Hydraulic and leaf reflectance alterations induced by Clavibacter michiganensis subsp. michiganensis on tomato plants

Abstract

Clavibacter michiganensis subsp. michiganensis causes bacterial wilt and canker in tomato. A detailed account of pathogen effects on growth and water parameters of affected plants is lacking. Besides, the long latent period of the disease represents a limitation for its management. Our objectives were to determine the physiological and morphological changes induced by this pathogen in tomato, in relation to water stress, and to evaluate two well-known leaf spectral reflectance indices for the diagnosis of this vascular disease before symptoms expression. Experiments were conducted in a greenhouse with pot-grown plants inoculated with a bacterial suspension (107 CFU/ml); controls were treated with sterile distilled water. Five weeks later plants were harvested and several growth parameters, the number and size of xylem vessels and the hydraulic conductivity of the stems were determined. Inoculated plants were shorter, with smaller leaf area and lower biomass than those of mock-inoculated plants. While the number and hydraulic radius of xylem vessels were not affected, the stem-specific (Ks) and the leaf-specific (KL) conductivity were significantly reduced by inoculation. Wilting of leaves of diseased plants was associated to the reduction of KL. Inoculated plants showed leaf spectral reflectance changes 1 week after inoculation, and 2 weeks before symptom expression, as shown by the index NDWI, an indicator of the water content of the leaves. We conclude that C. michiganensis subsp. michiganensis reduced the growth and altered plant-water relations of tomato plants by reducing the stem hydraulic conductivity. The NDWI index could be used for the early, non- destructive, diagnosis of bacterial canker.