected from sugar beet fields in Northern Greece. Fitness of these isolates was compared to that of DMI-sensitive isolates. The parameters measured were competitive ability both under growth chamber and field conditions, mycelial growth, spore germination, germ tube length, incubation period, virulence and spore production. The competitive ability under growth chamber conditions was measured for 4 pairs of one resistant and one sensitive isolate. Results showed that after 4 disease cycles, in 2 out of 4 pairs tested, the resistant isolates competed well with the sensitive isolates, but in the remaining two pairs the frequency of the resistant isolate decreased significantly. The competition experiment in the field was carried out by inoculating field plots with a conidial suspension consisting of a spore mixture from all the resistant and all the sensitive isolates used in this study. Results showed that at the end of the growing period the frequency of the resistant isolates had slightly decreased (P < 0.05). The measurements of fitness components of individual isolates showed that the resistant isolates had significantly lower (P < 0.05) virulence and spore production than the sensitive isolates, while no significant differences (P > 0.05) to the remaining 4 fitness components, were detected. With correlation analysis it was determined whether there is a relationship between values of each fitness component and the level of sensitivity to flutriafol of individual isolates. The correlation coefficients for virulence (r = 0.45) and spore production (r = 0.41) were significantly different from 0 (P > 0.05), indicating that resistance to DMIs affected, to some degree, the fitness of the resistant isolates.     

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.     

Climatic Zoning and Plant Disease Potential – Examples from the Near and Middle East1

Actual plant disease and pest occurrence depends on many genetic and environmental factors, and frequently obscures the basic suitability of a given location to support or prevent epidemic development. In order to allow the demarcation of climatic zones related to the potential of disease or pest occurrence, we have used long-term average climatic data, especially monthly average temperatures and monthly average rainfall. If applied to sugar beet leaf pathogens such as Cercospora beticola and Erysiphe betae in the Near and Middle Eastern region, some interesting zoning became possible, which could be verified by extended field studies. Other examples that have been analysed in the region are apple scab, Venturia inaequalis, and downy mildew of grapes, Plasmopara viticola. A recent and ongoing analysis of the factors controlling chickpea anthracnose caused by Ascochyta rabiei indicates that the same principle may be applied for very different pathogens. Large-scale planning and control strategies as tried by the International Agricultural Research Centers should therefore be based on careful climatic zoning for plant pest and disease potential, to avoid waste of the limited genetic and financial resources available.     

Molecular diagnostic for detecting the cytochrome b G143S - QoI resistance mutation in Cercospora beticola

The mutation G143S has been associated with high-level strobilurin resistance in laboratory mutant strains of Cercospora beticola, one of the most destructive pathogens in sugar beet plants. By using allele specific primers (PASA-PCR) and agarose gel visualization, a molecular diagnostic was developed for the detection of the G143S resistance mutation. This assay is simple and applicable in low tech laboratory settings, with high reliability when a relatively large proportion of mutated mitochondrial alleles are present in the resistant strains. To achieve detection of resistant alleles at low frequencies, a more sensitive Real Time PCR based assay capable of discriminating resistant (S143) genotypes in frequencies as low as 1:10,000 resistant:sensitive alleles was developed. Both diagnostics were successfully validated in laboratory strains. Subsequently, a large number of C. beticola isolates from QoI-treated sugar beet experimental fields in Greece were screened for resistance to Qo fungicides using these diagnostics and classic bioassays. No proportion of the 143S resistant allele was detected in all field isolates tested, which was in agreement with the phenotypes revealed by the biotests confirming that the efficacy of QoIs against C. beticola has been sustained in Greece 7 years after their introduction.     

Automated identification of sugar beet diseases using smartphones

Cercospora leaf spot (CLS) poses a high economic risk to sugar beet production due to its potential to greatly reduce yield and quality. For successful integrated management of CLS, rapid and accurate identification of the disease is essential. Diagnosis on the basis of typical visual symptoms is often compromised by the inability to differentiate CLS symptoms from similar symptoms caused by other foliar pathogens of varying significance, or from abiotic stress. An automated detection and classification of CLS and other leaf diseases, enabling a reliable basis for decisions in disease control, would be an alternative to visual as well as molecular and serological methods. This paper presents an algorithm based on a RGB‐image database captured with smartphone cameras for the identification of sugar beet leaf diseases. This tool combines image acquisition and segmentation on the smartphone and advanced image data processing on a server, based on texture features using colour, intensity and gradient values. The diseases are classified using a support vector machine with radial basis function kernel. The algorithm is suitable for binary‐class and multi‐class classification approaches, i.e. the separation between diseased and non‐diseased, and the differentiation among leaf diseases and non‐infected tissue. The classification accuracy for the differentiation of CLS, ramularia leaf spot, phoma leaf spot, beet rust and bacterial blight was 82%, better than that of sugar beet experts classifying diseases from images. However, the technology has not been tested by practitioners. This tool can be adapted to other crops and their diseases and may contribute to improved decision‐making in integrated disease control.     

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.