Rapid Screening of Cacao Genotypes for Field Resistance to Phytophthora palmivora Using Leaves, Twigs and Roots

Black pod, caused by Phytophthora spp. is one of the most important diseases of cacao occurring worldwide. Losses due to black pod caused by P. palmivora are still moderate in Côte d'Ivoire but P. megakarya causes high losses in Ghana and other Central African countries. Variation in field attack has been observed between cacao genotypes, but evaluation of pod losses is unsuitable for obtaining rapid progress in breeding. Results of inoculation tests using young detached leaves, twigs and roots, obtained from field and nursery plants, are presented here and compared to field resistance of similar genotypes observed over a 10-year period. Nine different Upper Amazon Forastero genotypes were tested together with progenies obtained by crossing these with the susceptible check IFC5 (Amelonado genotype). Rank correlations between the early screening tests and the level of field attack were positive and mostly significant (r=0.58–0.95). The coefficient of correlation was slightly higher for leaves (r=0.88) and roots (r=0.89) than for twigs (r=0.76). Also, resistance of the different plant organs was correlated (r=0.6–0.9). Resistance of the Upper Amazon parents was well correlated with the resistance of their cross progenies (r=0.7–0.9), suggesting that resistance is highly heritable. Resistance of leaves and twigs from the nursery was better correlated with field resistance than resistance of leaves and twigs from the field, which might result from more uniform growing conditions in the nursery. Inoculation of leaves appears the most suitable early screening method for black pod resistance. Application of this test in breeding more resistant cacao cultivars is discussed.     

Foliar resistance of cacao (Theobroma cacao) to Phytophthora palmivora as an indicator of pod resistance in the field: the effect of light intensity and time of day of leaf collection

Resistance of cacao leaves to Phytophthora palmivora was studied with regard to the time of leaf collection (morning, afternoon) and the degree of exposure of the leaves to light in the field (low, medium and high). The efficiency of leaf disc inoculations in predicting field resistance of nine clones was compared with that of detached and attached pod inoculations. Significant effects were observed, with leaves exposed to high light intensity and collected early in the afternoon showing highest susceptibility. The effect of time of leaf collection was reduced when leaves were stored overnight and leaf discs prepared and inoculated the following day, as compared to inoculations on the day of collection. Interactions between the main factors were significant, though less substantial than the clone effects. The most significant correlations with pod resistance ( r  = 0·70 to 0·97) were obtained for leaves collected early in the morning and exposed to intermediate shade conditions in the canopy. For other treatments, the correlations with pod resistance were still positive ( r  = 0·23 to 0·83) but often not significant. Pod inoculations in the laboratory were better correlated with field resistance ( r  = 0·92) than pod inoculations in the field ( r  = 0·72). Detached pod inoculations were also better correlated with leaf disc inoculations than those of attached pods. The results confirm the validity of laboratory inoculations of leaves and pods to assess field resistance to Phytophthora . Standardization of the leaf disc test is essential to obtain reliable results.     

Foliar resistance of cacao (Theobroma cacao) to Phytophthora palmivora as an indicator of pod resistance in the field: interaction of cacao genotype, leaf age and duration of incubation

The effects were studied of four leaf development stages (LDS) and three durations of incubation (DI) on the accuracy of leaf-disc tests on eight cacao (cocoa) clones (C) for predicting field resistance to phytophthora pod rot caused by Phytophthora palmivora . The clones were known to possess different general combining abilities (GCA) for pod resistance in the field, evaluated monthly at harvest over a 9-year period. Disease severity (DS) was affected strongly by C, DI and LDS, with increasing levels of significance. Two- and three-way interaction effects were smaller than the clone effect, but still significant. Clone effects were most significant for LDS3 (i.e. leaves 50–60 days old) and for DI5 and DI7 (observations made 5 and 7 days after inoculation, respectively). Coefficients of rank correlation between DS and field results were significant for seven of the 12 treatments, with highest values obtained again for treatments LDS3/DI5 ( r = 0·87) and LSD3/DI7 ( r = 0·93). Pooling of data for different LDS and DI treatments did not further improve the correlation with field results. However, these correlations were improved (from an average of 0·74 to 0·88) when the GCA values for field resistance were based on weekly observations, carried out in one year, including losses of pods and cherelles. It was concluded that, when carried out in a standardized manner and under optimal conditions, the leaf-disc test may explain 75–90% of the genetic variation for field resistance of cacao genotypes to P. palmivora .     

A disease and production index (DPI) for selection of cacao (Theobroma cacao) clones highly productive and tolerant to pod rot diseases

Frosty pod rot (FPR) (Moniliophthora roreri), along with black pod rot (Phytophthora species) and witches’ broom disease (Moniliophthora perniciosa) constitute the main phytosanitary problems limiting cacao (Theobroma cacao) production causing severe yield losses. One of the main sought after methods of pod rot management is the selection of tolerant cacao genotypes. Typically, the selection is carried out through the quantification of the percentage of diseased pods (PDP). However, PDP does not consider the relative productivity, or production potential (PT) of the genotype. Production potential can vary among cacao genotypes. Consequently, genotypes with similar PT can have similar or vastly different disease tolerance levels as measured by PDP. The disease and production index (DPI) was developed to integrate a genotype's tolerance to M. roreri and other diseases as measured by PDP, with its PT. Here, we evaluated the number of healthy pods, number of diseased pods, and weight of fresh seed for 29 clones grown in replicated five-tree plots over 4 years. The data obtained was used to calculate PDP and DPI for each clone for three different disease combinations: frosty pod rot alone, pod rots other than frosty pod rot, and the combination of all pod rots. Multivariate analysis verified that DPI discriminated between clones based on productivity and disease tolerance. Surprisingly, there was a close ranking of clones between resistance to FPR and resistance to all other pod rots. The DPI can be used in breeding programmes focused on the selection of high yielding disease-tolerant cacao genotypes.     

Evaluation of cacao (Theobroma cacao) clones against seven Colombian isolates of Moniliophthora roreri from four pathogen genetic groups

Artificial pod inoculation was used to compare the relative aggressiveness of seven Colombian isolates of Moniliophthora roreri (the causal agent of moniliasis or frosty pod disease), representing four major genetic groupings of the pathogen in cacao (cocoa), when applied to five diverse cacao genotypes (ICS-1, ICS-95, TSH-565, SCC-61 and CAP-34) at La Suiza Experimental Farm, Santander Department, Colombia. The following variables were evaluated 9 weeks after inoculation of 2- to 3-month-old pods with spore suspensions (1·2 × 10⁵ spores mL⁻¹): (i) disease incidence (DI); (ii) external severity (ES); and (iii) internal severity (IS). IS was found to be of greatest value in classifying the reaction of the host genotype against M. roreri . Genetic variation reported between isolates and cacao genotypes was not matched by similar diversity in their aggressiveness. All isolates were generally highly aggressive against most cacao genotypes, with only two isolates showing reduced IS and ES reactions. There was considerable variation between clones in the IS and ES scores, but one cultivated clone (ICS-95) displayed a significant level of resistance against all seven isolates. This clone may be useful in cacao breeding initiatives for resistance to moniliasis of cacao.     

Validation of a modified Petri-dish test to quantify aggressiveness of <Emphasis Type="Italic">Fusarium graminearum</Emphasis> in durum wheat

Fusarium graminearum is a common agent causing Fusarium head blight (FHB) on wheat throughout the world. Aggressiveness is crucial for understanding the interaction between host-pathogen in the FHB-wheat system. In this paper, we modified and validated the Petri-dish test originally described by Mesterhazy (Phytopathologische Zeitschrift 93:12–25, 1978) to quantify the aggressiveness of 25 F. graminearum strains using four durum wheat cultivars with different resistance levels for FHB. The results were highly significant and correlated with those obtained using adult plants in the growth chamber and in the field (r = 0.94, P < 0.001 and r = 0.65, P < 0.001, respectively). The Petri-dish test was further investigated for its repeatability and stability in different durum wheat cultivars and highly significant correlation coefficients were obtained (r = 0.90–0.91 (P < 0.001), 0.89–0.95 (P < 0.001), respectively). In this study, we also demonstrated that germination rate reduction and coleoptile length reduction are parameters involved with aggressiveness of F. graminearum. The mean of three disease parameters from the modified Petri-dish method is introduced in this paper as a new parameter for aggressiveness and named “Petri-dish aggressiveness index”. The results obtained reveal that this modified Petri-dish test is rapid, reliable and stable with different durum wheat cultivars, and yields highly significant correlation coefficients with floret and ear inoculations, thus it is suitable to be used for quantification of aggressiveness of F. graminearum.     

On-farm selection for quality and resistance to pest/diseases of cocoa in Sulawesi: (ii) quality and performance of selections against Phytophthora pod rot and vascular-streak dieback

The cocoa industry in Sulawesi, the main region of cocoa production in Indonesia, is threatened by destructive diseases, including vascular-streak dieback (VSD) caused by the basidiomycete Oncobasidium theobromae and stem canker and Phytophthora pod rot (PPR) or black pod, caused by Phytophthora palmivora. Using the considerable genetic diversity of cocoa on farms, host resistance was identified and tested with the participation of farmers. Forty-nine local and international cocoa selections with promising resistance characteristics (as well as susceptible controls) were side-grafted onto mature cocoa in a replicated trial with single-tree plots. Developing grafts were assessed in the dry season for severity of VSD infection, scored from 0 (no infection) to 4 (graft death). All of the 49 clones in the trial became infected with VSD in at least some replicates. Average severity varied from 0.2 to 1.6. Potential VSD-resistance was found in eight clones, including DRC 15, KA2 106 and a local Sulawesi selection, VSD2Ldg. Some of the most susceptible clones were local Sulawesi selections from areas with a history of little or no VSD. Thirty-four pod-bearing clones were evaluated over a 2-year period for yield, quality and resistance to natural infections of PPR. Cumulative PPR incidence for all clones was 22% but varied from 8.6 to 43% among clones. Clones with less than 15% PPR incidence were designated as resistant, including DRC 16 and local Sulawesi selections, Aryadi 1, Aryadi 3 and VSD1Ldg. Scavina 12 was moderately resistant in the trial with a PPR incidence of 23%. Cumulative incidences of the mirid, Helopeltis spp., determined in the same evaluation period, indicated that DRC16 was the most susceptible clone with an incidence of 52% in ripe pods and 23% in immature pods. In comparison, KKM4 showed evidence of resistance to Helopeltis spp., with incidences of 34 and 0.8% in ripe and immature pods, respectively. The impact of diseases and pests (including cocoa pod borer) on bean losses and bean quality varied between clones but generally the bean size (or bean count) was affected more than the fat content or shell content.     

Components of partial disease resistance detected using a detached leaf assay in CIMMYT Fusarium head blight resistant wheat germplasm

One hundred and nineteen entries from the CIMMYT International Wheat and Maize Improvement Centre 2004/05 Fusarium head blight (FHB) resistance screening nursery were evaluated as possible sources of novel components of partial disease resistance (PDR), against FHB and Microdochium nivale snow mould, detected using a detached leaf assay. In addition the FHB resistant cvs Arina, Alsen and Frontana and 21 European wheat genotypes were included for comparison. There was wide variation among CIMMYT entries for the PDR components incubation period, latent period and lesion length (P < 0.001) and European lines for incubation and latent periods (P < 0.001). The CIMMYT entries with the longest latent periods were not superior to cv. Arina, the best European source of this PDR component identified to date. Notably the CIMMYT lines exhibiting the longest latent periods had Aegilops squarrosa (878) in their pedigree, indicating that Ae. squarrosa (878) may be a source of enhanced resistance detected by latent period. Macroscopic observation suggested that the underlying mechanisms contributing to latent period may differ among the CIMMYT germplasm and European sources of long latent period such as cv. Arina. Among the CIMMYT germplasm, incubation period was only weakly correlated with latent period (r = 0.25; P < 0.01); this also was the case among European genotypes (r = 0.36; P < 0.05) supporting previous findings that these PDR components are largely under separate genetic control. However, the correlation was higher on a subset of the most resistant and susceptible lines for latent period (r = 0.73 and r = 0.44; incubated at 10 °C and 15 °C, respectively). While a number of the European lines had latent periods that were comparable to cv. Arina many were significantly shorter indicating potential for improvement in this PDR component. Adaptations to the experimental design utilized in the present experiments for the efficient evaluation of large numbers of genotypes utilizing the detached leaf assay are discussed.     

Relationship between the extent of colonization by Verticillium dahliae and symptom expression in strawberry (Fragaria × ananassa) genotypes resistant to verticillium wilt

Eleven strawberry (Fragaria × ananassa) genotypes from the University of California breeding programme known to be resistant to verticillium wilt were inoculated with Verticillium dahliae. Individual plants were given a resistance score based on the severity of visual symptoms, and the extent of colonization was quantified as the percentage of petioles not colonized by the pathogen. Both resistance scores and the percentage of pathogen‐free petioles decreased significantly from May to June (P < 0·05) during each of two growing seasons, indicating a progression of both colonization and symptom expression. Even the most resistant genotypes had plants with some infected petioles, and manifested some symptoms of verticillium wilt. Significant (P < 0·05) genotypic variance was detected for the percentage of pathogen‐free petioles, but not for resistance score. The percentage of pathogen‐free petioles had a strongly positive genotypic correlation (r_g = 0·77, P < 0·01) with resistance score, indicating that about 60% of the genotypic variation for visual symptoms in this set of resistant genotypes was explained by the extent of colonization of individual plants by V. dahliae. Conversely, the genotypic correlation between the percentage of pathogen‐free petioles and the resistance score for plants sampled in May (r_g = 0·74, P < 0·01) was smaller than that for plants harvested in July (r_g = 0·93, P < 0·01). Together, these results suggest that the overall performance of strawberry genotypes in the presence of V. dahliae can be enhanced by both resistance and tolerance, but that tolerance may be less stable over the course of a season. Distinguishing between these two mechanisms may require evaluations that supplement visual assessments of resistance.     

Cacao resistance to Phytophthora: Effect of pathogen species, inoculation depths and pod maturity

Two species of Phytophthora (P. palmivora and P. capsici) and inoculations at two depths (3 mm and 9 mm) were tested each on 10 clones of Theobroma cacao to determine their effects on pod resistance. Ripe and unripe pods were also assessed to determine the influence of physiological status of the pod on the expression of resistance. The two pathogens tested (P. palmivora and P. capsici) differed significantly in their reactions on pods, with P. palmivora being more aggressive than P. capsici. However, the lack of interaction between clones and pathogen species and the similarity in the ranking of clones based on lesion size suggested that selection for resistant clones can be based on one of the two pathogens, preferably the more aggressive one. Pod reactions differed between inoculation depths (3 mm and 9 mm), and between pod maturity stages (ripe and unripe pods) with relatively larger lesions being recorded at 9 mm depth and on unripe pods as compared to those observed at 3 mm depth and on unripe pods, respectively. The magnitude of increase in lesion sizes, however, varied with genotypes, indicating that inoculation depth and pod maturity stage should be standardized in screening cacao germplasm for resistance to Phytophthora.     

Influence of multiple herbicide resistance on growth in Amaranthus tuberculatus

Plant defence traits, such as herbicide resistance mutations, may incur a fitness cost to plants that become evident when the trait is not needed. However, individuals with multiple herbicide resistance traits may decrease fitness beyond that of plants with a single herbicide resistance mutation. Multiple herbicide‐resistant (MHR) Amaranthus tuberculatus populations are becoming more prevalent in Midwest United States agroecosystems. The objective was to determine whether selected MHR A. tuberculatus populations express differential development when grown in a herbicide‐free environment. The hypothesis was that MHR A. tuberculatus populations become increasingly less fit when additional herbicide resistances evolve. Multiple herbicide‐resistant and herbicide‐susceptible A. tuberculatus populations were grown in a herbicide‐free field for 20 weeks for two seasons. Differences (P < 0.001) in apical growth were detected 5 and 7 weeks after transplanting for all populations in 2016 and 2017 respectively. Gender and population influenced (P < 0.001) flowering date, with males flowering up to 1.5 weeks earlier than females, but did not cause pollination asynchrony. Shoot biomass was not different (P = 0.84) across A. tuberculatus populations, but there were differences (P < 0.001) for gender and year. Seed production was different amongst A. tuberculatus populations (P = 0.001), but was not influenced by the number of MHR traits. Conversely, a negative quadratic relationship between seed mass and the number of MHR traits was observed (r² = 0.32; P < 0.001). The experiment results demonstrate that MHR in A. tuberculatus populations is not incurring a fitness penalty that will remove the populations in the immediate future.     

Oomycete species associated with Theobroma cacao crops in Colombia

The study of oomycetes associated with crops is highly important due to the economic losses they might cause. In cacao, the genus Phytophthora has been extensively studied, but little is known about other genera and species of oomycetes associated with this plant. This study aimed to determine the oomycetes’ diversity present in Colombian cacao crops. A total of 146 isolates were obtained from diseased plants and soil in 11 departments. Analysis of internal transcribed spacer (ITS) and cytochrome oxidase subunit I (coxI) sequences was performed along with the assessment of morphological characteristics. Nine species were identified, distributed in four genera: Phytophthora (P. palmivora, 54%; P. nicotianae, 1%), Phytopythium (Phy. chamaehyphon, 15%; Phy. cucurbitacearum, 9%; Phy. vexans, 7%; Phy. helicoides, 1%), Globisporangium (G. splendens, 3%), and Pythium (Py. delicense, 1%; Py. inflatum,1%). Additionally, an unidentified and possibly new species of Phytophthora (5%) and three unidentified species of Phytopythium (3%) were found. This is the first report of Globisporangium, Phytopythium, and Pythium in cacao crops of Colombia and the first report of the species Phy. chamaehyphon in the country. Interestingly, some isolates of Phytopythium spp. were isolated from necrotic leaves and vascular section of stems, which may suggest a role in cacao diseases traditionally associated with Phytophthora. Also, it is proposed that the new species of Phytophthora may be contributing significantly to black pod disease in Colombian cacao crops, and we highlight that the study of P. palmivora is urgent because of its distribution all over the country.     

Evaluation of mulberry germplasm for resistance to powdery mildew in the field and greenhouse

Powdery mildew caused by the ascomycete Phyllactinia guttata (syn. P. corylea) is a major foliar disease worldwide of the unique mulberry (Morus spp.) for silkworm feed. Genetic resistance to powdery mildew, the most sustainable and economical strategy for disease control, is still elusive for tropical mulberry. About 147 germplasm sources, representing 18 countries of origin, were screened for resistance to P. guttata in six seasonal fields and greenhouse trials after exposure to natural and artificial inoculum, respectively. In the field, the level of plant responsiveness to disease was assessed from 30 to 62 days after pruning in each season as variations in the disease severity index (DSI), disease incidence (DI%) and area under the disease progress curve (AUDPC). These measures differed significantly among the germplasm. Of 147 germplasm sources, ~6.8% had useful resistance (two high and nine moderately resistant) to the powdery mildew pathogen on the basis of DSI. The AUDPC values were 13.5-fold higher in the most susceptible accession—(Philippines) than the least responsive (Vietnam-2). The results of DSI were strongly correlated with the obtained DI values (r = 0.92; P < 0.01) and AUDPC (r = 0.89; P < 0.01). Moreover, field screening results were highly correlated (R ² = 0.839) with values from the greenhouse evaluation using artificial inoculum. However, the DSI values in field and greenhouse screenings for three sources (Non-nayapati, Nao-khurkul and Tista Valley) varied significantly. A relatively low disease reaction of 09 resources (Vietnam-2, Ankara and 07 others) using different assessment scales after natural and artificial inoculation prove, for the first time, that they have potential in breeding for resistance in tropical mulberry to powdery mildew.     

Vavilov wheat accessions provide useful sources of resistance to tan spot (syn. yellow spot) of wheat

Host genetic resistance is the most effective and sustainable means of managing tan spot or yellow spot of wheat. The disease is becoming increasingly problematic due to the adoption of minimum tillage practices, evolution of effector‐mediated pathogenicity, and widespread cultivation of susceptible cultivars from a narrow genetic base. This highlights the importance of broadening the diversity of resistance factors in modern breeding germplasm. This study explored 300 genetically diverse wheat accessions, originally sourced from the N. I. Vavilov Institute of Plant Genetic Resources (VIR), St Petersburg, Russia. The collection was screened for resistance to tan spot at seedling and adult stage under controlled conditions, and in the field across 2 years. The phenotypic datasets, coupled with ToxA bioassay screening, identified a number of accessions with useful sources of resistance. Seedling disease response corresponded well with ToxA sensitivity (r = 0.49, P < 0.000), but not adult responses (r = ?0.02 to ?0.19, P < 0.002), and overall reactions to ToxA appeared to show poor correspondence with disease response at the adult stage. ToxA‐insensitive accessions were generally found resistant across different growth stages (all‐stage resistance, ASR) in all experiments (seedling and adult stage under controlled conditions and field). ToxA‐sensitive accessions that were susceptible at seedling stage, but resistant at both adult‐plant stages, were deemed to carry adult‐plant resistance (APR). This study provides detailed information on the degree of tan spot resistance in the Vavilov wheat collection and discusses strategies to harness these sources to boost the diversity of resistance factors in modern wheat breeding germplasm.     

Pathogenicity of morphologically different isolates of <Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis> with <Emphasis Type="Italic">Brassica napus</Emphasis> and <Emphasis Type="Italic">B. juncea</Emphasis> genotypes

Sclerotinia stem rot caused by Sclerotinia sclerotiorum is a serious threat to oilseed production in Australia. Eight isolates of S. sclerotiorum were collected from Mount Barker and Walkway regions of Western Australia in 2004. Comparisons of colony characteristics on potato dextrose agar (PDA) as well as pathogenicity studies of these isolates were conducted on selected genotypes of Brassica napus and B. juncea. Three darkly-pigmented isolates (WW-1, WW-2 and WW-4) were identified and this is the first report of the occurrence of such isolates in Australia. There was, however, no correlation between pigmentation or colony diameter on PDA with the pathogenicity of different isolates of this pathogen as measured by diameter of cotyledon lesion on the host genotypes. Significant differences were observed between different isolates (P ≤ 0.001) in two separate experiments in relation to pathogenicity. Differences were also observed between the different Brassica genotypes (P ≤ 0.001) in their responses to different isolates of S. sclerotiorum and there was also a significant host × pathogen interaction (P ≤ 0.001) in both experiments. Responses between the two experiments were significantly correlated in relation to diameter of cotyledon lesions caused by selected isolates (r = 0.79; P < 0.001, n = 48). Responses of some genotypes (e.g., cv. Charlton) were relatively consistent irrespective of the isolates of the pathogen tested, whereas highly variable responses were observed in some other genotypes (e.g., Zhongyou-ang No. 4, Purler) against the same isolates. Results indicate that, ideally, more than one S. sclerotiorum isolate should be included in any screening programme to identify host resistance. Unique genotypes which show relatively consistent resistant reactions (e.g., cv. Charlton) across different isolates are the best for commercial exploitation of this resistance in oilseed Brassica breeding programmes.     

Development and Evaluation of an in vitro Detached Leaf Assay forc Pre-Screening Resistance to Fusarium Head Blight in Wheat

An in vitro detached leaf assay, involving the inoculation of detached leaves with Microdochium nivale, was further developed and used to compare with whole plant resistance ratings to Fusarium head blight (FHB) of 22 commercial cultivars and published information on 21 wheat genotypes, identified as potential sources for FHB resistance. An incubation temperature of 10 °C and isolates of M. nivale var. majus of intermediate pathogenicity were found to be the most suitable for the differential expression of several components of partial disease resistance (PDR), namely incubation period, latent period and lesion length, in wheat genotypes used in the detached leaf assay. There were highly significant differences (P < 0.001) for each component of PDR within commercial cultivars and CIMMYT genotypes. Positive correlations were found between incubation period and latent period (r = 0.606; P < 0.001 and r = 0.498; P < 0.001, respectively, for commercial cultivars and CIMMYT genotypes), inverse correlations between incubation period and lesion length (r = -0.466; P < 0.01 and r = –0.685; P < 0.001, respectively) and latent period and lesion length (r = –0.825; P < 0.001 and r = –0.848; P < 0.001, respectively). Spearman rank correlations between individual PDR components and UK 2003 recommended list ratings were significant for incubation period (r_s = 0.53; P < 0.05) and latent period (r_s = 0.70; P < 0.01) but not for lesion length (r _s = –0.26). Commercial cultivars identified with high resistances across all three PDR components in the detached leaf assay also had high whole plant FHB resistance ratings, with the exception of cv. Tanker which is more susceptible than the results of the detached leaf assay suggested, indicating an additional susceptibility factor could be present. Agreement between resistances found in the detached leaf assay and resistance to FHB suggests resistances detected in detached leaves are under the same genetic control as much of the resistances expressed in the wheat head of the commercial cultivars evaluated. In contrast, high resistances in each of the PDR components were associated with higher susceptibility across 19 CIMMYT genotypes previously evaluated as potential breeding sources of FHB resistance (incubation period: r = 0.52; P < 0.01, latent period: r = 0.53; P < 0.01, lesion length: r = –0.49; P < 0.01). In particular, the CIMMYT genotypes E2 and E12 together with Summai #3, known to have high levels of whole plant FHB resistance, showed low levels of resistance in each PDR component in the detached leaf assay. Such whole plant resistances, which are highly effective and not detected by the detached leaf assay, do not appear to be present in Irish and UK commercial cultivars. The most resistant Irish and UK commercial cultivars were comparable to the genotype Frontana and the most resistant CIMMYT germplasm evaluated in the leaf assay.     

A Review on Options for Management of Cowpea Pests: Experiences From Uganda

More than 100 field pests of cowpea (Vigna unguiculata L. Walp.) can be found in most of the crop production agroecologies in Africa, but four of these – aphids (Aphis craccivora Koch), flower thrips (Megalurothrips sjostedti Trybom), the legume pod borer (Maruca vitrata Fab. Syn. Maruca testulalis Geyer) and pod sucking bugs – are commonly encountered and are of economic importance in Uganda. The diverse cowpea pest complex dictates that a single control strategy is unlikely to produce satisfactory control.Earlier field studies done in eastern and northern parts of Uganda demonstrated that close spacing (30 × 20cm) effectively reduces aphid infestation (early season pest) but seems to promote thrips, legume pod borers and pod bugs infestation. The other option for management of early season pests and nematodes is seed dressing, especially with carbofuran (Furadan 5G). Late season pests are more effectively controlled by the use of foliar sprays, the type of pesticide depending on the pest profile. Intercropping also offers remedial control, but the crop combination must consider the pest profile, cowpea/sorghum intercrop being effective against aphids and thrips, and cowpea/greengram against legume pod borers and pod sucking bugs. Selected combinations of agronomic, chemical and cultural control measures (Integrated Pest Management), especially when combined with early planting, offer better management options than the use of sole treatments. The success of these packages is highly dependent on the degree and level of farmer involvement and to what extent they are tailored to meet his/her production goals.     

Phytophthora resistance in cacao (Theobroma cacao): Influence of pod morphological characteristics

Twelve diverse cacao ( Theobroma cacao ) genotypes were assessed for pod resistance to Phytophthora palmivora at the penetration and post-penetration stages of infection using two inoculation methods. Correlation analysis between a number of pod characteristics (stomatal frequency, stomatal pore length, surface wax, thickness of exocarp/endocarp, hardness of exocarp/mesocarp, moisture content) and resistance indicated a strong relationship between resistance to lesion establishment (lesion frequency) and the joint effect of stomatal frequency and pore length. The epidermal impressions of the pod surfaces bearing germinating zoospores of P. palmivora provided evidence that penetration occurs through stomata, epidermal hair base, scar and by direct penetration. A poor correlation was obtained between the pod characteristics studied and post-penetration resistance, suggesting that this resistance, assessed by lesion size, is not governed by morphological or physical characteristics of the pod, but probably by biochemical factors. The importance of these findings in breeding of cacao for resistance to P. palmivora is discussed.     

Trichoderma harzianum produces nonanoic acid, an inhibitor of spore germination and mycelial growth of two cacao pathogens

An isolate of Trichoderma harzianum Rifai from an infected cacao pod produces and secretes nonanoic (pelargonic) acid into a liquid culture medium. Nonanoic acid (NA) was very inhibitory to spore germination and mycelial growth of two cacao pathogens, Crinipellis perniciosa Stahel and Moniliophthora roreri Cif. H.C. Evans. It was highly active causing 75% inhibition of spore germination in an in vitro assay at a rate as low as 0.09 μM for M. roreri and 0.92 μM for C. perniciosa. Mycelial growth was comparatively less sensitive to inhibition, but still there was a 75% reduction in growth with 0.62 μM in M. roreri and 151 μM NA in C. perniciosa. In contrast, NA did not affect Trichoderma mycelial growth or spore germination at concentrations that were inhibitory to the pathogens. 6-pentyl-α-pyrone was also produced and secreted into the medium by T. harzianum, however; it was not antagonistic to the cacao pathogens. Although a number of metabolites produced by Trichoderma spp. have been identified in the past, this is the first report of NA production and secretion by any Trichoderma. The results suggest that NA may play a role in the successful use of some Trichoderma spp. isolates in the biocontrol of fungal diseases of plants.     

Geographic location and year determine virulence,and year determines genetic change,in populations of Neopseudocercosporella capsellae

White leaf spot (Neopseudocercosporella capsellae) can be severe and problematic worldwide across both horticultural and oilseed Brassicaceae, including susceptible rapeseed. In this study, 82 isolates from 2015 and 106 isolates from across Australia in 2016 were first assessed for their virulence against three different rapeseed (Brassica napus) cultivars. For both years there were significant (P < 0.001) differences. Also, there were significant (all P < 0.001) differences between isolates in each year, and between cultivars. For 2016 isolates, there were also significant differences (P < 0.001) between isolates across three different Australian states, and a significant interaction (P < 0.001) between isolates with cultivars. Of the three Australian states, isolates from Victoria were most virulent. Among tested cultivars, cv. Scoop was most susceptible. Subsequently, phylogenetic analysis of 114 of these same 2015 and 2016 isolates showed current isolates clustered separately from the majority of 2005 N. capsellae isolates collected from Western Australia a decade earlier, confirming significant genetic change within N. capsellae populations over the past decade. However, isolate clusters showed no association with geographical location. The results suggest that phylogenetic association among 2005 and 2015–2016 N. capsellae isolates is complementary with pathogenicity variations explained by geographically different N. capsellae pathogen populations. Neopseudocercosporella capsellae populations are evolving rapidly, challenging management through host resistance at a time of increasing incidence and severity of white leaf spot disease over the past decade. The outcome is well illustrated by cv. Scoop, previously resistant to 2005 isolates but moderately susceptible to 2015 and highly susceptible to 2016 isolates.