Читать книгу Plant Pathology and Plant Pathogens - John A. Lucas - Страница 29

It was first necessary to determine if characteristic elements occurred in diseased parts of the body, which do not belong to the characteristics of the body, and which have not arisen from body characteristics.

(Robert Koch, 1843–1910)

Heterotrophic microorganisms, unlike autotrophs, are entirely dependent upon an external supply of organic carbon compounds. The ultimate source of most carbon compounds is green plants, but there are a variety of routes by which microbes can obtain these nutrients.

A large number of microorganisms are decomposers. These organisms utilize substrates in dead tissues and their activities eventually lead to the disappearance of plant and animal remains. Such decomposers play a key role in the ecosystem by releasing nutrients which would otherwise remain locked up in plant litter. Some microbes have, in addition, 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.