3.4.3
USE OF A LEAKAGE ELECTROLYTE ASSAY FOR RAPID IDENTIFICATION OF FUNGAL DISEASE RESISTANCE IN PLANTS

G CRISTINZIO, A TESTA and C IANNINI

Department of Arboriculture, Botany and Plant Pathology, University of Naples Federico II, 80055 Portici, (NA), Italy

Background and objectives
Important plants diseases such as the downy mildew and grey mould of the grape, the late blight of potato and tomato etc., are even today mainly controlled with chemical fungicides. Besides, in many countries resistant strains have become prevalent, and the treatments are becoming more and more numerous. The use of pesticides, however, has to be reduced, since they produce huge ecological hazards. All the alternative methods or those to support the chemicals are welcome. Genetic control, utilising either classical resistance strategies or new molecular approaches, acquire even more importance than in the past. Usually the screening of germoplasm for disease resistance is carried out under field conditions by artificial inoculation, but this method is time-consuming and frequently unrepeatable. In plant breeding the efficiency of the selection depends on the speed and accuracy of the screening method. The purpose of this work is the utilisation of a very simple and rapid method, LEA (leakage electrolyte assay) as a first step in a stepwise selection for resistance in some important plant-pathogen systems.

Materials and methods
LEA is based on the evaluation of electrolyte leakage from vegetables caused by culture filtrate (CF) of the pathogen. The selected plant tissues are put in 20 ml of 15% CF at 25C with magnetic stirring. Distilled water served as control. Different tissues were employed; callus on the Solanum spp.- Verticillium dahliae system; stem cuttings on Lycopersicon spp.-Phytophthora nicotianae; leaf-disks on vine-Botrytis cinerea; leaf- and tuber-disks on potato-P. infestans. The conductance of the solution is measured at 1-h intervals using a conductivity meter with 20 electrodes (range 20 mS-200 mS/cm; K=1.0) connected to a computer with a specific program. Each value is, at least, the mean of five replicate samples and is calculated as the difference to the initial reading at the beginning of the assay, corrected by subtracting those of the control.

Results and conclusions
In previous works with LEA, the following results have been obtained: (i) high levels of resistance to verticillium wilt in one accession of the wild species Solanum torvum and in two genotypes (one variety and one hybrid F1) of S. melongena [1]; (ii) several accessions, of three wild and two cultivated species, of the genus Lycopersicon with different behaviours toward P. nicotianae [2]. More recently, statistically appreciable differences has been emphasised among ten potato varieties to P. infestans both on the leaves and the tubers. In the system vine-B. cinerea the rootstock's influence on the sensitivity to the grey mould of a vine-cultivar has been evidenced for the first time.

Although some results must be considered preliminary, the answers obtained as a whole show that the method is applicable to plant-pathogen systems. LEA may be very useful in early selection of a wild species population, for the qualitative (specific) resistance that can be utilised for the control of pathogens stables (with slow multiplication); or in early mapping of the quantitative (general) resistance into commercial cultivars and/or hybrid accessions, against unstable pathogens, as the P. infestans that shows a great variability worldwide. LEA may be useful even in research on new methods of fungal plant-disease control, as the utilisation of rootstocks for the grapevine. It is worth noting that, in this plant, the primary way to increase resistance to biotic and abiotic disease is the selection of resistant (or tolerant) rootstocks.

References
1. Cristinzio G et al., 1994. Plant Pathology 44, 704-709.
2. Cristinzio G et al., 1994. Journal of Genetics & Breeding 48, 201-202.