Sweet Orange Groves Research Articles

Sugar cane processing residue, bagasse, enhances decomposition of citrus leaves and could contribute to citrus black spot management

Citrus black spot (CBS) is a quarantine disease, reported in Florida in 2010. The pathogen, Phyllosticta citricarpa (Pc), survives in twigs and leaf litter during the off-season. Speeding up leaf litter decomposition could contribute to reducing disease pressure and potential spread. Objectives were to investigate (a) effects of bagasse, cellulolytic microbes and urea on leaf litter decomposition; (b) relations between litter composition, moisture content and decomposition rate; and (c) effects of the treatments on Pc survival. Two laboratory and two field experiments were conducted in Northern Florida, where the pathogen is absent. In microcosms, the effects of bagasse ± cellulolytic microbes, just microbes, urea, and a control treatment were compared with respect to decomposition of Pc-inoculated mature sweet orange leaves on soil, Pc reisolation and detection by qPCR. In two disease-free sweet orange groves, effects of bagasse and urea on decomposition of nylon-bagged sweet orange leaves on soil were compared to a control at various distances from micro-sprinklers. Bagasse resulted in a leaf dry weight reduction of 93% in microcosms and 67% in groves compared to 36% in both controls after two months. Dry weight reductions declined with distance from sprinklers, except in the bagasse treatment which maintained leaf moisture content. Pc was not reisolated, but its DNA was detected in all inoculated but not in non-inoculated samples after one month. Bagasse enhanced decomposition of sweet orange leaves on soil. Additional experiments are needed in CBS affected groves to investigate effects of bagasse on Pc survival in leaf litter.

Living on the Edges: Spatial Niche Occupation of Asian Citrus Psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), in Citrus Groves.

The spatial niche occupation of the Asian citrus psyllid, Diaphorina citri Kuwayama, 1908, was evaluated to determine its field colonization and food resource exploitation strategies in citrus groves. Mature grapefruit and sweet orange groves were surveyed as part of an area-wide program in 2009–2010 to determine D. citri population densities and between-tree distribution. In both cultivars, significantly more psyllids were found on perimeter trees throughout the study period suggesting a strong edge effect in D. citri distribution in the groves. D. citri densities and infestation levels gradually declined from the edge to the center of grove. Higher numbers of D. citri were recorded on trees located on the east and south sides of the groves than those on the west and north sides. Citrus groves located at the outer edge of the study with at least one side non-surrounded to other citrus groves harbored significantly more D. citri than groves located within the block cluster and entirely surrounded by other groves. In detailed field studies during 2012, infestation of D. citri started from border trees in the grove where possibly one generation is completed before inner trees become infested. In addition, psyllid densities decreased significantly with increasing distance from the grove edge. Using the selection index, D citri exhibited a strong niche occupation preference for border trees.

Survey of Xylella fastidiosa and citrus variegated chlorosis in São Paulo state, Brazil.

Contrary to the well-documented incidence and severity of Citrus variegated chlorosis (CVC) throughout the sweet orange groves of Sao Paulo state (Brazil), the incidence and distribution of its causal agent, Xylella fastidiosa, is not well known. Sweet orange samples selected according to variety and age were collected from commercial groves in northern, central and southern regions of Sao Paulo state and analysed visually for the presence of symptoms and by PCR for the presence of X. fastidiosa. A decreasing gradient of both infected and symptomatic trees from northern to southern regions of the state was observed. However, there was a higher incidence of latent infection (symptomless trees) in the southern (32%) than in the northern and central regions (16%). The survey disclosed an increasing gradient of CVC and X. fastidiosa incidence according to the age of the trees. Infected trees younger than two years were observed only in the central region. CVC symptom expression is related to environmental conditions and the incidence of symptomatic trees does not correspond with that of trees infected by X. fastidiosa.

PROGRESS TOWARDS THE DEVELOPMENT OF TRANSGENIC DISEASE RESISTANCE IN CITRUS

Florida is the world's third largest producer of citrus, behind Brazil and China. This 9 billion dollar industry is now under siege by two important diseases - citrus greening, also known as huanglongbing (HLB), and citrus canker. Both these diseases are caused by gram negative bacteria. Canker is primarily affecting the grapefruit industry and poses problems with the establishment of new sweet orange groves. It can be managed by following a canker suppression program, whereas HLB affects all cultivated citrus varieties and cannot currently be controlled. Resistance to either HLB or canker is also not present in commercial orange and grapefruit cultivars. A major strategy is to develop resistant cultivars via genetic engineering by incorporating resistance genes not found in citrus. We have successfully cloned several natural and synthetic antibacterial genes and genes that have the potential to turn on SAR (Systemic Acquired Resistance) and made significant progress in introducing them into commercial sweet orange and grapefruit cultivars. We have regenerated hundreds of transgenic citrus plants, using both the standard Agrobacterium-mediated transformation system and the protoplast/GFP transformation system developed previously in our program. Genetic constructs containing promoters that target expression of the above gene(s) exclusively to the phloem tissue, where HLB resides, are also being utilized in efforts to minimize foreign gene expression in fruit or juice subsequently going to market. Progress towards the development and testing of such transgenic disease resistance in commercial citrus cultivars will be discussed.

Abundance of Asian Citrus Psyllid on Yellow Sticky Traps in Florida, Puerto Rico, and Texas Citrus Groves

Population dynamics of Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae), were studied using yellow sticky traps in citrus groves in Florida, Texas, and Puerto Rico. Four groves were studied in Florida (three sweet orange groves and one grove of mixed Citrus spp.) and Texas (four grapefruit groves), and three groves were studied in Puerto Rico (one grove of mandarin oranges and two sweet orange groves). Abundance of Asian citrus psyllid based on trap captures of adults at two groves in Puerto Rico were similar to those in Florida, although several peaks in numbers trapped at one Florida grove far exceeded those at any other grove. Few adults were captured on traps at one of the Puerto Rico groves and at each of the four Texas groves. Less abundance of the psyllid was attributed to the grove in Puerto Rico being higher altitude and cooler. Fewer psyllids captured at two of the four Texas groves during some trapping periods were attributed to applications of insecticides but, overall, fewer adults captured on traps at the Texas groves seemed to be a consequence of differences in abundance of the psyllid. Environmental, biological control, or host plant factors may be less favorable for increases in abundance of Asian citrus psyllid in Texas than in Florida.

Dinâmica temporal da população do ácaro Brevipalpus phoenicis e da leprose dos citros sob condições naturais de epidemia

A densidade populacional do ácaro vetor de Citrus leprosis virus (CiLV), Brevipalpus phoenicis, num talhão, é o principal indicador para a tomada de decisão de medidas de controle da leprose dos citros. Há pouca informação sobre o crescimento da incidência de plantas com leprose dos citros isoladamente ou em conjunto com a população do ácaro. Este trabalho teve por objetivo caracterizar o progresso temporal da população de B. phoenicis e da incidência de leprose dos citros e a relação entre essas populações, sob condições naturais de epidemia. Dois talhões de laranja doce, cvs. Valência e Natal, foram monitorados de 2002 a 2004, em intervalos de 22 dias, em média. O crescimento da incidência da doença foi lento e estimativas da taxa de progresso da doença foram bastante baixas, variando de 0,0126 a 0,0448 para 'Valência' e de 0,0044 a 0,0525 para 'Natal'. A quantidade de inóculo inicial nos ramos cresceu significativamente de um ciclo para outro. Ao final do período, a incidência foi de 32% das plantas de 'Valência' e de 6,8% na cv. Natal. Apesar de não ser sistêmica, a leprose dos citros comporta-se como poliética com acúmulo de inóculo de ano para ano, especialmente nos ramos. A incidência não esteve correlacionada com a presença de ácaros na planta em levantamentos anteriores, mas apresentou correlação positiva significativa (P<0,001) com a própria incidência da doença registrada anteriormente. Isto indica que a presença de sintomas, e não somente a de ácaros, deve ser considerada em amostragens visando controle da doença.

ASSESSMENT OF COTTON AS AN ALTERNATIVE HOST PLANT FOR THE BROWN CITRUS APHID, TOXOPTERA CITRICIDA(HOMOPTERA: APHIDIDAE)

Abstract Seven populations of Toxoptera citricida (Kirkaldy) were sampled in central Florida sweet orange groves in 2001. All populations contained individuals that accepted cotton seedlings as a host in a no-choice situation; many of these matured and deposited nymphs that also developed and became reproductive on the same plant. Significant differences were noted among populations with respect to the proportion of nymphs accepting, maturing, and ultimately reproducing on cotton. Differences in aphid survival were largely a function of differences in host plant acceptance, rather than differential mortality on the plant. A significant proportion of the apterous adults maturing on cotton abandoned the plant without reproducing. Second and third instars transferred from laboratory colonies maintained on sweet orange were more accepting of cotton than were either first or fourth instars. Apterous adults accepted cotton at rates similar to second and third instars. Alate adults settled on cotton seedlings in...

Agricultura de precisão: mapeamento da produtividade em pomares cítricos usando geoestatística

A variabilidade espacial de produtividade e tamanho de frutos foi avaliada em pomares de laranja irrigados e não irrigados, localizados no município de Luiz Antônio - SP, utilizando-se de geoestatística. Através dos mapas de krigagem, podem-se determinar as áreas de alta e baixa produtividade dos talhões. Verificou-se maior variabilidade para produtividade e tamanho de frutos nas quadras irrigadas e não irrigadas. Portanto, a geoestatística mostrou-se uma ferramenta extremamente útil para auxiliar em Programas de Agricultura de Precisão.

`Tahiti' lime fruit set related to gibberellic acid application on out-of-season flowering and the accumulation of degree days

Introduction. The production of `Tahiti' limes in southern Brazil has increased in the last 20 years and has become a good alternative for citrus growers in regions where Citrus Variegated Chlorosis has affected sweet orange groves. One of the main goals to insure success in growing `Tahiti' limes is to apply the right techniques to induce out-of-season flowering and winter fruit harvest when the fruit prices are the highest. This experiment evaluated the effects of gibberellic acid application on fruit set of `Tahiti' lime trees related to out-of-season flowering and accumulated degree days. Materials and methods. Eight-year-old plants were treated with five concentrations of gibberellic acid [ (0, 10, 25, 50 and 100) mgL-1 ] applied during anthesis of out-of-season bloom. Accumulation of degree days for fruit set and fruit development in the conditions of Sao Paulo State, Brazil, were calculated. The percentage of fruit set determined by weekly counts of the number of flowers from anthesis to the maturation of the fruits were determined on each previously marked branch. Data were submitted to regression analysis. Results and discussion. In regard to fruit set, it was observed that 10 mg gibberellic acidL-1 was enough to retain more than 10% of fruits in relation to the number of formed flowers. Starting from anthesis, (837.7 and 1 115.2) accumulated degree days were necessary for the stabilization of the fruit set and fruit maturation, respectively.

Preliminary Analysis of Citrus Greening (Huanglungbin) Epidemics in the People's Republic of China and French Reunion Island

Gottwald, T. R., Aubert, B., and Xue-Yuan, Z. 1989. Preliminary analysis of citrus greening (Huanglungbin) epidemics in the People's Republic of China and French Reunion Island. Phytopathology 79:687-693. Epidemics of citrus greening disease in sweet orange groves were severity in the Reunion plot. Ordinary runs analysis indicated aggregation monitored for several years at two sites in the Guangxi Province of the of diseased trees in all plots during all years except in the case of one People's Republic of China and one site in French Reunion Island. Both plot in China when disease incidence exceeded 0.99. Aggregation along the exponential and the logistic models adequately described the increase rows of the Reunion plot was associated with prevailing winds. Isopaths of greening expressed as disease incidence over time in two plots in of greening disease incidence demonstrated the highest level of disease China and when the disease was expressed as both disease incidence and was present in the western half of the Reunion plot. This spatial pattern disease severity for the Reunion Island plot. The logistic apparent infection suggested the pathogen was introduced from the west against the prevailing rates, r, were 2.53, 1.87, and 0.99/yr for disease incidence at the two winds, probably by psyllid vectors. Chinese and one Reunion plots, respectively, and 1.04/yr for disease