University of Strathclyde, Glasgow, UK

Background and objectives
Plasmodiophora brassicae is a plasmodial fungus which parasitizes the root hairs and cortical cells of Brassicas. Infection leads to massive morphological, biochemical and molecular changes which result in important economic losses. One traditional treatment for P. brassicae is the application of lime (calcium carbonate) which was thought to operate through changes to soil pH. However, research has shown that calcium itself has a role in controlling the growth and reproduction of P. brassicae. Field experiments have shown that the use of calcium nitrate leads to a reduction in P. brassicae infection. Calcium itself has a wide range of effects on plants, from strengthening cell walls to involvement in signalling [1]. Research is therefore attempting to establish how calcium affects this fungus and its subsequent pathogenesis in Brassicas.

Materials and methods
Split root experiments have been used to establish whether calcium effects are translocated. Seedlings of Chinese cabbage cv. Mariko were germinated in vermiculite and watered with a low calcium nutrient solution. After 5 days the root tips were excised. This resulted, after 2 weeks, in a bifurcation of the roots. The bifurcated root systems of each seedling were divided between two 15 ml bijoux bottles each containing differently constituted nutrient solutions, one with a high calcium content and the other with a low calcium content.

Plants were established in this system for 7 days. Each plant was then inoculated with 1 ml of a 106 resting spores/ml suspension. Microscope assessments of root hair infection were made daily.

Results and conclusions
The preliminary results suggest that the effect of calcium on P. brassicae was not translocated from one part of the root system to another. Infection was significantly decreased on the root portion treated with calcium. The degree of disease reduction was also dependent upon the form of calcium in the nutrient solution.

It has been established that applying calcium to the root leads to a reduction in pathogen growth and reproduction [2]. The manner by which this is achieved requires to be determined. Use of fluorescence microscopy will allow assessment of calcium affects on spore viability and atomic absorption spectroscopy will allow assessment of host calcium content. Use of further techniques will enable observation of P. brassicae development at a range of calcium concentrations, aiding identification of the infection stages at which calcium is most effective. To advance our knowledge of the effects of calcium the following questions must be addressed.
(i) Are the effects of calcium due to an increase in pH immediately in the root zone, leading to variation in the uptake of selected minerals?
(ii) Is calcium an inhibitor of the disease processes?
(iii) As the effects of calcium are not translocated, is the effect limited to the rhizosphere?
(iv) Does the presence of calcium affect the resting spores or primary stages of infection?
(v) What effect does calcium have upon the secondary stages of infection?
(vi) Is calcium strengthening plant cell walls against invasion by the fungus?
(vii) Is calcium, or other minerals whose uptake is increased by the presence of calcium, inducing a hypersensitive response?
(viii) Is the presence of calcium encouraging the development of antagonistic organisms?

1. Roux SJ, Slocum RD, 1982. Calcium and Cell Function, III, pp. 409-453.
2. Dixon GR, Page LP, 1998. Acta Horticulturae (in press).