Effects of calcium on predisposition of leaves and citrus petals to phytopathogens: anatomy/ultrastructure of infected tissue

Abstract

Citrus canker (Xanthomonas citri pv. citri) and postbloom floral drop (Colletotrichum acutatum) are two of the major citrus diseases and, if not controlled or pressured by environmental conditions unfavorable to the stages of infection and/or colonization of pathogens, can cause significant production losses. In addition, intensive use of chemical pesticides for disease control poses potential risks to the environment and society. Mineral nutrients may directly or indirectly affect the health of the plant, whose interaction is due to the alteration of plant tissue susceptibility to infection by pathogens. Therefore, the efficient use of fertilizers is highlighted by the role of plant nutritional status in disease resistance/tolerance. Calcium (Ca) is a structural and functional component of cell walls and medium lamellae and, consequently, the nutrient concentrations affect the anatomy of tissues. Anatomical characteristics of plant tissue can favor the resistance/tolerance of plants to diseases. In a long-term field experiment, orange trees fertirrigated with calcium nitrate (CaN) accumulated higher concentrations of Ca in the tissues and developed leaves and petals with cell walls and lamellae thicker than fertirrigated orange trees with ammonium nitrate (AN). In this way, the present research project proposes to test the hypothesis that leaves and petals of citrus with different higher levels of Ca present a lower predisposition to citrus diseases. Thus, we aim to evaluate the effects of the supply of Ca to citrus plants grown with three levels of the fertilization and inoculated by Xanthomonas citri pv. citri (leaves) and Colletotrichum acutatum (flower petals) and to describe anatomical/ultrastructural alterations of the tissues infected by both diseases. (AU)