Molecular and Biochemical Characterization of Carbendazim-Resistant Botryodiplodia theobromae Field Isolates

Identity, Pathogenicity, and Genetic Diversity of Lasiodiplodia Species 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.; four Lasiodiplodia species were identified, namely, L. pseudotheobromae (59.41%), L. theobromae (24.75%), L. mahajangana (7.92%), and 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 92.08% of the isolates exhibited a high level of virulence on avocado (disease index >70), related to the disease severity on avocado fruits. All tested isolates grew well under the temperature ranging from 23 to 33°C. There was a significant difference in mycelial growth between the four species of Lasiodiplodia after treatment with high or low temperatures. The growth of L. pseudotheobromae was the fastest at 13 to 18°C but was the lowest at 38°C (P < 0.05). The red pigment could be produced by all tested isolates after culturing for 7 days at 38°C. The mycelial growth rate was the fastest on PDA medium, and the slowest on the OMA medium but promoted spore formation (P < 0.05). In addition, the genetic diversity of pathogenic Lasiodiplodia species associated with SER collected from avocado, mango, guava, and soursop fruits was determined. A total of 74 isolates were clustered into four main ISSR groups by the unweighted pair-group method with arithmetic mean 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, survey their occurrence and 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 the epidemic of SER disease caused by Lasiodiplodia species, which will help in developing strategies for the management and control of SER in avocado.

Biochemical and Molecular Characterization of Prochloraz Resistance in Lasiodiplodia theobromae Field Isolates

Stem-end rot (SER), caused by Lasiodiplodia theobromae, is one of the most critical diseases of mango in China. The demethylation inhibitor fungicide prochloraz has been widely used in China to control mango diseases. Isolates (n = 139) of L. theobromae were collected in 2019 from six mango-producing regions in Hainan Province, China. The fungicide sensitivity of L. theobromae isolates to prochloraz revealed that the EC₅₀ (50% effective concentration) values ranged from 0.0006 to 16.4131 µg/ml. In total, 21 of the 139 isolates were categorized as resistant to prochloraz. The resistant isolates sprayed with prochloraz could not be effectively controlled in detached fruit. The mycelial growth, conidia germination, and ability to grow at temperatures ranging from 12 to 35°C of resistant isolates decreased, suggesting fitness penalties. The experiment showed that, after treatment with prochloraz at 10 µg/ml, the content of ergosterol in the mycelia of the sensitive isolate decreased by 80.23%, whereas the resistant strain decreased by only 57.52%. The damage to membranes in the sensitive isolates was more serious than for resistant isolates. The target gene CYP51 and the ATP-binding cassette (ABC) subfamily ABCG gene were cloned but no mutation was found. When treated with prochloraz, the expression of CYP51 and ABCG in resistant isolates was significantly higher than in the sensitive isolates. Thus, induced expression of its target gene combined with the induction of expression drug efflux transporters appeared to mediate the prochloraz resistance of L. theobromae.

Resistance characterization of the natural population and resistance mechanism to pyraclostrobin in Lasiodiplodia theobromae

Lasiodiplodia theobromae is the main pathogen of mango stem-end rot disease, causing mango fruit decay and major economic loss. QoI resistance has been found in field populations of L. theobromae. The characterization and resistance mechanism of pyraclostrobin-resistant L. theobromae was investigated by using a combination of bioassays and biochemical and molecular methods. The pyraclostrobin resistance among the L. theobromae population samples from Hainan was 93.41%. The resistant isolates were stable after successive subculturing for 10 times on PDA. Cross-resistance was observed only between the Qols pyraclostrobin and azoxystrobin. The alternative oxidase (AOX) inhibitor SHAM notably decreased the EC₅₀ values of pyraclostrobin for all tested L. theobromae isolates. Induction of AOX by pyraclostrobin was observed in mycelia cells of L. theobromae. After treatment with pyraclostrobin, the final ATP and AOX contents of all sensitive isolates were significantly lower than those of resistant isolates. The relevant mutation and high expression of the cytochrome b gene were not detected in resistant isolates. However, there were 4 mutations in the AOX gene, which were only observed in highly resistant isolates. Pretreatment with pyraclostrobin resulted in a significant upregulation of AOX gene expression, and the average expression level of the highly resistant isolates was 33-fold that of the control group. These results suggested that the AOX pathway is responsible for resistance to pyraclostrobin, and that the AOX-related resistance mechanism is common in field populations of L. theobromae in Hainan mango orchards.

Occurrence and Detection of Carbendazim Resistance in Botryosphaeria dothidea from Apple Orchards in China

Botryosphaeria dothidea causes white rot, which is among the most devastating diseases affecting apple crops globally. In this study, we assessed B. dothidea resistance to carbendazim by collecting samples from warts on the infected branches of apple trees or from fruits exhibiting evidence of white rot. All samples were collected from different orchards in nine provinces of China in 2018 and 2019. In total, 440 B. dothidea isolates were evaluated, of which 19 isolates from three provinces were found to exhibit carbendazim resistance. We additionally explored the fitness and resistance stability of these isolates, revealing that they were no less fit than carbendazim-sensitive isolates in terms of pathogenicity, sporulation, and mycelial growth and that the observed carbendazim resistance was stable. Sequencing of the β-tubulin gene in carbendazim-resistant isolates showed the presence of a substitution at codon 198 (GAG to GCG) that results in an alanine substitution in place of glutamic acid (E198A) in all 19 resistant isolates. A loop-mediated isothermal amplification (LAMP) method was then developed to rapidly and specifically identify this E198A mutation. This LAMP method offers value as a tool for rapidly detecting carbendazim-resistant isolates bearing this E198A mutation and can thus be used for the widespread monitoring of apple crops to detect and control the development of such resistance.

Molecular characterization and overexpression of the difenoconazole resistance gene CYP51 in Lasiodiplodia theobromae field isolates

Stem-end rot (SER) caused by Lasiodiplodia theobromae is an important disease of mango in China. Demethylation inhibitor (DMI) fungicides are widely used for disease control in mango orchards. The baseline sensitivity to difenoconazole of 138 L. theobromae isolates collected from mango in the field in 2019 was established by the mycelial growth rate method. The cross-resistance to six site-specific fungicides with different modes of action were investigated using 20 isolates randomly selected. The possible mechanism for L. theobromae resistance to difenoconazole was preliminarily determined through gene sequence alignment and quantitative real-time PCR analysis. The results showed that the EC₅₀ values of 138 L. theobromae isolates to difenoconazole ranged from 0.01 to 13.72 µg/mL. The frequency of difenoconazole sensitivity formed a normal distribution curve when the outliers were excluded. Difenoconazole showed positive cross-resistance only with the DMI tebuconazole but not with non-DMI fungicides carbendazim, pyraclostrobin, fludioxonil, bromothalonil, or iprodione. Some multifungicide-resistant isolates of L. theobromae were found. Two amino acid substitutions (E209k and G207A) were found in the CYP51 protein, but they were unlikely to be related to the resistance phenotype. There was no alteration in the promoter region of the CYP51 gene. However, difenoconazole significantly increased the expression of the CYP51 gene in the resistant isolates compared to the susceptible isolates. These results are vital to develop effective mango disease management strategies to avoid the development of further resistance.

Resistance mechanisms and fitness of pyraclostrobin-resistant isolates of Lasiodiplodia theobromae from mango orchards

Background

Stem-end rot, caused by Lasiodiplodia theobromae (Pat.) Griffon & Maubl is a serious postharvest disease in mango. In China, a high prevalence of the QoI fungicides resistance has been reported in the last decade. The study aimed to discuss factors determining rapid development of pyraclostrobin-resistance and its resistance mechanisms.

Methods

To determine the resistance stability and fitness of pyraclostrobin resistance in L. theobromae, three phenotypes of pyraclostrobin resistance were compared and analyzed for the EC₅₀ values, mycelial growth, virulence and temperature sensitivity and osmotic stress sensitivity. The relative conductivity and enzyme activities of different phenotypes were compared under fungicide stress to explore possible biochemical mechanisms of pyraclostrobin resistance in L. theobromae. The Cytb gene sequences of different phenotypes were analysed.

Results

All isolates retained their original resistance phenotypes during the 10 subcultures on a fungicide-free PDA, factor of sensitivity change (FSC) was approximately equal to 1. The resistance-pyraclostrobin of the field isolates should be relatively stable. Two pyraclostrobin-resistant phenotypes shared similar mycelial growth, virulence and temperature sensitivity with pyraclostrobin-sensitive phenotype. After treated by pyraclostrobin, the relative conductivity of the sensitive phenotype was significantly increased. The time of Pyr-R and Pyr-HR reached the most conductivity was about 8–10 times than that of Pyr-S, the time for the maximum value appearance showed significant differences between sensitive and resistant phenotypes. The activities of Glutathione S-transferase (GST), catalase (CAT) and peroxidase (POD) of Pyr-HR were 1.78, 5.45 and 1.65 times respectively, significantly higher than that of Pyr-S after treated by 200 mg/l pyraclostrobin.

Conclusion

The results showed that the pyraclostrobin-resistant phenotypes displayed high fitness and high-risk. The nucleotide sequences were identical among all pyraclostrobin-resistant and -sensitive isolates. The pyraclostrobin resistance was not attributable to Cytb gene alterations, there may be some of other resistance mechanisms. Differential response of enzyme activity and cell membrane permeability were observed in resistant- and sensitive-isolates suggesting a mechanism of metabolic resistance.