Root mat of tomato caused by rhizogenic strains of Agrobacterium biovar1 in the UK

Since 1993 many hydroponic cucumber (Cucumis sativus) crops in the UK have been affected by a root disorder known as root mat, a condition first described in soil and straw bed crops in the 1970s. The predominant symptom is extensive root proliferation within the rockwool propagation cube and across the rockwool slab surface. In severe cases a loss in marketable yield has been reported. Recently the causal agent of this disease was shown to be wild-type Agrobacterium biovar 1 strains harbouring an Ri-plasmid (Weller et al., 2000). Since 1997, sporadic cases of a similar disorder have also been observed in commercial tomato (Lycopersicon esculentum) crops.

Tomato plants affected by root mat were collected from crops across the UK. All crops were grown hydroponically in rockwool and the incidence of affected plants ranged from <1% to 50%. Rhizogenic strains of Agrobacterium biovar 1 were isolated from affected but not from healthy roots, of symptomatic crops. Suspensions of three isolates of the bacterium (NCPPB¹ 4060, 4061, 4062) were inoculated, as in previous experiments on cucumber (Weller et al., 2000), onto six rockwool slabs each containing four young plants of one of two tomato varieties (cv. Pronto - a round variety; cv. Favorita - a cherry variety). Symptoms were initially seen five weeks after inoculation and at three months sixteen plants from five of the six slabs were symptomatic.

Symptoms were observed as increased root production across the propagation cube surface and swelling of the cube and/or root proliferation across the slab surface. Although root growth was excessive compared with normal tomato plants, it was less than that seen in root mat affected cucumbers. Rhizogenic Agrobacterium bv. 1 strains were re-isolated from four of the five slabs with symptoms and from the one slab without symptoms. No symptoms developed and no rhizogenic Agrobacterium strains were isolated from uninoculated control plants.

Investigations on a heavily infected cherry tomato crop (cv. Favorita) at a commercial nursery indicated that fruit diameter was reduced by 2-3 mm in infected compared to healthy crops, leading to a higher proportion of unmarketable fruit (<28 mm). Fruit numbers did not appear to be affected, though overall fruit yield was reduced by 15%.

This is the first UK record of rhizogenic Agrobacterium causing root mat symptoms in tomato. Although tomato is susceptible to crown gall producing Agrobacterium strains, crown gall is a rare disease of glasshouse tomatoes. Non-pathogenic strains of Agrobacterium (without the plasmid) are commonly found on hydroponic tomato crops.

This work was funded by the Horticultural Development Council (HDC) Project PC 149.