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1.
ABSTRACT Eradication of Asiatic citrus canker (ACC) has become increasingly difficult over the last decade, following the introduction of the Asian leafminer into Brazil and Florida, which has led to changes in the eradication protocols. The present study, undertaken in Brazil, was aimed at characterizing the spatial patterns of ACC in commercial citrus plantings to gain better understanding of the dynamics of the disease subsequent to introduction of the leafminer. The spatial patterns of ACC were mapped in 326 commercial citrus plantings and statistically assessed at various spatial dimensions. The presence of "within-group" aggregation in each plot was examined via beta-binomial analysis for groups of trees parsed into three-by-three-tree quadrats. The relative intensity of aggregation was expressed as a binomial index of dispersion (D) and heterogeneity among plots expressed as the intracluster correlation coefficient, rho. The population of data sets was found to fall into three D categories, D < 1.3, 1.3 3.5. These categories then were related to other spatial characteristics. The binary form of Taylor's power law was used to assess the overdispersion of disease across plots and was highly significant. When the overall population of plots was parsed into D categories, the Taylor's R (2) improved in all cases. Although these methods assessed aggregation well, they do not give information on the number of foci or aggregations within each plot. Therefore, the number of foci per 1,000 trees was quantified and found to relate directly to the D categories. The lowest D category could be explained by a linear relationship of number of foci versus disease incidence, whereas the higher two categories were most easily explained by a generalized beta function for the same relationship. Spatial autocorrelation then was used to examine the spatial relationships "among groups" composed of three-by-three-tree quadrats and determine common distances between these groups of ACC-infected trees. Aggregation was found in >84% of cases at this spatial level and there was a direct relationship between increasing D category and increasing core cluster size, and aggregation at the among-group spatial hierarchy was generally stronger for the within-row than for the across-row orientation. Clusters of disease were estimated to average between 18 and 33 tree spaces apart, and the presence of multiple foci of infection was commonplace. The effectiveness of the eradication protocol of removing all "exposed" trees within 30 m surrounding each "ACC-infected tree" was examined, and the distance of subsequent infected trees beyond this 30-m zone from the original focal infected tree was measured for each plot. A frequency distribution was compiled over all plots to describe the distance that would have been needed to circumscribe all of these outliers as a theoretical alternative protocol to the 30-m eradication protocol. The frequency distribution was well described by a monomolecular model (R(2) = 0.98) and used to determine that 90, 95, and 99% of all newly infected trees occurred within 296, 396, and 623 m of prior-infected trees in commercial citrus plantings, respectively. These distances are very similar to previously reported distances determined for ACC in residential settings in Florida. 相似文献
2.
The spatial spread of citrus tristeza virus (CTV) disease was studied using data from two infected groves. Statistical analysis indicated a spatial grouping of infected trees. A gamma distribution was fitted to the empirical distribution of the distances between nearest infected neighbors. The findings suggest that, after locating a newly infected tree, additional inspections to detect CTV should be carried out mainly on trees within 14 m of that tree. 相似文献
3.
M. B. Spósito L. Amorim R. B. Bassanezi A. Bergamin Filho B. Hau 《Plant pathology》2008,57(1):103-108
Guignardia citricarpa , the causal agent of citrus black spot, forms airborne ascospores on decomposing citrus leaves and water-spread conidia on fruits, leaves and twigs. The spatial pattern of diseased fruit in citrus tree canopies was used to assess the importance of ascospores and conidia in citrus black spot epidemics in São Paulo State, Brazil. The aggregation of diseased fruit in the citrus tree canopy was quantified by the binomial dispersion index ( D ) and the binary form of Taylor's Power Law for 303 trees in six groves. D was significantly greater than 1 in 251 trees. The intercept of the regression line of Taylor's Power Law was significantly greater than 0 and the slope was not different from 1, implying that diseased fruit was aggregated in the canopy independent of disease incidence. Disease incidence ( p ) and severity ( S ) were assessed in 2875 citrus trees. The incidence-severity relationship was described ( R 2 = 88·7%) by the model ln( S ) = ln( a ) + b CLL( p ) where CLL = complementary log-log transformation. The high severity at low incidence observed in many cases is also indicative of low distance spread of G. citricarpa spores. For the same level of disease incidence, some trees had most of the diseased fruit with many lesions and high disease severity, whereas other trees had most of the fruit with few lesions and low disease severity. Aggregation of diseased fruit in the trees suggests that splash-dispersed conidia have an important role in increasing the disease in citrus trees in Brazil. 相似文献
4.
G. J. Silva‐Junior M. B. Spósito D. R. Marin P. J. Ribeiro‐Junior L. Amorim 《Plant pathology》2014,63(3):519-529
Citrus postbloom fruit drop (PFD) is caused by Colletotrichum acutatum and C. gloeosporioides. These pathogens attack the flowers and cause premature fruit drop and the retention of fruit calyces. This study was designed to characterize the spatial and temporal dynamics of PFD in commercial citrus‐growing areas to better understand the disease spread. Experiments were carried out in three young orchards (500 trees each) in two municipalities in Sao Paulo State, Brazil. Symptoms of PFD on the flowers and presence of persistent calyces were assessed in each of three orchards for three years. Logistic, Gompertz and monomolecular models were fitted to the incidence data over time from the trees with symptoms. The spatial pattern of diseased trees was characterized by a dispersion index and by Taylor′s power law. An autologistic model was used for the spatiotemporal analysis. The logistic model provided the best fit to the disease incidence data, which had a fast progress rate of 0·53 per day. During the early epidemic of PFD, the spatial pattern of diseased trees was random, which suggested that inoculum spread was due to mechanisms other than rain splash. As the disease incidence increased (up to 12·6%), the spatial pattern of diseased trees became aggregated. The rapid rate of disease progress and the distribution of PFD suggest that dispersal of the pathogen is possibly related to a mechanism other than splash dispersal, which is more typical of other fruit diseases caused by Colletotrichum spp. 相似文献
5.
ABSTRACT Aphid vector species population composition is known to affect the spatial patterns of citrus tristeza virus (CTV) and the changes in these patterns over time. However, the biological processes that are associated with virus spread have not been well defined. The spatiotemporal dynamics of CTV were examined using data collected from research plots in the Dominican Republic and Costa Rica, where the brown citrus aphid (BCA), Toxoptera citricida, was the predominant species, and in Florida, where the BCA was absent and the melon aphid, Aphis gossypii, was the predominant vector. Data were analyzed using a spatiotemporal stochastic model for disease spread, and parameter values were evaluated using Markov chain Monte Carlo stochastic integration methods. Where the melon aphid was the dominant species, the model parameter likelihood values supported the hypothesis that the disease was spread through a combination of random background transmission (transmission originating from inoculum sources outside the plot) and a local interaction (transmission from inoculum sources within the plot) operating over short distances. Conversely, when BCA was present, results often suggested a local short-range transmission interaction that was not restricted to nearest-neighbor interactions and that the presence of background infection was not necessary to explain the observations. 相似文献
6.
ABSTRACT Since its first detection in southwest Washington state 30 years ago, eastern filbert blight, caused by Anisogramma anomala, has spread slowly southward ( approximately 2 km/year) into the Willamette Valley of Oregon, an important hazelnut production region. Experiments were conducted to measure gradients of disease spread, rates of disease increase as affected by distance from an inoculum source and variation in host plant resistance, and dispersal of ascospores of A. anomala from diseased orchards. In each of 3 years, 1-year-old hazelnut trees placed from 0 to 150 m north of diseased orchards were infected uniformly and slopes of disease gradients were not significantly different from zero. In 1 year when trees also were placed south of an orchard, the disease gradient was significant (P < 0.05), with disease incidence high at the edge of the orchard and few trees infected at 10 m south of the orchard. Disease gradients were shallower and the magnitude of the area under the disease progress curve (AUDPC) greater in 0.1-ha mini-orchards of highly susceptible cv. Ennis than in mini-orchards of moderately susceptible cvs. Barcelona or Casina. Lower AUDPC values were observed in mini-orchards of Barcelona interplanted with a moderately resistant pollenizer Hall's Giant compared with the highly susceptible pollenizer Daviana. Fungicides applied biweekly starting at bud break reduced AUDPC values in Ennis mini-orchards to values observed in Barcelona and Casina mini-orchards. Data from aspirated spore samplers placed on towers adjacent to severely diseased hazelnut orchards indicated that spores of A. anomala dispersed horizontally and vertically away from the canopy during periods of extended branch wetness and, thus, show potential to be transported long distances in wind currents. Weather patterns in the Pacific Northwest may account for the relatively slow, southward spread of eastern filbert blight within Oregon's Willamette Valley. Of 196 precipitation events greater than 10 h in duration recorded from 1974 to 1995, conditions most favorable for ascospores discharge, periods with wind from the north were rare, representing <6% of total hours. 相似文献
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ABSTRACT Citrus tristeza virus (CTV) was monitored for 4 years by monoclonal antibody probes via enzyme-linked immunosorbent assay in four citrus orchards in northern Costa Rica and four orchards in the Dominican Republic following the introduction of the brown citrus aphid, Toxoptera citricida. The Gompertz nonlinear model was selected as the most appropriate in most cases to describe temporal increase of CTV. Ordinary runs analysis for association of CTV-positive trees failed to show a spatial relationship of virus status among immediately adjacent trees within or across rows. The beta-binomial index of dispersion for various quadrat sizes suggested aggregations of CTV-positive trees for all plots within the quadrat sizes tested. Spatial autocorrelation analysis of proximity patterns suggested that aggregation often existed among quadrats of various sizes up to four lag distances; however, significant lag positions discontinuous from the main proximity pattern were rare. Some asymmetry was also detected for some spatial autocorrelation proximity patterns. These results were interpreted to mean that, although CTV-positive trees did not often influence immediately adjacent trees, virus transmission was common within a local area of influence that extended two to eight trees in all directions. Where asymmetry was indicated, this area of influence was somewhat elliptical. The spatial and temporal analyses gave some insight into possible underlying processes of CTV spread in the presence of T. citricida and suggested CTV spread was predominantly to trees within a local area. Patterns of longer-distance spread were not detected within the confines of the plot sizes tested. Longer-distance spread probably exists, but may well be of a complexity beyond the detection ability of the spatial analysis methods employed, or perhaps is on a scale larger than the dimensions of the plots studied. 相似文献
9.
L. Batista K. Velázquez I. Estévez I. Peña D. López M. L. Reyes D. Rodríguez F. F. Laranjeira 《Plant pathology》2008,57(3):427-437
The spatiotemporal dissemination of Citrus tristeza virus (CTV) was evaluated by DASI-ELISA in orange and grapefruit fields of six citrus producing regions in Cuba, and aphid populations were evaluated in two selected areas. The aphid species found in these areas were Toxoptera citricida (the most efficient vector of CTV), T. aurantii and Aphis spiraecola . A logistic model was the most appropriate to explain the temporal increase in the proportion of CTV-infected plants for almost all the fields. Although nearly all areas showed an increase in this proportion, there were regions of low CTV spread, which was slower in grapefruit than in orange fields. 2DCORR analysis indicated spatial dependence among immediately adjacent trees, higher in the within-row direction than in the across-row direction. Aggregation was also detected by dispersion index and binary power law fitting within quadrats of all sizes. For most fields, autocorrelation analysis showed a significant edge effect and spatial dependence among infected plants of different sub-areas. Based on these results a new tristeza management strategy was proposed for each region of the country. 相似文献
10.
Eunice Golda Danièle Ndo Faustin Bella-Manga Sali Atanga Ndindeng Michel Ndoumbe-Nkeng Ajong Dominic Fontem Christian Cilas 《European journal of plant pathology / European Foundation for Plant Pathology》2010,128(3):385-397
In Cameroon and most countries of tropical Africa where Phaeoramularia leaf and fruit spot disease of citrus is reported, a total yield loss of 50–100% is common. For effective control of this
disease, it is imperative to understand how it originates, and which factors contribute to its development. To this end, the
environmental and biological characteristics of each tree from 39 sites located in 13 citrus production basins in the humid
zones of Cameroon were collected by means of a survey. Information was collected by interviewing heads of households, and
by visual inspection of trees and their environment. The independent variable was severity of the disease while the dependent
variables were environmental and biological characteristics. Climatic characteristics varied from one basin to another. The
13 citrus production basins were regrouped in five categories based on disease severity. Altitude, tree species and soil type
were the main factors influencing the disease severity. Thus the higher is the altitude, the more important is the disease
severity. Also, disease severity increased with increasing number of grape fruit, orange and pummelo trees. However, disease
severity was lower on trees growing on volcanic soils as compared with the other soil types. Further analysis of these factors
could lead to the development of a risk assessment model for Phaeoramularia leaf and fruit spot of citrus. 相似文献
11.
ABSTRACT The spatial pattern of Sharka disease, caused by Plum pox virus (PPV) strain M, was investigated in 18 peach plots located in two areas of southern France. PPV infections were monitored visually for each individual tree during one to three consecutive years. Point pattern and correlation-type approaches were undertaken using the binary data directly or after parsing them in contiguous quadrats of 4, 9, and 16 trees. Ordinary runs generally revealed a low but variable proportion of rows with adjacent symptomatic trees. Aggregation of disease incidence was indicated by the theta parameter of the beta-binomial distribution and related indices in 15 of the 18 plots tested for at least one assessment date of each. When aggregation was detected, it was indicated at all quadrat sizes and tended to be a function of disease incidence, as shown by the binary form of Taylor's power law. Spatial analysis by distance indices (SADIE) showed a nonrandom arrangement of quadrats with infected trees in 14 plots. The detection of patch clusters enclosing quadrats with above-average density of symptomatic trees, ellipsoidal in shape and generally extending from 4 to 14 trees within rows and from 4 to 10 trees perpendicular to the rows, could be interpreted as local areas of influence of PPV spread. Spatial patterns at the plot scale were often characterized by the occurrence of several clusters of infected trees located up to 90 m apart in the direction of the rows. When several time assessments were available, increasing clustering over time was generally evidenced by stronger values of the clustering index and by increasing patch cluster size. The combination of the different approaches revealed a wide range of spatial patterns of PPV-M, from no aggregation to high aggregation of symptomatic trees at all spatial scales investigated. Such patterns suggested that aphid transmission to neighboring trees occurred frequently but was not systematic. The mechanism of primary virus introduction, the age and structure of the orchards when infected, and the diversity of vector species probably had a strong influence on the secondary spread of the disease. This study provides a more complete understanding of PPV-M patterns which could help to improve targeting of removal of PPV-infected trees for more effective disease control. 相似文献
12.
ABSTRACT The structure of Heterobasidion annosum populations was studied in 15 mixed-conifer sites in central and northern California. Study sites displayed mortality of white fir trees in enlarging discrete patches (mortality centers). At each site, fungal genotypes were defined by somatic compatibility tests. In two sites, further genetic and molecular analyses were performed on field genotypes and on homokaryons obtained by dedikaryotization of field heterokaryons. Isolates were found to be colonizing mostly the roots and the bole sapwood of white fir trees, and no significant infections of other tree species were observed. Each mortality center was characterized by the presence of several fungal genotypes, all belonging to the S intersterility group. Both homokaryotic and heterokaryotic strains were present in all sites. Multiple genotypes were retrieved in individual trees or stumps. Out of 228 fungal genotypes, 86% were found only within a single tree or stump, while 14% had spread to adjacent trees. The two largest genotypes had diameters of 9 and 10 m, and had colonized five and nine trees, stumps, or both, respectively. The maximum distance between two adjacent trees colonized by the same genotype was 6 m, and a highly significant correlation was found between tree diameter and distance of fungal "vegetative" spread. The largest clones were found in areas characterized by high tree and stump densities, and secondary spread of the fungus was more significant in denser stands. In most cases, original infection courts of existing genotypes could be traced to standing trees and not to stumps. The genetic analysis performed in two mortality centers revealed that most local genotypes had different mating alleles, and thus originated from unrelated basidiospores. In a few cases, the same mating allele was shared by two heterokaryons (n+n genome) or by a homokaryon (n genome) and a heterokaryon. Molecular analysis showed that nuclei bearing the same mating allele were identical, providing evidence that the two nuclei forming heterokaryons can act independently in the field and can be shared among isolates, presumably via di-mon mating or by separate matings of different portions of widespread homokaryons. 相似文献
13.
ABSTRACT We evaluated the impact of roguing on the spread and persistence of the aggressive Plum pox virus strain M (PPV-M) in 19 peach orchard blocks in Southern France. During a 7- to 10-year period, orchards were visually inspected for PPV symptoms, and symptomatic trees were removed every year. Disease incidence was low in all orchards at disease discovery and was <1% in 16 of the 19 orchard blocks. The spread of Sharka disease was limited in all 19 blocks, with an annual disease incidence between 2 and 6%. However, new symptomatic trees were continuously detected, even after 7 to 10 years of uninterrupted control measures. An extended Cox model was developed to evaluate to what extent tree location, orchard characteristics, environment, and disease status within the vicinity influenced the risk of infection through time. Eleven variables with potential effect on tree survival (i.e., maintenance of a tree in a disease- free status through time) were selected from survey data and databases created using a geographical information system. Area of the orchard, density of planting, distance of a tree from the edge of the orchard block sharing a boundary with another diseased orchard, and distance to the nearest previously detected symptomatic tree had a significant effect on the risk for a tree to become infected through time. The combined results of this study suggest that new PPV-M infections within orchards subjected to roguing resulted from exogenous sources of inoculum, disease development of latent infected trees, as well as infected trees overlooked within the orchards during visual surveys. A revision of the survey and the roguing procedures used for more effective removal of potential sources of inoculum within the orchards and in the vicinity of the orchards would improve disease control suppression of PPV. 相似文献
14.
McLedwaba Nkgobe Baby Phahladira Ronel Viljoen Gerhard Pietersen 《European journal of plant pathology / European Foundation for Plant Pathology》2012,134(1):39-47
Recent studies in citrus orchards confirmed that Citrus Greening, a heat sensitive citrus disease, similar to Huanglongbing (HLB), is associated with the presence of ??Candidatus Liberibacter africanus?? (Laf) in South Africa. Neither ??Candidatus Liberibacter asiaticus?? (Las), associated with HLB, ??Candidatus Liberibacter americanus??, nor ??Candidatus Liberibacter africanus ssp. capensis?? (LafC), previously detected in the Western Cape, South Africa on an indigenous Rutaceous species, Calodendrum capense (L. f.) Thunb. (Cape Chestnut), were detected in citrus. The current study aims to determine the potential role of C. capense in the epidemiology of Citrus Greening in South Africa and whether LafC poses a risk to citriculture. A total of 278?C. capense samples were collected throughout South Africa and tested for Liberibacters using real-time PCR. While LafC was found in 100 samples, distributed from all areas where collected, no evidence of Laf infection in any sample was found . The identity of the LafC present was confirmed by sequencing the amplicon derived from conventional PCR of the ?-operon of the ribosomal protein gene region of the first 17 infected trees found and of a representative sample from each district. The Liberibacter status of 44?C. capense and 272 citrus (Midnight Valencia) trees growing in close proximity to each other for over 15?years was determined. Out of 44?C. capense specimens, 43 were infected with LafC, but none of the citrus trees were infected with LafC. Based on the results of this it appears that natural spread of LafC to citrus does not occur. 相似文献
15.
2018年—2019年,采用田间小区试验评价了噻虫胺4种剂型(30%悬浮剂、18%包埋颗粒剂、13.5%壳聚糖微球和0.5%颗粒剂)及2种施用方式(喷雾和沟施)对柑橘木虱的防治效果。结果表明:叶面喷施30%噻虫胺悬浮剂60 mg/kg处理对幼年树及成年树上柑橘木虱的控制效果好且持效期约30 d,而药后46~96 d的防治效果大幅下降,难以控制其为害。在幼年树根部周围拌毒土沟施时,噻虫胺18%包埋颗粒剂和13.5%壳聚糖微球0.1~0.2 g/株处理对柑橘木虱的控制效果较好,且持效期长达96 d;0.5%噻虫胺颗粒剂0.1 g/株处理对柑橘木虱的效果好且持效期达46 d,而药后66~96 d的防治效果下降明显,难以控制其为害;在0.1 g/株剂量下,0.5%噻虫胺颗粒剂药后7~30 d的效果最好、18%包埋颗粒剂次之、13.5%壳聚糖微球稍差,而18%包埋颗粒剂药后46~96 d的效果最佳、13.5%壳聚糖微球次之、0.5%颗粒剂稍差。在成年树根部周围沟施时,噻虫胺18%包埋颗粒剂、13.5%壳聚糖微球0.6~0.8 g/株处理对柑橘木虱的控制效果好且持效期长达89 d,而0.5%噻虫胺颗粒剂0.2 g/株处理的效果较好且持效期长达89 d;在0.2 g/株剂量下,不同剂型噻虫胺对柑橘木虱的防效差异与幼年树基本一致。此外,当剂型、剂量及施用方式均相同时,噻虫胺对柑橘木虱的控制作用略低于噻虫嗪。综上所述,在柑橘生产实践中,若田间柑橘木虱发生量大,应喷施1次30%噻虫胺悬浮剂60 mg/kg;为有效控制幼年树柑橘木虱为害,应于初发期拌毒土沟施1次,使用药剂为18%噻虫胺包埋颗粒剂,用量为0.2 g/株;针对成年树柑橘木虱,则需将其用量增至0.6 g/株。 相似文献
16.
Ed Etxeberria Pedro Gonzalez Diann Achor Gene Albrigo 《Physiological and Molecular Plant Pathology》2009,74(1):76-83
The citrus disease Huanglongbing (HLB or citrus greening) is characterized, among other symptoms, by extraordinary levels of starch accumulation in leaves. This condition denotes imbalances in carbohydrate source sink relationship which in turn may have direct implications in the overall health of HLB-trees and in future strategies to manage the disease. Using light, scanning, and transmission electron microscopy we investigated the extent of carbohydrate partitioning imbalances throughout the tree. In all aerial tissues, starch accumulation in HLB-affected trees far exceeded that of HLB-negative control trees. Starch accumulated extensively in photosynthetic cells as well as phloem elements and vascular parenchyma in leaves and petioles. In stems, starch was commonly observed in xylem parenchyma and in the phelloderm of HLB-affected trees but absent from control samples. In contrast, roots from HLB-affected trees were depleted of starch whereas roots from control trees contain substantial starch deposits. The data supports the notion that the substantial changes in carbohydrate partitioning observed throughout the citrus tree may not only be a result of HLB infection, but in itself, a cause for the rapid decline and death of infected trees. 相似文献
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柑桔黄龙病在我国北部产区及南部高山发生得很轻微或完全不发生。在南部的重病区中,由于地势、防风林、间作果树等所造成的遮蔭条件也不利于病害的发生。大气候和小气候与黄龙病的相关有多种可能的原因。但首先应当研究它们对病毒传播媒介和侵染后发病的影响。用病芽作为接穗所长出的新梢在相当长时间内没有病状。最容易发病的是着果很多的幼树。初期发病的柑桔树通过重修剪和靠接辅助砧木的方法治疗有成功的实例。这些现象都可以从树冠树根比例与发病的关系来解释。柑桔类的种和品种间显然存在着耐病力的差异。这种差异也可能存在于同一品种的不同株系和个体间,因为用病区老树的接穗比幼树的接穗育成的苗木发病率较低。在黄龙病传播和流行上的许多不一致的现象还可能决定于病毒浓度和株系间毒力的差异及其相互关系。上述问题的研究可能导致栽培免疫、品种免疫和人工接种免疫方法的应用。 相似文献
19.
Renato Beozzo Bassanezi Luiz Henrique Montesino Eduardo Sanches Stuchi 《European journal of plant pathology / European Foundation for Plant Pathology》2009,125(4):565-572
Citrus huanglongbing (HLB), associated with Candidatus Liberibacter asiaticus and Ca. L. americanus and transmitted by the Asian psyllid Diaphorina citri, is the most serious disease of citrus worldwide because of crop devastation and difficulty to control. Since 2004, approximately 3 million trees were eliminated in attempts to limit its spread in Brazil. Where HLB becomes endemic, the disease progression in the orchard and the increasing symptom severity throughout the tree canopy can be relatively fast, greatly reducing the economic life of affected orchards because of tree decline and yield reduction. Although the majority of the fruit from symptomatic branches drop before harvest, a significant amount of affected fruit remain attached, are available for harvest, and can affect juice quality. To quantify and compare the effects of HLB on fruit quality of the most important sweet orange cultivars grown in São Paulo State, 4–6 year-old sweet orange trees from 26 blocks (two of ‘Valencia Americana’, eight of ‘Hamlin’, four of ‘Westin’, seven of ‘Pera’, and five of ‘Valencia’) were selected prior to harvest. In each block, 14–21 HLB-symptomatic trees were chosen. In each tree, the quality of 20 fruit normal in appearance from asymptomatic branches and 20 symptomatic fruit from symptomatic branches were assessed. In general, compared to normal fruit, the symptomatic fruit were small, light, more acidic, and had lower juice percentage, Brix, total soluble solids per box, total soluble solids per fruit, and Brix/acidity ratio. These effects of fruit quality were less pronounced on early and mid season sweet orange cultivars than on late season cv. Valencia. 相似文献
20.
Microcyclus ulei, the fungus causing South American leaf blight (SALB) on rubber tree leaves, produces two main types of spores: ascospores and conidia. To assess their respective epidemiological role, a field experiment was conducted in French Guiana over 3 years. Tree phenology, disease severity and climate variables were recorded while airborne spores were trapped and quantified. Ascospores were shown to play an essential role in the perpetuation of the disease outside the host's growth periods, in the resumption of epidemics, and in the spread of the disease to disease‐free zones. Conidia were trapped in visibly infected plots only, during periods of host growth. Disseminated over short distances and present only temporarily on leaves, the conidia enabled the disease to spread stepwise when the climate was conducive. Segmentation analysis revealed that the duration of high relative humidity was the climatic variable most related to ascospore trapping. Ascospore release did not require low temperatures. Considering the essential role of the ascospores in the initiation and spread of disease, artificial defoliation as a means of reducing the inoculum pressure during tree refoliation is proposed to control SALB. To check the validity of this method, a survey over several years of natural defoliation–refoliation in relation to climate and other leaf diseases is needed. 相似文献