In 2009 a field trial was conducted in Henderson County, NC on a grower’s field where Ralstonia solanacearum, causal agent of bacterial wilt of tomato, was discovered the previous year. Because soilborne diseases are difficult to control and this grower uses organic practices, the field trial involved grafting the desired tomato variety onto disease resistant rootstocks to determine if this management strategy would help to control bacterial wilt.
The experiment was a randomized complete block design with five treatments and four repetitions. Plants were planted 24 inches apart on black plastic mulch with drip irrigation. The treatments were, hybrid variety ‘Mountain Fresh’, ‘Mountain Fresh’ grafted onto rootstock ‘RST-04-105-T’, heirloom variety ‘Cherokee Purple’, ‘Cherokee Purple grafted onto ‘Maxifort’ rootstock, heirloom variety ‘Brandywine’ and ‘Brandywine’ grafted onto ‘Maxifort’ rootstock.
Plants were transplanted on 5 June 2009. On 4 August an epiphytotic of late blight, caused by Phytophthora infestans, began in Henderson County and tomato crops throughout the region were devastated. This included this organically-managed, tomato grafting field trial. As a result, data on the effect of grafting on bacterial wilt control was not collected. However, numerical (not statistically analyzed) data shows that the hybrid variety ‘Mountain Fresh’ was more resistant to P. infestans than the heirloom varieties.
In western North Carolina (WNC) one of the biggest obstacles for any tomato grower is disease problems. Historically, foliar diseases, such as late blight (Phytophthora infestans) and early blight (Alternaria solani) have been the major disease issues in WNC. Over the past few years there has been a rise in diseases caused by soilborne plant pathogens. Diseases such as southern bacterial wilt (Ralstonia solanacearum), verticillium wilt (Verticillium spp.) and fusarium wilt (Fusarium oxysporum f. sp. lycopersici) are becoming a widespread issue for tomato producers throughout the region. This increase is thought to be a result of flooding of rivers and streams due to recent hurricane activity. Soilborne plant pathogens are difficult to control even with the use of resistant tomato varieties, fumigation and conventional fungicides. Growers who chose to grow heirloom tomatoes in an organic system face an even more challenging situation because fumigation, synthetic fungicides and disease resistant varieties are not available as a management strategy.
In August of 2008, bacterial wilt (R. solanacearum) was identified on heirloom tomatoes growing in Henderson County. The farmer was concerned that bacterial wilt would halt the production and income of heirloom tomatoes on his farm, which provides a steady market year to year. Though the farmer is not certified organic, he runs his operation following organic standards. Because there is limited to no genetic disease resistance in heirloom tomatoes and fumigation is not an option for his system, management recommendations were limited to rotating the field with a non-host and finding another location for the heirloom tomato production. Concerned that soilborne disease problems would continue to be a problem even in other fields, and the very high price of land in WNC, other methods of managing disease situations in heirloom tomato production need to be identified and evaluated.
This farmer’s situation is not uncommon in WNC and it is expected that the disease problems will continue to increase, especially as the incidence of soilborne disease pressures increases. The use of grafted vegetable plants on resistant rootstock as a management strategy for soilborne disease problems has the attention of the organic community in WNC looking for ways to successfully produce tomatoes for their markets.
The objective of this research project was to determine the benefits and challenges of using grafted tomato transplants to control soilborne disease problems. Unfortunately, the foliar disease problem, late blight (Phytophthora infestans) was detected early in the cropping season and progressed rapidly as the cool, wet weather dominated the growing season. As a result, data was not reliable and appropriate for the determination of the benefit of grafting. Though it was not an objective for this trial, numerical results show that the variety ‘Mountain Fresh’ was more resistant to late blight than the heirloom varieties used. Data included as a spreadsheet.
The experiment was a randomized complete block design with five treatments and four repetitions. Plants were planted 24 inches apart on black plastic mulch with drip irrigation. The treatments were, hybrid variety ‘Mountain Fresh’, ‘Mountain Fresh’ grafted onto rootstock ‘RST-04-105-T’, heirloom variety ‘Cherokee Purple’, ‘Cherokee Purple grafted onto ‘Maxifort’ rootstock, heirloom variety ‘Brandywine’and ‘Brandywine’ grafted onto ‘Maxifort’ rootstock.
Areas needing additional study
The benefit, challenges, viability and economics of tomato grafting for the control of soilborne diseases still need to be studied in western NC on growers’ fields.