Does apple canker develop independently on leaf scars of a single apple shoot?

European apple canker, caused by Neonectria ditissima, causes serious damage to apple trees, particularly young trees. Canker management is difficult because of the limited availability of effective fungicides, the long latency period, inoculum abundance and host resistance in commercial cultivars as well as the need for costly manual pruning interventions. To understand disease aggregation for more effective pruning management, we assessed whether canker infection and subsequent lesion development on leaf scars are independent from each other on the same shoot. Four inoculation experiments were conducted: one in glasshouse, and three in orchards. On each shoot, 10 consecutive leaf scars were inoculated and assessed for visible cankers over time in situ. Number of cankers developed per shoot as well as spatial distribution of these cankers within a shoot was statistically analysed. Most data of the number of visible canker lesions on a single shoot failed to fit binomial distributions (indicator for independence) and were fitted much better by beta binomial distributions. In a number of cases (4–20%), there appeared to be positive association between lesion development on neighbouring leaf scars. However, in one experiment where laboratory incubation and isolation of N. ditissima from inoculated but asymptomatic leaf scars (after eight months’ field incubation) were used the results suggested independence of canker development on a single shoot. We conclude that apparent aggregation of canker lesions on individual shoots is likely to originate from host responses. Such aggregation of canker lesions on individual shoots should be taken into consideration for field disease assessment and management.

Proof of Concept for Shoot Blight and Fire Blight Canker Management with Post-Infection Spray Applications of Prohexadione-Calcium and Acibenzolar-S-Methyl in Apple

To reduce the severity of shoot blight and prevent the resulting development of cankers on perennial apple wood, we evaluated eight fire blight post-infection spry programs of prohexadione-calcium (PCA) alone or with acibenzolar-S-methyl (ASM) over two years. On mature trees of cv. Royal Court, a single application of the high PCA rate (247 mg/L) at 2 to 3 days after inoculation resulted in 89.5 and 69.5% reduction of shoot blight severity after inoculation. Two applications of PCA 247mg/L, 12 or 14 days apart, with the first one applied 2 to 3 days after inoculation, resulted in 78.8 and 74.5% reduction of shoot blight severity in both years. A 100% control of canker incidence on perennial wood from infected shoots in both years was achieved with a single application of PCA (247mg/L) applied at 2 or 3 days after the inoculation, while three applications of PCA (125 mg/L) + ASM (25mg/L) 12 – 16 days apart, reduced canker incidence by 83.5 and 69% in the two years. The other programs with lower PCA rates and frequencies of application reduced shoot blight severity for 50.8 and 51.8% (PCA) and for 62.6 – 72% and 59.3% (PCA + ASM), over two years, respectively. Reduction of canker incidence on wood by other programs was 66.5% and 69 – 90.4% in the two years, respectively. As fire blight cankers lead to death of dwarf apple trees and serve as primary sources of inoculum, our effective PCA and PCA + ASM programs could serve as viable post-infection management options. These treatments can reduce or prevent canker development and thus significantly abate tree losses in high-density apple orchards after fire blight epidemics occur.

Screening of Commercially Available Satsuma (Citrus unshiu and C. reticulata) Cultivars for Their Susceptibility to Citrus Canker Caused by Xanthomonas axonopodis pv. citri

Citrus canker presents major challenges in citrus production worldwide. The disease spreads and develops rapidly under conducive environmental conditions, and Louisiana’s hot and humid weather provides highly favorable conditions for citrus canker development in both commercial and backyard citrus production. Despite being considered eradicated in late 1940s, citrus canker was reconfirmed in Louisiana in 2013. Citrus is the most popular fruit tree grown commercially and in backyards in Louisiana, and satsumas dominate with 63% of total citrus acreage. The results from an annual citrus cooperative agricultural pest survey from 2014 to 2016 revealed that satsumas are highly tolerant to citrus canker with only 2.5% disease incidence, but the survey lacked information on the types of satsuma cultivar. The main objective of this study was to screen commercially available satsuma cultivars including Brown’s Select, Louisiana Early, Miho, Owari, and St. Ann for their susceptibility to citrus canker under natural field disease inoculum conditions with a goal to provide citrus growers with alternate disease management tactics. The results showed that there were significant differences in disease severity (average number of lesions per leaf) among the satsuma cultivars. Of the five satsuma cultivars, Brown’s Select and Miho consistently showed less disease severity with delayed incidence. Additionally, both Brown’s Select and Miho exhibited significantly smaller lesion size, which could lower canker inoculum production for secondary infections. This work provides Louisiana growers with scientific field-based data to make sound decisions in selecting less-susceptible satsuma cultivars for future plantings to mitigate disease spread and development in the state.

Progression of symptoms caused by Botryosphaeria dothidea on apple branches

Until recently, the causal agent of Botryosphaeria canker was assumed to differ from that causing ring rot on fruit and warts on branches on apple trees in China and East Asia. However, recent research documented that Botryosphaeria dothidea caused both disease symptoms on apple. Inoculations with strains isolated from cankers and warts on branches were conducted to investigate symptom progression caused by B. dothidea and conditions inducing the two symptom types. The results confirmed that both cankers and warts are caused by B. dothidea. Warts are the results of hyperplasia and suberization of bark tissues induced by fungal infection, whereas cankers result from the rapid growth of hyphae from inside warts, lenticels or wounds. Resistance to B. dothidea exists in living apple branches. When a living branch is infected via lenticels, the pathogen induces proliferation and suberization of cortical cells that restricts the growth and expansion of the hyphae, leading to wart symptom. However, under certain stress conditions such as drought, the hyphae inside host tissues expand rapidly and kill cortical cells, leading to canker development. Host resistance may recover during active growth periods, which suppresses or even stops rapid expansion of the hyphae, leading to the intermediate symptom of canker warts. Abiotic factors, such as drought or high temperature in early spring, can result in rapid extension of colonized hyphae in branches and conversion of warts to cankers. Preventing this transition can be an important measure in managing Botryosphaeria canker on apple.

Systematic Analysis and Functional Validation of Citrus Pectin Acetylesterases (CsPAEs) Reveals that CsPAE2 Negatively Regulates Citrus Bacterial Canker Development

The present study was designed to serve as a comprehensive analysis of Citrus sinensis (C. sinensis) pectin acetylesterases (CsPAEs), and to assess the roles of these PAEs involved in the development of citrus bacterial canker (CBC) caused by Xanthomonas citri subsp. citri (Xcc) infection. A total of six CsPAEs were identified in the genome of C. sinensis, with these genes being unevenly distributed across chromosomes 3, 6, and 9, and the unassembled scaffolds. A subset of CsPAEs were found to be involved in responses to Xcc infection. In particular, CsPAE2 was identified to be associated with such infections, as it was upregulated in CBC-susceptible variety Wanjincheng and inversely in CBC-resistant variety Calamondin. Transgenic citrus plants overexpressing CsPAE2 were found to be more susceptible to CBC, whereas the silencing of this gene was sufficient to confer CBC resistance. Together, these findings provide evolutionary insights into and functional information about the CsPAE family. This study also suggests that CsPAE2 is a potential candidate gene that negatively contributes to bacterial canker disease and can be used to breed CBC-resistant citrus plants.

Different Responses in Mandarin Cultivars Uncover a Role of Cuticular Waxes in the Resistance to Citrus Canker

‘Okitsu’ is a mandarin cultivar showing substantial resistance to X. citri subsp. citri (X. citri). We have previously shown that this cultivar has significantly lower canker incidence and severity than ‘Clemenules’, particularly during early stages of leaf development in the field. This differential response is only seen when the leaves are inoculated by spraying, suggesting that leaf surface contributes to resistance. In this work, we have studied structural and chemical properties of leaf surface barriers of both cultivars. Ultrastructural analysis showed a thicker cuticle covering epidermal surface and guard cells in young ‘Okitsu’ leaves than in ‘Clemenules’. This thicker cuticle was associated with a smaller stomatal aperture and reduced cuticle permeability. These findings correlated with an accumulation of cuticular wax components, including primary alcohols, alkanes, and fatty acids. None of these differences were observed in mature leaves, where both cultivars are equally resistant to the bacterium. Remarkably, mechanical alteration of cuticular thickness of young ‘Okitsu’ leaves allows canker development. Furthermore, cuticular waxes extracted from young ‘Okitsu’ leaves have higher antibacterial activity against X. citri than ‘Clemenules’. Taken together, these data suggest that a faster development of epicuticular waxes in ‘Okitsu’ leaves play a central role in its resistance to X. citri.

CitrusKB: A Comprehensive Knowledge Base for Transcriptome and Interactome of Citrus spp. Infected by Xanthomonas citri subsp. citri at Different Infection Stages

Citrus canker type A is a serious disease caused by Xanthomonas citri subsp. citri (X. citri), which is responsible for severe losses to growers and to the citrus industry worldwide. To date, no canker-resistant citrus genotypes are available, and there is limited information regarding the molecular and genetic mechanisms involved in the early stages of the citrus canker development. Here, we present the knowledge base for transcriptome of in vivo citrus interactome, the CitrusKB. This is the first in vivo interactome database for different citrus cultivars, and it was produced to provide a valuable source of information on citrus and their interaction with the citrus canker bacterium X. citri. The database provides tools for a user-friendly web interface to search and analyze a large amount of information regarding eight citrus cultivars with distinct levels of susceptibility to the disease and their interaction, at different stages of infection, with the citrus canker bacterium X. citri. Currently, CitrusKB comprises a reference citrus genome and its transcriptome, expressed transcripts, pseudogenes and predicted genomic variations (SNPs and SSRs). The updating process will continue by incorporating annotations and analysis tools. We expect that CitrusKB may substantially contribute to the area of citrus genomics. CitrusKB is accessible at http://bioinfo.deinfo.uepg.br/citrus. Users can download all the generated raw sequences and generated datasets by this study from the CitrusKB website.

CitrusKB: a comprehensive knowledge base for transcriptome and interactome of Citrus spp. infected by Xanthomonas citri subsp. citri at different infection stages

Citrus canker type A is a serious disease caused by Xanthomonas citri subsp. citri (X. citri), which is responsible for severe losses to growers and to the citrus industry worldwide. To date, no canker-resistant citrus genotypes are available, and there is limited information regarding the molecular and genetic mechanisms involved in the early stages of the citrus canker development. Here, we present the CitrusKB knowledge base. This is the first in vivo interactome database for different citrus cultivars, and it was produced to provide a valuable resource of information on citrus and their interaction with the citrus canker bacterium X. citri. CitrusKB provides tools for a user-friendly web interface to let users search and analyse a large amount of information regarding eight citrus cultivars with distinct levels of susceptibility to the disease, with controls and infected plants at different stages of infection by the citrus canker bacterium X. citri. Currently, CitrusKB comprises a reference citrus genome and its transcriptome, expressed transcripts, pseudogenes and predicted genomic variations (SNPs and SSRs). The updating process will continue over time by the incorporation of novel annotations and analysis tools. We expect that CitrusKB may substantially contribute to the field of citrus genomics. CitrusKB is accessible at http://bioinfo.deinfo.uepg.br/citrus. Users can download all the generated raw sequences and generated datasets by this study from the CitrusKB website.

Can phosphorous acid be used to control Neonectria ditissima in New Zealand grown apples?

European canker, Neonectria ditissima, is a worldwide apple tree disease killing shoots, branches and trees, and treatment with phosphorous acid is a possible control option. The effect of six postharvest phosphorous acid (PA) treatments on fruit residues the following season was studied in Tasman on two trial sites growing ‘Scifresh’ or ‘Scilate’ apple trees. Spray treatments consisted of number (0–3) and timing (early, mid and/or late) of PA applications. Additionally, leaf-scar wounds were artificially inoculated with N. ditissima spores at the ‘Scilate’ site on 1 and 8 June 2017 to determine disease control. Symptom expression was checked regularly between October 2017 and February 2018. None of the treatments caused a statistically significant reduction in the incidence of canker development compared with the control. Two or more PA applications resulted in PA residues in fruit, at harvest, the following season. Higher PA residues were found in fruit following early applications than with late applications. More applications of PA resulted in higher residues. This finding has important implications for exporting fruit to markets that have no tolerance for PA residues.