Lilian Amorim

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.

Angular leaf spot of Phaseolus beans : Relationships between disease, healthy leaf area, and yield

ABSTRACT Five field experiments were conducted to investigate the relationship between the severity of visible disease (X), area under the disease progress curve (AUDPC), healthy leaf area index on any given day (HLAI), radiation intercepted by healthy leaf area on any given day (HRI), healthy leaf area duration (HAD), total healthy leaf area absorption (HAA), and yield of Phaseolus beans, cultivars Rosinha and Carioca, inoculated with Phaeoisariopsis griseola at several doses. In general, yield was not related to disease severity (X) or AUDPC. In contrast, the highest yields were always related to the highest values of HAD and HAA. The relationship between yield and HAD was linear in each of five trials (29.9 < R(2) < 70.2%, P < 0.001). The relationship between yield and HAA was linear in four of the trials (52.3 < R(2) < 70.3%, P < 0.001) and exponential in one of them (in which the plant canopy was the largest). Singlepoint models using HRI to estimate yield at various times during the crop season were developed. The slope of the yield-HRI relationship proved to be stable (26.8 +/-2.4 g MJ(-1)), regardless of cultivar, locale, planting date, and bean growth stage (from R5 to R8). The yield-HLAI relationship proved to be less consistent. HRI is proposed as a key explanatory variable for a transportable system of disease management; it may be useful in producing precise recommendations at the farm level.

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.

Colletotrichum gloeosporioides, a new causal agent of citrus post-bloom fruit drop

Citrus post-bloom fruit drop (caused by Colletotrichum acutatum) frequently occurs in the southwestern region of São Paulo State, Brazil. A survey of Colletotrichum isolates associated with symptoms of post-bloom fruit drop in São Paulo State showed C. gloeosporioides in addition to C. acutatum. The objectives of this study were to confirm the identification of C. gloeosporioides isolated from symptomatic citrus flowers, to test the pathogenicity of C. gloeosporioides isolates, to compare the development of disease caused by C. gloeosporioides and C. acutatum, and to determine the frequency of C. gloeosporioides in a sample of isolates obtained from symptomatic flowers in different regions of São Paulo State. Through the use of species-specific primers by PCR, 17.3% of 139 isolates were C. gloeosporioides, and the remaining 82.7% were C. acutatum. The pathogenicity tests, carried out in 3-year old potted plants of sweet oranges indicated that both species caused typical symptoms of the disease including blossom blight and persistent calyces. Incubation periods (3.5 and 3.9 days, respectively, for C. acutatum and C. gloeosporioides) and fruit sets (6.7 and 8.5%, respectively for C. acutatum and C. gloeosporioides) were similar for both species. The incidences of blossom blight and persistent calyces were higher on plants inoculated with C. acutatum than in those inoculated with C. gloeosporioides. Conidial germination was similar for both species under different temperatures and wetness periods. Under optimal conditions, appressorium formation and melanisation were higher for C. gloeosporioides than for C. acutatum. These results indicated that Colletotrichum gloeosporioides is a new causal agent of post-bloom fruit drop.

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.

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

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.

Injuries caused by citrus leafminer (Phyllocnistis citrella) exacerbate citrus canker (Xanthomonas axonopodis pv. citri) infection

After the introduction of citrus leafminer in Sao Paulo State, an increase in the number of new plants infected with citrus canker has been observed. The interaction between these two organisms is known, but there is no information about how the leafminer damage intensifies citrus canker incidence and severity. The objectives of this paper were to evaluate the effects of leafminer damage in citrus canker infection and its influence on the monocyclic components of the disease on Citrus limonia. Higher incidence of diseased plants, AUDPC (area under the disease progress curve), disease severity and shorter incubation periods were observed in plants inoculated after insect infestation. These factors explain the association found between the higher citrus canker intensity and the damage caused by the insect and show, albeit partially, the consequences of these changes in the spread of the pathogen under natural conditions of infection.

Incidência de danos pós-colheita em goiabas no mercado atacadista de São Paulo e sua relação com a prática de ensacamento dos frutos

The incidence of post harvest damages in guavas was quantified from April 2005 to August 2006 in four wholesalers from the terminal wholesale market of Sao Paulo (CEAGESP). The sampling was stratified by fruit size, fruit origin, flesh color and bagging the fruit. All fruit from 323 guava boxes were visually assessed. Post harvest mechanical injuries and diseases were quantified. Five thousand and eighty one fruit were assessed, 51.1% of which came from orchards where fruit received paper bags some time before harvesting. Post harvest mechanical injuries were observed in 63% of fruit but only 5.5% of fruit showed symptoms of post harvest diseases. These diseases incidence was correlated to the incidence of post harvest mechanical injuries only in bagged fruit (R=0.20, p< 0.05). These variables were not correlated in fruit not bagged (R=0.09). Black spot (Guignardia psidii) was observed in 3.5 % of fruit and anthracnose (Colletotrichum spp.), in 1.1 % of them. Post harvest rots caused by fungi from genera Fusicoccum, Rhizopus, and Pestalotia occurred in less than 1 % of fruit. The incidence of post harvest diseases caused by quiescent pathogens was significantly greater in bagged fruit (7.7 % of fruit) than in non-bagged fruit (2.1 % of fruit). The opposite was observed for wound-pathogens, with averages of 0.3 % and 0.8 % of symptomatic guavas for bagged and non-bagged fruit, respectively. There was no significant difference in the incidence of quiescent diseases in bagged fruit from varieties with white flesh (7.8 %) or red flesh (7.3 %).

Dinâmica Temporal da Clorose Variegada dos Citros em Três Regiões do Estado de São Paulo

CVC is considered the most important disease of brazilian citrus industry but many aspects of its epidemiology are still unknown. This work aimed to characterize the spatial pattern of CVC affected trees in three regions (Northwest, Center and South) of Sao Paulo state, Brazil. Three orchards of Pera sweet orange (Citrus sinensis) grafted on Rangpur lime (Citrus limonia) were evaluated twice a month by visual assessments from July 1998 to December 2000. For each evaluation all plants were inspected and assigned as diseased or healthy and cumulative maps were produced. The following analyses were performed: ordinary runs, isopath areas, Taylor law, dispersion index and foci structure and dynamics analysis. Ordinary runs indicated a trend to randomness. Isopath areas analysis showed few compact foci and a trend to uniform incidence in all areas. The other analysis showed few if any differences between regions and results that could classify CVC pattern as slightly aggregated.