Fruit Drop Of Citrus Research Articles

Weather-Based Fruit Fly Population Dynamics Prediction Model for the Mid-Hills of Eastern Himalayan Region of India

Across the globe, it is widely observed that current change in climate or weather pattern has marked effect on insect population dynamics and behavior, though with varied aspects and intensity. Understanding the correlations of insect population dynamics with weather parameters is the fundamental first step in formulating an effective integrated pest management programme. In the major citrus growing mid-hill regions of Arunachal Pradesh, fruit fly, Bactrocera dorsalis, is one of the chief reasons for heavy loss leading to pre-harvest fruit drop in citrus. The current study aims to determine how changes in weather parameters influence the population dynamics of fruit flies and to develop an optimized weather-based population prediction model for fruit flies infesting citrus in the mid-hills of Arunachal Pradesh. The population was monitored through standard methyl eugenol para-pheromone traps and a model was developed using stepwise multiple regression technique. The study revealed that the population was highest during the fruit development and ripening stage and it was found that the number of flies per trap and meteorological parameters (temperature and humidity) is positively correlated with appreciable statistical significance. The optimized regression model was developed with variables temperature and humidity, and was found to be 79 percent accurate in the study site. This model will serve as an important tool in pre-estimation of the fruit fly population in the citrus growing mid-hills of Arunachal Pradesh and for formulation of effective preventive management strategies.

Colletotrichum abscissum: Detection on symptomless citrus leaves and symptomatic citrus flowers using high‐resolution melting analysis

AbstractHigh‐resolution melting (HRM) analysis has been recently applied for the diagnosis of plant diseases. This cost‐effective method is advantageous over standard PCR as it does not require DNA band visualization or the use of probes necessary for quantitative PCR. Colletotrichum abscissum causes postbloom fruit drop of citrus (PFD) and survives asymptomatically on vegetative tissue. Quiescent appressoria on citrus leaves are the primary source of inoculum between flowering periods. Early PFD symptoms may be misidentified as physical damage on citrus flowers. Our objectives were to validate an HRM‐based method to rapidly identify C. abscissum on citrus leaves and flowers. We screened seven previously published primer pairs and concluded that the most effective set of primers was CaITS‐F and CaITS‐R815. We evaluated three different DNA extraction methods, two with a purification step (DNeasy Plant Mini and PowerSoil kits) and a faster method without a purification step (buffer GEB2). We inoculated citrus leaves with C. abscissum conidial suspensions of varying concentrations and HRM analysis detected as few as 200 C. abscissum spores using DNeasy Plant Mini or DNeasy PowerSoil kits for DNA extraction. As expected, samples extracted with GEB2 buffer reduced HRM sensitivity. We further collected field samples from areas with high and low C. abscissum inoculum levels. The HRM method could detect the pathogen in high inoculum areas from both leaves and flowers regardless of the extraction method used. In summary, we describe a new molecular diagnostic tool to detect C. abscissum on citrus during its asymptomatic and symptomatic stages.

Evaluation of disease alert systems for postbloom fruit drop of citrus in Florida and economic impact of adopting the Citrus Advisory System

Gama, A. B., Peres, N. A., Singerman, A., and Dewdney, M. M. 2021. Evaluation of disease alert systems for postbloom fruit drop of citrus in Florida and economic impact of adopting the Citrus Advisory System. Crop Protec. XX: xx-xx.Postbloom fruit drop of citrus (PFD) is caused by Colletotrichum abscissum in the Colletotrichum acutatum species complex. PFD does not occur annually but some growers tend to apply fungicides during flowering every year, wasting resources and negatively impacting the environment. Due to its sporadicity, the Fungicide Application Decision system (PFD-FAD) and the Citrus Advisory System (CAS) were developed in Florida and Brazil in 2001 and 2017, respectively, to aid in PFD management. Our objectives were to compare the recommendations from CAS and PFD-FAD, a weekly fungicide treatment schedule, and a nontreated control. We also aimed to assess the economic impact of adopting CAS over the other fungicide schedule treatments with low- and high-risk aversion profile scenarios. Weather conditions were not favorable to PFD in seven field trials. We performed one fungicide application based on CAS over four seasons. An average of approximately 2 and 3 fungicide applications per trial was performed based on PFD-FAD and the weekly treatments, respectively. Average cost to manage PFD following CAS, PFD-FAD, and the weekly treatments were, respectively, $17.52, $186.87, and $272.35. The reduction in management costs related to CAS also translates to reduced use of commercial fungicide formulations in small properties by 382 and 573 L compared to PFD-FAD and the weekly treatments, respectively. CAS recommendations greatly reduced production costs and fungicide usage without compromising disease control and citrus yield in years of low weather favorability for PFD in Florida.

Identification and Abundance of Fruit Fly Species Responsible for Fruit Drop of Sweet Orange (Citrus sinensis L. Osbeck) in Benue State, Nigeria

Background and Objective: Citrus is one of the most important and among the top ten widely cultivated fruits in Nigeria. It is produced more in the Guinea and Sudan Savanna zones of the country with Benue State having the highest annual production. In the South of Savanna region of Africa, particularly in Nigeria; despite the economic, nutritional and health benefits of citrus, limited research work has been carried out on the identification and control of fruit flies of economic importance to the crop. The objectives of this study are to identify and determine the abundance of the fruit fly species responsible for fruit drop of citrus in Benue State, Nigeria.
 Materials and Methods: Fruit fly identification was done through fruit culture experiment carried out in the College of Agronomy Teaching and Research Farm, Federal University of Agriculture, Makurdi in October 2014 and October 2015.The experiment was a 2 x 3 factorial in a completely randomized design with four replications. The two factors were Zones (Zone A and B) and Varieties (Ibadan Sweet, Valencia and Washington Navel). Ten naturally infested orange fruits from four randomly selected citrus trees of each variety in each zone were weighed and placed in each plastic rearing box with dimension 39 x 27 x 26 cm containing sterilized moist soil securely covered with 1 mm mesh net for pupation and adult insect emergence. Emerged adults were killed using Mobil insecticide (Cyphenothrin), counted, sexed and stored in specimen bottles with 70% alcohol for preservation and later identification.
 Results: Fruit fly species identified from citrus fruit culture were: Bactrocera invadens (Drew), Ceratitis capitata (Weid) and Dacus bivittata (Biggot). Bactrocera invadens were the most abundant species and accounted for 63.70% in Washington navel and 63.10% in Valencia in 2014 and 2015 respectively. The varieties showed no significant differences (p>0.05) on the abundance of Ceratitis capitata and Dacus bivittata in 2014. In 2015 however, Ibadan Sweet variety showed significant difference (p < 0.05) on the abundance of Ceratitis capitata (28.30%) when compared with other varieties (15.8%) each.
 Conclusion: Fruit fly species responsible for citrus fruit drop in Benue State were Bactrocera invadens (Drew), Ceratitis capitata (Weid) and Dacus bivittata (Biggot) with Bactrocera invadens (Drew) recorded as the most abundant species.

New insight in Huanglongbing-associated mature fruit drop in citrus and its link to oxidative stress

As Huanglongbing (HLB) becomes epidemic throughout Florida, there is a sharp rise in mature fruit drop prior to harvest in affected citrus, leading to a substantial reduction in yield. To investigate how HLB increases mature fruit abscission, this research evaluated the drop rate and compared the global gene expression using RNA sequencing in HLB-susceptible ‘Hamlin’ sweet orange (Citrus sinensis) versus HLB-tolerant ‘LB8-9’ mandarin {‘Clementine’ mandarin (C. reticulata) and ‘Minneola’ tangelo [hybrid of ‘Duncan’ grapefruit (C. paradisi) and ‘Dancy’ tangerine (C. reticulata)]}. Consistent with their susceptibility to HLB documented in the literature, the drop rate of mature fruit was significantly higher in ‘Hamlin’ sweet orange than ‘LB8-9’ mandarin. Between the two citrus, there were 368 differentially expressed genes (DEGs); 310 and 58 were upregulated and downregulated, respectively, in the abscission zone (AZ) of ‘Hamlin’ sweet orange compared to ‘LB8-9’ mandarin fruit. Although at the time of collection, fruits of both citrus were still attached to the tree branch (fruit detachment force > 6 kgf), upregulation of the DEGs related to cell wall loosening in ‘Hamlin’ sweet orange indicates that the process of abscission was already initiated in the AZ of this citrus. The pattern of DEGs related to the metabolism and signal transduction of hormones, specifically ethylene, auxin, cytokinin, abscisic acid, and gibberellic acid, also suggests that HLB likely brings about changes in endogenous hormone balance that promote mature fruit abscission. Interestingly, DEGs encoding antioxidants were upregulated, suggesting a demand for detoxification mechanisms against oxidative stress in the AZ of ‘Hamlin’ sweet orange fruit. Additionally, gene ontology (GO) terms for cell death and senescence, two of consequences of oxidative stress, were highly enriched. Together, the results of transcriptome analysis presented herein provide evidence that for HLB-susceptible ‘Hamlin’ sweet orange, oxidative stress caused by the pathogen infection likely results in cell wall modification, leading to cell separation and eventually fruit abscission. The results further suggest that ‘LB8-9’ mandarin may have an advanced antioxidant system to mitigate the pathogen-induced oxidative stress, thereby contributing to its tolerance to HLB.

カンキツの早期生理落果における果実の離脱誘導と離脱特性

カンキツの早期生理落果における果実の離脱誘導と離脱特性

Bacillus subtilis based-formulation for the control of postbloom fruit drop of citrus.

Postbloom fruit drop (PFD) caused by Colletotrichum acutatum affects flowers and causes early fruit drop in all commercial varieties of citrus. Biological control with the isolate ACB-69 of Bacillus subtilis has been considered as a potential method for controlling this disease. This study aimed to develop and optimize a B. subtilis based-formulation with a potential for large-scale applications and evaluate its effect on C. acutatum in vitro and in vivo. Bacillus subtilis based-formulations were developed using different carrier materials, and their ability to control PFD was evaluated. The results of the assays led to the selection of the B. subtilis based-formulation with talc+urea (0.02%) and talc+ammonium molybdate (1mM), which inhibited mycelial growth and germination of C. acutatum. Studies with detached citrus flowers showed that the formulations were effective in controlling the pathogen. In field conditions, talc+urea (0.02%) provided 73% asymptomatic citrus flowers and 56% of the average number of effective fruit (ANEF), equating with fungicide treatment. On the contrary, non-treated trees had 8.8% of asymptomatic citrus flowers and 0.83% ANEF. The results suggest that B. subtilis based-formulations with talc as the carrier supplemented with a nitrogen source had a high potential for PFD control.

Cell wall related gene expression during secondary physiological fruit drop in ponkan (Citrus reticulataBlanco) and hyuganatsu (C. tamuranahort. ex Tanaka)

Physiological fruit drop in citrus is the deeper problem for fruit production and many researches have suggested that decreasing of carbohydrate translocation to fruit involved in fruit abscission. The fruit abscission process requires 3 stages for successful abscission, which include “induction”, “decision”, and “execution”. However, there is almost no information in the molecular mechanism of secondary physiological fruit drop in citrus. Here, anatomical observation and microarray analysis demonstrated the candidate genes involved in the fruit abscission process after induction of fruit abscission by incubating the detached ponkan (Citrus reticulata Blanco) and hyuganatsu (C. tamurana hort. ex Tanaka) fruit on agar medium. In our study, the cumulative ratio of fruit abscission was very high in ponkan, but very low in hyuganatsu. Morphology of abscission zone (AZ) cells was drastically changed in ponkan, but almost not changed in hyuganatsu. From the 60 K microarray analysis at 0, 12, 24, or 36 h after the induction, the specific up-regulated genes in ponkan AZ tissue were identified. The expressions of many cell wall degrading related genes, such as beta-galactosidase, polygalacturonase and beta-glucosidase etc. increased rapidly at 24 h. These results indicated that “execution” stage of fruit abscission process would be initiated until 24 h after the induction of fruit abscission in citrus.

Effect of Plant Growth Regulators and Fungicides on Huanglongbing-related Preharvest Fruit Drop of Citrus

The severe citrus (Citrus sp.) disease Huanglongbing (HLB), associated with Candidatus Liberibacter asiaticus, has resulted in widespread tree decline in Florida and overall citrus production is now the lowest it has been in 50 years. More than 80% of Florida citrus trees are HLB affected, and most growers attempt to sustain production on infected trees through good asian citrus psyllid (Diaphorina citri) control and enhanced fertilization and irrigation management. Although production appears to benefit from these treatments, preharvest fruit drop is considerably greater than on uninfected trees. U.S. Department of Agriculture (USDA) data indicate that Florida statewide fruit drop has increased by 10% to 20% of the entire crop in the last three growing seasons, essentially doubling the historical levels. Extensive research is underway to identify solutions to HLB, but it is essential to maintain production on existing trees to sustain the industry in the near term. For decades, several plant growth regulators (PGRs) have been labeled to reduce preharvest fruit drop in commercial citrus. Trials of these materials, other nonlabeled PGRs, and some fungicides were conducted in two seasons to determine if fruit drop could be reduced. Randomized complete block design experiments were established using four to six replications of four- to six-tree groups as experimental units, blocked spatially. In 2013–14, sprays of gibberellic acid (GA), 2,4-dichlorophenoxyacetic acid (2,4-D), 1-naphthaleneacetic acid (NAA), S-abscisic acid (S-ABA), aminoethoxyvinylglycine (AVG), and 1-methylcyclopropene (1-MCP) were applied once or twice alone or in some combinations at standard rates to trees in various mature blocks of ‘Valencia’ and ‘Pineapple’ sweet orange (Citrus sinensis), ‘Star Ruby’ grapefruit (Citrus paradisi), or ‘Murcott’ tangor (Citrus reticulata ×C. sinensis) in central Florida in the Indian River area. Only 1 of the 10 individual trials had treatments with significantly lower drop rates than controls; and when pooled across all experiments, GA + 2,4-D reduced number of fruit dropped per tree 4%, but only at P = 0.10. NAA, S-ABA, AVG, and 1-MCP had no effect and were not used the following year. Starting in 2014, treatments were initiated earlier in the season with greater effort to minimize variability: GA; 2,4-D; GA + 2,4-D; a natural GA, indolebutyric acid, cytokinin mix; and strobilurin fungicides were applied to 22 mature blocks of ‘Hamlin’ and ‘Valencia’ sweet orange trees. In 2014–15, only three of the 11 individual ‘Hamlin’ trials and one of the 11 ‘Valencia’ trials included a treatment with significant drop reduction compared with controls. However, when all the tests on ‘Hamlin’ were pooled, there was a significant 5% reduction in total crop drop for GA + 2,4-D and significant reductions with many of these PGRs alone, but in only one case with fungicide treatments. When all tests on ‘Valencia’ were combined, 2,4-D reduced drop significantly but only by 2% of the total crop (14% drop vs. 16% drop), but fruit drop in ‘Valencia’ blocks was near the historical average in control trees. Soil conditions and tree conditions were similar across all test sites and there were no apparent relationships between product efficacy and observed tree condition or any other grove characteristics. In addition, four ‘Hamlin’ and four ‘Valencia’ blocks were treated with 1/4 rates of 2,4-D + 6-benzyladenine every 45 days during the growing season (six sprays) and three of the eight individual trials showed significant reductions in drop: when pooled, these treatments reduced drop by 3% in ‘Valencia’ and 6% in ‘Hamlin’. At this time, PGRs cannot be recommended as a consistent way to reduce fruit drop related to HLB, but further work needs to be conducted to refine the most promising treatments.

Where have all the flowers gone? Postbloom fruit drop of citrus in the Americas.

Postbloom fruit drop (PFD), caused primarily by Colletotrichum acutatum, is a serious disease annually in the humid tropical citrus areas of the Americas and occurs more sporadically in the humid subtropics. The fungus infects flowers of all citrus species producing orange-brown lesions on petals that result in abscission of the fruitlets leaving the persistent calyx and floral disk attached to the twigs. C. acutatum also causes Key lime anthracnose and is morphologically identical to PFD, but the strains can be differentiated by molecular means and pathogenicity tests. C. acutatum produces abundant conidia on infected petals that are dispersed primarily by rain splash. After the bloom season, the fungus persists as appressoria on persistent calyces and other vegetative plant parts. Those appressoria are stimulated to germinate by flower extracts and produce secondary conidia to initiate a new disease cycle. Some cultural measures are useful in reducing disease severity, but control is based principally on application of fungicides. Benzimidazole fungicides and captafol were the best materials in the past but are no longer available. The QoI fungicides, folpet and tebuconazole + trifloxystrobin are the most effective among currently available materials. Forecasting systems based primarily on the availability of inoculum and recent rainfall were developed to aid growers on timing fungicide applications. The expert system, PFD-FAD, based on environmental conditions, inoculum, disease history, and varietal susceptibility is the most effective means currently available for scheduling fungicide applications.

Application of microorganisms, alone or in combination, to control postbloom fruit drop in citrus

Isolates of Trichoderma spp. (ACB-14, ACB-33, ACB-37, and ACB-40) and Bacillus subtilis (ACB-66, ACB-69, ACB-77, and ACB-83) were tested separately or in mixtures for suppression of postbloom fruit drop in citrus, caused by Colletotrichum acutatum. This work aimed at: (i) determining the incubation time and temperature for production of cells of biocontrol agents; (ii) determining the effect of the isolates, separately or in mixture on the germination conidia of C. acutatum; (iii) evaluating the efficiency of antagonistic isolates on detached citrus flowers and under field conditions. The results of the interactions in vitro showed that there was little differentiation in cell production among the species, and the optimum temperature was 27oC. The best time for multiplication of bacterial cells was 36 hours, whereas for Trichoderma, the production of conidia continued to increase up to 120 hours of incubation. The mixtures of the ACB-77 plus ACB-66, ACB-33, or ACB-37 inhibited pathogen germination from 84% to 89%. Studies with detached citrus flowers showed that ACB69 alone gave 99% control. The use of mixture ACB-69 plus ACB-37 proved to be viable in the control of disease under field conditions, but the efficiency of the control was lowest than the obtained by applications of ACB-69 alone.

Controle alternativo de Colletotrichum acutatum agente causal da queda prematura dos frutos cítricos

A queda prematura dos frutos cítricos (QPFC), causada por Colletotrichum acutatum, dados os grandes prejuízos que têm causado aos produtores, constitui-se numa doença de grande importância econômica. O controle da doença é feito predominantemente mediante uso de fungicidas, que eleva o custo de produção e afeta negativamente o meio ambiente. Diante disso, este trabalho teve por objetivo buscar um método alternativo de controle da QPFC, mediante o uso de agentes de biocontrole ou de biofertilizantes. Diferentes concentrações de biofertilizantes (originários de duas fontes distintas e denominados de Bio1 e Bio 2); três isolados de Bacillus subtilis (ACB-69; 72 e 77) e três isolados de Trichoderma spp. (ACB-14; 37 e 39) foram testados, isoladamente ou em combinação, sob condições de laboratório, quanto à capacidade inibitória da germinação de conídios de C. acutatum. Estudaram-se, ainda, a produção de metabólitos termoestáveis por B. subtilis e o efeito sobre a germinação do patógeno. Quinze isolados de B. subtilis foram testados quanto à capacidade de prevenir a infecção por C. acutatum em flores destacadas de lima- ácida 'Tahiti' e, no campo, foram instalados dois experimentos, visando a testar ACBs e biofertilizantes no controle da doença. Verificou-se que o isolado ACB-72 (B. subtilis) e ACB-37 (T. pseudokoningii) foram os que mais inibiram a germinação do patógeno. Quanto à produção de metabólitos termoestáveis, ACB-69 e 77 foram os mais eficientes em produzir substâncias antifúngicas, e em quantidades suficientes para inibirem a germinação do patógeno. A mistura dos quatro isolados de Bacillus (ACBs: 69; 72; 77 e AP3) foi o que apresentou maior porcentagem de inibição (73%). Os biofertilizantes (Bio1 e Bio2), em concentrações acima de 10% e, quando em associação com isolados de Trichoderma spp., promoveram maiores inibições na germinação de C. acutatum. Em testes com flores destacadas, verificou-se que, onde foram aplicados os ACBs 69; 76; 74 e 77, as porcentagens de pétalas sem sintomas de infecção por C. acutatum foram de 83; 92; 92 e 97%, respectivamente. Mediante avaliações a campo, verificou-se a potencialidade de B. subtilis e de biofertilizantes em controlar a doença.

The Colletotrichum acutatum Gene Encoding a Putative pH-Responsive Transcription Regulator Is a Key Virulence Determinant During Fungal Pathogenesis on Citrus

Postbloom fruit drop of citrus and Key lime anthracnose (KLA) are caused by different pathotypes of Colletotrichum acutatum. Both pathotypes are pathogenic to citrus flowers, resulting in blossom blight and induction of young fruit abscission. Two fungal mutants defective in pathogenicity were recovered from a KLA pathotype after Agrobacterium-mediated mutagenesis. A PacC(KLAP2) gene encoding a polypeptide that resembles many pH-responsive PacC/ Rim101 transcription regulators in fungi was identified from one of the mutants, and functionally characterized to play a crucial role in pathogenesis to both Key lime leaves and citrus flowers. Gene disruption at the Pac(KLAP2) locus created fungal mutants that were hypersensitive to alkaline pH, altered in conidium and appressorium production and germination, and concomitant with reduced virulence to both tissues. The pacC(KLAP2) null mutants had lower alkaline phosphatase and protease activities, but increased pectolytic and lipolytic activities. The mutants initiated penetration and incited lesion formation on Key lime, indistinguishable from the wild type, when a functional copy of PacC(KLAP2) was reintroduced or the leaves were wounded prior to inoculation. The null mutants were blocked at the penetration stage and, thus, failed to initiate the necrotrophic phase. The PacC(KLAP2) transcript was barely detectable when the fungus was grown on medium buffered to pH 3 or 4, yet accumulated to high levels at a pH between 5 and 7. The Pac(KLAP2) transcript was detected 2 days postinoculation on Key lime leaves, correlating with the time of lesion formation. We conclude that PacC(KLAP2) is essential for C. acutatum pathogenesis by regulating multiple physiological and developmental processes.

Lifestyles of Colletotrichum acutatum.

Lifestyles of Colletotrichum acutatum.

Evaluation of Systems for Timing of Fungicide Sprays for Control of Postbloom Fruit Drop of Citrus in Brazil.

Postbloom fruit drop (PFD) of citrus, caused by Colletotrichum acutatum, infects petals of citrus flowers and produces orange-brown lesions that induce the abscission of young fruitlets and the retention of calyces. Proper timing of fungicide applications is essential for good disease control. Different systems for timing of fungicide applications for control of PFD in a major citrus-growing region in southern São Paulo state in Brazil were evaluated from 1999 to 2002. The following programs were compared to an unsprayed control using counts of diseased flowers, persistent calyces, or fruit: (i) a phenology-based program currently recommended in Brazil with one application at early and another at peak bloom; (ii) the Florida PFD model; (iii) the post-bloom fruit drop-fungicide application decision system (PFD-FAD), a new computer-assisted decision method; and (iv) grower's choice. In 1999, no disease developed, sprays applied with the phenology-based program had no effect, and the Florida PFD model saved two sprays compared with the phenology-based program. In 2000, PFD was moderate and the phenology-based and grower's choice treatments had a significantly lower number of persistent calyces and higher fruit numbers than the control, but no differences were found between those treatments and the PFD model. In 2001, PFD was severe with considerable yield loss. The PFD model, the phenology-based program, and the grower's choice reduced flower blight and the number of persistent calyces, and improved fruit yields with two to three applications, but the PFD-FAD achieved comparable yields with only one spray. In 2002, the disease was mild, with no yield loss, and the Florida PFD model and the PFD-FAD saved one spray compared with the other systems. The PFD model and the PFD-FAD were equally effective for timing fungicide applications to control PFD in Brazil. Scouting of trees is simpler with PFD-FAD; therefore, this system is recommended and should eliminate unnecessary sprays and reduce costs for growers.

Outbreak of postbloom fruit drop of citrus, caused by Colletotrichum acutatum, in Santa Catarina State, southern Brazil

Postbloom fruit drop (PFD), caused by Colletotrichum acutatum , is an important disease of citrus, especially in orchards planted in humid areas of the Americas (Timmer et al. , 1994). In Brazil, PFD was first reported in 1979 and causes variable yield losses depending on the weather conditions (Feichtenberger, 1994). Until recently, the disease was not considered serious in Santa Catarina, a southern Brazilian state, in which about 100 km 2 is planted with citrus. In 2002, disease symptoms of PFD were reported in numerous orchards located in the west area of Santa Catarina. These were reddish brown spots on the petal tissues followed by abscission of the fruitlets at the base of the ovary, with the basal disc, calyx and peduncle remaining firmly attached to the tree. These persistent calyces (buttons) are diagnostic for the disease and may persist for many months after flowering (Timmer et al. , 1994). In September 2002, during the main flowering period, the weather conditions were particularly suitable for the disease, as the mean temperature during the month was low (11·1 ° C) and associated with long periods of leaf wetness. The severity of the infection was estimated by the number of infected flowers and persistent calyces observed on 20 trees randomly distributed in a 7-year-old ‘Rubi’ sweet orange orchard. On each tree, the number of buttons in a 1 m 2 quadrat was counted. An average of 58·6 persistent calyces/m 2 was calculated and 100% yield losses were observed. Five isolates of a slow-growing fungus were recovered from calyces plated on potato-dextrose agar. After 7 days’ incubation at 25 ° C in the dark, there was an abundant production of orange conidial masses. The conidia were mostly fusiform, without setae, and were 11·5 ± 1·1 μ m long by 4·0 ± 0·5 μ m wide. The morphological characteristics of these isolates fit the description of C. acutatum (Agostini et al. , 1992; Brown et al. , 1996). Flowers of 3to 4-year-old potted sweet orange trees in a screenhouse were inoculated with a conidial suspension (10 5 conidia mL − 1 ). A control treatment was sprayed with distilled water only. The characteristic reddish brown lesions on the petals were observed on the inoculated flowers and C. acutatum was reisolated from the infected tissues fulfilling Koch’s postulates. This is the first report of a significant outbreak of PFD for the Santa Catarina state, Brazil.

Indole derivatives produced by the fungus Colletotrichum acutatum causing lime anthracnose and postbloom fruit drop of citrus

Postbloom fruit drop (PFD) of citrus and Key lime anthracnose (KLA) are caused by Colletotrichum acutatum. Both fungal isolates can infect flower petals, induce young fruit abscission and result in severe yield loss on many citrus cultivars. Previous studies revealed that infection of citrus flowers by C. acutatum caused higher levels of indole-3-acetic acid (IAA), which could be synthesized from the host plant and/or the fungal pathogen. The ability for IAA production by C. acutatum isolates was investigated. Similar to many microorganisms, the production of indole compounds in the medium by C. acutatum was dependent solely on the presence of tryptophan (Trp). In total, 14 PFD and KLA fungal isolates were tested, and revealed that they all were capable of utilizing Trp as a precursor to synthesize IAA and other indole derivatives. High-performance liquid chromatography analysis and chromogenic stains after a fluorescence thin-layer chromatography separation unambiguously identified IAA, tryptophol (TOL), indole-acetaldehyde, indole-acetamide (IAM), indole-pyruvic acid, and indole-lactic acid (ILA) from cultures supplemented with Trp. The data suggest that C. acutatum may synthesize IAA using various pathways. Interestingly, increasing Trp concentrations drastically increased the levels of TOL and ILA, but not IAA and IAM. The ability of C. acutatum to produce IAA and related indole compounds may in part contribute to the increased IAA levels in citrus flowers after infection.

Controle biológico de Colletotrichum acutatum, agente causal da queda prematura dos frutos cítricos

O trabalho teve por finalidade estudar a potencialidade antagonística de isolados de Bacillus subtilis a Colletotrichum acutatum, agente causal da queda prematura dos frutos cítricos (Citrus spp.) (QPFC), sob condições de laboratório e de campo. Foram estudados 64 isolados de B. subtilis, quatro isolados de Bacillus spp. e um isolado de B. thuringiensis quanto à capacidade de inibir o desenvolvimento do patógeno em cultura pareada e quanto à produção de metabólitos com atividade antimicrobiana. Os isolados mais promissores foram testados em condições de campo para controle da doença. In vitro, todos os isolados de Bacillus spp. inibiram o crescimento de C. acutatum, não havendo diferenças significativas entre eles. Os isolados de Bacillus spp. produziram, in vitro, metabólitos capazes de inibir o crescimento micelial de C. acutatum, os quais mantiveram suas atividades capazes de causar a inibição, após autoclavagem a 120 ºC, durante 20 min. Dentre os sete isolados de B. subtilis testados para o controle da QPFC, em condições naturais, o ACB-69 diferiu da testemunha e de vários outros isolados, porém equiparou-se estatisticamente ao benomyl, proporcionando menor porcentagem de flores com sintomas e maior número médio de frutos efetivos. Ainda, sob condições de campo, isolados de cada uma das espécies Trichoderma viride, T. pseudokoningii e T. aureoviride foram ineficientes, apresentando o mesmo comportamento da testemunha. Em relação aos métodos de avaliação da doença, a porcentagem de flores com sintomas foi mais eficiente do que o número médio de frutos efetivos (NMFE), uma vez que esses resultam do efeito direto do patógeno.

Engineering a genetic transformation system for Colletotrichum acutatum, the causal fungus of lime anthracnose and postbloom fruit drop of citrus

Postbloom fruit drop (PFD) of citrus is caused by Colletotrichum acutatum. PFD isolates infect flower petals, induce abscission of small fruit and can cause severe yield loss on most citrus cultivars. Isolates from Key lime anthracnose (KLA) cause that disease on the Mexican lime, but also cause PFD on sweet orange. Both PFD and KLA isolates exhibited resistance to the common selection agents including hygromycin, bialaphos, benomyl and geneticin/G418. A genetic transformation system was developed for C. acutatum to confer resistance to sulfonylurea (chlorimuron ethyl) by expressing an acetolactate synthase gene ( sur) cassette from Magnaporthe grisea. The protocol was tested on 11 different KLA and PFD isolates. The transformation frequencies were highly variable among isolates and among experiments (0–17.9 per μg circular DNA using 10 7 protoplasts). Southern blot analysis of transformants indicated that the plasmid vector was randomly integrated in multiple copies into the genome of C. acutatum. Addition of restriction enzymes or use of a vector with homologous sequences did not change the transformation frequencies, but tended to reduce the number integrated. Over 97% of the transformants retained the sulfonylurea resistance phenotype under non-selective conditions. Of 300 transformants tested, three were unable to cause necrotic lesions on detached Key lime leaves. The transformation method opens up opportunities for the genetic manipulation of C. acutatum.

A Fungicide Application Decision (FAD) Support System for Postbloom Fruit Drop of Citrus (PFD)

Surveys were conducted to identify fungi associated with postemergence cotton seedling disease in Missouri. Samples consisted of 10 cotton seedlings, 2 to 3 weeks after emergence, with symptoms of seedling diseases collected from a 0.25 ha area in each of 60 fields in 1997 and 1998. Four genera of fungi were cultured from the roots (Fusarium, Pythium, Rhizoctonia, and Thielaviopsis) and three species were identified: Rhizoctonia solani, Thielaviopsis basicola, and Pythiumultimum. Rhizoctonia solani, T. basicola, and P. ultimum were cultured from seedlings in 70%, 47%, and 15% of fields sampled in 1997, respectively, and 55%, 17%, and 5% of fields sampled in 1998, respectively. Repeated tests of pathogenicity confirmed that R. solani AG-4, T. basicola, and P. ultimum were major causal agents of postemergence cotton seedling disease in Missouri. This study provides the first documentation on the distribution and frequency of fungi associated with postemergence seedling disease complex of cotton based on a survey of randomly selected fields. Accepted for publication 10 July 2002. Published 31 July 2002.