Detached Leaf Tests Research Articles

Sensitivity of dominant UKPhytophthora infestans genotypes to a range of fungicide active ingredients

AbstractLate blight, caused by Phytophthora infestans, is a serious disease of potatoes worldwide and is predominantly controlled by repeated prophylactic use of fungicides throughout the growing season. Effective blight management and integrated pest management strategies rely on knowledge of the efficacy of available fungicides to control contemporary genotypes of P. infestans. Between 2019 and 2022, representative isolates of the newer dominant genotypes EU36, EU37 and the older dominant genotype EU6 were sampled from GB crops and tested for sensitivity to seven commonly used fungicide active ingredients (cyazofamid, fluopicolide, mandipropamid, propamocarb, oxathiapiprolin, amisulbrom and mancozeb) used preventatively in detached leaf tests and zoospore motility assays where appropriate. Dose–response curves based on lesion area (mm2) were constructed, and EC50 values calculated. Isolates of P. infestans genotype EU37 were insensitive to fluazinam as previously reported. No insensitivity, or progression towards resistance over time, to any of the other fungicides was observed in any isolates of the genotypes tested, and EC50 values were generally in line with previous testing. Zoospore motility tests with fluopicolide revealed significantly higher MIC values for isolates of genotype EU36 compared with EU37 and EU6, which, whilst not indicative of resistance at the low concentrations of active ingredient tested, may be a factor contributing to the overall dominance of that genotype in the wider population of P. infestans. The evidence suggests that the fungicides tested, with the exception of fluazinam and EU37, are effective for the control of P. infestans genotypes dominant in the GB population up until 2022.

Genetic mapping of the powdery mildew resistance gene Pm7 on oat chromosome 5D

Key messageThree independent experiments with different genetic backgrounds mapped the resistance gene Pm7 in the oat genome to the distal part of the long arm of chromosome 5D.Resistance of oat to Blumeria graminis DC. f. sp. avenae is an important breeding goal in Central and Western Europe. In this study, the position of the effective and widely used resistance gene Pm7 in the oat genome was determined based on three independent experiments with different genetic backgrounds: genome-wide association mapping in a diverse set of inbred oat lines and binary phenotype mapping in two bi-parental populations. Powdery mildew resistance was assessed in the field as well as by detached leaf tests in the laboratory. Genotyping-by-sequencing was conducted to establish comprehensive genetic fingerprints for subsequent genetic mapping experiments. All three mapping approaches located the gene to the distal part of the long arm of chromosome 5D in the hexaploid oat genome sequences of OT3098 and ‘Sang.’ Markers from this region were homologous to a region of chromosome 2Ce of the C-genome species, Avena eriantha, the donor of Pm7, which appears to be the ancestral source of a translocated region on the hexaploid chromosome 5D.

Pseudomonas chlororaphis CP07 strain reduces disease severity caused by Phytophthora palmivora in genotypes of Theobroma cacao

Black pod rot is the most important disease affecting Theobroma cacao in Cuba. Plant growth promoting bacteria (PGPB) are considered an alternative for controlling plant disease in the context of the integrated management. The present work was aimed to evaluate Pseudomonas chlororaphis strain CP07, isolated from T. cacao rhizosphere, as potential PGPB against P. palmivora in cacao plants cultivated under controlled conditions. Three genotypes of Cuban traditional cacao of the group Trinitario and the genotype UF 677 were used. The effect of CP07 was evaluated by using detached leaf tests and in planta assays with plants obtained in vitro from both seeds and micrografting. Inoculation of P. palmivora in control plants showed that genotypes EICB-371, EICB-384 and UF 677 were highly susceptible to the disease (71.4 to 99% disease severity) while EICB-385 was significantly less susceptible (39 to 54.5%). In EICB-371 and EICB-385 genotypes, disease symptoms were significantly reduced in presence of CP07 compared to the control plants in all assays. In contrast, for EICB-384 and UF 677 genotypes, there was not reduction of disease severity in plants pre-treated with CP07. Results show the protective effect of treating cacao plants with CP07 against P. palmivora. Plant genotype significantly influenced the protective ability of bacteria affecting the CP07-mediated disease control. Results suggest that depending on cacao genotype, CP07 has a potential for inducing plant defense against P. palmivora which can be used in the control of black pod rot.

Düzce ili çilek üretim alanlarında görülen fungal hastalıklar

Surveys in Düzce province indicated that strawberry cultivation was made in small open fields changing between 0.5 and 5.0 da with mulching. Four strawberry cultivars; Camarosa, Kabarla, Sweet Ann and Osmanlı have been commonly grown in the province. Fungal diseases of strawberry which has gained importance recently in Düzce province of Turkey were studied with the surveys conducted in three periods (15.03.2012; 20.05.2012; 15.09.2012). In the first period for leaf diseases; in the second period for leaf, petiol, fruit, and root diseases and in the third period mainly for root diseases were investigated. Alternaria spp., Botrytis cinerea, Hainesia lythri, Mycosphaerella fragariae, Phoma sp., Phomopsis sp., Rhizoctonia solani were isolated from the leaves and petioles; B. cinerea from the fruits and Alternaria spp., Fusarium oxysporum, Macrophomina phaseolina, Phytophthora spp., and R. solani were isolated from the roots. All the fungi obtained caused necrosis on detached leaves at the pathogenicity tests. Detached leaf tests were clearly reflected the pathogenicity of the fungi. Botrytis cinerea and Mycosphaerella fragariae were the most widespread diseases found in all the fields surveyed. The other fungi showed various rates of disease occurrence.

Fungicide sensitivity and monocyclic parameters related to the Phakopsora pachyrhizi–soybean pathosystem from organic and conventional soybean production systems

The failure of chemical control of soybean rust has been related to the selection of less sensitive isolates, and the infection capacity of such isolates could have implications for the management of the disease. The aims of the present study were to compare the sensitivity to tebuconazole and azoxystrobin and the monocycle of soybean rust using isolates of Phakopsora pachyrhizi from two soybean fields with different production systems (organic and conventional) in 2012/13 and 2013/14 seasons, and to monitor mutations in the CYP51 gene. To assess the sensitivity to tebuconazole and azoxystrobin, detached leaf tests and in vitro germination, respectively, were used. To evaluate the monocycle, detached leaves were inoculated with a urediniospore suspension and evaluated daily by counting the number of uredia. The occurrence of the mutations in CYP51 was investigated by a pyrosequencing assay. In both 2012/13 and 2013/14 seasons, the EC50 to tebuconazole was lower for the population from the organic system (0.41 and 0.10 μg mL−1, respectively) compared to the conventional system (1.60 and 4.44 μg mL−1, respectively), while the EC50 to azoxystrobin was similar for both populations. The lower sensitivity to tebuconazole and azoxystrobin was associated with F120L + Y131H mutations in CYP51, and the F129L mutation in CYTB, respectively. The monomolecular model fitted to monocycle data and parameters related to the maximum asymptote and the AUDPC were superior for organic than the conventional system.

Two different R gene loci co-evolved with Avr2 of Phytophthora infestans and confer distinct resistance specificities in potato.

Late blight, caused by the oomycete pathogen Phytophthora infestans, is the most devastating disease in potato. For sustainable management of this economically important disease, resistance breeding relies on the availability of resistance (R) genes. Such R genes against P. infestans have evolved in wild tuber-bearing Solanum species from North, Central and South America, upon co-evolution with cognate avirulence (Avr) genes. Here, we report how effectoromics screens with Avr2 of P. infestans revealed defense responses in diverse Solanum species that are native to Mexico and Peru. We found that the response to AVR2 in the Mexican Solanum species is mediated by R genes of the R2 family that resides on a major late blight locus on chromosome IV. In contrast, the response to AVR2 in Peruvian Solanum species is mediated by Rpi-mcq1, which resides on chromosome IX and does not belong to the R2 family. The data indicate that AVR2 recognition has evolved independently on two genetic loci in Mexican and Peruvian Solanum species, respectively. Detached leaf tests on potato cultivar ‘Désirée’ transformed with R genes from either the R2 or the Rpi-mcq1 locus revealed an overlapping, but distinct resistance profile to a panel of 18 diverse P. infestans isolates. The achieved insights in the molecular R – Avr gene interaction can lead to more educated exploitation of R genes and maximize the potential of generating more broad-spectrum, and potentially more durable control of the late blight disease in potato.

Virulence Differences in Blumeria graminis f. sp. tritici from the Central and Eastern United States.

Wheat powdery mildew is a disease of global importance that occurs across a wide geographic area in the United States. A virulence survey of Blumeria graminis f. sp. tritici, the causal agent, was conducted by sampling 36 wheat fields in 15 U.S. states in the years 2013 and 2014. Using a hierarchical sampling protocol, isolates were derived from three separated plants at each of five separated sites within each field in order to assess the spatial distribution of pathotypes. In total, 1,017 isolates from those fields were tested individually on single-gene differential cultivars containing a total of 21 powdery mildew resistance (Pm) genes. Several recently introgressed mildew resistance genes from wild wheat relatives (Pm37, Pm53, MlAG12, NCAG13, and MlUM15) exhibited complete or nearly complete resistance to all local B. graminis f. sp. tritici populations from across the sampled area. One older gene, Pm4b, also retained at least some efficacy across the sampled area. The B. graminis f. sp. tritici population sampled from Arkansas and Missouri, on the western edge of the eastern soft red winter wheat region, had virulence profiles more similar to other soft wheat mildew populations than to the geographically closer population from hard wheat fields in the Plains states of Oklahoma, Nebraska, and Kansas. The Plains population differed in that it was avirulent to several Pm genes long defeated in the soft-wheat-growing areas. Virulence complexity was greatest east of the Mississippi River, and diminished toward the west. Several recently introgressed Pm genes (Pm25, Pm34, Pm35, and NCA6) that are highly effective against mildew in the field in North Carolina were unexpectedly susceptible to eastern-U.S. B. graminis f. sp. tritici populations in detached-leaf tests. Sampled fields displayed a wide range of pathotype diversity and spatial distribution, suggesting that epidemics are caused by varying numbers of pathotypes in all regions. The research confirmed that most long-used Pm genes are defeated in the eastern United States, and the U.S. B. graminis f. sp. tritici population has different virulence profiles in the hard- and soft-wheat regions, which are likely maintained by host selection, isolation by distance, and west-to-east gene flow.

Mapping of powdery mildew resistance gene Pm53 introgressed from Aegilops speltoides into soft red winter wheat.

A powdery mildew resistance gene was introgressed from Aegilops speltoides into winter wheat and mapped to chromosome 5BL. Closely linked markers will permit marker-assisted selection for the resistance gene. Powdery mildew of wheat (Triticum aestivum L.) is a major fungal disease in many areas of the world, caused by Blumeria graminis f. sp. tritici (Bgt). Host plant resistance is the preferred form of disease prevention because it is both economical and environmentally sound. Identification of new resistance sources and closely linked markers enable breeders to utilize these new sources in marker-assisted selection as well as in gene pyramiding. Aegilops speltoides (2n = 2x = 14, genome SS), has been a valuable disease resistance donor. The powdery mildew resistant wheat germplasm line NC09BGTS16 (NC-S16) was developed by backcrossing an Ae. speltoides accession, TAU829, to the susceptible soft red winter wheat cultivar 'Saluda'. NC-S16 was crossed to the susceptible cultivar 'Coker 68-15' to develop F2:3 families for gene mapping. Greenhouse and field evaluations of these F2:3 families indicated that a single gene, designated Pm53, conferred resistance to powdery mildew. Bulked segregant analysis showed that multiple simple sequence repeat (SSR) and single nucleotide polymorphism (SNP) markers specific to chromosome 5BL segregated with the resistance gene. The gene was flanked by markers Xgwm499, Xwmc759, IWA6024 (0.7 cM proximal) and IWA2454 (1.8 cM distal). Pm36, derived from a different wild wheat relative (T. turgidum var. dicoccoides), had previously been mapped to chromosome 5BL in a durum wheat line. Detached leaf tests revealed that NC-S16 and a genotype carrying Pm36 differed in their responses to each of three Bgt isolates. Pm53 therefore appears to be a new source of powdery mildew resistance.

No Evidence of QoI Resistance in Apple Powdery Mildew (Podosphaera leucotricha)

AbstractSensitivity monitoring studies using detached leaf tests with isolates of Podosphaera leucotricha showed no adaptation to pyraclostrobin in the last years. Sequence analysis of the target gene of QoIs, cytochrome b, of different isolates of P. leucotricha showed the presence of an intron directly after codon 143. This makes the occurrence of the G143A mutation unlikely. On the other hand, intron sequences have not been detected in immediate vicinity to the codons 129 and 137; therefore, the occurrence of those two mutations cannot be excluded. As the effects of these mutations on field performance on QoI fungicides are rather low, the overall resistance risk of P. leucotricha to this fungicide class is estimated to be low. The amplified cytochrome b gene fragments (exons and introns) for samples from different European countries and Australia were highly conserved.

Citrus canker: a new disease of Mexican lime (Citrus aurantifolia) and sour orange (C. aurantium)in Ethiopia

Introduction . Citrus canker, caused by the bacterium Xanthomonas axonopodis pv. citri, is a serious disease of most commercial citrus cultivars and some citrus relatives. Surveys were conducted in small-scale farms, commercial plantations, backyard orchards and nurseries in order to quantitatively study the occurrence and distribution of the disease and determine the intensity and field host range. Materials and methods . Seventeen locations in four regional states of Ethiopia were surveyed. Incidence on leaves was determined by counting total number of leaves and expressed as a proportion of leaves with at least one lesion. Incidence on fruits was determined on attached fruits, recorded as presence or absence of the symptoms. Severity was also measured on leaves and fruits. The identity of the pathogen was confirmed by morphological, biochemical and physiological characterization and detached leaf tests of representative isolates. Results . The surveys indicated the occurrence of citrus canker in Ethiopia. Overall incidence on leaves was 71.4% of leaves with at least one lesion and severity was 26.8% of leaf area infected. Incidence on fruits was 30% of fruits infected and severity was 21.25% fruit area infected. Morphological, biochemical and physiological characterization and detached leaf tests of isolates indicated characteristics similar to X. axonopodis pv. citri. The host range for citrus canker in Ethiopia was limited to Mexican lime (C. aurantifolia ) and sour orange (C. aurantium ). Based on the field host range and detached leaf tests, it appears that the X. axonopodis pv. citri variant that occurs in Ethiopia has similar host ranges to that of atypical Asiatic (Xac -A*) form. Canker severity significantly correlated with temperature but not with rainfall, elevation or tree age. Conclusion . This is the first confirmed report of the disease in Ethiopia.

Leaf-rust resistance in rye (Secale cereale L.). 2. Genetic analysis and mapping of resistance genes Pr3, Pr4, and Pr5.

Three dominant resistance genes, Pr3, Pr4, and Pr5, were identified by genetic analysis of resistance to leaf rust in rye (Puccinia recondita f. sp. secalis). Each of the three genes confers resistance to a broad scale of single-pustule isolates (SPIs), but differences could be observed for specific Pr gene/SPI combinations. Resistance conferred by the three genes was effective in both detached-leaf tests carried out on seedlings and in field tests of adult plants. Molecular marker analysis mapped Pr3 to the centromeric region of rye chromosome arm 1RS, whereas Pr4 and Pr5 were assigned to the centromeric region of 1RL. Chromosomal localization and reaction patterns to specific SPIs provide evidence that the three Pr genes represent distinct and novel leaf-rust resistance genes in rye. The contributions of these genes to resistance breeding in rye and wheat are discussed.

Greenhouse, Detached-leaf, and Field Testing Methods to Determine Cucumber Resistance to Gummy Stem Blight

The effects of leaf age, guttation, stomata and hydathode characteristics, and wounding on the symptom development of gummy stem blight [Didymella bryoniae (Auersw.) Rehm] of cucumber (Cucumis sativus L.) were studied to develop a useful germplasm screening method. Older cucumber leaves were more susceptible than younger leaves in field, greenhouse, and detached-leaf tests. Compared to seedlings with true leaves, seedlings at the cotyledon stage were less susceptible, had a smaller variance for ratings, and were more likely to escape infection. Stomata density and hydathode counts were not correlated with field ratings; but, stomata length on older leaves was highly correlated with susceptibility y. In greenhouse and field tests, susceptibility y increased as guttation increased and actively guttating plants were more susceptible than nonguttating plants. Phylloplane moisture and/or nutrition were more important in the infection process than was stomata] opening. Although important, guttation was not necessary for infection. Dawn inoculation of field or greenhouse tests increased leaf symptoms compared with dusk inoculation. The increase was likely due to the free water and nutrients provided by guttation. Genotype ranks and ratings for detached-leaf tests were not correlated with field results. A useful method, highly correlated (r = 0.82 to 0.96) with field ratings. for screening germplasm in the greenhouse was developed.

Pathogenic variability of Erysiphe graminis f.sp. hordei in South Australia, 1981-1985

Two techniques (mobile seedling nursery and detached leaf) were used to study the pathogenic variability of Erysiphe gramznis DC. ex Merat f.sp. hordei Em. Marchal in four barley growing areas of South Australia (S.A.). The mobile nurseries were conducted over 5 years (1981-1985) to monitor changes in the spectrum of virulence and individual gene frequencies. The race-specific resistance genes M1-a6, M1-k, M1-v and M1-ra were found to be susceptible to the pathogen populations in all surveyed areas. The same virulence genes spectrum (V-a6, V-k, V-v and V-ra) was present in the pathogen populations in the surveyed areas throughout the period of 1981-1984. During the 1985 season, one new virulence matching the resistance gene in cv. Forrest was detected. The resistance in cv. Galleon did not 'break down' over the cultivation period when its cultivation rose to 55% of the barley areas of S.A. The probable reasons for apparent durability of Galleon resistance are discussed. The relative frequencies of the matching virulence genes varied only slightly over time and space. The relative frequency of V-k was always almost 100%. The relative frequencies of V-a6, V-ra and V-v occurred at a higher rate than expected, since the matching resistance genes were not deployed in barley cultivars. The results of the detached leaf experiments (1982-1984) confirmed the virulence spectrum of the pathogen populations found in the mobile nursery experiments. Three individual single colony isolates from different cultivars (Clipper, Sonja and Goldmarker) were isolated and purified. Each isolate can produce a susceptible infection on other cultivars having different resistance genes in addition to its matching cultivar. Thus, each isolate carries more than one virulence gene. In the mobile nursery tests, the resistance gene M1-v (Varunda and LaMi) showed variable infection types (I.T.l-3), but in most cases it was moderately susceptible (I.T.3). Under the controlled conditions of the detached leaf tests, it gave an I.T.3 indicating the presence of virulence gene for M1-v in the pathogen populations. Reasons for this variable reaction are discussed. Midas with an ineffective gene (M1-a6) produced a resistant reaction to all isolates, suggesting the presence of additional resistance genes in the cultivar. The Mildew resistance genes M1-a, M1-a7, M1-a9, 141-al2, M1-g, M1-h, M1-(CP) and ml-o3 were found to be resistant in all surveyed areas in both experiments throughout S.A. Either the matching virulence genes for these resistance genes were absent or they were present at a very low frequency in the pathogen populations that could not be detected by the sampling techniques used.

SEEDLING AND DETACHED LEAF SCREENING METHODS FOR RESISTANCE TO GUMMY STEM BLIGHT IN CUCUMBER

Gummy stem blight (Didymella bryoniae (Auersw.) Rehm) is one of the major cucumber diseases, causing the second highest loss of any disease in North Carolina. Published methods of screening for resistance to this fungus are poorly correlated with field resistance. The objective of this study was to develop seedling or detached-leaf screening methods that are correlated with field resistance. Seedling tests examined the effects of: seedling age (1, 2 or 3 true leaves), days in humidity chamber, inoculum concentration (1×105, 1×106 or 1×107 spores per ml), time of inoculation (am vs. pm), fungal isolates, and cultigens. Detached leaf tests examined the effects of leaf age (1st, 2nd or 3rd true leaf), inoculum concentration (1×104, 1×105 or 1×106 spores per ml), and light levels during incubation (dark vs. 12h light/12h dark). Correlations between seedling tests and field data were moderate to high (r = 0.5 to 0.7). However, the coefficients of variation were also high. Correlations between detached leaf tests and field data were very low or negative.

Field and greenhouse evaluation of the components of partial resistance to early leafspot in peanut

Ten peanut (Arachis hypogaea L.) genotypes were evaluated fro components of partial resistance to Cercospora arachidicola Hori in the field and in two detached leaf tests in the greenhouse. The relationships among the resistance components in the two environments were examined by computing rank correlations among the components measured in the greenhouse and among those measured in the field. A comparison of field and greenhouse testing for partial resistance was made by determining the correlations between components measured in the two testing methods. The genotypes were significantly different for all the components of resistant measured in the field and for most of the components measured in the greenhouse. In the field study necrotic area/10 cm2 leaf area was moderately correlated (r=0.58) with lesion number/10 cm2 leaf area and highly correlated (r=0.71−0.76) with total lesion number, predicted number of days after planting (X) to reach a standard lesion count, and defoliation. In the greenhouse only the correlation between necrotic area (mm2)/10 cm2 leaf area and sporulation/leaf was highly significant (r=0.71−0.83) in both greenhouse tests. Necrotic area (mm2)/10 cm2 leaf area measured in the field was significantly correlated with that measured in the greenhouse (r=0.66). Sporulation/leaf measured in the greennhouse was significantly correlated (r=0.66) with lesion increase in the field (measured as predicted X). It should thus be possible to evaluate and select for components of partial resistance in the greenhouse to develop resistant lines for the field.

RACE‐SPECIFIC RESISTANCE IN GROUNDSEL (SENECIO VULGARIS) TO THE POWDERY MILDEW ERYSIPHE FISCHERI

SUMMARYTwo‐hundred‐and‐fifty lines of groundsel (Senecio vulgaris L.) were tested for their reactions to five different isolates of the powdery mildew fungus Erysiphe fischeri Blumer. Fifty‐one of the lines were also tested for their reactions to three further isolates. All the lines were tested using detached leaves but a number of them were also tested using seedlings. The seedling tests gave esentially the same results as the detached leaf tests. Over 50 % of the host lines were highly resistant (gave low infection types) to one or more of the mildew isolates and this resistance was isolate‐ or race‐specific. Eight specific resistance factors were differentiated by the eight mildew isolates used in the tests. Some of the resistance factors appear to be present in species of Senecio other than S. vulgaris. All eight mildew isolates were complex races with virulence for all but one of the specific resistance factors, the isolates differing from each other in the possession of avirulence for a unique specific resistance factor. The interactions between the host lines and mildew isolates are indicative of a gene‐for‐gene interaction but the genetic analysis required to establish this was not attempted. The majority of lines which were susceptible to each isolate developed the highest or most susceptible infection type. It is considered that tolerance of the parasite may operate together with race‐specific resistance in an overall survival strategy which has evolved to enable groundsel to cope with mildew attack.

Combining Ability and Heritability of Resistance to Early and Late Leafspot of Peanut1

Abstract Four parental lines with resistance to early leafspot caused by Cercospora arachidicola Hori and four parental lines with resistance to late leafspot caused by Cercosporidium personatum (Berk. & Curt.) Deighton and the F1 hybrid progeny from crosses between the two groups of parents were evaluated for resistance to both leafspot diseases in the greenhouse using a detached leaf technique. The subsequent F2 plants of all crosses were evaluated in the field for resistance to early leafspot in order to estimate combining ability effects for components of partial resistance and to identify parents useful in developing lines resistant to both diseases. General combining ability, attributed largely to additive genetic variance, accounted for the largest portion of the variability among the F1 and F2 generations for most parameters of resistance to both early and late leafspots. Reciprocal effects and heterosis toward the susceptible parents were also significant for parameters of resistance to the two pathogens. GP-NC 343 and FESR 5-P2-B1 were the best parents for incorporating genes for resistance to both early and late leafspots. Progenies of NC 17090 had a high level of resistance to late leafspot in detached leaf tests and progeny of PI 350680 had reduced defoliation from early leafspot in the field. Broad-sense heritabilities ranged from 0.2 to 0.4 for parameters of resistance to early leafspot estimated from the pooled variances of F2 plants of all crosses planted in the field. Parameters of resistance evaluated in the greenhouse for F1 hybrids were compared to parameters evaluated in the field for the F2 population by rank correlation of entry means. Latent period and sporulation of the fungus on detached leaves of F1 generation plants correlated (r = −0.46 and 0.54, respectively) with defoliation of F2 plants in the field.

An assessment of varietal resistance to chocolate spot (Botrytis fabae) infection of field beans (Vicia faba L.), with some indications of its heritability and mode of inheritance

SUMMARYVarietal differences in resistance to non-aggressive chocolate spot infection were detected in the field, although complete resistance was not observed. Following artificial inoculation under growth-room conditions, a parent–offspring regression of disease scores for the heterogeneous stock Chivers was estimated as 0·51 ± 0·24, and although this cannot be regarded as a reliable heritability estimate, it demonstrates a degree of additive genetic control. AnF2diallel cross, under field conditions, indicated resistance to be controlled mainly by dominant alleles within a system of partial dominance. Detached-leaf tests indicated that within virtually complete dominance systems, dominant alleles facilitate fungal penetration and induce a hypersensitive response within the leaf. Control of further lesion development was indicated to be effected by a virtually complete additive system.