Host plant resistance is the most sustainable option in managing soilborne disease , but like other crops it is believed there is little resistance to southern blight within commercial processing tomato cultivars. However, some resistance is available in the tomato germplasm. The Texas A&M breeding program released several breeding lines that have shown superior resistance to southern blight under field conditions . The mechanism of resistance is associated with the development of secondary tissue on the basal mainstem called the phellem barrier. The six Texas A&M selections 5635M, 5707M, 5719M, 5737M, 5876M, and 5913M were screened for two years in fields infested with A. rolfsii and showed resistance commensurate to a resistant wild accession PI 126432 . Additionally, the six selections showed field resistance to Fusarium oxysporum f. sp. lycopersici W. C. Snyder & H.N. Hansen race 1 including good average plant yields for 5719M and 5876M . The relative susceptibility of commonly grown commercial processing tomato cultivars to southern blight is unknown but would be beneficial for disease management. Grafting is another option for management of soilborne diseases. Disease control by grafting has already shown to be a beneficial alternative to the soil fumigant methyl bromide in Asia and much of Europe . Grafting is a fusion of two plant segments, 25 liter pot the shoot of the plant with desired fruit quality called the ‘scion’ and the root system with desired root traits as the ‘rootstock,’ that functions as a single plant . Grafting is commonly used for perennial crops and has since the early 20th century become a technique for vegetable production in Cucurbitae and Solanaceae species . Grafting to a resistant rootstock has previously been shown to reduce diseases causes by soilborne pathogens and has potential to be a sustainable alternative to fumigants for the control of many soilborne diseases .
In tomatoes, grafting has been used to augment growth under low potassium environments , improve tomato resistance to root-knot-nematodes , and increase tolerance to drought . The main mechanism of disease control by grafted plants is speculated to be by avoidance by having the resistant rootstock come into contact with the pathogen instead of the susceptible scion tissues . Maxifort is an interspecific hybrid of tomato and a wild Solanum species developed as a rootstock for greenhouse tomato . In a study in the southeastern United States, heirloom tomato grafted to the rootstock specific Maxifort exhibited 0 to 5% southern blight incidence whereas incidence in nongrafted plants was 27 to 79% . To our knowledge grafting processing tomatoes to a southern blight-resistant rootstock has not been explored in processing tomatoes in the San Joaquin Valley. Although rootstocks like Maxifort are highly resistant, some plants often develop southern blight symptoms . In our preliminary work, we observed that the graft union was planted below the soil line, possibly rendering the susceptible scion vulnerable to infection by A. rolfsii in the field. To our knowledge, raising the height of the graft union in other crops has yet to be evaluated in processing tomato. The use of resistant rootstock for an annual crop has been studied in fresh-market and heirloom tomatoes for improvement on yield but has yet to be explored for disease resistance for processing tomato in California. The objectives of this study were to: evaluate susceptibility of commercial processing tomato cultivars to southern blight; and evaluate grafting and increased height of the graft union with the resistant rootstock Maxifort for southern blight management in processing tomato.Athelia rolfsii sclerotia were produced in culture media using the oat seed method . The three Athelia rolfsii isolates used were each obtained from a different processing tomato field in Kern County, California in 2017.
Briefly, for each trial the isolates were grown from infested filter paper maintained at – 80ºC, hyphal tipped from mycelium actively growing on potato dextrose agar, and incubated at 25ºC under continuous light for approximately six days. Two plugs from the edge of the purified colonies were inoculated into Erlenmeyer flasks containing oat seeds and 1% water agar that had been autoclaved twice for 60 min on a 24 hr interval. Flasks were then incubated at room temperature for approximately 33 days. The sclerotia grown on oats were moved into sterile 5.7 L plastic containers placed in a biosafety cabinet to dry for approximately 14 days, and sclerotia were separated from oats by pressing the dried oat-sclerotia mixture with a gloved hand over a 2.0 mm and 850 µm sieves. Sclerotia were stored at room temperature in a plastic Ziploc bag until experiment set up. Viability of the inoculum was evaluated by germinating surface disinfested sclerotia on water agar.The susceptibility of 19 commercial processing tomato cultivars to A. rolfsii was evaluated in a greenhouse study in 2018 . In 2019, 19 commercial cultivars and six processing tomato breeding lines from Texas A&M were evaluated. The commercial cultivars were chosen based on highest total yield in California counties affected by southern blight. Treatments consisted of the 19 of these commercial cultivars grown in inoculated soil and a selection of 6 of the 19 commercial cultivars grown in non-inoculated soil as negative controls. The 2018 trial included two hybrid tomato cultivars grown in inoculated soil as positive controls, but were not included in 2019 due to poor germination. The rate of inoculum was 10 sclerotia per 100 cm3 soil based on recommendations by Punja and Rahe . The plants were started from seed using an organic seed starter soil mix in a tray with 200 22 mL, 2.22 cm x 2.22 cm cells. Two seeds were planted per cell.
The trays were placed on a clear plastic-lined chamber in the greenhouse on a warming mat set at 24ºC and misted three times per day for 15 seconds. Emergence began five days post seeding. Eleven days post seeding the trays were moved to an open misting bench where the plants could receive more sunlight, thinned to one plant per cell using sterile metal scissors, and sprinkled with one tablespoon of granular Osmocote Flower and Vegetable fertilizer 14-14-14 per 90 cells on the trays. The Osmocote rate used for germination was recommended by colleagues with tomato germination experience. Three weeks post seeding the plants were transplanted into trays with 36 166 mL, 5.72 cm x 5.08 cm cells to allow for advanced root development. In these larger trays, the soil substrate used was UC Soil Mix III, composed of 50:50 plaster sand:peat moss that was pasteurized at 100ºC for two hours. Inoculation and transplanting occurred five weeks post seeding. The day before inoculation, sclerotia were surface disinfested using 0.5% sodium hypochlorite solution for 1 minute, subsequently rinsed twice in sterile deionized water, and dried with sterile paper towels. The number of sclerotia needed to total 1 g was determined by manually counting, the amount to be added was weighed and incorporated to the top 10.2 cm of soil in 15.2 cm diameter pots to reach the target sclerotia count per 100 cm3 soil. Plants were then transplanted into 15.2 cm , 2.7 L pots with UC Soil Mix III at one plant per pot. Plants were grown in a greenhouse with the temperature set at 33ºC. Temperature data loggers were installed, but in 2018 they malfunctioned. In 2018, 25 liter plant pot the plants were arranged in a randomized complete block design with seven replications across four benches and with six replications across three benches in 2019 oriented east-west, one or two blocks per bench. Blocking was designed to capture potential confounding factors of light, temperature, and watering differences across the different benches. All benches had their own irrigation sub-line connected to a main line. A drip system was installed 15 days post transplanting with one JAIN Twist Weight emitter per pot and was set to water daily for 2 minutes early in the morning. Each plant was fertilized once per week for 3 weeks with 15 mL of a solution containing Jack’s Classic Professional Water Soluble Plant Food 20-20-20 at the recommended rate of 1 tablespoon per gallon of water. The volume of fertilizer was chosen based on observation of adding a volume of liquid that would not leach through the openings of the pots. Four days after the drip system was installed approximately one tablespoon of granular Osmocote Flower and Vegetable fertilizer 14- 14-14 was added around the drip emitter of each pot. The same growing methods from 2018 were used for 2019 with adjustments in using only granular Osmocote Flower and Vegetable fertilizer 14-14-14. The plants were maintained in the greenhouse for 126 days in 2018 and the plants from 2019 were maintained in the greenhouse for 107 days.In 2018, because little disease development was observed in blocks 5 and 6, additional inoculum was added to the inoculated pots in these blocks 77 days after initial inoculum was added. Inoculum for each individual pot was calculated by multiplying the volume of the top four inches of the pots by the rate of inoculum , then divided by the average number of sclerotia from one gram of sclerotia. The sclerotia from all three isolates were evenly mixed. The mixed sclerotia were then weighed to 0.12 g for each individual pot, inoculum per pot were placed in a ziplock bag, then one bag of inoculum was carefully poured around the previously inoculated tomato stem. Plant material for grafting experiments.
The processing tomato cultivars used as scions or non-grafted controls in grafting experiments were Heinz 5608 and Heinz 8504, which are commonly grown in the San Joaquin Valley of California. The hybrid cultivar Maxifort served as the rootstock in grafted treatments. For all greenhouse and field grafting experiments, transplants and grafting were produced by Growers Transplanting Inc. in Salinas, CA using the tube grafting technique with the modification of using a clip that applies minimal pressure on the graft union. Grafting for the high-union grafted treatment consisted of plants with a union approximately 2.54 cm above the standard graft. These high-union grafted plants were produced by stretching the rootstock 2.54 cm to 5.08 cm before the grafting process, applying extra fertilizer to the rootstock, and cutting the rootstock approximately 6.35 cm to 7.62 cm from the plug .Grafting greenhouse experiments. In 2017 a preliminary study was conducted that evaluated two cultivars , two graft treatments , and four inoculum levels in a full factorial treatment arrangement. On June 5, 2017 a single plant was transplanted into each 2733 mL pot with UC Soil Mix III that was inoculated as described above for the cultivar trial and grown in a greenhouse at 32ºC. One-plant pots were arranged in a randomized complete block design with 8 replications across two benches oriented east-west, four blocks per bench. The pots were watered via a drip system beginning 24 days post planting. The plants were fertilized every 2 to 3 weeks with 100 mL of Jack’s Classic Professional Water Soluble Plant Food 20-20-20. The treatment structure was modified based for 2018 and 2019 to include a grafted treatment with a high-union, referred to as ‘tall’. These studies evaluated two cultivars , three graft treatments , and two inoculum levels in a full factorial arrangement . Plants were transplanted into one-plant pots on July 18 in 2018 and April 29 in 2019. Plants were arranged in a randomized complete block design with 6 and 8 replications in 2018 and 2019, respectively, on benches oriented east-west with two or three blocks per bench. The 2018 experiment was conducted in a greenhouse set to 21ºC for 120 days, which was increased to 26ºC for 34 days due to low disease pressure. The 2019 experiment was conducted in a greenhouse set to 35ºC for 83 days. Hobo MX2301 data loggers monitored temperature in the greenhouse and reported the average temperature maximum 38ºC and minimum 15ºC in 2018. In 2018, four days post planting drip irrigation was used to water daily with fertilized water for 21 days before switching to industrial water. After adjusting the drip system, the plants were fertilized once every week then adjusted to fertilizing twice a week with 100 mL solution of Jack’s Classic Professional Water Soluble Plant food. In 2019, approximately one tablespoon of granular Osmocote Flower and Vegetable fertilizer 14-14-14 was added around the drip emitter of each pot. In 2019 additional inoculum was added to the inoculated pots 64 days after transplant to encourage disease development.