The visible symptoms of disease that develop on the sward are generally our initial indication that the turf has become infected by a pathogen. However, prior to the development of these symptoms, the pathogen has had to go through a series of different processes both on the surface of the plant and inside the plant cells, in order for the plant to show these detrimental symptoms. This article is intended to provide a general look at the processes that take place before disease becomes apparent and to look more closely at the ways in which the pathogens have been able to overcome the plants natural defences.
Melanised appressorium of C. cereale showing the pore (hole) for the penetration peg
Breaching plant DEFENCES
W Fungi
The fungi that cause disease are a diverse group of microscopic organisms that, for the most part, complete their life cycle by producing spores (by either sexual or asexual reproduction) that germinate to produce a mycelium which, in turn, produces further spores or specific resting structures that remain on the plant or in the turf. The conditions that encourage fungal activity and spore production will vary depending on the individual species and the details are well documented in text books such as the Compendium of Turfgrass Diseases, 3rd Ed (APS Press).
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hen we talk about the pathogen, we are referring to the organism that is responsible for causing disease. In amenity turfgrasses, the most common pathogens are fungi but, increasingly, we are becoming aware of the role of plant parasitic nematodes (henceforth referred to as nematodes) in the development of disease symptoms. All pathogens are able to overcome the plants defences, enter the plant cells and adversely affect the normal functioning of the plant through induced changes in the plant’s biochemistry. These biochemical changes are the cause of symptom development in the plant.
Nematodes
The nematodes that are involved in the parasitism of turfgrasses are also a large and diverse group of organisms that will not be visible without magnification. In addition to their small size, they are virtually colourless, enabling them to become invisible when living in the rootzone or on plant tissues. There are two main types of nematodes that can potentially cause damage to turf, and these are the endoparasitic (endo- = inside) and the ectoparasitic (ecto- = outside) nematodes. Whether they tend to live and feed inside or outside of the plant, they may also be migratory in their habit, often moving to new feeding sites. Some nematodes are what we call sedentary, where the adult remains in one feeding location for the duration of its life. However diverse the group is as a whole, nematodes of all species tend to follow the same life cycle pattern. They emerge from the egg as juveniles, often having completed their first moult inside the egg and go through a series of three further moults until they become reproductive adults.
Disease can only develop once the pathogen has entered the plant and, in the case of the fungi, it is either the spore or the mycelium that acts as the
infective propagule, allowing the fungus to begin its infection of the plant. In the case of the nematodes, it is generally the emerging juveniles or, in some cases all of the developing stages of the nematode, that initiate the infection but, in both cases, the fungi and the nematodes have developed several ways in which they can enter the plant cells. Before infection can occur, the turf must become inoculated with the pathogen. The process of inoculation simply means the coming together of the pathogen and the turfgrass host. Fungal spores can be deposited on the leaf tissue, for example, by being wind-blown from previously infected areas, being washed from the air by rainfall or irrigation, transported in clippings or on machinery, or washed across an area in the mass movement of water. Mycelium can grow through the rootzone or across the sward towards to the target plant and can develop from the saprophytic survival of certain so-called facultative fungi (able to live as both saprophytes and parasites), from current infections or from the germination of specific resting structures in the turf. Nematodes are likely to be naturally
present in most rootzones, coming in from adjacent areas, migrating up in to the rootzone from the lower soil profile
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