Survival of Corynebacterium michiganense subsp. michiganense within host debris in soil

Bacterial can ker caused by Corynebacterium michiganense subsp. michiganense (Smith 1910) Jensen 1934 (3) is an important disease in both staked and ground tomato (Lycopersicon esculentum Miller) crops in Queensland. The tomato cultivars grown commercially have no resistance to this bacterium and control has been based on the planting of pathogenfree seed. Although the use of such seed has reduced disease Incidence, outbreaks of bacterial canker occur. Growers are concerned that the pathogen survives in the soil between consecutive crops thus negating the benefit of planting pathogen-free seed. In the United States. Echandi (5) reported that C. michiganense subsp. michiganense survived for only two weeks when the organism was introduced into the soil as free cells. He also found that it was able to overwinter In decomposing plant material. Survival in host tissue in soil exposed to winter freezing was reported aiso by other workers (6, 7, 9). Winter freezing comparable to that in the United States does not occur in Queensland so the pathogen buried in the soil in plant debris from the previous crop is affected by different environmental factors. The work reported here investigated the survival of C. michiganense subsp. michiganense in decomposing tomato tissue buried in the field and in soil held at two constant moisture regimes. Soil types used were representative of two important tomato growing districts in Queensland. They were a ciay ioam from the Redland Bay district in south-east Queensland and a granitic sandy loam from Applethorpe on the Granite Belt of Southern Queensland. Artificially inoculated tomato plants provided the infected plant material for this experiment. A pathogenic mutant (1079SR) of C. michiganense subsp. michiganense resistant to 1,000 Jl g mlstreptomycin sulphate was used to inoculate the tomato plants. Growth from a 48 h culture on glucose yeast extract calcium carbonate agar medium (4) containing 500 Jl g ml1 streptomycin sulphate (GYCASS) was suspended in sterile distilled water and the concentration adjusted to 10' viable cells ml• Plants of the cultivar Floradade raised In the glasshouse were inoculated at the fourth leaf stage by excising the cotyledon at the node with a scalpel dipped in inoculum. Inoculated plants were placed on the glasshouse bench and symptoms developed two weeks later. Plants were harvested at the 2nd inflorescence stage. The roots, leaves and apical 4 em of the stems of the diseased tomato plants were discarded and the remaining stem cut into 4 em lengths. Four stem pieces were placed in 6 x 6 em square nylon mesh bags which