Managing pests in carrot production is challenging. Endophytic microbes have been demonstrated to improve the health and productivity of many crops, but factors affecting endophyte dynamics in carrot is still not well understood. The goal of this study was to determine how crop management system and carrot genotype interact to affect the composition and potential of endophytes to mitigate disease caused by Alternaria dauci, an important carrot pathogen. Twenty-eight unique isolates were collected from the taproots of nine diverse genotypes of carrot grown in a long-term trial comparing organic and conventional management. Antagonistic activity was quantified using an in vitro assay, and potential for individual isolates to mitigate disease was evaluated in greenhouse trials using two carrot cultivars. Results confirm that carrot taproots are colonized by an abundant and diverse assortment of bacteria and fungi representing at least distinct 13 genera. Soils in the organic system had greater total organic matter, microbial biomass and activity than the conventional system and endophyte composition in taproots grown in this system were more abundant and diverse, and had greater antagonistic activity. Carrot genotype also affected endophyte abundance as well as potential for individual isolates to affect seed germination, seedling growth and tolerance to A. dauci. The benefits of endophytes on carrot growth were greatest when plants were subject to A. dauci stress, highlighting the importance of environmental conditions in the functional role of endophytes. Results of this study provide evidence that endophytes can play an important role in improving carrot performance and mediating resistance to A. dauci, and it may someday be possible to select for these beneficial plant-microbial relationships in carrot breeding programs. Implementing soil-building practices commonly used in organic farming systems has potential to promote these beneficial relationships and improve the health and productivity of carrot crops.
Fungal endophytes can influence production and post-harvest challenges in carrot, though the identity of these microbes as well as factors affecting their composition have not yet been determined, which prevents growers from managing these organisms to improve crop performance. consequently, we characterized the endophytic mycobiome in the taproots of three carrot genotypes that vary in resistance to two pathogens grown in a trial comparing organic and conventional crop management using Illumina sequencing of the internal transcribed spacer (ITS) gene. A total of 1,480 individual operational taxonomic units (OTUs) were identified. Most were consistent across samples, indicating that they are part of a core mycobiome, though crop management influenced richness and diversity, likely in response to differences in soil properties. There were also differences in individual OTUs among genotypes and the nematode resistant genotype was most responsive to management system indicating that it has greater control over its endophytic mycobiome, which could potentially play a role in resistance. Members of the Ascomycota were most dominant, though the exact function of most taxa remains unclear. Future studies aimed at overcoming difficulties associated with isolating fungal endophytes are needed to identify these microbes at the species level and elucidate their specific functional roles. Carrot (Daucus carota L. subsp. sativus (Hoffm.) Arcang.) is one of the most important vegetable crops in the world, providing a good source of beta-carotene, fiber, Vitamin A and other vitamins and minerals to the human diet 1,2. Carrot taproots are often consumed raw, with per person consumption averaging 3.8 kg in 2015 3. Organic carrot production now accounts for 14% of the U.S. market 4 , and price premiums average 15% 4 , representing an opportunity for growers to transition to organic production. However, both organic and conventional carrot growers face many challenges to produce quality crops while protecting the environment. For example, while carrots are considered a nitrogen (N) scavenging crop, a substantial amount of N fertilizers are lost to the environment 5,6. Carrots are also subject to attack by many pests and diseases including Alternaria dauci 7 , and root knot nematodes 8 , as well as those that contribute to post-harvest storage losses 9. Endophytes, which are now commonly defined as microbes that spend at least part of their life cycle living inside plant tissues 10 , are one component of the plant microbiome that could help address these challenges. These microbes have been demonstrated to help plants acquire nutrients 11-13 , withstand abiotic stress 14,15 , and possibly even enhance the nutritional quality of crops. For example, some endophytes can produce or stimulate production of secondary metabolites 16,17 , indicating that they could play a role in the nutritional quality and organoleptic properties of plants 18. In addition, many endophytic taxa, especially fungi, have been shown to reduce disease caused by ...
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