Identity, pathogenicity and genetic diversity of Lasiodiplodia associated with stem-end rot of avocado in China

Stem-end rot (SER) causes brown necrotic lesions in the pulp near the base of the fruit pedicel and is one of the most devastating postharvest diseases of avocados in all avocado growing regions of the world. China's avocado industry is growing very rapidly, and the planting area is expanding, but little is known about the pathogens and genetic diversity of avocado SER. To determine the causal agents of SER, avocado fruits were sampled from the main avocado-producing areas in China during 2020 and 2021. Fungal isolates were obtained from SER symptomatic avocado fruits and identified by morphology combined with phylogenetic analysis of internal transcribed spacer (ITS), translation elongation factor 1-α (EF1-α) and β-tubulin (TUB2) gene sequences. All 101 isolates belonged to Lasiodiplodia spp., and four Lasiodiplodia species were identified, namely L. pseudotheobromae (59.41%), L. theobromae (24.75%), L. mahajangana (7.92%), L. euphorbiaceicola (1.98%), and six others are classified as Lasiodiplodia sp. (5.94%). There were only slight morphological differences in colonies and conidia of these four species of Lasiodiplodia. The pathogenicity tests showed symptoms of SER, and the 92.08% of the isolates exhibited a high level of virulence on avocado (disease index > 70), related to the disease severity on avocado fruit. All tested isolates grew well under conditions from 23 to 33℃. There was a significant difference in mycelial growth between the four species of Lasiodiplodia after treatment with high temperature or low temperature. L. pseudotheobromae growth was the fastest at 13 to 18℃, but was the lowest at 38℃ (P < 0.05). Red pigment could be produced by all tested isolates after culturing for 7 days at 38℃. The mycelial growth rate was the fastest on PDA medium, and the slowest on OMA medium but promoted spore formation (P < 0.05). In addition, was determined the genetic diversity of Lasiodiplodia pathogenic species associated with SER collected from avocado, mango, guava and soursop fruits was determined. A total of 74 isolates were clustered into 4 main ISSR groups by unweighted pair-group method with arithmetic mean (UPGMA) analysis, and the classification of this group was related to the host. Extensive diversity was detected in the Lasiodiplodia populations. The diverse geographical origins and host species significantly influenced the population differentiation, and most of the genetic variation occurred within populations (P < 0.001). This is the first study to identify the major pathogens of avocado SER in China and to survey their occurrence, pathogenicity and include a comparative analysis of genetic diversity with Lasiodiplodia spp. causing SER on other fruit hosts. Collectively, the Lasiodiplodia species complex affecting avocado showed high pathogenicity and diversity, while L. pseudotheobromae was the most frequently isolated species in China. The results of this study provide insights into the aspects of epidemic of SER disease caused by Lasiodiplodia species, which will help in developing strategies for the management and control of stem end-rot in avocado.

Application of Thymol Vapors to Control Postharvest Decay Caused by Penicillium digitatum and Lasiodiplodia theobromae in Grapefruit

Two of the major postharvest diseases impacting grapefruit shelf life and marketability in the state of Florida (USA) are stem-end rot (SER) caused by Lasiodiplodia theobromae and green mold (GM) caused by Penicillium digitatum. Here, we investigated the in vitro and in vivo efficacy of vapors of thymol, a natural compound found in the essential oil of various plants and the primary constituent of thyme (Thymus vulgaris) oil, as a potential solution for the management of GM and SER. Thymol vapors at concentrations lower than 10 mg L-1 significantly inhibited the mycelial growth of both pathogens, causing severe ultrastructural damage to P. digitatum conidia. In in vivo trials, the incidence and lesion area of GM and SER on inoculated grapefruit were significantly reduced after a 5 d exposure to 50 mg L-1 thymol vapors. In addition, the in vitro and in vivo sporulation of P. digitatum was suppressed by thymol. When applied in its vapor phase, thymol had no negative effect on the fruit, neither introducing perceivable off-flavor nor causing additional weight loss. Our findings support the pursuit of further studies on the use of thymol, recognized as safe for human health and the environment, as a promising strategy for grapefruit postharvest disease management.

Using Trichoderma asperellum to Antagonize Lasiodiplodia theobromae Causing Stem-End Rot Disease on Pomelo (Citrus maxima)

Stem-end rot disease has been causing damage to the production of pomelos in Vietnam. The cur-rent study aimed to (i) isolate fungal pathogens causing pomelo stem-end rot disease (PSERD) and (ii) discover Trichoderma spp. that had an antagonistic ability against pathogens under in vitro conditions. Fungi causing PSERD were isolated from pomelo fruits with symptoms of stem-end rot disease and collected from pomelo farms in Ben Tre province, Vietnam. Moreover, 50 fungal strains of Trichoderma spp. also originated from soils of these pomelo farms in Ben Tre province and were dual-tested with the fungal pathogen on the PDA medium. The results demonstrated that 11 pathogenic fungi causing PSERD were isolated from the fruit and showed mycelial growth of roughly 5.33-8.77 cm diameter at 72 h after inoculation. The two fungi that exhibited the fast-est growth, namely, S-P06 and S-P07, were selected. ITS sequencing of the S-P06 and S-P07 fungi resulted in Lasiodiplodia theobromae. All the 50 Trichoderma spp. strains were allowed to antago-nize against the S-P06 and S-P07 strains under in vitro conditions. The greatest antagonistic effi-ciency was found in Trichoderma spp. T-SP19 at 85.4-86.2% and T-SP32 at 84.7-85.4%. The two antagonists were identified as Trichoderma asperellum T-SP19 and T-SP32. The selected strains of Trichoderma asperellum were potent as a biological control for fruit plants.

First report of stem-end rot caused by Botryosphaeria fabicerciana on mango fruits in China

Mango (Mangifera indica L.), belongs to the family Anacardiacea, and is one of the most popular tropical fruits in the world. Stem-end rot is a major postharvest disease of mango fruit, causing severe losses during storage in China (Chen et al., 2015). In July 2021, the mango fruits harvested from Baise Municipal National Agricultural Science and Technology Park (23.683568 N, 106.986325 E) of Guangxi province in China developed stem-end rot during storage. The disease incidence reached ca. 8.3%. The initial symptoms appeared as light brown lesions surrounding the peduncle, which quickly expanded becoming large dark-brown lesions. Small pieces of epidermis (5 mm × 5 mm) from 8 typical diseased friuts were cut from the edges of lesions surface-sterilized with 2% sodium hypochlorite and rinsed with sterile distilled water. The tissue was plated on potato dextrose agar (PDA) and incubated at 28 ℃ in the dark for 3 days. Fifteen, similarcolonies were isolated from the symptomatic tissue. The representative isolates DF-1, DF-2 and DF-3 were selected for morphological characterization, molecular identification, and pathogenicity testing. The colonies were circular with fluffy aerial mycelium, initially white turning to smoke-gray from the center in upper side and greenish black in reverse side, covering the 90 mm diameter Petri dish after 4 days of incubation on PDA at 28 ℃ in dark. Pycnidia were produced on the surface of the colony after 30 days. Conidia were fusiform, aseptate, hyaline, thin-walled with granular contents, apex sub-obtuse, base subtruncate to bluntly rounded, 14.0-20.3 (16.8±1.6) μm × 3.1-7.2 (5.1±0.9) μm (n=50). The sexual stage was absent. Based on morphology, isolates were preliminarily identified as Botryosphaeria speices. To accurately identify the pathogen, genomic DNA was extracted from the mycelium of the three isolates DF-1, DF-2 and DF-3. The internal transcribed spacer of rDNA region (ITS), elongation factor 1-alpha (EF-1α) and beta-tubulin gene (TUB) genes were amplified using primers ITS1/ITS4, EF1-728F/EF1-986R and Bt2a/Bt2b, respectively (Slippers et al., 2004). The nucleotide sequences were all deposited in GenBank (ITS: OP729176-OP729178 EF-1α: OP758194-OP758196 and TUB: OP758197-OP758199). Based on the BLASTn analysis, the ITS, EF1-α and TUB sequences of three isolates were 100%, 99% and 99% similar to the Botryosphaeria fabicerciana MFLUCC 10-0098 sequences (ITS: JX646789, EF-1α: JX646854 and TUB: JX646839). Multi-locus phylogenetic analyses (ITS, EF-1α and TUB) showed that the isolate DF-1, DF-2 and DF-3 were clustered within Botryosphaeria fabicerciana clade based on the maximum likelihood , Bayesian inference, and maximum parsimony methods. The pathogenicity test was performed by placing discs mycelium around the peduncle of mature mango fruits by pin-prick method. Each treatment carried out with 12 fruits. The inoculated fruits were placed in plastic boxes at 28 ℃ with three replicates. Three days after inoculation, typical symptoms of stem-end rot were observed. The control fruits were inoculated with sterile PDA discs, and remained symptomless. The same fungus was re-isolated from the symptomatic tissue to complete Koch's postulate. Botryosphaeria fabicerciana (basionym: Fusicoccum fabicercianum) was first reported as pathogen causing senescent twig of Eucalyptus spp. in China (Chen et al., 2011; Phillips et al., 2013). To our knowledge, this is the first report of Botryosphaeria fabicerciana causing stem-end rot of Mangifera indica in China.

The Antifungal Effect of Gaseous Ozone on Lasiodiplodia theobromae Causing Stem-End Rot in 'Keitt' Mangoes

This study evaluated the antifungal activity of ozone (O₃) against stem-end rot of mango fruit (cv. Keitt). Mango fruit were exposed to gaseous ozone (0.25 mg/L) for 24 or 36 h during cold storage, and control fruit were untreated. Experimental fruit were stored at 90% relative humidity and 10 ± 0.5 °C for three weeks and ripened at ambient temperature for one week. Ozone treatment (24 h) inhibited the mycelial growth of Lasiodiplodia theobromae by 60.35%. At day twenty-eight of storage, fruit treated with O₃ for 36 h had low mass loss (%) and high firmness compared to the untreated control fruit. Treating mango fruit with O₃ (36 h) maintained the color and concentration of total flavonoids throughout the storage time. At the end of storage, peroxidase activity under the O₃ 24 h treatment was significantly higher (0.91 U min^-1 g^-1 DM) compared to O₃ (36 h) and control, which, respectively, had 0.80 U min^-1 g^-1 DM and 0.78 U min^-1 g^-1 DM. Gaseous ozone for 24 h is recommended as a cost-effective treatment for controlling stem-end rot. These findings suggest that gaseous ozone effectively controlled stem-end rot and enhanced the postharvest quality of mango fruit.

The Potential of Gaseous Chlorine Dioxide for the Control of Citrus Postharvest Stem-End Rot Caused by Lasiodiplodia theobromae

The focus of this study was to develop technologies using chlorine dioxide (ClO₂) gas to control postharvest stem-end rot of citrus caused by Lasiodiplodia theobromae. Mycelial growth of L. theobromae on potato dextrose agar (PDA) plugs was completely inhibited by a 24-h ClO₂ exposure provided by 0.5 g of solid ClO₂ generating granular mixture in a 7.7-liter sealed container. In vivo experiments were conducted on artificially inoculated Tango and naturally infected U.S. Early Pride mandarins. When ClO₂ treatments were initiated 0 to 6 h after inoculation, decay development was significantly reduced as compared with the control, and higher ClO₂ doses were more effective. A ClO₂ treatment (using 3 g of generating mixture per 7.7-liter sealed container) administered 0 h after inoculation resulted in 17.6% Diplodia stem-end rot incidence compared with 95.6% in the control, whereas the same treatment administered 24 h after inoculation was much less effective, resulting in 63.0% incidence compared with 85.4% in the control. Diplodia stem-end rot incidence of naturally infected fruit after using 6 or 9 g of generating mixture per 24-liter sealed box was 23.8 or 25.7%, respectively, compared with 47.9% for control fruit. The ClO₂ treatments had no negative effects on fruit quality characteristics including weight loss, firmness, puncture resistance, titratable acids (TAs), total soluble solids (TSSs), and rind color. Albedo pH at wounds was significantly reduced from 6.0 to 4.8 by the ClO₂ treatments, whereas undamaged albedo remained at 5.8. In addition, no visible physiologic defects, such as peel browning and bleaching, were observed on ClO₂-treated fruit. These results indicate that ClO₂ gas has the potential to be developed as a component of an integrated citrus postharvest decay control system to minimize fruit losses.

Characterization of Botrytis cinerea and B. prunorum From Healthy Floral Structures and Decayed 'Hayward' Kiwifruit During Post-Harvest Storage

Gray mold is the primary postharvest disease of 'Hayward' kiwifruit (Actinidia deliciosa) in Chile, with a prevalence of 33.1% in 2016 and 7.1% in 2017. Gray mold develops during postharvest storage, which is characterized by a soft, light to brown watery decay that is caused by Botrytis cinerea and B. prunorum. However, there is no information on the role of B. prunorum during the development and storage of kiwifruit in Chile. For this purpose, asymptomatic flowers and receptacles were collected throughout fruit development and harvest from five orchards over two seasons in the Central Valley of Chile. Additionally, diseased kiwifruits were selected after storage for 100 days at 0°C and 2 days at 20°C. Colonies of Botrytis sp. with high and low conidial production were consistently obtained from apparently healthy petals, sepals, receptacles, and styles and diseased kiwifruit. Morphological and phylogenetic analysis of three partial gene sequences encoding glyceraldehyde-3-phosphate dehydrogenase, heat shock protein 60, and DNA-dependent RNA polymerase subunit II were able to identify and separate B. cinerea and B. prunorum species. Consistently, B. cinerea was predominantly isolated from all floral parts and fruit in apparently healthy tissue and diseased kiwifruit. During full bloom, the highest colonization by B. cinerea and B. prunorum was obtained from petals, followed by sepals. In storage, both Botrytis species were isolated from the diseased fruit (n = 644), of which 6.8% (n = 44) were identified as B. prunorum. All Botrytis isolates grew from 0°C to 30°C in vitro and were pathogenic on kiwifruit leaves and fruit. Notably, B. cinerea isolates were always more virulent than B. prunorum isolates. This study confirms the presence of B. cinerea and B. prunorum colonizing apparently healthy flowers and floral parts in fruit and causing gray mold during kiwifruit storage in Chile. Therefore, B. prunorum plays a secondary role in the epidemiology of gray mold developing in kiwifruit during cold storage.

Effect of Biological and Chemical Treatments During Flowering on Stem-End Rot Disease, and Mango Yield

Pathogenic fungi, as the Botryosphaeriaceae family, can penetrate during flowering and endophytically colonize the stem of mango fruit (Mangifera indica) without causing any visible symptoms. Those fungi become active during abiotic stress or fruit ripening and cause stem and inflorescence dieback or fruit stem-end rot (SER) fungal disease. We hypothesized that anti-fungal treatments during the main event of Botryosphaeriaceae penetration would reduce the disease. Initially, we showed that treatments with the fungicide "Switch" (fludioxonil and cyprodinil) during orchard flowering (cv. Keitt and Shelly) reduced the occurrence of pathogenic fungi in the fruit stem-end and significantly reduced fruit's incidence of SER disease. As mango orchards are sprayed weekly against powdery mildew (PM) disease during flowering, we combined two treatments against PM disease with two treatments against both PM- and SER-causing pathogens. Application of biological treatments of the fungicide "Serenade" (Bacillus subtilis) or chemical treatments of the fungicides "Luna Tranquility" (fluopyram and pyrimethanil) or "Switch" during flowering in 'Shelly' and 'Keitt' mango orchards significantly reduced inflorescence/stem dieback (up to 50%) and fruit drop and significantly increased the number of fruit per tree, which led to a significant increase in yield, up to 41%, in heavily infected orchards. In addition, this application during flowering (March to April) affected post-harvest fruit quality (August to September) by a significant (P < 0.005) reduction of the incidence and the severity of stem-end rot disease and even fruit side-rot disease, without affecting fruit ripening and other quality parameters. While all fungicides were effective, the chemical fungicides were more effective than the biological fungicide. Thus, changing the PM fungicide regime to control Botryosphaeriaceae penetration during mango orchard flowering led to reduced inflorescence/stem dieback, reduced fruit drop, increase in yield, and minimized post-harvest decay.

Draft Genome Sequence Resource of the Citrus Stem-End Rot Fungal Pathogen Lasiodiplodia theobromae CITRA15

Lasiodiplodia theobromae is a fungal pathogen associated with perennial tropical fruit plants worldwide. In citrus, L. theobromae causes stem-end rot (Diplodia stem-end rot), a damaging postharvest disease that is aggravated when trees are also infected with the citrus greening bacteria 'Candidatus Liberibacter asiaticus'. Due to the latent infection of L. theobromae during the preharvest stage, it becomes difficult to control the disease by chemical or physical treatment. In the current study, we sequenced and assembled strain CITRA15, the first genome of L. theobromae obtained from diseased Citrus paradise 'Flame' grapefruit in Florida, and thereby provided a genomic resource for future research on diagnostics, and postharvest and preharvest disease management of citrus and other fruit crops.

Identification of Botryosphaeriaceae associated with stem-end rot of mango (Mangifera indica L.) in Malaysia

AIMS

To identify Botryosphaeriaceae fungal species that are associated with stem-end rot of mango, and to study their pathogenicity on mango fruit.

METHODS AND RESULTS

Based on the sequences of internal transcribed spacer (ITS), TEF1-α and β-tubulin, as well as on the phylogenetic analysis of combined sequences, four species of Lasiodiplodia (L. theobromae,L. pseudotheobromae, L. iranensis, L. mahajangana) and two species of Neofusicoccum (N. ribis, N. parvum) were identified. Pseudofusicoccum violaceum, Neoscytalidium dimidiatum and three species of Botryosphaeria (B. scharifii, B. dothidea, B. ramosa) were identified based on sequences of ITS and TEF1-α. Pathogenicity test of selected isolates were tested on Chok Anan, Waterlily and Falan mango cultivars. Generally, all species were observed to be pathogenic on the three tested mango cultivars on wounded fruits, except for N. ribis and N. parvum, which were pathogenic on both wounded and unwounded fruits. However, N. ribis was only pathogenic on cultivar Falan, whereas B. ramosa were pathogenic on cultivars Waterlily and Falan.

CONCLUSIONS

Eleven species of Botryosphaeriaceae were associated with mango stem-end rot in Malaysia. To the best of our knowledge, four species, namely L. mahajangana, B. ramosa, N. ribis and P. violaceum are the first recorded Botryosphaeriaceae fungi associated with stem end rot of mango.

SIGNIFICANCE AND IMPACT OF THE STUDY

The identification of Botryosphaeriaceae fungi is important to establish suitable control measures and quarantine requirements. Many species have a wide host range, which means that there is a possibility of cross infection from other infected plants.

Harvesting Mango Fruit with a Short Stem-End Altered Endophytic Microbiome and Reduce Stem-End Rot

Stem-end rot (SER) is a serious postharvest disease of mango fruit grown in semi-dry area. Pathogenic and non-pathogenic microorganisms endophytically colonize fruit stem-end. As fruit ripens, some pathogenic fungi switch from endophytic colonization to necrotrophic stage and cause SER. Various pre/post-treatments may alter the stem-end community and modify SER incidence. This study investigates the effects of harvesting mango with or without short stem-end on fruit antifungal and antioxidant activities, the endophytic microbiome, and SER during fruit storage. Our results show that harvesting mango with short stem significantly reduced SER during storage. At harvest, fruit harvested with or without stem exhibit a similar microorganisms community profile. However, after storage and shelf life, the community of fruit without stem shifted toward more SER-causing-pathogens, such as Lasiodiplodia, Dothiorella, and Alternaria, and separated from the community of fruit with stem. This change correlated to the high antifungal activity of stem extract that strongly inhibited both germination and growth of Lasiodiplodia theobromae and Alternaria alternata. Additionally, fruit that was harvested with stem displayed more antioxidant activity and less ROS. Altogether, these findings indicate that harvesting mango with short stem leads to higher antifungal and antioxidant activity, retaining a healthier microbial community and leading to reduced postharvest SER.

Characterization of difenoconazole resistance in Lasiodiplodia theobromae from papaya in Brazil

BACKGROUND

Stem-end rot caused by Lasiodiplodia theobromae is one of the most important diseases of papaya in northeastern Brazil. It can be controlled effectively by demethylation inhibitor (DMI) fungicides, but the occurrence of DMI resistance may decrease fungicide efficacy.

RESULTS

Detached fruit studies revealed that isolates with EC₅₀ values of 6.07 and 6.28 μg mL^-1 were not controlled effectively, but reduced virulence and ability to grow at temperatures ranging from 12 to 32 °C suggesting fitness penalties were observed. Cross-resistance was observed only between difenoconazole and propiconazole. The entire cytochrome P450 sterol 14α-demethylase (LtCYP51) gene and its flanking regions were cloned. The gene was 1746 bp in length and contained three introns. The predicted protein contained 525 amino acids. Phylogenetic tree analysis showed that the LtCYP51 belongs to the CYP51B clade. No amino acid variation was found between sensitive and resistant isolates; however, the gene was constitutively more highly expressed in resistant isolates.

CONCLUSION

Resistance to DMI fungicides in L. theobromae is based on LtCYP51 gene overexpression and fitness penalties may be present in difenoconazole-resistant isolates. © 2019 Society of Chemical Industry.

Postharvest Fungicide for Avocado Fruits: Antifungal Efficacy and Peel to Pulp Distribution Kinetics

Postharvest application of fungicides is commonly applied in order to reduce food loss. Prochloraz is currently the only postharvest fungicide registered in Israel and Europe in avocado fruits. Due to its unfavorable toxicological properties, prochloraz will be banned from the end of 2020 for future postharvest usage and therefore a substitute candidate is urgently warranted. Fludioxonil, a relatively safe, wide spectrum fungicide, is approved in Europe and Israel for postharvest use in various fruits, but not avocado. Hence, fludioxonil has been evaluated in the present study as a potential substitute for prochloraz in avocado. The objectives of the present study were to determine fludioxonil efficacy against common fungal infestations in avocado and distribution kinetics between peel and pulp in comparison to prochloraz. At the same concentration range (75-300 µg/L), fludioxonil was as effective as prochloraz in inhibiting postharvest decay, while in the early season cultivars, suffering mainly from stem-end rot, it exhibited a better decay control than prochloraz. Fludioxonil and prochloraz displayed negligible and undetected pulp levels, respectively, due to low peel penetrability. Taken altogether, fludioxonil was found to be a suitable candidate for replacing prochloraz as a postharvest fungicide in avocado.