Plant Pathology and Plant Pathogens. John A. Lucas

Plant Pathology and Plant Pathogens - John A. Lucas


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an ability to parasitize living plants; if, during invasion of the plant, they kill host cells this ensures a supply of dead tissues on which they can continue to grow. Other microorganisms are only able to obtain nutrients from living host cells, and establish more balanced relationships which may be of mutual benefit. The effects of microbes on plants therefore vary from severe damage and even death, to diversion of nutrients, to associations in which both partners gain some advantage. Hence, heterotrophic microorganisms are involved in a variety of ways in the movement of fixed carbon between different trophic levels in the ecosystem.

      A comprehensive analysis of plant disease caused by microorganisms requires several different types of information. First, the causal agents must be identified. However, the usual criteria employed for distinguishing between microbial species are of limited value when dealing with microorganisms isolated from plants. Different isolates of the same species may vary widely in their ability to cause disease. It is important to understand the genetic basis of such variation, and the corresponding variations in the plant's response. Second, the nature of the host–parasite relationship needs to be considered; the biology of infection, sources of nutrients, the basis of damage to the host, and the effects of the environment. The diversity of relationships is enormous, but identifying some common features is helpful in providing basic guidelines for the control of contrasting types of pathogens.

      Considerable confusion surrounds the terms pathogen and parasite. While they are generally used to describe microbial disease agents, in particular the fungi, bacteria, and viruses, the distinction between the two terms has often been overlooked. They are not synonymous; a parasite is an organism having a particular type of nutritional relationship with a host, while the term pathogen refers to the ability of an organism to cause disease. They may be defined as follows.

       Parasite: an organism or virus living in intimate association with another living organism (host) from which it derives some or all of its nutrients, while conferring no benefit in return.

       Pathogen: an organism or virus able to cause disease in a particular host.

      The allied term pathogenesis describes the complete process of disease development in the host, from initial infection to production of symptoms.

      At first sight, the distinction between a parasite and a pathogen might appear subtle; indeed, in many cases the parasitic activities of an organism automatically lead to it being a pathogen as well. The diversion of nutrients from the host will cause some metabolic stress which will normally be expressed as disease. However, in other host–microorganism associations this stress may be offset by the microbe contributing nutrients in return. This is the case with root nodules of legumes, where the bacterium Rhizobium obtains carbohydrates from the host but also fixes atmospheric nitrogen, some of which the host subsequently utilizes. Mycorrhizal fungi infect plant roots but actually stimulate growth by assisting the uptake of scarce nutrients, especially phosphates, from the soil. The definition of a parasite given above takes account of situations such as these.

      The advantage of this scheme is that it can accommodate relationships where the balance may shift from mutual benefit, termed mutualism, to injurious effects on one partner.

Diagram displaying a bar at the top labeled Symbiosis with arrows connecting to 2 boxes to the bottom labeled Antagonistic and Parasitism + - and Mutualistic and Mutualism + +.

      Biotrophs and Necrotrophs

      Although there is an enormous variety of pathogens, an important distinction can be made between those which rapidly kill all or part of their host and others which co‐exist with host tissues for an extended period without inflicting severe damage. The former category, referred to as necrotrophs, are often opportunist pathogens which invade wounds and juvenile or debilitated plant tissues. They grow intercellularly, producing cytolytic factors and then utilize the dead host tissues as a resource. The ability to attack a living host distinguishes these organisms from the saprotrophs which subsist exclusively on organic debris. In contrast, biotrophs do not kill their host immediately. They are, in fact, dependent upon viable host tissue to complete their development. Extreme biotrophy resembles mutualism in that it is difficult to discern any marked pathogenic effects.

Necrotrophs Biotrophs
Morphological and biochemical features
Host penetration via wounds or natural openings Host penetration direct or via natural openings
Few special parasitic structures formed Special parasitic structures, e.g., haustoria, typically formed
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