Effects of salinity on plant diseases development

Abstract

In arid and semi arid areas, irrigation water must be brought to the land to increase crop production. In these areas, poor quality irrigation water and inadequate soil drainage often result in the accumulation of salts. Many physiological processes are altered in plants grown in saline soils which ultimately reduce their growth and yield.While salinity stress can cause severe disruptions in the plant physiology and morphology, very little is known about how such disturbances might affect the plant susceptibility to pathogens. The threat of increasing salinity in many irrigated areas of the world as well as field observations motivated some researches on the possible interactions between salinity stress and disease development. The number of these studies is still limited. They were particularly conducted in Morocco on tomatoFusariumandVerticilliumwilt and in the United States on somePhytophthora.Some investigators worked also on the relationships between salinity and the Vesicular-Arbuscular (VA) mycorrhizal fungi.Irrigation water and soil salinity increase the population ofFusarium oxysporumf.sp.lucopersiciin the soil, the sporulation of the fungus in the plant vessels, the formation of chlamydospores and finally they also allow the development of the fungus even in very hot conditions. When the salinity of irrigation water is high, even the tomatoFusariumresistant varieties are attacked and show severe symptoms. High salinity can therefore breakdown the resistance of theFusariumresistant cultivars. Verticillium dahliaeis a serious problem particularly on tomatoes grown under plastic tunnels. The salinity of the soil and the irrigation water has a great effect both on the virulence of the pathogen and on the susceptibility of the plant. When the salinity of the medium increases, the mycelial growth of the pathogen, the number of conidia and of microsclerotia of the fungus increase. Tomato seedlings irrigated with saline water are more susceptible to the pathogen than the tomato seedlings irrigated with non saline water. The race 1 resistant tomato hybrids are not infected by the pathogen and do not show any symptoms when the plants are grown in non saline soil and irrigated with non saline water. However, when salt is added to the substrate and to the irrigation water, the race 1 resistant tomato cultivars become susceptible to this race.In most major citrus growing areasPhytophthoragummosis caused by Phytophthora parasiticaand P. citrophthorais a serious problem. When the soil and water salinities are high, the disease was observed to be unusually severe. The Phytophthoraisolates have a greater tolerance of soil salinity than the crop plants they attack. They produce more sporangia in saline soils than in non saline ones. The salinity stress inhibits also root initiation and growth and increases tissue susceptibility.Chrysanthemum plants grown in saline conditions are pre-disposed to severe root infection byPhytophthora cryptogea.Following the salinity stress, a greater number of zoospores attached to roots immediately. In addition to the increase in the numbers of initial infection sites on roots, stress also alters the normal active responses of the plants.In some saline soils of California, it was shown that Pythium root rots may be suppressed by the soil salinity.Vesicular-arbuscular (VA) mycorrhizal fungi are beneficial microorganisms which are commonly associated with the roots of the majority of crops growing throughout the world. VA mycorrhizal colonization enhances salt tolerance of selected varieties.