Premature Fruit Drop Research Articles

Highly specific qPCR and amplicon sequencing method for detection of quarantine citrus pathogen Phyllosticta citricarpa applicable for air samples

AbstractThe fungus Phyllosticta citricarpa is a quarantine pathogen in the EU and is of high economic importance in many parts of the world where favourable climate conditions drive the development of citrus black spot (CBS) disease. Disease symptoms include necrotic lesions on leaves and fruits. Low disease pressure can reduce crop marketability, while higher disease pressure can cause premature fruit drop, significantly increasing crop losses. The wind‐dispersed spores of P. citricarpa are especially problematic for rapid pathogen dispersal, but also provide an opportunity for early detection of the disease spreading into a new area. In this study we have developed and validated a quantitative PCR (qPCR) assay based on the TEF1‐α sequence. Specificity testing demonstrated that it is currently the only qPCR assay that does not cross‐react with closely related Phyllosticta species. The assay is sensitive and can detect a single copy of the TEF1 gene in a reaction, it is highly repeatable and reproducible and can be used for testing of the sticky tapes from spore traps as well as citrus fruit samples. High‐throughput sequencing (HTS) of the DNA barcodes ITS1 and TEF1 was also explored for the detection and discrimination of P. citricarpa. The limit of detection of the HTS was 1000 spores on a daily spore trap tape. This study makes an important improvement to the diagnostics of the CBS and the methods developed can also be applied to improve the surveillance and early detection of the pathogen when linked to spore samplers in the field.

Effectiveness of snow harvesting and water productivity practices in combatting climate change‐induced drought in a Himalayan district of Nepal

Effectiveness of snow harvesting and water productivity practices in combatting climate change‐induced drought in a Himalayan district of Nepal

Pollination system and reproductive failure in an endangered semi-evergreen tree, Syzygium myhendrae (Bedd. ex Brandis) Gamble

• The relationship between the phenology of Syzygium myhendrae and climatic factors was studied. • Visitation frequency and visitor activity index were calculated for each floral visitor to confirm its pollination efficiency. • The flowers were exclusively visited by honeybees, rockbee, wasps, psyllids and beetles. • Eulophid and pteromalid parasitic wasps were limited the fruit set rate and induced premature fruit drop. • Higher bud drop, flower wastage and insect infestation on fruits decreased the fruit production. The present investigation has attempted to analyze the phenology, floral biology, pollination biology, fruit set and insect infestation on Syzygium myhendrae , an endemic and endangered semievergreen tree in the Western Ghats, India. This tree exhibited a bi-annual mode of flowering and fruiting behavior. The flowers lack strong fragrance, colorful and attractive petals, or sepals; however, the dense stamens functioned as a major floral advertisement. The flowers were exclusively visited by insects ( Apis dorsata, A. mellifera, A. cerana, A. florea, Tetragonula iridipennis , wasps, psyllids and beetles). Among these, A. dorsata was the most frequent and effective pollinator. The infestation by Eulophid and Pteromalid wasps has limited the fruit set rate and induced premature fruit drop in S. myhendrae. The damage incidence has ranged from 82% to 100%. Higher bud drop, flower wastage and insect infestation on fruits have resulted in lower fruit production in S. myhendrae . This endangered tree has reproduced via outcrossing using stamen advertisement and pollen rewards. Eventually, higher flower wastage and fruits infestation have resulted in lesser fruit production, seed maturation and seedling establishment. These factors might have jointly resulted in the reproductive failure and limited distribution of this endemic and endangered species.

Bacterial Symbionts in Ceratitis capitata.

Simple SummaryThe Mediterranean fly (Medfly), Ceratitis capitata, is considered one of the world’s most destructive fruit pests, as it can attack commercially important fruit, thus causing considerable economic damages, estimated to be more than 2 billion dollars annually. The yield reductions are mainly due to the damage incurred when larvae feed directly on the pulp, inducing the premature fruit drop. Additionally, oviposition holes facilitate secondary fungal and bacterial infections, further reducing the yields. Integrated pest management (IPM) strategies for medfly control are highly dependent on the use of insecticides, which, however, pose environmental concerns. Alternative strategies include the Sterile Insect Technique (SIT), which aims to eliminate or suppress pest insects without using pesticides. Lately, the medfly microbiota has been explored to develop new control strategies for insect pests and insect vectors. Here, we report the characterization of the microbial communities associated with selected organs of three different populations of C. capitata to identify possible candidates for a Symbiotic Control approach. Our findings provide new knowledge about the microbiota associated with C. capitata and stress the characterization of microbial symbionts as possible tools for a Symbiotic Control approach to implementing the pest management programs.Ceratitis capitata (Diptera: Tephritidae) is responsible for extensive damage in agriculture with important economic losses. Several strategies have been proposed to control this insect pest including insecticides and the Sterile Insect Technique. Traditional control methods should be implemented by innovative tools, among which those based on insect symbionts seem very promising. Our study aimed to investigate, through the 16S Miseq analysis, the microbial communities associated with selected organs in three different medfly populations to identify possible candidates to develop symbiont-based control approaches. Our results confirm that Klebsiella and Providencia are the dominant bacteria in guts, while a more diversified microbial community has been detected in reproductive organs. Concertedly, we revealed for the first time the presence of Chroococcidiopsis and Propionibacterium as stable components of the medfly’s microbiota. Additionally, in the reproductive organs, we detected Asaia, a bacterium already proposed as a tool in the Symbiotic Control of Vector-Borne Diseases. A strain of Asaia, genetically modified to produce a green fluorescent protein, was used to ascertain the ability of Asaia to colonize specific organs of C. capitata. Our study lays the foundation for the development of control methods for C. capitata based on the use of symbiont bacteria.

Citrus Canker—Distribution, Taxonomy, Epidemiology, Disease Cycle, Pathogen Biology, Detection, and Management: A Critical Review and Future Research Agenda

Xanthomonas citri subsp. citri, a causative agent of the citrus canker (CC) disease, belongs to one of the essential groups of the bacterial phytopathogen family, Xanthomonadaceae. It has been a potential threat to the globally significant citrus fruit crop, which has remained under investigation for disease management and epidemiology since the 1980s. In Pakistan, the average yield of citrus is 11 t/ha, which is lower than other countries, including China, Brazil, and India, having average productions of 27, 26, and 22 tons/hectare, respectively. Citrus canker is one of the most devastating diseases, posing a significant threat to crop yield and fruit quality. To date, five distinct types (or forms) of the citrus canker have been recognized; the Asiatic (Canker A) form is most destructive and affects most citrus cultivars. Severe infection outcomes include dieback, defoliation, severely blemished fruit, premature fruit drop, and reduced fruit quality. The infection increases under humid, warm, cloudy climate, wind, and heavy rainfall. The analysis of plasmid and chromosomal DNA of X. citri subsp. citri depicted an evolutionary relationship among pathovars of Xanthomonas. The extensive study on the genome of X. citri subsp. citri has contributed to the current knowledge of plant host recognition of pathogens, host specificities, dissemination, and propagation. Regulatory programs, i.e., quarantine or exclusion, continued to be practiced, prohibiting infected citrus plant material into the existing stock. Other measures include removal of inoculums sources, resistant hosts, protective copper-containing sprays, and windbreak systems. In this review, we explored the latest trends in the areas of epidemiology, pathogenome, detection, host–pathogen interaction, biofilm formation, and management of X. citri subsp. citri.

Impact of diseases and pests on premature fruit drop in sweet orange orchards in São Paulo state citrus belt, Brazil.

Despite technical improvements in the citrus chain and leadership in orange production achieved in the past decades, premature fruit drop remains a major component of crop loss in São Paulo state citrus belt, the largest sweet orange production area in the world. The present study aimed to determine, during five consecutive seasons, the impact of the diseases and pests on premature fruit drop in the orange belt. Fruit drop due to the main diseases and pests averaged approximately 11.0%, which corresponded to approximately 63% of the annual fruit drop. The average fruit drop rate due to fruit borer and fruit flies combined was 4.0%, Huanglongbing (HLB) 3.3%, black spot 2.6%, leprosis 1.0% and citrus canker 0.3%. The average amount of fruit drop (million 40.8kg boxes) and value of crop losses (million US$ dollars), in five seasons, were 12.7 and 66.2 for fruit borer/fruit flies, 11.0 and 57.9 for HLB, 8.1 and 42.2 for black spot, 3.1 and 15.6 for leprosis, and 0.9 and 4.9 for citrus canker, respectively. Fruit borer and fruit flies (combined), HLB, black spot, leprosis and citrus canker are, in this order, the main diseases and pests in the orange belt of São Paulo state. All of these causes significantly increased the overall fruit drop rate in the evaluated seasons. The results will contribute to the development of the Brazilian citrus industry, while showing to other citrus-growing regions the potential that diseases and pests have to jeopardize production. © 2022 Society of Chemical Industry.

Pre‐dispersal seed predation could help explain premature fruit drop in a tropical forest

Abstract Pre‐dispersal seed mortality caused by premature fruit drop is a potentially important source of plant mortality, but one which has rarely been studied in the context of tropical forest plants. Of particular interest is premature fruit drop triggered by enemies, which—if density dependent—could contribute to species coexistence in tropical forest plant communities. We used a long‐term (31 year) dataset on seed and fruit fall obtained through weekly collections from a network of seed traps in a lowland tropical forest (Barro Colorado Island, Panama) to estimate the proportion of seeds prematurely abscised for 201 woody plant species. To determine whether enemy attack might contribute to premature fruit drop, we tested whether plant species abscise more of their fruit prematurely if they (a) have attributes hypothesised to be associated with high levels of enemy attack and (b) are known to be attacked by one enemy group (insect seed predators). We also tested (c) whether mean rates of premature fruit drop for plant species are phylogenetically conserved. Overall rates of premature fruit drop were high in the plant community. Across all species, 39% of seeds were abscised before completing their development. Rates of premature seed abscission varied considerably among species and could not be explained by phylogeny. Premature seed abscission rates were higher in species which are known to host pre‐dispersal insect seed predators and species with attributes that were hypothesised to make them more susceptible to attack by pre‐dispersal enemies, namely species which (a) have larger seeds, (b) have a greater average height, (c) have temporally predictable fruiting patterns and (d) are more abundant at the study site. Synthesis. Premature fruit drop is likely to be a major source of seed mortality for many plant species on Barro Colorado Island. It is plausible that pre‐dispersal seed enemies, such as insect seed predators, contribute to community‐level patterns of premature fruit drop and have the potential to mediate species coexistence through stabilising negative density dependence. Our study suggests that the role of pre‐dispersal enemies in structuring tropical plant communities should be considered alongside the more commonly studied post‐dispersal seed and seedling enemies.

EVALUATION OF ROMANIAN APPLE CULTIVARS IN LATVIA

Commercial apple growing in Latvia feels some lack of climate adapted modern cultivars. Development of fruits and trees is limited by active growth period (temperatures over 10 °C) of 135-150 days and active temperature sum of 1700-2100 °C. Winter hardiness also is essential, as temperatures below −30°C occur each 10 years, and winter thaws are frequent. Annual precipitation 692 mm favours development of fungal diseases. In 2016, a trial of four Romanian apple cultivars resistant to scab - ‘Aura’, ‘Ciprian’, ‘Jonaprim’ and ‘Romus 3’ was established in Latvia. Trees were grafted on dwarfing rootstock B.396 and planted as 1-year old whips in 3 replications with 2-3 trees in each replication, and trained as slender spindle. Commercial cultivars ‘Auksis’ (midseason) and ‘Ligol’ (late) were used as controls. Most trees of all cultivars in the 2nd year (2017) developed a well branched frame, from 37.5% trees of ‘Ciprian’ to 90% trees of ‘Jonaprim’. The first fruits were harvested in 2017, and commercially significant yield was achieved in 2018, except ‘Jonaprim’, which had the slowest yield increase. By the sum of yield in 2017-2021 ‘Aura’ and ‘Jonaprim’ surpassed commercial cultivar ’Auksis’, but were less productive than ‘Ligol’. Only slightly lower summary yield was obtained from ‘Ciprian’, while ‘Romus 3’ showed the lowest productivity and strong premature fruit drop. Very good taste panel results were obtained for ‘Aura’ and ‘Ciprian’, 7 to 8 points in average. Their fruits also kept well, to 5 months in common storage at 2 ±1oC. Fruits of the other two cultivars were too small and had mediocre or poor taste. The weather conditions in the trial years allowed to test susceptibility to spring frosts and drought. All cultivars showed good tree health after over-wintering. ‘Jonaprim’ and ‘Auksis’ in 2019 had the highest spring frost injury, 60-70% of fruits, while least injured were ‘Aura’ (21.5%) and ‘Ciprian’ (27.7%). Extreme drought in summer of 2021 led to low average fruit mass of all cultivars including the normally large fruited ‘Ligol’ (120-130 g), while in other years ‘Aura’ had fruits over 180 g, very attractive yellow with red stripes. Fruits of ‘Ciprian’ also had good size, but in some years too dark over-colour. ‘Aura’ and ‘Ciprian’ showed good adaptation to the climate of Latvia and can be recommended for wider trials at farms.

Citrus black spot severity related to premature fruit drop in sweet orange orchards

AbstractCitrus black spot (CBS) may cause substantial yield loss in sweet orange orchards. Severity and distribution of CBS symptoms on fruit may be variable and not all affected fruit may fall. However, the relationship between CBS symptoms and yield loss has not yet been investigated. This study aimed to determine CBS characteristics associated with premature fruit drop. A set of fruit was removed from the tree to measure the detachment force and another set of dropped fruit was collected from Pera and Valencia sweet orange orchards in three municipalities in Sao Paulo state, Brazil. CBS severity, distance of the nearest lesion from the peduncle, and fruit peel colour were assessed in both fruit sets. CBS severity was consistently related with the force to remove fruit and hence associated to the premature fruit drop, irrespective of the cultivar and location. The segmented linear regression showed that the fruit detachment force was reduced abruptly, at a rate of 7% for every 1% increase in disease severity up to 4.2 ± 1.0% CBS severity and, above this breakpoint, the force decreased slightly at a rate of 1.5%. Taking into account all orchards, the proportion of dropped fruit with CBS severity higher than the breakpoint was approximately 90%. This study provides a better understanding of the relationship between CBS severity and premature fruit drop of sweet orange. CBS severity breakpoint may be used as a reference to anticipate harvest and reduce CBS‐related yield loss in orchards intended for juice processing.

Economics of Mango Production, Marketing System and Constraints Faced by Growers in Lucknow District of Uttar Pradesh

Background: Mango is an important commercially grown fruit crop and it is very popular due to its wide range of adaptability and high nutritive value. The present study was conducted in Lucknow district of Uttar Pradesh to know the economics of mango production, marketing system and constraints faced by growers in production and marketing.Methods: The study was based on primary data collected from 200 respondents with the help of personal interview using a pre-tested questionnaire, semi-structured interview schedule and open discussion method. Estimate the cost and return of mango cultivation by using simple cost and return equation. The marketing system of mango involved market channels, margin, cost, price spread, producer share in consumer’s rupee and marketing efficiency was worked out by tabular analysis. The Garrett’s ranking technique was adopted to rank the constraints faced by the growers in production and marketing of mango.Result: The total cost of mango cultivation was found Rs.23450.67/ha. Out of which, the variable cost was constituted Rs.18316.39/ha (78.11%) and fixed cost was Rs.5134.28/ha (21.89%). The net return over the cost of cultivation was found to be highest as Rs. 62811.59/ha at the 4th growing stage of orchard. It was observed that net price received by growers, per cent share in consumer’s rupees and market efficiency was highest in channel-III followed by channel-II and III, where the numbers of marketing intermediaries were involved lesser. The market margin, cost and price spread were highest in channel-I followed by Channel-II and III. The channel-III was found to be most efficient and profitable for farmers in the study area. Among the production and marketing constraints faced by mango growers, the shattering of flowers and premature fruit drop were cited as the most serious constraints. While alternate and irregular bearing creates glut with excess production in one year and results in low production in the next year was reported as second major constraint. Usually, contractors were not paying money to the orchard owner in the lump sum amount and at the appropriate time. Thus, the second major constraint was delayed and irregular payments to producers.

Cold stress triggers premature fruit abscission through ABA-dependent signal transduction in early developing apple.

Fruit abscission is a complex physiological process that is regulated by internal and environmental factors. During early development, apple fruit are exposed to extreme temperature fluctuations that are associated with premature fruit drop; however, their effect on fruit abscission is largely unknown. We hypothesized that fruit abscission is triggered by cold stress and investigated the molecular basis of premature fruit drop using RNA-Seq and metabolomics data from apple fruit undergoing abscission following cold stress in the field. Genes responsive to abscisic acid signaling and cell wall degradation were upregulated during abscission, consistent with the increased abscisic acid concentrations detected by liquid chromatography-mass spectrometry. We performed ex vivo cold shock experiments with excised tree subunits consisting of a branch, pedicel, and fruit. Abscission induction occurred in the cold-stressed subunits with concurrent upregulation of abscisic acid biosynthesis (MdNCED1) and metabolism (MdCYP707A) genes, and ethylene biosynthesis (MdACS1) and receptor (MdETR2) genes in the pedicel. Another key finding was the activation of cytoplasmic streaming in abscission-zone cells detected by electron microscopy. Our results provide a novel insight into the molecular basis of fruit abscission physiology in response to cold stress in apple.

Effect of enhanced nutritional programs and exogenous auxin spraying on huanglongbing severity, fruit drop, yield and economic profitability of orange orchards

Citrus trees infected with ‘Candidatus Liberibacter asiaticus’, the bacterium associated with huanglongbing (HLB), present asymmetric chlorosis and yellow veins on leaves, nutritional deficiencies due to impaired water and nutrient uptake by a reduced root system, and also premature fruit drop and progressive yield reduction. This motivates growers to adopt additional foliar nutritional and auxin plant hormone derivative 2,4-D sprays to try to mitigate HLB damage. However, long-term studies that compare the yield of both well-nourished asymptomatic (AT) and symptomatic trees (ST) under these programs are absent in the literature. This work presents a long-term field study that compared the effect of several enhanced nutritional programs (ENP) and exogenous auxin spray on the progress of HLB severity and on the fruit drop and yield of AT and ST. Moreover, the benefit–cost ratio of each program was estimated. Programs did not contribute to reduce HLB severity over time (mean final HLB severity > 50%). None of the programs contributed to reduce the percentage of fruit drop (remained higher than 40% in symptomatic branches) nor to increase fruit production of ST. Benefit/cost ratio was, on average, 1.49 times higher in AS than in ST. Grower standard nutritional program had the higher benefit-cost ratio (132.8 and 84.7 for AT and ST, respectively) than the others more costly programs. These observations suggest that ENP and 2,4-D sprays are not important in controlling HLB nor mitigating its damages, and their use, instead of removing diseased trees, can build up inoculum within a region. Diseased trees are inoculum source that psyllid vector Diaphorina citri can acquire the bacteria, transmit to a healthy tree, and, consequently, spread HLB. Rigorous vector control by insecticide sprays and eradication of diseased plants are still the best long-term HLB control.

Antifungal activity of some alternative control against mango anthracnose in Senegal

Mango production in the South of Senegal is exposed to intensive rainfall from late May to October, with high temperature and moisture levels. These conditions are conducive for the development of anthracnose caused by Colletotrichum gloeosporioides (sensu lato) and leading to an absolute necessity for adequate control measures for good quality mango production. Anthracnose disease causes both pre- and postharvest fruit spots and fruit rot as well as premature fruit drop. The purpose of this study was to test the efficacy of several fungicide alternatives (fertilizers and biological control agents) against mango anthracnose in Senegal comparatively to standard fungicides. Field trials were set up in contrasting climates conditions and involved three orchards in the Northern part of the country with a short rainy season and dry climate over 9 to 10 months a year and three other orchards in the Southern part of Senegal with in a humid tropic environment. No anthracnose was recorded in the orchards in northern Senegal. In the south, in contrast, the disease was actual. Among the treatments tested for the control of anthracnose, Sodium molybdate were found effective after fungicides (azoxystrobin and thiophanate methyl). All the alternative treatments to fungicides provided a statistically significant control to the disease as compared to the control

First Report of Alternaria alternata and Fusarium spp. Causing Brown Apical Necrosis in Walnut Fruit in Southern Chile

This is the first report of Alternaria alternata and Fusarium spp. causing brown apical necrosis (BAN) in walnut fruit in southern Chile. English walnut (Juglans regia L.) is the second most widely grown fruit in Chile. The bacterial pathogen Xanthomonas arboricola pv. juglandis affects walnut production in Chile and has been associated with apical necrosis symptom and premature fruit drop; this research focused on determining if fungal pathogens were associated with this damage. The presence of BAN in commercial walnut orchards in southern Chile reveals the need for improved phytosanitary programs to control this disease.

A Brief Review ofResseliella citrifrugis(Diptera: Cecidomyiidae), a Lesser-Known Destructive Citrus Fruit Pest

AbstractThe gall midge, Resselielia citrifrugis Jiang (Diptera: Cecidomyiidae), is a major citrus pest in China. The pest occurs widely in regions with tropical, subtropical, and temperate climates. Larvae feed inside the fruit, leading to premature fruit drop or damaged fruits. An infested fruit can have hundreds of larvae in it. The extent of losses varies, usually between 10 and 100%, depending on the grove management level. Resselielia citrifrugis hosts include common citrus varieties. China has no area-wide management program against the pest. Field pest management measures include grove sanitation, fruit bagging, and pesticide applications. This review identifies three scientific and technological gaps that need to be filled to protect the U.S. citrus industry from this pest. First, the taxonomical and systematic status of R. citrifrugis needs to be clarified and validated before the pest can be effectively regulated. Second, traps and/or lures for early detection of the pest need to be developed before the pest arrival. Third, pest risk mitigation measures against the pest need to be evaluated and strengthened.

EXPRESIÓN DIFERENCIAL DEL COMPLEJO ARF8-IAA25-like-TIR1 DURANTE EL SÍNDROME POST-POLINIZACIÓN EN DOS GENOTIPOS DE VAINILLA

La transición de flor a fruto (TFF) es un proceso crítico en la reproducción de las angiospermas. Su regulación responde a un mecanismo hormonal que involucra genes de las familias ARF, Aux/IAA y TIR1; sin embargo, aún no se conoce con detalle su funcionamiento en plantas con síndrome post-polinización como las orquídeas. El conocimiento acerca de los procesos moleculares que dirigen la TFF permitirá atender problemáticas como la ’caída prematura de fruto’, que afectan al cultivo de vainilla a nivel global. Con el objetivo de describir el proceso de TFF en vainilla (Vanilla planifolia Andrews) se analizó, mediante qPCR, el nivel de expresión relativa (ER) de los genes VpARF8, VpIAA25-like y VpTIR1 durante 45 días después de la polinización (ddp), en un genotipo ‘tolerante a caída’ (CH-I) y otro ‘susceptible’ (CH-VI). Se realizó purificación de ARN, síntesis de ADNc y se cuantificó la ER. Los datos de expresión se analizaron mediante ANOVA de dos factores (genotipo y tiempo de desarrollo) y análisis de conglomerados tipo Heatmap (genes, genotipo, fase del desarrollo). Se detectaron diferencias significativas en los niveles de ER de los genes VpARF8 y VpTIR1 durante los 45ddp, sin variación entre genotipos; mientras que el gen VpIAA25-like presentó diferencias únicamente entre genotipos. En el análisis multivariado se distinguieron dos perfiles de expresión que pueden estar relacionados con las etapas de polinización y fecundación en vainilla, mismas que ocurren en momentos distintos debido al síndrome post-polinización. En el genotipo CH-I la fecundación ocurre alrededor de 45ddp, mientras que en el genotipo CH-VI no se observa. Los tres genes presentaron cambios en los niveles de expresión durante el síndrome post-polinización en CH-I, pero no en CH-VI, lo que sugiere que el fenómeno conocido como ’caída prematura de fruto’ es un efecto genotipo dependiente, definido por una diferencia en el mecanismo de regulación del metabolismo de auxinas.

Comparative Genomics of Xanthomonas citri pv. citri A* Pathotype Reveals Three Distinct Clades with Varying Plasmid Distribution.

Citrus bacterial canker (CBC) is an important disease of citrus cultivars worldwide that causes blister-like lesions on host plants and leads to more severe symptoms such as plant defoliation and premature fruit drop. The causative agent, Xanthomonas citri pv. citri, exists as three pathotypes—A, A*, and Aw—which differ in their host range and elicited host response. To date, comparative analyses have been hampered by the lack of closed genomes for the A* pathotype. In this study, we sequenced and assembled six CBC isolates of pathotype A* using second- and third-generation sequencing technologies to produce complete, closed assemblies. Analysis of these genomes and reference A, A*, and Aw sequences revealed genetic groups within the A* pathotype. Investigation of accessory genomes revealed virulence factors, including type IV secretion systems and heavy metal resistance genes, differentiating the genetic groups. Genomic comparisons of closed genome assemblies also provided plasmid distribution information for the three genetic groups of A*. The genomes presented here complement existing closed genomes of A and Aw pathotypes that are publicly available and open opportunities to investigate the evolution of X. citri pv. citri and the virulence factors that contribute to this serious pathogen.

Mechanical Crop Load Management (CLM) Improves Fruit Quality and Reduces Fruit Drop and Alternate Bearing in European Plum (Prunus domestica L.)

(1) Background: With ca. 10 million tons of annual production worldwide, the plum (Prunus ssp.) ranks as a major fruit crop and can suffer from small fruit size, premature fruit drop and alternate bearing, which are addressed in this paper using a range of crop load management (CLM) tools. (2) Methods: Sixty 10-year-old European plum cv. “Ortenauer” trees on dwarfing St. Julien INRA GF 655/2 rootstock (slender spindle; 4.25 × 2.80 m) in a commercial orchard near Bonn (50°N), Germany, were thinned in 2 years and flower intensity assessed in the following year. Thinning was performed either mechanically (type Bonn/Baum) or chemically, with ATS (ammonium thiosulfate) or ethephon (Flordimex), or by a combination of mechanical and chemical methods, to improve fruit quality and the proportion of Class 1 fruit. Adjacent un-thinned trees served as controls. (3) Results: Natural fruit drop in June was reduced from 290 fruits per tree in the un-thinned controls to 265 fruits after ATS blossom treatment, and to 148 fruits after mechanical thinning at 380 rpm at a 5 km/h tractor speed at full bloom. The un-thinned control trees developed a large number of small, undersized fruits. The yield of Class 1 fruits increased per tree from 47% in the un-thinned controls, up to 69% after crop load management. Sugar content and fruit firmness were unaffected. (4) Conclusions: The study has shown that fruit quality (i.e., fruit size) and financial returns could be improved by either mechanical (380 rpm at 5 km/h) or chemical thinning, or a combination of both.

Inoculum Dynamics and Infection of Citrus Fruit by Phyllosticta citricarpa.

Citrus black spot, caused by Phyllosticta citricarpa, is characterized by fruit blemishes and premature fruit drop, resulting in significant economic losses in summer rainfall areas. The pathogen forms both conidia and ascospores during its life cycle. However, the occurrence of these spores and their contributions to infection of fruit in field conditions are not well understood. Our research using direct leaf litter monitoring and volumetric spore trapping in Queensland orchards revealed that pseudothecia and ascospores in leaf litter as well as trapped ascospores had low abundance, while pycnidia and conidia were highly abundant. Both P. citricarpa and endophytic Phyllosticta spp. were identified, with P. citricarpa being dominant. In replicated field trials, we determined that infection of Imperial mandarin fruit by P. citricarpa occurred from fruit set until week 20 of fruit development, with the key infection events taking place between weeks 4 and 16 in Queensland subtropical conditions. These results demonstrate that protecting fruit during weeks 4 to 16 significantly reduced P. citricarpa infection. We found no significant correlation between the disease incidence in fruit and P. citricarpa conidial abundance in leaf litter or ascospore abundance measured by volumetric spore trapping. Therefore, it is suggested that inoculum sources in the tree canopy other than those detected by spore trapping and direct leaf litter monitoring may play a major role in the epidemiology of citrus black spot. Improved knowledge regarding epidemiology of P. citricarpa and an understanding of propagules causing infection may aid in development of more effective disease management strategies.

Premature Apple Fruit Drop: Associated Fungal Species and Attempted Management Solutions

The aim of this research was to determine the incidence and possible causal pathogen(s) of premature apple fruit drop (PAFD), and also to assess some fungicides for controlling the disease organisms, in order to promote a sustainable system in orchards. The prevalence and natural incidence of apple fruit drop in cv. Anna was assessed during the 2017–2018 growing seasons in Nubaria and Cairo–Alexandria regions, Egypt. Phytopathogenic fungi were isolated from dropped fruit, and four fungicides, pyraclostrobin + boscalid, difenoconazole, carbendazim, and thiophanate methyl, were tested against the diseases in vitro and under naturally occurring infections in the field. Several phytopathogenic fungi, including Alternaria alternata, Cladosporium cladosporioides, Fusarium semitectum, and Penicillium spp., were associated with apple fruit drop. A. alternata was the most frequently isolated fungus occurring during the investigation. Pathogenicity tests confirmed that the maximum percentage of apple fruit drop was noted when petioles and fruits were inoculated with mixed fungal pathogens using branch sections with fruit. In vitro tests showed that the fungicides had a variable effect against the fungal isolates depending on the concentration used. All fungicides completely inhibited the growth of A. alternata, C. cladosporioides, and F. semitectum at 400 mg·L−1. Under naturally occurring infections, thiophanate methyl applied at fruit set had the greatest effect (81.68%) against PAFD, followed by difenoconazole (73.76%), pyraclostrobin + boscalid (70.29%), and carbendazim (66.34%). The results indicated that PAFD may in part be a result of diseases caused by certain phytopathogenic fungi, which could be controlled using a number of fungicides applied at the beginning of fruit set.