Project Overview

Reports

Commodities

• Vegetables: tomatoes

Target Pests

• Target Pests: bacterial diseases

Practices

• Pest Management: genetic resistance, host resistance

• Crop Production: plant breeding and genetics

i) The problem or opportunity

Bacterial wilt (BW) is one of the most damaging pathogens causing significant loss of solanaceous crops, especially tomato (Solanum lycopersicum) in the southeastern U.S. states (SE-USA). The disease is caused by Ralstonia solanacearum/pseudosolanacearum, a soil bacterium that causes BW by blocking vascular movement of water, leading to rapid plant death. The bacterium can stay virulent for months or years in deep soil layers, travel with groundwater, be transmitted via field machinery, irrigation systems, and infected crops/weeds, and survive on many different host plants. Thus, mitigation of BW in tomato fields has proven difficult in the SE-USA.

Current IPM strategies to manage BW lack efficacy, are not cost-effective, and/or pose a danger to human health and the environment. These IPM tactics include chemical (bactericides, plant activators), biological (the seeding of avirulent strains or antagonistic bacteria), and cultural (crop rotation, avoidance of infested land areas, soil amendments) methods. These tactics can mitigate the level of BW disease incidence to some extent, but there is no mechanism to limit losses once the pathogen successfully begins to colonize the plant. In the end, many Southern region growers abandon infested land and plant less profitable crops.

Host resistance is the most efficient, cost-effective, and environmentally friendly IPM approach to control BW infection and spread. However, breeding resistant cultivars has proven inefficient and ineffective because BW resistance genes have never been identified and acceptable resistance is tightly linked to small fruit size. Therefore, no resistant commercial cultivars are grown in the SE-USA. Grafting marketable scions (tops) to highly resistant rootstocks is an efficacious IPM tactic but is costly. The majority of SE-USA conventional (mature green or vine ripe) tomato production systems do not have the profit margins to utilize grafted plants.

A recent breakthrough discovered a candidate BW resistance gene; a turning point for IPM if this gene can be validated, introgressed into future hybrids and complemented with other IPM tactics.

ii) The approach in simple terms that can be understood by the general public, university personnel, private organizations, and budget staff

The most critical seed work needed is to validate the efficacy of this gene so it can be confidently introgressed into future IPM programs in superior cultivars. We cloned the candidate gene from the BW resistance tomato Hawaii7996 and transformed the expressed gene into the BW susceptible tomato Heinz1706. Our preliminary data showed that the expressed gene did not interfere with fruit size. The transgenic plants will be advanced to homozygous lines, and molecular and phenotypic analysis will be conducted to confirm the presence and expression of the candidate gene in the transgenic lines. The most important hypothesis is: superior resistance can be achieved and fruit size will not decrease. Through USDA permits, two virulent Ralstonia strains indigenous to NC (and on hand) will be used to screen transformed Heinz1706 compared to negative (non-transformed Heinz1706) and positive controls (Hawaii7996). The knowledge learned will be transferred to breeders to advance host resistance-based IPM.

iii) The anticipated impact

Two tiers of major impacts will emerge. First, if our hypothesis is negative i.e. that the gene confers resistance AND small fruit size, our work will save breeders years of frustrating work and enable them to focus on other genes or IPM tactics e.g. breed less-expensive rootstocks. If our hypothesis is validated, breeders can confidently introgress the gene (using non-GMO methods) into future hybrids. Based on the depth of work we have done with the industry, we are confident many growers will utilize emergent hybrids to reclaim infested land lost to BW, reduce crop losses, and enhance IPM-based profitability.

1. Generation of homozygous transgenic tomato plants in BW susceptible tomato Heinz1706 overexpressing the candidate BW resistance gene from Hawaii7996;
2. Molecular and phenotypic analysis of the transgenic lines;
3. Bacterial screening with virulent solanacearum;
4. Transfer knowledge to breeders to advance host resistance-based IPM tactics.