“Dull, improperly adjusted equipment not only destroys the aesthetic value of the turf by discoloration and bruising of the leaf tips but also provides more ready access for disease organisms and insects through the damaged leaves” J Harper, PennState University
or they could potentially be introduced during
construction or maintenance. However they arrive on the plant surface, inoculation does not mean that disease is inevitable. Pathogens must enter the plant to have a chance of initiating disease and there are three main ways in which pathogens can enter plant cells.
Through natural openings
Stomata, the openings on the leaf surface that facilitate gaseous and water exchange, are an ideal entry point for certain plant pathogens. The stomata open to allow gaseous exchange and to enable water to move out from the leaf to aid cooling and to maintain upward movement of nutrients from the root. Once open, these stomatal pores can be used by certain fungi to allow them to grow down inside the leaf and initiate growth between and through the plant cells. Some nematodes will also enter through these open pores but, compared to the number and diversity of nematodes that penetrate the plant through the root, these so-called foliar nematodes are relatively less important - or at least, are considered so at present.
Through wounds
Entry through wounds on the leaf or root tissues is also a well used entry port for various turfgrass pathogens. Once the leaf tissue is wounded or cut during mowing, the cell content which is high in nutrients, leaks out on to the surface of the sward. Fungal spores may be stimulated in to germination in the presence of this nutrient-rich medium and mycelium may be chemically attracted to the wounded areas. The quicker the plant can heal itself, the less time the cells are vulnerable to infection, and this is why a clean cut is
crucial, especially on turf where disease is already active or when environmental conditions are optimal for fungal growth. Confirmation of this is summarised in a publication by J Harper for PennState University (2008) which reads ‘It is essential that all types of mowing equipment be kept sharp and in good operating condition. Dull, improperly adjusted equipment not only destroys the aesthetic value of the turf by discoloration and bruising of the leaf tips but also provides more ready access for disease organisms and insects through the damaged leaves’.
Direct penetration
Although certain pathogens can take advantage of natural openings or wounds on the plant tissues to gain access to the plant cell and begin their colonisation of the tissues, many pathogens have developed methods of direct penetration of intact plant cells. This method of entry means that the pathogen can enter the plant when the conditions are right for the growth of the pathogen and not only when an opportunity is offered by the plant. Certain fungi and nematodes can enter through direct penetration and their methods of achieving this are often quite elaborate and well adapted.
In the case of Microdochium nivale, the spores of the pathogen germinate on the leaf surface by producing one to several germ (germination) tubes within twelve hours from inoculation. The germ tubes then develop in to mycelial networks on the surface of the plant which eventually penetrate the host tissue through growth of specific infection hyphae (Kang et al, 2004). Other pathogens produce specific infection structures, appressoria, which develop from the germ tube and
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