Depending on the pathogen species and the host, plants may become stunted, wilt partially or completely, and ultimately die. A large range of symptoms is caused by vascular wilt pathogens, and the same pathogen may cause different symptoms on different host plants. Epinasty is the primary disease symptom, followed by flaccidity, chlorosis, vascular browning, and necrostress tolerance when compared with non-stress tolerance when compared with non-sis of the terminal leaflets ( Agrios, 2005). Most of the symptoms caused by vascular wilt pathogens develop in acropetal direction: from bottom to top. Vascular wilt diseases occur worldwide and affect annual crops as well as woody perennials, thus not only impacting food and feed production, but also natural ecosystems. Vascular wilt pathogens are among the most destructive plant pathogens that can wipe out entire crops. Xylem-invading pathogens comprise bacterial, fungal, and oomycete microorganisms that cause vascular wilt diseases.
Consequently, phloem pathogens comprise rickettsias, spiroplasmas, and phytoplasmas that are introduced by vectors such as phloem feeding insects, or by cultural practices like grafting. This may be explained by the accessibility of both types of vessel elements, as the phloem is characterized by living cells with a high osmotic pressure which makes penetration difficult, while the xylem is composed of dead tracheary elements with relatively low osmotic pressure. Paradoxically, although the phloem is rich in sugars, most vascular pathogens colonize the nutrient-poor xylem vessels.
Specific pathogens target the vascular system that is composed of xylem vessels, tracheary elements that transport water and minerals that are absorbed by the roots to the photosynthetic organs, and phloem elements, the living tissue that transports organic photosynthesis products. While many of these organisms evolved to infect aerial plant parts, such as leaves, stems, flowers, and fruits, others target below-ground organs, such as roots and tubers. Plants are continuously confronted with potential pests and pathogens that include insects, nematodes, viruses, bacteria, fungi, and oomycetes. In this review, we discuss the current knowledge on interactions of vascular wilt pathogens with their host plants, with emphasis on host defense responses against this group of pathogens. However, to design novel strategies to combat vascular wilt diseases, understanding the (molecular) biology of vascular pathogens and the molecular mechanisms underlying plant defense against these pathogens is crucial. Owing to the fact that vascular wilt pathogens live deep in the interior of their host plants, studies into the biology of vascular pathogens are complicated.
Cultural, chemical, and biological measures to control this group of plant pathogens are generally ineffective, and the most effective control strategy is the use of genetic resistance. As a consequence, leaves wilt and die, which may lead to impairment of the whole plant and eventually to death of the plant. These diseases are generally caused by soil-borne bacteria, fungi, and oomycetes that infect through the roots and enter the water-conducting xylem vessels where they proliferate and obstruct the transportation of water and minerals. Vascular wilts are among the most destructive plant diseases that occur in annual crops as well as in woody perennials.
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