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.

Effect of moisture on efficiency of microwaves to control plant--parasitic nematodes in soil.

Laboratory studies were conducted to evaluate effect of microwave irradiation of sandy loam soil on thermal energy absorption and control of plant-parasitic nematodes when air dry soil layers were placed on top of less moist, moist, and wet soil layers. The soil was packed in 12 cm high and 10 cm dia columns to a bulk density of 1.4 g/cm3. Moisture contents of air dry, less moist, moist, and wet soils were 0.75, 4.50, 6.00, and 10.30%, respectively, on dry mass basis. The top air dry soil was 4.0 cm thick and the bottom layer was 8.0 cm thick. Temperature measurements and thermal radiation absorption data were monitored in both soil layers and showed that the use of a top dry soil both increased depth of penetration of microwave radiation and it provided insulation for better absorption of thermal energy in the lower layer of soil. An exposure of 65 seconds resulted in soil temperatures high enough to cause significant decrease in nematode population in soil infested with Rotylenchulus reniformis nematodes. No such effect was observed in combination where dry soil layer was placed over dry soil at the bottom. These results are helpful in sterilizing soil used for greenhouses and nurseries.

Potential of microwaves to control plant-parasitic nematodes in soil.

Microwave radiation of 2450 MHz frequency was used to irradiate sandy loam soil placed in 12 cm high and 10 cm dia columns as a function of exposure times of 30, 45, 60, and 120 s. This was done to evaluate the effect of radiation on the highest soil temperature attained and subsequent temperature patterns in relation to time. Soil columns were packed to a field bulk density of approximately 1.4 g/cm3, and treatments consisted of moist soil, dry soil, and layers of moist and dry soil of varying thicknesses. Moisture contents of moist and dry soil were 10% and 2%, respectively, on a dry mass basis. An exposure time of 45 seconds was the most efficient in yielding soil temperatures high enough to kill plant-parasitic nematodes. Irradiation of soil infested with Rotylenchulus reniform nematodes for 45 seconds resulted in a 99% extermination of the organisms in all treatments. However, radiation proved to be most effective in nematode control with 6.0 cm dry soil placed over 6.0 cm moist soil.

Development of an Integrated Approach for Managing Bacterial Wilt and Root-Knot on Tomato Under Field Conditions.

A 2-year field study was conducted to develop a field application method using thymol as a preplant soil treatment for controlling bacterial wilt and root-knot nematode on tomato (Lycopersicon esculentum). In addition, acibenzolar-S-methyl (ASM), which induces plant systemic resistance, was applied in conjunction with thymol to determine whether combining these tactics could improve bacterial wilt management. The test sites were artificially infested with Ralstonia solanacearum and Meloidogyne arenaria, and thymol was applied as preplant fumigation through drip irrigation lines under polyethylene mulch at a rate of 73 kg/ha in both 2004 and 2005. ASM was applied primarily as foliar spray at a concentration of 25 mg/liter. Application of thymol significantly reduced incidence of bacterial wilt on tomato in both years of the trial. In thymol-treated plots, 26.0 and 22.6% of the plants wilted in 2004 and 2005, respectively; whereas, in untreated plots, more than 95% of the plants wilted in each year. Number of root-knot nematode juveniles was significantly reduced in field plots treated with thymol and ASM for both years. The combined use of thymol and ASM provided the greatest reduction of root galling among the treatments. Tomato yield (cv. FL47) was evaluated only in the 2005 trial; thymol-treated plots produced significantly higher marketable yield than untreated plots, and the thymol treatment in combination with ASM significantly increased tomato yield compared with thymol or ASM alone. These results indicate that use of thymol and ASM was beneficial in controlling bacterial wilt and root-knot. We developed an effective method for applying thymol through drip irrigation lines for managing these diseases in tomato production.