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.

We have investigated the detection of mechanical and disease stresses in citrus plants (Citrus limonia [L.] Osbeck) using laser-induced fluorescence spectroscopy. Due to its economic importance we have chosen to investigate the citrus canker disease, which is caused by the Xanthomonas axonopodis pv. citri bacteria. Mechanical stress was also studied because it plays an important role in the plant's infection by such bacteria. A laser-induced fluorescence spectroscopy system, composed of a spectrometer and a 532 nm 10 mW excitation laser was used to perform fluorescence spectroscopy. The ratio of two chlorophyll fluorescence bands allows us to detect and discriminate between mechanical and disease stresses. This ability to discriminate may have an important application in the field to detect citrus canker infected trees.

No abstract

Citrus huanglongbing (HLB) is a severe disease caused by ‘Candidatus Liberibacter asiaticus’ and vectored by Diaphorina citri. In commercial orchards, where the disease is controlled by removal of trees with symptoms and by frequent vector control, epidemics of HLB are mostly driven by primary infections from inoculum sources external to the orchard. It has been previously shown that the density of D. citri populations is usually higher around the edges than the inner sections of HLB‐affected citrus orchards. Consequently, diseased trees are also concentrated on the edges of orchards. However, there is a lack of quantitative data on HLB gradient dynamics over long periods of time. The objective of this study was to characterize temporal HLB progress and spatiotemporal gradient dynamics over six years in a large citrus farm where the disease was managed according to standard recommendations. Disease incidence and annual disease progress rates were higher at the citrus blocks closer to the edge than those located around inner sections of the farm. A decreasing logistic function provided a good description of the spatiotemporal dynamics of HLB gradients. A fixed slope of disease gradients was estimated over time and diseased trees were observed up to 1500 m from the orchard border. These results confirm that primary infection is the main factor for the progress of HLB epidemics in an orchard under the recommended three‐pronged system management. Therefore, HLB management should be intensified at the blocks located on the edges of the orchards and extended to external sources of inoculum.

The objectives of this work were (i) to determine the influence of temperature on infection of citrus by 'Candidatus Liberibacter asiaticus' and 'Candidatus Liberibacter americanus', the two bacterial species associated with citrus huanglongbing (HLB) in Brazil, and (ii) to determine the influence of temperature on citrus colonization by 'Ca. L. asiaticus', which has taken over from 'Ca. L. americanus' as the predominant species in Brazil since 2008. Two experiments were carried out with graft-inoculated Valencia oranges on Rangpur lime rootstocks. Immediately after inoculation the plants were maintained for 423 days in growth chambers under the following night ⁄ day temperature conditions: 17 ⁄ 22, 22 ⁄ 27 or 27 ⁄ 32°C, with a dark ⁄ light photoperiod of 8 ⁄ 16 h. Infection and colonization of plants were determined using quantitative PCR (qPCR). 'Candidatus Liberibacter americanus' did not infect the plants maintained at 27 ⁄ 32°C; however, infection by 'Ca. L. asiaticus' occurred at all studied temperatures. Two months after inoculation, 'Ca. L. asiaticus' was distributed throughout the inoculated plants, with mean C t values in the range of 30-31 for leaves and 25-28 for roots. Over time, 'Ca. L. asiaticus' reached the highest titres in mature leaves (mean C t value = 26AE7) of citrus plants maintained at 22 ⁄ 27°C. 'Candidatus Liberibacter asiaticus' colonization of citrus plants was negatively affected by the daily temperature regime of 27 ⁄ 32°C (mean C t value in mature leaves = 33AE6).