The Use and Impact of Antibiotics in Plant Agriculture: A Review.

Growers have depended on the specificity and efficacy of streptomycin and oxytetracycline as a part of their plant disease arsenal since the middle of the 20th century. With climate change intensifying plant bacterial epidemics, the established success of these antibiotics remains threatened. Our strong reliance on certain antibiotics for devastating diseases eventually gave way to resistance development. Although antibiotics in plant agriculture equal to less than 0.5% of overall antibiotic use in the United States, it is still imperative for humans to continue to monitor usage, environmental residues, and resistance in bacterial populations. This review provides an overview of the history and use, resistance and mitigation, regulation, environmental impact, and economics of antibiotics in plant agriculture. Bacterial issues, such as the ongoing Huanglongbing (citrus greening) epidemic in Florida citrus production, may need antibiotics for adequate control. Therefore, preserving the efficacy of our current antibiotics by utilizing more targeted application methods, such as trunk injection, should be a major focus. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Trunk Injection of Oxytetracycline for Huanglongbing Management in Mature Grapefruit and Sweet Orange Trees.

Huanglongbing (HLB) is a devastating bacterial disease associated with 'Candidatus Liberibacter asiaticus'. The location of the pathogen within the vasculature of the tree has left growers with limited options for the effective management of the disease. Trunk injection is a crop protection technique that applies therapeutics directly into the xylem of woody tree species and allows for their systemic uptake and transport, which may provide more effective management of vascular diseases such as HLB. In this study, mature 'Valencia' and 'Hamlin' sweet orange (Citrus sinensis) and 'Duncan' grapefruit (C. paradisi) trees were injected with oxytetracycline (OTC) in the spring and/or fall to evaluate the effects of injection timing and response to injection. In addition to seasonal evaluations of tree health and bacterial titer, preharvest fruit drop, yield, and fruit quality were measured at harvest to determine the effects of OTC injection. The benefits associated with injection included a reduction in fruit drop, an increase in fruit yield and fruit size, and improvements in juice quality. However, results varied due to the timing of injection and were not consistent across all three varieties. Residue analysis at different time points after injection suggests that trunk injection effectively delivers therapeutics to mature citrus trees. This study provides fundamental information on the short-term benefits associated with trunk injection of OTC for HLB management in citrus groves. The potential for use of trunk injection at the commercial scale and the possible risks are discussed.

First Report of Bacterial Wilt Disease of Tomato, Pepper and Gboma Caused by the Ralstonia solanacearum Species Complex in Togo.

Tomato (Solanum lycopersicum), pepper (Capsicum annum), and gboma (Solanum macrocarpon) are major vegetables in Togo, with many people depending on these crops for their livelihood. In December 2018, during the dry season with temperatures between 21°C to 35°C, tomato ('Petomech'), pepper ('Gboyebesse') and gboma (local landrace) showing wilt symptoms without foliar yellowing were collected from two locations, Tchouloum and CECO-AGRO sites in the Sotouboua Prefecture of Togo, ~300 km from the capital city of Lome. Disease incidence ranged between 10% to 50% in multiple fields. Cut stems of most wilting tomato, pepper and gboma plants produced bacterial ooze in water and vascular discoloration was visible in longitudinal stem sections. Ground cut stem tissue tested positive with Rs ImmunoStrips specific to the Ralstonia solanacearum species complex (RSSC) (Agdia Inc., Elkhart, IN, USA). Collected samples were stored at ambient temperature and cultured within 36 hr. Culturing sap from cut stems plated on modified SMSA medium (Engelbrecht 1994) yielded colonies with typical RSSC morphology: slow-growing, irregular, mucoid, and white with red centers. Genomic DNA was extracted from thirteen isolates: two from gboma, five from tomato and six from pepper. The expected 280-bp band was amplified from all 13 genomic DNAs following polymerase chain reaction (PCR) using the 759/760 RSSC-specific primer pair (Opina et al. 1997). PCR with the 630/631 primers, which identify the Race 3 biovar 2 RSSC subgroup, did not yield a product from any Togo isolate (Opina et al. 1997). The phylotype multiplex PCR identified all Togo isolates as belonging to the phylotype I subgroup, also called R. pseudosolanacearum (Prior et al. 2016; Fegan and Prior 2005). Phylotype control DNAs were from strains GMI1000 (phylotype I, Asia), K60 (phylotype II, Americas), CMR15 (phylotype III, Africa), and PSI07 (phylotype IV, Indondesia). Comparative genomic analysis of the partial endoglucanase (egl) gene, amplified with the Endo primer pairs (Poussier et al. 2000), revealed all Togo strains belonged to sequevar 17, a group known to cause bacterial wilt of peanut in China. (Xu et al. 2009). The egl sequences are in NCBI GenBank accessions MT572393 to MT572405. Koch's postulates were completed by inoculating 28-day-old bacterial wilt-susceptible 'Bonny Best' tomato plants by soil soak (Khokhani et al. 2018). Briefly, soil around each unwounded plant was drenched with 50 ml of a 108 CFU/mL suspension of bacteria grown from a single colony. Five plants were inoculated with each of four randomly selected Togo strains. RSSC phylotype I strain GMI1000 served as a positive control and water treated plants as negative controls. Plants were kept in a 28°C growth chamber with a 12 hr photoperiod. All RSSC inoculated plants were fully wilted within a week; symptoms resembled to those observed in the field. Water treated control plants did not wilt. Culturing sap from all inoculated plants on SMSA medium yielded colonies with typical RSSC morphology that tested positive with the Rs ImmunoStrips. This is the first identification of RSSC in Togo. These results will guide development of disease management strategies and regionally appropriate breeding of vegetable lines with resistance to the phylotype I RSSC strains present in Togo.

Genetic dissection of heat-responsive physiological traits to improve adaptation and increase yield potential in soft winter wheat.

BACKGROUND: Climate change, including higher temperatures (HT) has a detrimental impact on wheat productivity and modeling studies predict more frequent heat waves in the future. Wheat growth can be impaired by high daytime and nighttime temperature at any developmental stage, especially during the grain filling stage. Leaf chlorophyll content, leaf greenness, cell membrane thermostability, and canopy temperature have been proposed as candidate traits to improve crop adaptation and yield potential of wheat under HT. Nonetheless, a significant gap exists in knowledge of genetic backgrounds associated with these physiological traits. Identifying genetic loci associated with these traits can facilitate physiological breeding for increased yield potential under high temperature stress condition in wheat. RESULTS: We conducted genome-wide association study (GWAS) on a 236 elite soft wheat association mapping panel using 27,466 high quality single nucleotide polymorphism markers. The panel was phenotyped for three years in two locations where heat shock was common. GWAS identified 500 significant marker-trait associations (MTAs) (p ≤ 9.99 × 10- 4). Ten MTAs with pleiotropic effects detected on chromosomes 1D, 2B, 3A, 3B, 6A, 7B, and 7D are potentially important targets for selection. Five MTAs associated with physiological traits had pleiotropic effects on grain yield and yield-related traits. Seventy-five MTAs were consistently expressed over several environments indicating stability and more than half of these stable MTAs were found in genes encoding different types of proteins associated with heat stress. CONCLUSIONS: We identified 500 significant MTAs in soft winter wheat under HT stress. We found several stable loci across environments and pleiotropic markers controlling physiological and agronomic traits. After further validation, these MTAs can be used in marker-assisted selection and breeding to develop varieties with high stability for grain yield under high temperature.

Transgenic Expression of EFR and Bs2 Genes for Field Management of Bacterial Wilt and Bacterial Spot of Tomato.

Field trials were conducted at two locations in Florida to evaluate transgenic tomato expressing the ELONGATION FACTOR TU RECEPTOR (EFR) gene from Arabidopsis thaliana, the Bs2 gene from pepper, or both Bs2 and EFR (Bs2/EFR) for managing bacterial wilt caused by Ralstonia solanacearum and bacterial spot caused by Xanthomonas perforans. Expression of EFR or Bs2/EFR in the susceptible genotype Fla. 8000 significantly reduced bacterial wilt incidence (50 to 100%) and increased total yield (57 to 114%) relative to lines expressing only Bs2 or the nontransformed Fla. 8000 control, although the marketable yield was not significantly affected. Following harvest, surviving symptomatic and nonsymptomatic plants were assessed for colonization by R. solanacearum. There were no significant differences in the population at the lower stem. Interestingly, in the middle stem, no bacteria could be recovered from EFR or Bs2/EFR lines but viable bacterial populations were recovered from Bs2 and nontransformed control lines at 102 to 105 CFU/g of stem tissue. In growth-chamber experiments, the EFR transgenic tomato lines were found to be effective against seven different R. solanacearum strains isolated from the southeastern United States, indicating utility across the southeastern United States. In all of the bacterial spot trials, EFR and Bs2/EFR lines had significantly reduced disease severity (22 to 98%) compared with the Fla. 8000 control. The marketable and total yield of Bs2/EFR were significantly higher (43 to 170%) than Fla. 8000 control in three of four field trials. These results demonstrate for the first time the potential of using the EFR gene for field management of bacterial wilt and bacterial spot diseases of tomato.

Foliar Applications of Acibenzolar-S-Methyl Negatively Affect the Yield of Grafted Tomatoes in Fields Infested with Ralstonia solanacearum.

Three field experiments were conducted in Florida from 2012-2014 to assess the impact of acibenzolar-S-methyl (ASM), a systemic acquired-resistance inducer, applied as foliar spray or through drip-irrigation lines, on bacterial wilt incidence and yield of grafted tomatoes. The experiments were conducted in a field with race 1, biovar 1 strain of Ralstonia solanacearum, causal agent of tomato bacterial wilt. In all three experiments, the susceptible tomato variety BHN 602, grafted onto a resistant rootstock BHN 998, was compared with nongrafted BHN 602, treated with or without foliar applications of ASM and with grafted plants treated with foliar applications of ASM. In two experiments, an additional treatment of drip applications of ASM on grafted and nongrafted plants was evaluated. Grafting alone or in combination with drip applications of ASM (178.6 µM) significantly reduced disease incidence and increased total marketable yield relative to nongrafted treatments. There were no significant differences between grafted plants with or without drip ASM applications in terms of bacterial wilt incidence or total marketable yield. However, we demonstrate for the first time that foliar ASM applications on grafted plants negatively affects the total marketable yield compared with drip ASM applications on grafted plants or nontreated grafted control.

Grafting Using Rootstocks with Resistance to Ralstonia solanacearum Against Meloidogyne incognita in Tomato Production.

Root-knot nematodes (RKNs; Meloidogyne spp.) and Ralstonia solanacearum, the causal agent of bacterial wilt, are major soilborne pathogens in U.S. tomato production. Methyl bromide has been used for decades to effectively manage RKN but its phase-out and the high cost of other effective fumigants such as 1,3-dichloropropene has resulted in a need to develop sustainable alternatives. Many of the commercially popular varieties used by the tomato industry do not have resistance to RKNs and R. solanacearum. Recent studies worldwide have shown the potential for grafting using resistant rootstocks as a sustainable and ecofriendly practice for R. solanacearum management. However, the effectiveness of R. solanacearum-resistant rootstocks on RKN management is not known. In this study, three commercially available R. solanacearum-resistant tomato rootstocks ('RST-04-106-T', 'BHN 998', and 'BHN 1054') were evaluated for resistance to Meloidogyne incognita in field tomato production in four field trials conducted for two consecutive years in two geographical locations: Florida and Virginia. Grafting rootstocks onto 'BHN 602' a tomato scion susceptible to bacterial wilt and RKNs, significantly reduced root galling caused by RKNs in all four field trials and increased yield in two of the trials compared with the nongrafted treatment. This study demonstrates the potential of grafting for managing multiple soilborne pathogens using the same rootstocks.

Nanotechnology in plant disease management: DNA-directed silver nanoparticles on graphene oxide as an antibacterial against Xanthomonas perforans.

Bacterial spot caused by Xanthomonas perforans is a major disease of tomatoes, leading to reduction in production by 10-50%. While copper (Cu)-based bactericides have been used for disease management, most of the X. perforans strains isolated from tomatoes in Florida and other locations worldwide are Cu-resistant. We have developed DNA-directed silver (Ag) nanoparticles (NPs) grown on graphene oxide (GO). These Ag@dsDNA@GO composites effectively decrease X. perforans cell viability in culture and on plants. At the very low concentration of 16 ppm of Ag@dsDNA@GO, composites show excellent antibacterial capability in culture with significant advantages in improved stability, enhanced antibacterial activity, and stronger adsorption properties. Application of Ag@dsDNA@GO at 100 ppm on tomato transplants in a greenhouse experiment significantly reduced the severity of bacterial spot disease compared to untreated plants, giving results similar to those of the current grower standard treatment, with no phytotoxicity.