Search Results (18)

Alternaria brown spot is an important fungal disease in citrus. The infection of young citrus organs usually coincides with strong rainfall, which causes low efficiency of fungicides and the outbreak of this disease. Here, the microbiomes of the asymptomatic peels, the spot edge, and the center of citrus fruits were compared to reveal the commensal microbes as alternative control methods for the Alternaria pathogen. As the disease severity increased from the asymptomatic peels to the spot edge and the center, the bacterial communities were more severely changed than the fungal communities. Both the bacterial diversity, represented by the Shannon diversity index, and the bacterial composition and structure significantly decreased and altered, respectively. Increased Alternaria, in relative abundance, correlated positively with bacterial genera like Massilia and Sphingomonas, while negatively correlating with bacterial genera like Delftia and Pantoea. In addition, Alternaria fASV1 positively correlated with several top ASVs of 1174_901_12 and Sphingomonas. These results suggest that the bacterial communities respond to Alternaria brown spot by Alternaria–bacteria cross-kingdom interactions; these responsive bacteria are worth testing experimentally. Full article

The Alternaria alternata tangerine pathotype causes Alternaria brown spot, a devastating disease of susceptible tangerine varieties and their hybrids. Alternaria citri toxin (ACT) is the primary virulence factor, but the regulatory mechanisms governing ACT synthesis remain unclear. Deubiquitinating enzymes maintain ubiquitination homeostasis and regulate fungal pathogenicity, yet their role in A. alternata remains unexplored. We characterized 13 ubiquitin-specific protease (UBP) family members in A. alternata tangerine pathotype. Six UBP genes (Aaubp2, Aaubp3, Aaubp4, Aaubp6, Aaubp14, and Aaubp15) regulated mycelial growth. Aaubp14 deletion abolished sporulation, while mutations of Aaubp3, Aaubp4, Aaubp6, Aaubp8, and Aaubp15 altered conidial morphology. qRT-PCR demonstrated distinct host-induced expression patterns among Aaubp genes. Pathogenicity tests showed that ΔAaubp6, ΔAaubp14, and ΔAaubp15 mutants failed to produce lesions on Citrus reticulata cv. Hongjv leaves. Moreover, Aaubp14 deletion significantly suppressed ACT biosynthesis gene expression and blocked ACT production. Comparative proteomics showed Aaubp14 regulates ACT biosynthesis by modulating protein ubiquitination in metabolic pathways and controls pathogenicity via a complex network. Our findings elucidate Aaubp gene function in development and pathogenicity, particularly the Aaubp14-mediated regulation mechanism, providing insights into ubiquitination-mediated pathogenicity in phytopathogenic fungi. Full article

Alternaria Brown Spot (ABS) is one of the most critical diseases affecting susceptible mandarins worldwide, being a limiting factor in their cultivation. Although there are numerous reports on effective plant protection products against the disease, field control is failing. In the literature, some of the results are contradictory, depending on the study and experimental scale. Therefore, this paper aimed to analyze the empirical evidence to answer the following questions: (i) What plant protection products have been used to control ABS? (ii) What are the methodologies used to test the substances? (iii) Why is ABS field control failing? An extensive literature search was performed in five databases: WoS, Scopus, Google Scholar, PubMed, and SciELO. The search string used was “Alternaria alternata” AND “Citrus”. Records were classified into ten groups according to their main topic. Group 3 “microorganisms and natural substances” and group 4 “fungicides” were full-text reviewed for data extraction (98 reports). Details of the microorganisms, natural substances, and fungicides used against A. alternata, as well as summaries of the methodologies, are provided. During this research, we highlighted significant aspects that may be hindering the control of Alternaria alternata in citrus: long periods of fruit sensitivity, abundance and floatability of inoculum, rapid infections, the appearance of resistance to fungicides, moderate effectiveness inhibiting the germination of conidia, uncertainty about the times of application, and persistence of the products. Full article

Citrus alternaria brown spot, caused by the tangerine pathotype of Alternaria alternata, is one of the most severe fungal diseases affecting citrus crops. Currently, there is a critical need for rapid and visual detection techniques to identify the tangerine pathotype of A. alternata. In this study, a novel detection system was developed by combining recombinase polymerase amplification (RPA) with CRISPR/Cas12a technology, targeting the ACTT3 gene specific to the tangerine pathotype of A. alternata. Through optimization of reaction time and component concentrations, the assay demonstrated a detection sensitivity of 1 pg μL⁻¹ within 40 min at a constant temperature of 37 °C. The results can be visually interpreted using nucleic acid test strips, offering advantages in specificity, sensitivity, and speed. This system has been successfully validated for the rapid detection of the pathogen within plant tissues, including leaves and fruits, providing an efficient and practical solution for real-time field detection of the tangerine pathotype of A. alternata. Full article

Citrus Alternaria brown spot caused by the necrotrophic fungal pathogen of the tangerine pathotype of Alternaria alternata causes yield losses in global tangerine production. In this study, we focus on a cytochrome P450 monooxygenase encoding gene, Aacp1, for its role in the sporulation, toxin production, and virulence of the tangerine pathotype of Alternaria alternata. Aacp1-deficient mutants (∆Aacp1) produced significantly fewer conidia than the wild-type strain. Chemical assays demonstrated that Aacp1 plays a negative role in resistance to oxidant stress and biosynthesis of ACT toxin. Virulence assays revealed that ΔAacp1 fails to induce necrotic lesions on detached Hongjv leaves. Transcriptomic analyses of WT and ΔAacp1 revealed that many metabolic process genes were regulated. Furthermore, our results revealed a previously unrecognized Aacp1 affected the expression of the gene encoding a naphthalene dioxygenase (AaNdo1) for sporulation and full virulence. Overall, this study revealed the diverse functions of cytochrome P450 monooxygenase in the phytopathogenic fungus. Full article

Mandarin fruits with good physicochemical quality are desired for the fresh fruit market. The ease of crosses among citrus taxa has been used to expand genetic variability and select new varieties for different fruit markets. With the aim of selecting new commercial varieties, eleven hybrids obtained from the cross between Murcott tangor and Ponkan mandarin were grafted on Rangpur lime rootstock and evaluated for genetic and phenotypical characteristics. It was observed that there are significant differences in the fruit shape, content, and organoleptic characteristics in juice and differential responses to Alternaria alternate, the causal agent of the Alternaria brown spot (ABS) disease. The phenotypic and molecular characterization showed two groups of hybrids, those that resemble Ponkan and others with characteristics closer to the parental Murcott. There are significant differences in the period of fruit maturation among the hybrids and their parents. Considering the parents stand out as the main commercial varieties in the Brazilian mandarin fresh fruit market, these new hybrids have great potential for commercial acceptance and could be an excellent option to extend the period of supply of mandarins. Genetic and phenotypical information will be fundamental for the registration and/or protection of new varieties, which are essential steps that precede the commercial release to the producer. Full article

Alternaria brown spot (ABS), caused by Alternaria alternata, is one of the main citrus diseases that causes heavy production losses and reductions in fruit quality worldwide. The application of chemical fungicides has a key role in the management of ABS. In this study, 48 isolates of A. alternata collected from citrus orchards since 2014 were tested in vitro for their sensitivity to pyraclostrobin and fludioxonil, the latter being temporarily registered in Italy since 2020. Pyraclostrobin sensitivity was determined using spore germination and mycelial growth assays. The effective concentration inhibiting 50% of fungal growth (EC₅₀) was determined for each isolate. The sensitivity assays showed that the majority of A. alternata isolates tested were sensitive to pyraclostrobin. EC₅₀ values of fludioxonil in a mycelial growth assay indicated that 100% of isolates were sensitive to this fungicide. The analysis of the cytochrome b gene showed that none of the 40 isolates with a different sensitivity profile had the G143A mutation, and the subgroup of 8 isolates analyzed by real-time PCR did not carry the G137R and F129L mutations. A subset of four more sensitive and two reduced-sensitive isolates was chosen to assess sensitivity on detached citrus leaves treated with pyraclostrobin at the maximum recommended label rate. Disease incidence and symptom severity were significantly reduced, with a small reduction reported in leaves inoculated with the reduced-sensitive isolates. Furthermore, there was no correlation between sensitivity and fitness parameters evaluated in vitro (mycelium growth and sporulation rate). These findings help the development of monitoring resistance programs and, consequently, set up effective anti-resistance strategies for managing ABS on citrus orchards. Full article

Both of the two citrus diseases, Alternaria brown spot (ABS) and Anthracnose, caused by Alternaria and Colletotrichum spp., respectively, can produce leaf lesions which are hard to differentiate. These two diseases have been confused as causal agents of brown spot for over a decade in China. In this study, citrus leaves with or without brown spot were collected from Zhaoqing, Guangdong and Wanzhou, Chongqing, and were further used for the taxonomic and functional comparisons between the co-occurring Alternaria and Colletotrichum species. In the amplicon sequencing, the average relative abundance and the composition of Alternaria, but not Colletotrichum, increased (from 0.1 to 9.9, p = 0.059; and to 0.7, p < 0.05) and significantly altered (p < 0.01) with the brown spot in Zhaoqing and Wanzhou, respectively. Two representative isolates Alternaria sp. F12A and Colletotrichum sp. F12C, from the same brown spot, were proved with different virulence and host response activation to citrus leaves. F12A caused typical symptoms of brown spot with the average spot length expanded to 5 and 6.1 cm, and also altered the citrus global gene expression 48 and 72 h after inoculation. In addition, F12A enriched the expression of genes that were most frequently involved in plant defense. In comparison, F12C caused leaf spot limited to the wounded site, and its milder activation of host response recovered 72 h after inoculation. Our study indicates that the incidence of brown spot in China is caused by Alternaria species, and the ABS should be a fungal disease of major concern on citrus. Full article

Mandarins occupy the third group among citrus produced in Brazil. Primarily aimed at the fresh fruit market, commercial orchards have suffered significant production losses due to the high incidence of diseases, such as the alternaria brown spot (ABS) and huanglongbing (HLB). The fact that 80% of the orchards are composed of two varieties, Ponkan mandarin and Murcott tangor, which are highly susceptible to both diseases, demands intensive chemical control of the diseases in the field, which has resulted in a significant increase in production costs and a negative environmental impact. A total of one hundred and seventy-three mandarin accessions, including several species and hybrids belonging to the Collections of the Instituto Agronômico de Campinas (IAC), were evaluated in the field in an endemic location for the diseases. Evaluations of the incidence and severity of the diseases showed a range of genotypes tolerant to ABS that could replace the current commercial varieties, mainly within the clementine and willow leaf mandarin groups. Although, with differences in incidence and susceptibility to HLB, there are no tolerant varieties to this disease, which significantly impacts the loss of fruit quality. Full article

Alternaria brown spot (ABS) is a disease caused by the fungus A. alternata f. sp. citri, which results in lesions on the fruits, leaves, and branches of several mandarin varieties and their hybrids. Due to the high cost of fungicide application, alternative methods for controlling ABS need to be studied. Therefore, this study aimed to evaluate the use of essential oils (EOs) from different mandarin varieties to mitigate the effects of ABS. The inhibitory effect of different concentrations (1, 2, 4, 8, and 16 μL·mL⁻¹) of the EOs of Fremont IAC 543 mandarin, IAC 2019Maria mandarin, Murcott IAC 221 tangor, and Late IAC 855 willowleaf on the in vitro mycelial growth of the fungus A. alternata was evaluated. Additionally, the curative and preventive effects of these EOs on the ABS symptoms in detached leaves of Murcott IAC 221 tangor were also assessed. The EO of IAC 2019 Maria mandarin induced less mycelial growth, and consequently, a greater inhibition of the growth of the fungus A. alternata at a concentration of 16 μL·mL⁻¹. This EO was more effective for control than the other oils tested. In the detached leaf experiment, both the curative and preventive treatments at a concentration of 16 μL·mL⁻¹ showed lower values of disease severity. Full article

Host-pathogen interactions are the result of the continuously evolving dynamics of the genomic interphases between pathogens and the host plants. Alternaria brown spot (ABS) caused by the pathogen Alternaria alternata is a serious threat to tangerine production. Although recent studies have made significant advances in the characterization of A. alternata virulence factors, a gap exists in the regulation of virulent genes throughout the course of A. alternata infection on host plants. To gain a better understanding of the dynamic defense transcriptome in Alternaria alternata during Infection, we performed a comparative transcriptome approach. After inoculation on citrus, we found that 2142, 1964, 2359 genes were up-regulated, and 1948, 1434, 1996 genes were down-regulated at 12 hours-post-inoculation (hpi), 24 hpi and 48 hpi, respectively. Among these genes, 1333 genes were up-regulated at three time points, and 1054 genes were down-regulated, indicating that most of the differentially expressed genes at the early stage of infection tended to remain differentially expressed at the later stage of infection. In addition to the genes that are known to be part of the infection network in plant-pathogen interactions, many novel genes related to plant-pathogen interaction were identified. Interestingly, our results indicate that A. alternata is able to rapidly alter its gene expression pattern during infection process, which is vital for the successful colonization of the pathogen. Moreover, this rapid alteration of gene expression is likely to be an adaptive mechanism, enabling the pathogen to quickly respond to any changes in the environment and adapt to the host’s defense system. This ability to modify gene expression quickly in the face of environmental changes could play a critical role in the successful establishment of infection. RT-qPCR analysis confirmed that the expression pattern of nine randomly selected genes from the peroxisome pathway were consistent with the RNA-seq data. Our study provided a comprehensive study of the expression of genes during A. alternata infection of citrus, which may facilitate the understanding of host-plant interactions in A. alternata. Full article

Alternaria brown spot disease is caused by the Alternaria alternata tangerine pathotype, which relies on ACT-toxin for infection. At present, all identified ACT-toxin biosynthesis-related genes are multi-copy genes. In this study, we summarized the advances in important host-specific toxins (HSTs), and listed key genes required for the pathogenicity of the A. alternata tangerine pathotype. Toxin virulence test results revealed that different citrus species displayed distinctly different tolerances to ACT-toxin. The extraction method of ACT-toxin crude extract was described in schematic form to make the method easier to understand. In addition, target gene disruption of two copies of ACTT5 (∆∆ACTT5) displayed significantly reduced virulence, indicating that ACTT5 is essential for the pathogenicity of the A. alternata tangerine pathotype. Full article

This paper reviews the threat of citrus pathogens during citrus production, with a focus on two pre-harvest diseases, citrus leaf spot, caused by Alternaria alternata (Fr.) Keissl. (1912) and brown rot, caused by Phytophthora citrophthora (R.E. Sm. and E.H. Sm.) Leonian, (1906) as well as green and blue mold post-harvest disease, caused by Penicillium digitatum (Pers.) Sacc. and P. italicum Wehmer, (1894), respectively. Furthermore, it reviews the role of soluble silicon, Si nutrition in biotic stress mitigation and potential mitigation mechanisms. Previous studies on the use of Si fertilizers have focused on high accumulator Si crops. These have demonstrated the potential of Si to reduce the occurrence of biotic stresses, which takes place through both physical and biochemical mechanisms. However, few studies have demonstrated the potential of Si to mitigate biotic stress in citrus, or the mechanisms involved. There is a clear need for studies on the impact of Si on various stress biochemical pathways in plants generally, and specifically for citrus due to the huge loss caused by pre- and post-harvest pathogens. This will assist in deepening our understanding of the pathophysiology which is essential to develop resistant cultivars. Full article

Alternaria brown spot is a severe disease that affects leaves and fruits on susceptible mandarin and mandarin-like cultivars, and is produced by Alternaria alternata. Consequently, there is an urge to obtain new cultivars resistant to A. alternata, and mutation breeding together with tissue culture can help shorten the process. However, a protocol for the in vitro selection of resistant citrus genotypes is lacking. In this study, four methods to evaluate the sensitivity to Alternaria of mandarin ‘Fortune’ explants in in vitro culture were tested. The four tested systems consisted of: (1) the addition of the mycotoxin, produced by A. alternata in ‘Fortune’, to the propagation culture media, (2) the addition of the A. alternata culture filtrate to the propagation culture media, (3) the application of the mycotoxin to the intact shoot leaves, and (4) the application of the mycotoxin to the previously excised and wounded leaves. After analyzing the results, only the addition of the A. alternata culture filtrate to the culture media and the application of the mycotoxin to the wounded leaves produced symptoms of infection. However, the addition of the fungus culture filtrate to the culture media produced results, which might indicate that, in addition to the mycotoxin, many other unknown elements that can affect the plant growth and behavior could be found in the fungus culture filtrate. Therefore, the application of the toxin to the excised and wounded leaves seems to be the most reliable method to analyze sensitivity to Alternaria of ‘Fortune’ explants cultured in vitro. Full article

In the past decade Colletotrichum gloeosporioides, C. karstii, and Alternaria alternata represent emerging fungal pathogens on citrus in the Mediterranean basin. Selection of tolerant Citrus germplasm offers evaluable long-term solution and should be considered as promising alternative to limit synthetic fungicide application to manage Alternaria and Colletotrichum infections in Citrus groves. In this study, the high variability of pre-harvest disease symptoms among 37 Citrus accessions was investigated in Italy in the most representative Sicilian production districts covering 1500 ha and including 20 homogeneous areas over an eleven-year survey period (2010–2020). Early fruit drop, brown spot, and anthracnose on fruit and leaves, pre-harvest fruit drop, and twig blight and defoliation associated to Alternaria and Colletotrichum spp. were identified on oranges, mandarins, and lemons. Comprehensively, first extensive data obtained herein on field susceptibility within “Tarocco” blood orange group to above disease symptoms clearly indicate as “Tarocco Nucellare 57-1E-1,” “Tarocco Tapi,” “Tarocco Sant’Alfio,” and “Tarocco Catania” accessions should be preferred to remaining Tarocco ones. A broad degree of tolerance or susceptibility was also observed within other Citrus group as it happens for tolerant lemon “Femminello Zagara Bianca,” thus demonstrating a putative resource for further studies to employ in a breeding program for genetic improvement of Citrus. Full article