R. B. Bassanezi

Spatial and Temporal Analyses of Citrus Sudden Death as a Tool to Generate Hypotheses Concerning Its Etiology

ABSTRACT Citrus sudden death (CSD), a new disease of unknown etiology that affects sweet orange grafted on Rangpur lime, was visually monitored for 14 months in 41 groves in Brazil. Ordinary runs analysis of CSD-symptomatic trees indicated a departure from randomness of symptomatic trees status among immediately adjacent trees mainly within rows. The binomial index of dispersion (D) and the intraclass correlation (k) for various quadrat sizes suggested aggregation of CSD-symptomatic trees for almost all plots within the quadrat sizes tested. Estimated parameters of the binary form of Taylors power law provided an overall measure of aggregation of CSD-symptomatic trees for all quadrat sizes tested. Aggregation in each plot was dependent on disease incidence. Spatial autocorrelation analysis of proximity patterns suggested that aggregation often existed among quadrats of various sizes up to three lag distances; however, significant lag positions discontinuous from main proximity patterns were rare, indicating a lack of spatial association among discrete foci. Some asymmetry was also detected for some spatial autocorrelation proximity patterns, indicating that within-row versus across-row distributions are not necessarily equivalent. These results were interpreted to mean that the cause of the disease was most likely biotic and its dissemination was common within a local area of influence that extended to approximately six trees in all directions, including adjacent trees. Where asymmetry was indicated, this area of influence was somewhat elliptical. Longer-distance patterns were not detected within the confines of the plot sizes tested. Annual rates of CSD progress based on the Gompertz model ranged from 0.37 to 2.02. Numerous similarities were found between the spatial patterns of CSD and Citrus tristeza virus (CTV) described in the literature, both in the presence of the aphid vector, Toxoptera citricida. CSD differs from CTV in that symptoms occur in sweet orange grafted on Rangpur lime. Based on the symptoms of CSD and on its spatial and temporal patterns, our hypothesis is that CSD may be caused by a similar but undescribed pathogen such as a virus and probably vectored by insects such as aphids by similar spatial processes to those affecting CTV.

Elaboração e validação de escala diagramática para avaliação da severidade da mancha preta em frutos cítricos

Uma escala diagramatica, abrangendo os dois tipos prevalentes de sintomas de mancha preta em frutos citricos (Citrus spp.), os de mancha dura e de falsa melanose, foi desenvolvida para padronizar a avaliacao da severidade da doenca. A escala foi elaborada considerando os limites maximos e minimos de severidade da doenca observados no campo. Os valores intermediarios seguiram incrementos logaritmicos para os sintomas do tipo mancha dura (0,5; 1,7; 5,0; 11,5; 22,5 e 49,0%) e do tipo falsa melanose (1,1; 4,5; 15,0; 31,0; 53,0 e 68,0%). Para a validacao da escala, seis avaliadores quantificaram a severidade da doenca a partir das imagens digitalizadas de 50 frutos com diferentes niveis de doenca. Inicialmente, a estimativa da severidade foi feita sem auxilio da escala. Em seguida, os mesmos avaliadores, utilizando a escala diagramatica proposta, estimaram a severidade nos mesmos frutos avaliados anteriormente. As avaliacoes com a escala diagramatica foram mais precisas e acuradas nas estimativas de todos os avaliadores e proporcionaram maior reprodutibilidade entre avaliacoes de diferentes avaliadores. A escala diagramatica proposta foi considerada adequada para estimar a severidade da mancha preta nos frutos e sera usada em estudos epidemiologicos e de avaliacao de estrategias de controle desta doenca.

Escalas diagramáticas para avaliação da severidade do cancro cítrico

Diagrammatic scales are important tools for disease severity assessment. Four diagrammatic scales for isolated small (SL), medium (ML), and large (LL) lesions and for symptoms associated with the leaf miner injuries (LM), were developed to standardize the severity assessments of citrus canker caused by Xanthomonas axonopodis pv. citri on leaves of citrus (Citrus sinensis). Each scale has eight levels of disease severity (percentage of diseased leaf area): 0.2 to 16% for SL; 0.6 to 25% for ML; 1.8 to 30% for LL and 0.5 to 30% for LM. Initially, six persons evaluated the severity of 447 digitalized images of symptomatic leaves using the four scales. Training was carried out and later on, disease severity was assessed in 115 new images. Linear regressions between actual and estimated disease severity were calculated by each person. All the scales were validated together considering the accuracy, precision and reproducibility of the evaluations. The scales were adequate to quantify the severity of citrus canker on leaves.

Yield loss caused by huanglongbing in different sweet orange cultivars in São Paulo, Brazil

Citrus huanglongbing (HLB) reduces an affected orchard’s economic life. This work aimed to characterize yield loss due to HLB for different sweet orange cultivars and determine the relationship between disease severity and yield. Disease severity and yield were assessed on 949 individual trees distributed in 11 different blocks from sweet orange cultivars Hamlin, Westin, Pera and Valencia. In each block, plants showing a range of HLB severity levels and asymptomatic plants were selected. Total yield (weight of harvested fruit), mean weight of asymptomatic and symptomatic fruit, relative yield (symptomatic tree yield/mean yield of asymptomatic trees from the same block) and relative number of fruits (fruit number from symptomatic tree/mean number of fruits from asymptomatic trees from the same block) were determined. The weight of symptomatic fruit was lower than the weight of asymptomatic fruit, but the weights of asymptomatic and symptomatic fruit were not correlated with disease severity, indicating that the effects of HLB were restricted to symptomatic branches. The relationship of the relative yield with HLB severity can be satisfactorily described by a negative exponential model. The rates of yield decrease as a function of disease severity were similar for all assessed cultivars. A relative yield (up to 19%) was observed even for trees where disease severity was 100%. The strong linear relationship between relative number of fruits per tree and the relative yield per tree suggested that the yield reduction was due primarily to early fruit drop or lack of fruit set on affected branches.

Base científica para a erradicação de plantas sintomáticas e assintomáticas de Huanglongbing (HLB, Greening) visando o controle efetivo da doença

Instituto Agronomico do Parana - IAPAR, 86001-970, Londrina, PR, BrasilAutor para correspondencia: Jose Belasque Junior, e-mail: belasque@fundecitrus.com.br1. IntroducaoHuanglongbing (HLB), ou Greening, e a doenca mais importante e destrutiva da citricultura mundial (Bove, 2006; Gottwald et al., 2007a). Presente de forma endemica nos continentes asiatico e africano ha varias decadas, essa doenca foi recentemente detectada nos dois principais paises produtores de citros, Brasil e Estados Unidos (Colleta Filho et al., 2004; Halbert, 2005; Teixeira et al., 2005). O Brasil detem 30% da producao mundial de laranja e 59% da producao de suco de laranja (Neves et al., 2007). Sao Paulo e Florida dominam a oferta mundial, um caso raro em se tratando de commodities agricolas. O sistema agroindustrial citricola brasileiro movimenta anualmente R

Análise da dinâmica e estrutura de focos da morte súbita dos citros

9 bilhoes e gera mais de 400 mil empregos diretos e indiretos. Pelos danos que causa e pela dificuldade do seu manejo, o HLB coloca em risco esse importante setor do agronegocio brasileiro. Ha necessidade de acoes conjuntas urgentes envolvendo citricultores, industrias processadoras e os governos federal e estaduais. Em 17 de Outubro de 2008 entrou em vigor a Instrucao Normativa n

Spatial pattern of trees affected by black spot in citrus groves in Brazil

Citrus Sudden Death (CSD) is a new and destructive disease that affects sweet oranges (Citrus sinensis) and some mandarins (C. reticulata) grafted on Rangpur lime (C. limonia). Its etiology and transmission mechanisms are still unknown, epidemiologic study of the disease was recently initiated. Dynamics and structure analysis of foci technique (DSAF) was used to characterize CSD dissemination on 166 spatial distribution maps of CSD symptomatic trees from 51 blocks of northern Sao Paulo State and the southern of Triângulo Mineiro. According to DSAF, the CSD epidemic begins with unitary foci randomly distributed in the block. At an incidence up to 2%, most of the foci were composed by an unique plant. Later, the disease progresses more by increasing the number of foci than by increasing the size of the focus. This was indicated by an increase in the number of foci up to 18% of disease incidence and by the reduced number of plants per focus (less than four plants per focus at 20% of incidence). In most cases (71.5%), foci expanded more within-row than across-row. Compactness reduction of foci with increase in CSD incidence suggests that larger foci are less compact and, thus, that disease dissemination does not occur in a homogenous and continuous way around the first affected tree. The observed patterns are similar to those of diseases caused by an infectious agent, which is transmitted by a vector.

The importance of primary inoculum and area-wide disease management to crop health and food security

Citrus black spot (CBS), caused by Guignardia citricarpa, is the most important fungal disease of orange trees in Brazil. The spatial pattern of CBS-symptomatic trees was evaluated using the binomial dispersion index (D), Ripleys K function (K), and a Monte Carlo test for minimum mean distance (d) to understand the distribution of the pathogen. Disease was monitored in 7,790 citrus trees from four commercial groves. In one grove, disease incidence was assessed from 1999 to 2001 and, in the others, disease assessments were conducted only in 2002. Infected trees were aggregated based on the three statistical analyses used (D, K, and d) regardless of the CBS incidence. The binomial index of dispersion (D) indicated aggregation of CBS-affected trees for all groves and for various quadrat sizes (2 by 2, 3 by 3, 4 by 4… up to 10 by 10). According to Ripleys K function, the dependence among symptomatic trees comprised two to three neighboring trees. Disease dispersion occurred at distances below 24.7 m according to the test for d. This suggests that the dispersion of inoculum is highly important over short distances. As a consequence, the required sample size to achieve a level of accuracy of C = 20% increases exponentially with the decrease in incidence of CBS below 15% infected plants.

Quantificação do efeito do crestamento bacteriano comum na eficiência fotossintética e na produção do feijoeiro

In some epidemics that have devastating consequences, the primary inoculum plays an important role in both epidemic onset and intensification. This article documents the dynamics of such epidemics, and illustrates their importance using two examples: Huanglongbing of citrus and begomoviruses of tomato. The latter disease is a major constraint to tomato production in Brazil, while the former has become a threat to global citrus production and farmers’ livelihoods. In spite of their importance little is known of the characteristics of these diseases and their management. This is because classical botanical epidemiology considers two types of diseases: polycyclic diseases, where the inoculum that causes infections is produced during the epidemic in or on individual plants that had been previously infected in the course of that epidemic; or monocyclic diseases, where inoculum that causes infection is not produced in or on individual plants that had been infected in the course of the epidemic, but in the soil, on secondary hosts, or in infected crop plants of the same host in other fields. Diseases of the first type typically present a logistic disease progress curve and management is based on reducing the rate of infection, whereas diseases of the second type present a monomolecular disease progress curve and management is based on reducing the initial inoculum. This article deals with plant diseases that depart in their structure and behaviour from these two archetypes, because they borrow elements from both. We address polycyclic diseases in which the primary inoculum has a continuous and dynamic role, and in which the secondary inoculum contributes to epidemic build-up, i.e., polycyclic diseases with continuous primary spread. This epidemiological structure generates less clear-cut disease progress curves, but usually follows a monomolecular dynamic. Our focus on this type of disease is multifold because (1) this more complex, combined, pattern is actually quite common, often leading to grave plant diseases epidemics, with impacts at the farm, community, and country scales, and (2) such epidemics are among the most difficult to manage. Our analysis leads us to assess past errors and current courses of action. It allows us to recognize, in addition to the conventional tools for management with local effects, the critical importance of collective action. Collective management action – at the farm, community, or national scales – is congruent with the characteristics of many epidemics, because they also entail properties at successive and nested scales. The management of such epidemics needs to address both the primary and secondary inoculum. More importantly, these actions have to be performed in an area-wide, regional basis in order to be effective.

Modelo autologístico espaço-temporal com aplicação à análise de padrões espaciais da leprose-dos-citros

The effect of common bacterial blight on photosynthetic efficiency and on common bean (Phaseolus vulgaris) yield was quantified. Two experiments were carried out during the wet season of 1997 at Piracicaba-SP using two cultivars, IAC-Carioca and Rosinha. Different levels of severity were obtained by varying the number of inoculations with the pathogen. Disease severity, assessed with a diagrammatic scale, did not show a significant linear relationship (P>0.01) with yield, while the healthy leaf area duration (HAD) was significantly (P£0.01) linear in relation to yield in both experiments (R2 between 0,66 and 0,78). Photosynthesis was related to diseased leaf area by the equation Px/P0=(1-x )b, where Px is the net photosynthesis of the leaf with severity x, P0 is the mean net photosynthesis of the healthy leaves, x is the disease severity, and b is the relation between virtual and visual lesion. The values of b, determined by non-linear regression, were 3.08±0.18 and 3.19±0.14 for IAC-Carioca and Rosinha, respectively. In general, the use of b did not significantly improve the HAD-yield relationship.