Bacterial spot (BS) on tomato is a destructive disease that can reduce fruit quality and yield during hot and humid growing seasons. The disease is caused by four species of Xanthomonas. Xanthomonas gardneri and X. perforans are currently the predominant species in Ohio. Existing chemical strategies to manage BS lack effectiveness and no economic threshold level has been established for the disease. Field trials were conducted to assess the relationship between foliar disease severity and marketable fruit yield and the economic impact of bactericide application under low and high disease pressure for both fresh market and processing tomatoes. In 2018, the treatments in each field trial included a non-inoculated plot treated with an industry standard chemical spray program; plots inoculated with a high (~108 CFU/ml) or low (~104 CFU/ml) concentration of X. gardneri (FM) or X. perforans (processing) and no chemical treatments and; a non-inoculated, non-treated control plot. In 2019, both field trials were repeated, and two additional treatments were evaluated in the fresh market tomato trial. The additional treatments were an industry standard chemical spray program for the entire season or at bloom only in plots inoculated with a high (~108 CFU/ml) concentration of X. gardneri. Processing and FM tomato trials were evaluated independently, and each treatment was referred to as an economic scenario. For both trials in both years, disease severity was lowest in the plots managed using a standard chemical spray program with no inoculum and highest in the plots with a high level of inoculum but no chemical treatment (p<0.0001). For FM tomato, marketable yield was significantly lower in the economic scenario in which plots were inoculated with a high concentration of X. gardneri and were not treated (p<0.0001). No difference was observed in marketable yield among the processing tomato treatments in both years (2018, p=0.761; 2019, p=0.0983). Marketable yield negatively correlated with foliar disease severity for both fresh market tomato (2018, p<0.0001, r= -0.73; 2019, p<0.0001; r= -0.8) and processing tomato (2018, p<0.06, r= -0.34; 2019, p<0.022, r= -0.4). Bactericide application was economically profitable only when foliar disease severity was high (benefit-cost ratio (BCR) > 1). For the fresh market tomato trials, the BCRs of bactericide application in 2018 were 1.9 and 4.8 for the economic scenarios in which non-treated plots were inoculated with low or high inoculum of X. gardneri. In 2019, foliar disease severity was high and BCRs of bactericide application were greater than 1 for all the economic scenarios and ranged from 1.1 to 12.3. The BCRs of bactericide application ranged from -0.17 to 0.6 in 2018 and from -0.1 to 1.3 in 2019 for the processing tomato trial which had a low to moderate level of foliar disease severity. In addition to understanding the impact of foliar disease severity and bactericide application on farm profit, it is also important to understand how the load of Xanthomonas on the seed impacts BS disease outbreak in greenhouse.
Recent surveys indicate that X. perforans is now replacing X. gardneri, which was the predominant species in Ohio a decade ago. Laboratory and greenhouse experiments were conducted to determine if the longevity of Xanthomonas on tomato seed varies depending on the species and if the bacterial load on seed impacts the risk and severity of an outbreak during transplant production. Five strains of X. perforans and five strains of X. gardneri were used to prepare bacterial suspensions (~108 CFU/ml) and infest conditioned (hairless) tomato seed. Infested seed were stored at 10°C. Seed washing assays were performed to determine the number of viable bacterial cells per seed over time, seed plating assays to determine the number of infested seed per lot over time, and seed grow out assays were performed to determine the level BS incidence and severity on seedlings produced from infested seed. All assays were performed at one hour and 3, 7, 14, 21, 28, and 60 days post inoculation. Overall, significantly more viable cells of X. gardneri (0.15 Log10 CFU/seed) survived on the infested seed than X. perforans cells (p=0.0006). The number of bacterial cells for both species of Xanthomonas on the seed declined over time, however X. perforans populations declined faster than X. gardneri populations with a mean difference of 37 CFU/seed day-1 (p=0.0138). The number of detectable X. perforans-infested seed in the lot declined at a rate 0.1% faster per day compared to the lot infested with X. gardneri (p=0.0502). In contrast, the number of infested seed from seed lots infested with X. perforans was higher by 15.9% (p<0.0001) than the lots infested with X. gardneri. Seedlings produced from seed infested with X. perforans had 32% more foliar disease severity (p<0.0001), 7.2% more diseased seedlings (p<0.0001) and a higher AUDPC (1.4; p<0.0001) than those infested with X. gardneri. The understanding of the longevity of Xanthomonas on tomato seed is important to design effective management strategies at the seed stage.
Managing BS is very challenging and strains of predominant Xanthomonas species (X. gardneri and X. perforans) are resistant to current micron size copper (Cu) due to its prolonged use to manage BS on tomato. A holistic approach that targets seed, transplant production and field/greenhouse production stages is crucial to effectively manage BS on tomato. Therefore, a field experiment was conducted to assess the effectiveness of three formulations of coppers nanoparticles (CuNPs; MV-Cu, CS-Cu, FQ-Cu) for the control of BS caused by X. gardneri on processing tomato. All three CuNPs significantly reduced foliar disease severity and disease progression compared to the non-treated control (p<0.0001). No differences in yield amongst these treatments were observed (p=0.7638). Also, laboratory and greenhouse experiments were conducted to assess the efficacy of silver nanoparticles (AgNPs) and magnesium oxide nanoparticles (MgONPs) at reducing viable Xanthomonas cells on tomato seed and BS incidence and severity at the transplant stage. Conditioned tomato seed were inoculated with X. gardneri or X. perforans and treated with a high (10mg/ml) or low (2.5 mg/ml) dose of silver nanoparticles (AgNPs) or magnesium oxide nanoparticles (MgONPs). Seed washing, seed plating, and seed grow out assays were conducted to determine the efficacy of AgNPs and MgONPs to manage BS at the seed and transplant levels. Both doses of AgNPs and MgONPs significantly reduced BS incidence, severity, and disease progression on tomato seedlings produced from seed infested with X. perforans or X. gardneri compared to the infested non-treated control (p=0.0002, p=0.0009, p<.0001).
In this study we report the results of the economic impact of bactericide applications on the net income of tomato production, the longevity of Xanthomonas species on tomato seed, and the efficacy of CuNPs, AgNPs, and MgONPs to manage BS on tomato. More research about BS disease threshold and alternative management tools are needed to overcome some of the challenges related to managing bacterial spot on tomato.