Protected tomato: Evaluation of biological treatments, biocides and an improved diagnostic for control of root mat disease

Summary: 

Root mat disease of tomato caused by strains of Rhizobium radiobacter carrying a root-inducing (Ri) plasmid is an increasing problem in the UK and elsewhere. Current knowledge of disease biology and crop observations both indicate that infection probably occurs when plants are young, including during propagation, though symptoms can take many weeks to develop.  The disease causes excessive vegetative growth, reduced fruit size and quality and secondary root rots.  Together these result in significant crop losses estimated at 15% and additional management costs. 

There are no proven treatments for disease control.  Current efforts focus on biological treatments, crop management and hygiene; there are no approved bactericides.  An increasing number of biological products reported to increase plant health and/or resistance to disease are now available; the NatuGro programme, for example, is used quite widely although there is no evidence for effectiveness against root mat.   

A real-time PCR assay previously developed detects the pRi plasmid in isolates of Rhizobium radiobacter. It is unclear whether the assay detects all variant of pRi or can be reliably used to detect transformed roots before symptoms of root mat have developed.  The availability of a reliable test that detects pRi in plants as well as in bacteria would allow accurate early detection of infected plants and more reliable evaluation of control measures.

The specific objectives of this project are to: (1) review and summarise knowledge of root mat disease for production as a Factsheet; (2) develop and fully validate a rapid molecular test for detection of Ri plasmid DNA in transformed tomato roots prior to symptom occurrence; (3) evaluate biological-based products in propagation for prevention or reduction of root mat disease; (4) evaluate biological-based products applied after planting for continued suppression and control of root mat symptoms; (5) determine efficacy of biocides againstR. radiobacter and Ri plasmid for use at crop turnaround; (6) investigate interaction of root mat disease and PepMV; (7) communicate results to growers.

Benefits to Industry:

• Sound knowledge on the efficacy of biological products currently available as treatments to prevent / control tomato root mat disease.
• Potentially large financial savings through a reduction of infected plants – i.e. reduced production of unmarketable small fruit, improved fruit quality and reduced plant wilting/death through secondary root rots. 
•  Information on the relative benefit of biological-based treatments applied during plant propagation, compared with treatment applied after planting on slabs.
• Potentially large financial savings for growers where any treatments they currently use against root mat are shown to be ineffective.
• Potential for the treatments found effective to be used for control of root mat disease in cucumber.
• Improvement of diagnostic services currently used by propagators and tomato growers in the UK and the Netherlands with increased reliability of detection of root mat-causing bacteria and new opportunity to screen tomato and cucumber roots for evidence of transformation by these pathogens in advance of symptom development (eg prior to dispatch; at arrival on a nursery).
• Potential development of on-nursery diagnostic tests (e.g. LAMP assay) for direct application by propagators and producers; growers would be able to have samples tested at an early stage, when it is difficult to distinguish strong root growth from root mat disease based on symptoms alone.
•  Potential to elucidate growing media, light and factors which may influence symptom development.
• An enhanced understanding of the disease and control options for root mat disease.
• Observations on the interaction of root mat disease and PepMV.
•  Treatments found effective against tomato root mat could be short-listed for trials against the related disease crown gall, on a range of ornamental and cane fruit hosts.

Aims and Objectives:

(i)     Project aim(s):

To identify biological treatments and biocides that reliably control or suppress root mat disease by prevention of infection and transformation of protected tomato by bacteria carrying the root initiation plasmid (pRi) and to develop a rapid molecular test for early detection of infected plants.

(ii)    Project objective(s):

1. To review and summarise current knowledge of root mat disease in tomato and cucumber through production of text and photographs for an HDC Factsheet. 

2. To develop and fully validate a rapid molecular test for detection of T-DNA from different Ri plasmids in tomato roots prior to symptom occurrence;

3. To quantify the effect of biological-based products applied during propagation on infection and transformation of roots and incidence and severity of root mat disease.

4. To evaluate the effect of biological-based products applied after planting on infection and transformation of roots and incidence and severity of root mat disease;

5. To determine the efficacy of some biocides used at crop turnaround in reduction ofRhizobium populations and Ri plasmid;

6. To investigate interaction of root mat disease and PepMV;

7. To transfer knowledge to growers through articles, presentations, on-site visits and project reports.