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.     

Assessing resistance to Crinipellis perniciosa using cocoa callus

Callus cultures from cocoa clones reported to have different susceptibilities to Brazilian isolates of Crinipellis perniciosa , causal fungus of witches' broom disease, including Catongo (susceptible). Scavina-6, CAB 64 and CAB 67 (resistant) were inoculated with basidiospores of C. perniciosa and the subsequent development of mycelia was assessed. Generally, on Catongo callus, mycelium similar to that found in green brooms (parasitic type), developed abundantly but on callus from Scavina-6 and the two CAB clones, mycelial growth was relatively poor and tended to be similar to that in dry brooms and on agar media (saprotrophic type). There was often considerable variation in fungal development on individual callus cultures from the same clone, but there was an overall consistency between successive experiments in ranking the callus material in terms of fungal development. However, growth of isolates from Colombia and Trinidad was similar on callus from Scavina-6 which in the field is known to reac differentially to these two isolates. Experiments with isolates from Colombia indicated that inoculum from basidiocarps produced on brooms which were kept for long periods in cabinets to induce fruiting had a decreased ability to colonize callus.     

Analysis of resistance to witches’ broom disease (Moniliophthora perniciosa) in flower cushions of Theobroma cacao in a segregating population

This study evaluates resistance to witches’ broom disease in flower cushions of Theobroma cacao under field conditions. The aim was to determine optimal inoculation methods to evaluate the disease incidence using flower cushions in the field. A segregating mapping population of 580 trees (cultivar TSH 1188 × CCN 51) was analysed under two field conditions: high and low inoculum levels (in different years), corresponding respectively to trees with or without dried witches’ brooms hanging on the trees and producing basidiocarps. The number of newly formed cushion brooms in each tree was counted by the conventional method, and also the healthy and infected flower cushions in three 30 cm‐long regions along the trunk and the two main branches. The field inoculation methods discriminated between genotypes, with a 26% increase in disease incidence by Moniliophthora perniciosa at high inoculum. Two different segregation patterns were also observed: 27:27:9:1 under low, and 27:9:9:9:3:3:3:1 under high inoculum potential. It was also determined that at least 20 flower cushions were needed to accurately determine the percentage of infection. These methodologies allowed identification of the extreme phenotypes in this mapping population, and can therefore facilitate the detection of sources of resistance to witches’ broom disease.     

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.     

Development of a marker assisted selection program for cacao

ABSTRACT Production of cacao in tropical America has been severely affected by fungal pathogens causing diseases known as witches' broom (WB, caused by Moniliophthora perniciosa), frosty pod (FP, caused by M. roreri) and black pod (BP, caused by Phytophthora spp.). BP is pan-tropical and causes losses in all producing areas. WB is found in South America and parts of the Caribbean, while FP is found in Central America and parts of South America. Together, these diseases were responsible for over 700 million US dollars in losses in 2001 (4). Commercial cacao production in West Africa and South Asia are not yet affected by WB and FP, but cacao grown in these regions is susceptible to both. With the goal of providing new disease resistant cultivars the USDA-ARS and Mars, Inc. have developed a marker assisted selection (MAS) program. Quantitative trait loci have been identified for resistance to WB, FP, and BP. The potential usefulness of these markers in identifying resistant individuals has been confirmed in an experimental F(1) family in Ecuador.     

Factors influencing the production of basidiocarps and the deposition and germination of basidiospores of Crinipellis perniciosa, the causal fungus of witches' broom on cocoa (Theobroma cacao)

The production of basidiocarps by Crinipellis perniciosa on detached, dead witches'brooms from cocoa was assessed in relation to temperature, light, cocoa clone, age of broom and type of tissue, in cabinets with a daily cycle of 8 h wet and 16 h dry. More basidiocarps formed and matured at 20–25°C than at 25–30°C. In the latter regime the pilei were smaller and white, instead of the usual crimson colour, and the stipes were longer. No basidiocarps formed at 30–35°C. At 20–25°C. more basidiocarps formed and matured with light at 100 μE m^-2 s^-1 during the wet period than at 10 μE m^-2 s ^-1. Only one basidiocarp and five primordia developed on 20 brooms kept in the dark. Brooms from 10 cocoa clones at Pichilingue. Ecuador, differed in basidiocarp productivity. most basidiocarps forming on brooms from Seavina and least on ICS clones. The numbers of basidiocarps produced on brooms aged 1.2.3 or 4 months when detached from cocoa trees were similar but time to initiation of the first primordium differed considerably. More basidiocarps formed at nodes than internodes.
The discharge of basidiospores was optimal at 20–25°C and 80% RH: germination was optimal in water agar films. Neither process was dependent on light.     

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.     

Pathogenic variability amongst isolates of Crinipellis perniciosa from cocoa (Theobroma cacao)

Dry witches' brooms from cocoa were imported from various areas within Bolivia, Brazil, Colombia, Ecuador, Trinidad and Venezuela. Basidiocarps of Crinipellis perniciosa were induced to form on these brooms and seedlings of different types of cocoa were inoculated with basidiospores either on the hypocotyl or cotyledon bud. Host reactions were assessed mainly by recording stem base swelling and broom development at the cotyledon node (hypocotyl inoculations) or the extent of swelling and branching of shoots (cotyledon bud inoculations).
Results from 30 experiments indicated considerable diversity amongst isolates in inducing disease symptoms, but suggested that two groups or populations of C. perniciosa exist on cultivated cocoa. One group (A), comprising isolates from Bolivia and Pichilingue (Ecuador) and most isolates tested from Colombia, induced severe symptoms on cocoa with Scavina 6 as one parent; the other group (B), comprising isolates from Brazil, Trinidad and Venezuela, did not. Within these groupings variants could be further distinguished by particular host reactions. Isolates from Ecuador, especially from the Oriente, a centre of diversity for Theobroma cacao, showed a range of pathogenicity comparable to that found amongst isolates from cultivated cocoa over a much wider area.     

Production and Germination of Conidia of Trichoderma stromaticum, a Mycoparasite of Crinipellis perniciosa on Cacao

ABSTRACT Growth characteristics of the fungus Trichoderma stromaticum, a mycoparasite on the mycelium and fruiting bodies of Crinipellis perniciosa, the causal agent of witches'-broom disease of cacao, were evaluated under controlled environmental conditions. The ability of T. stromaticum to produce conidia and germinate on dry brooms was evaluated at three constant temperatures (20, 25, and 30 degrees C) and two constant relative humidities (75 and 100%). T. stromaticum produced abundant conidia on brooms at 100% relative humidity and incubation temperatures of 20 and 25 degrees C, but none at 30 degrees C. Sporulation of T. stromaticum was not observed at 75% relative humidity at any temperature. At 100% relative humidity and either at 20 or 25 degrees C, treatment of brooms with T. stromaticum suppressed C. perniciosa within 7 days. In contrast, at 30 degrees C, treatment with T. stromaticum had no effect on the pathogen in brooms maintained at either 75 or 100% relative humidity. Mycelium of C. perniciosa grew from brooms at all temperatures at 100% relative humidity. Conidial germination on broom tissue approximated 80% at temperatures from 20 to 30 degrees C. Results suggest that applying T. stromaticum under high-moisture conditions when the air temperature is below 30 degrees C may enhance the establishment of this mycoparasite in cacao plantations.     

Heritability of dollar spot resistance in creeping bentgrass

ABSTRACT The dollar spot disease incited by Sclerotinia homoeocarpa is an important disease of creeping bentgrass (Agrostis stolonifera). Genetic resistance is an important control strategy and could reduce fungicide use. Despite recent research, the genetic mechanism of dollar spot resistance in turfgrasses is still not fully understood. The objectives of this study were to (i) determine narrow-sense heritability and predicted gain from selection for dollar spot resistance in creeping bentgrass and (ii) evaluate inheritance characteristics of dollar spot disease resistance. Inheritance characteristics such as the detection of major genes, heterosis, maternal effects, and combining ability were determined by evaluating the disease severity of progeny from crosses between resistant and susceptible bent-grass clones. Parental clones and progenies from crosses were established in a field trial in a randomized complete block design and inoculated with one isolate of S. homoeocarpa applied at a rate of 0.25 g m(-2) of prepared inoculum. Differences in progeny means between crosses were observed over both years. Progeny from resistant x resistant crosses had significantly less disease severity than resistant x susceptible and susceptible x susceptible crosses. High narrow-sense heritability estimates (0.79 [2002], 0.79 [2003]) and large mean squares for general combining ability support the idea that additive gene action plays a significant role in disease resistance and support previous research that dollar spot resistance is most likely quantitatively inherited.     

Evaluation of oilseed rape seed yield losses caused by <Emphasis Type="Italic">Leptosphaeria biglobosa</Emphasis> in central China

Phoma stem canker of oilseed rape (Brassica napus), caused by Leptosphaeria maculans/L. biglobosa is a globally important disease. Severe phoma stem canker symptoms have been observed on winter oilseed rape in China but the seed yield loss caused by this disease remains unknown. In May 2012 and May 2013, 17 and 13 crops were surveyed, respectively, in seven counties of Hubei Province, central China. Stems with phoma stem canker disease symptoms were sampled for pathogen isolation and identification. Only L. biglobosa was identified by culture morphology and species-specific PCR; no L. maculans was found. To evaluate the yield losses, yield components (number of branches per plant, number of pods per plant, 1000-seed weight, number of seeds per pod) were assessed on healthy and diseased plants sampled from crops in four counties and on plants from inoculated pot experiments (plants of three cultivars were inoculated at the green bud stage by injecting L. biglobosa conidia into the stem between the first and second leaf scars). Results of the field surveys showed that diseased plants had 14–61% less branches and 32–83% less pods than healthy plants, respectively. The estimated seed yield loss varied from 10% to 21% and from 13% to 37% in 2012 and 2013, respectively. In the pot experiments, there were no differences in numbers of branches or pods but there were differences in number of seeds per pod between inoculated and control plants. For the three cultivars tested, the inoculated plants had yield losses of 29–56% compared with the control. This study indicates that L. biglobosa could cause substantial seed yield loss in China.     

Witches' broom disease on cocoa in Rondonia, Brazil: basidiocarp production on detached brooms in the field

Fruiting of Crinipellis perniciosa was assessed in relation to various climatic factors using dead witches' brooms detached from cocoa, either suspended in the canopy or laid on leaf litter under cocoa. Induction of basidiocarp production occurred over approximately 60 days with rain. Basidiocarps were finally observed on about 85% of brooms in all samples examined and induction of fruiting required at least 17 rainy days, although brooms with a minimum age of 2 years produced basidiocarps about 25 days before brooms with a minimum age of 1 year. Groups of brooms showed distinct cycles of basidiocarp production, and up to 50% of fruiting occurred on dead, attached leaves, Fruiting was reduced on brooms on leaf litter and basidiocarps survived for less time than when brooms were suspended in canopy. Brooms were most productive with moderate amounts of wetness per day, and less than 4 h or more than 20h was inhibitory. No fruiting occurred at mean air temperatures higher than 30°C or lower than 20°C. The microclimate of the litter layer is discussed in relation to reduction of fruiting caused by pruning of brooms.     

Apricot Chlorotic Leaf Roll

Apricot chlorotic leaf roll is a graft-transmissible disease, apparently a mycoplasm infection, which affects Japanese plum and sometimes peach. It is particularly important in Mediterranean areas. First symptoms are a rolling and a yellowing of the leaves, often localized on some twigs. Later, various disorders, often localized on a part of the tree, are observed: proliferation of branched and slender shoots, growth of buds in autumn and winter. Leaf buds break before flower buds. This growth in winter induces sensitivity to frost. Phloem tissues of affected branches are more or less damaged, taking on a brown discolouration. Some branches are destroyed, while on others, new shoots may develop. The presence of weak shoots with small leaves among normal shoots and irregularly distributed dried twigs on the branches is a characteristic aspect of the disease. Symptom expression and evolution of the disease vary much according to environmental conditions, age and rootstocks.     

Critical disease components of common bacterial blight to effectively evaluate resistant genotypes of snap bean

Common bacterial blight (CBB) caused by Xanthomonas axonopodis pv. phaseoli (Xap) is the main bacterial disease of snap bean. The present work aimed to select snap bean genotypes that are resistant to CBB based on three components of resistance: area under the disease progress curve, latent period, and lesion diameter on pods (DL). A completely randomized two-factor factorial design with six replications was used to evaluate leaves and pods of 14 snap bean and three common bean genotypes (PI 207262, BAC-6 and A-794) with high resistance to CBB. Two Xap isolates, 1394-98 and 775-90, were used to inoculate leaves and pods. Data were analyzed using nonparametric statistics. Significant differences were observed among the evaluated genotypes for all of the components of resistance, except for DL. The snap bean UENF 1482 demonstrated good performance in two disease resistance components. For this reason, this genotype can be recommended for use in breeding programs that aim to generate snap bean genotypes resistant to Xanthomonas axonopodis pv. phaseoli.     

东北岩高兰枝芽萌发及分枝生长特性研究

为探究濒危植物东北岩高兰枝芽萌发、分枝和枝条生长特性,以大兴安岭北部满归林业局高地林场石质山地东北岩高兰为研究对象,在疏林-林缘-石隙连续样带设置标准地,分别均等选取标准株和标准枝,调查枝芽形成的时间、类型、着生部位、萌发时间、1a枝长和枝径、样株1~5a枝的叶和不定根生长情况。结果表明:1)东北岩高兰越冬枝芽5月中下旬萌发,顶芽和近枝顶腋芽具早熟性,可在当年8月中下旬萌发,秋梢长度0.5~1.0cm,形成新枝的顶芽在密集叶包裹下越冬。2)顶端优势具多变性,形成"一优多强顶端优势"或"多强顶端优势",以适应多变的极端生境拓展种群生存空间。3)枝芽萌发抽枝分为5种类型,非近枝顶侧枝为偶见;各类型分枝率1.00~6.44,新枝数量差异较大。顶芽萌发对侧枝、特别是非近枝顶侧枝的抽生具有一定的抑制作用。4)3a枝匍匐生长且逐渐生根成不同匍匐茎克隆个体,形成与贫瘠生境中不同生存资源微生境相协调的生长发育响应策略,保证种群在总体干旱、贫瘠、多风极端生境下的不同微生境中生息繁衍。     

Homothallism in Peronospora viciae f.sp. pisi and the effect of temperature on oospore production

Monoconidial cultures derived from seven P. viciae f.sp. pisi isolates, obtained from different countries, were able lo produce oospores, Apparently, these isolates were homothallic. Oospore production of one isolate was studied at 5, 10, 15 and 20°C in systematically colonized shoots, and in local lesions on leaflets, stem parts and pods of the pisum sativum cv. Kelvedon Wonder. The number of oospores produced per gram systemically colonized tissue increased with temperature. In lesions of leaflets and of stem parts, including tendrils, petioles and main stem, most oospores were produced at 20°C. At 10°C, a few oospores were found in stem parts but none in leaflet lesions. At 5° C, no oospores were formed at all. In pods, moe oospores were produced at 15 and 20°C than at 10°C, but the numbers of oospores was smaller than in the other plant parts. Oospores formed at lower temperatures were larger than those formed at higher temperatures. At 20°C, similar oospore densities were found in leaflet lesions of three cultivars widely differing in resistance to downy mildew.     

Influence of host resistance and phenology on South American leaf blight of the rubber tree with special consideration of temporal dynamics

South American leaf blight (SALB), the most dangerous disease of the rubber tree, is responsible for the lack of significant natural rubber production in South America and is a major threat to rubber tree plantations in Asia and Africa. Although the selection of resistant clones is the preferred disease control method, greater knowledge is required of the relationship between host and pathogen, in order to construct more durable resistance. Based on small-scale trials, this study set out to compare the dynamics of SALB on two highly susceptible and one moderately susceptible clone and to analyse the effect of host phenology on disease severity, at leaflet and flush scales. Clonal resistance was found to have a noticeable effect on disease severity, asexual sporulation and stromatal density at both leaflet and flush levels, and on disease dynamics at a leaflet level; time for symptom and sporulation appearance were longer on the moderately susceptible clone than on the susceptible clones. On the moderately susceptible clone, the stromatal density was largely dependent on disease severity. The phenology did not differ among the three clones and could not be considered as a factor in genetic resistance to SALB. However, for the three clones, the position of the leaflet in the flush affected the duration of the immature stages and the disease: the shorter the duration of leaflet development, the lower the disease severity, the sporulation intensity and the stromatal density.     

麦瘟病研究新进展

麦瘟病是1985年在南美洲出现的一种小麦真菌病害,目前已成为南美洲的主要小麦病害之一。在麦瘟病发病严重区域该病害可导致小麦减产高达100%,抗麦瘟病的小麦资源和基因少,且麦瘟病菌对甲氧基丙烯酸酯(QoIs)类杀菌剂具有较强的耐受性,监测、延缓、抑制其传播对有效控制麦瘟病意义重大。2016年和2018年发生了两次重要的麦瘟病扩散事件,受到广泛关注,两次扩散事件导致麦瘟病分别蔓延至亚洲孟加拉国和非洲赞比亚,麦瘟病的持续蔓延对全球小麦生产安全带来严重威胁。我国科学家与世界同行一道,在病害监测、小麦抗病资源和基因发掘、抗病品种的培育、病原菌生物学等方面的研究均取得了进展。本文从麦瘟病的传播历史、防控方法、国际国内开展麦瘟病相关研究工作进展等方面对麦瘟病进行综述;同时,也为我国应对麦瘟病威胁提供了策略、提出了应对建议。     

桑枝枯菌核病的发病规律研究

桑枝枯菌核病是近年来在江苏发现的一种新的桑树病害,该病主要危害春季一年生枝条上已萌发的桑芽及新梢,常造成桑芽枯萎和病芽上方的枝条干枯死亡。经鉴定,病原菌为子囊菌亚门,核盘菌属真菌[Sclerotina sclerotiorum(Libert)de Bary]。病原菌在桑芽或新梢的寄生部位能形成黑色菌核。菌核随采叶、伐条等田间操作而掉入土中越冬越夏,成为翌年的初侵染源。病害发生与桑树品种关系密切,育2号为易感病品种。桑园间作是造成病害交互感染的重要原因。     

Spatial characteristics of white mould epidemics and the development of sequential sampling plans in Australian bean fields

To improve sampling efficiency and precision in the assessment of white mould (caused by Sclerotinia sclerotiorum) disease incidence on bean (Phaseolus vulgaris), the spatial characteristics of epidemics were characterized in 54 linear transects in 18 bean fields during 2008–2010 in northern Tasmania, Australia. The incidence of diseased pods and plants was assessed prior to harvest. Distributional and correlation‐based analyses indicated the incidence of diseased pods was characterized by a largely random pattern at the individual plant scale, with some patches of similar disease levels on pods occurring at a scale of 1·5 m or greater. Collectively, these results suggested epidemics may be dominated by localized sources of inoculum. Sequential sampling approaches were developed to estimate or classify disease incidence above or below provisional thresholds of 3, 5 and 15% incidence on pods near harvest. Achieving prespecified levels of precision by sequential estimation was possible only when disease incidence on pods was greater than approximately 4% and sampling was relatively intense (i.e. 10 pods evaluated on each of at least 64 plants). Using sequential classification, correct decisions on disease status were made in at least 95% of independent validation datasets after assessment of only 10·1–15 plants, depending on classification threshold and error rates. Outcomes of this research provide the basis for implementing more efficient sampling and management strategies for this disease in Australian fields.