Agriculture Reference
In-Depth Information
affected by epinasty caused by pathogens and leaves may be lost by abscission and
chlorosis. A more subtle disruption to growth results when there is simply a loss of
yield without any other associated symptoms.
Other pathogens, such as seedling blights, disrupt the mobilization of stored food,
whereas root and foot rots disrupt the absorption of water and minerals. Vascular wilts
disrupt water transport. Phyllody and decline follow the disruption of translocation.
Many pathogens cause disruption to many functions and it is these that are most
commonly mistaken for the consequences of abiotic agents. Among the most common
symptoms are yellowing, chlorosis caused by the break down of chlorophyll, water
soaking and exudation as well as necrotic reactions ranging from minute spotting to
completely blighted plants. More specialized pathogens disrupt the secondary
metabolism of the plants and it is these that result in pigmentation change - sometimes
with colourful results, as for example the viruses that cause colour breaking.
Since reliance on symptomology alone is such an imperfect means of identifying
any other than the most familiar pathogens, in many cases it is necessary to confirm
the identity of the causal agent by the use of microscope techniques, isolation or
both. If the putative pathogen that is isolated is unfamiliar then its pathogenicity
should be checked by satisfying Koch's postulates by reinoculating the isolate into
the host plant. This process is both time-consuming and fraught with problems. The
initial isolation of microorganisms from plants is beset with difficulties, due to
contamination during and after the putative agent has been targeted. Numerous
pathogens are biotrophs which cannot be grown
in vitro
;
many others are so
fastidious that a suitable medium is too troublesome to find and in practice isolation
is not feasible; and others grow so slowly or weakly that they are lost too readily for
reliable identification. Even before isolation, it is necessary to prepare the host
material appropriately to induce sporulation. Where sporulation is recalcitrant,
progress may be stifled at the onset.
Where conventional isolation methods are used, a number of specialized
techniques have had to be perfected in order to cope with microorganisms in specific
substrates such as soil, roots, aerial parts and seeds. Successful isolation from most
of these substrates involves considerable expertise in the special problems involved
and the normal structure and functioning of the plant host. Where such knowledge is
absent, it is all too easy for a novice to make serious mistakes. Selective media offer
one potential source of aid for the inexperienced, but in practice, suitable products
are rare. Nonetheless, where they are available, selective media and selection
techniques can greatly increase the chances of successful isolation, subsequent
purification and conservation of fungi and bacteria. Routines for the extraction and
purification of viruses require equally professional proficiency and skill. Once
isolation has been successful, other sorts of expertise are needed to confirm the
identity of the pathogenic agents.
Among the methods available once a pathogen has been isolated are a number
of biochemical and physiological ones which traditionally have been used mainly with
bacteria but may be used more widely in future. However, in most cases microscope
techniques are preferred as the most conventional method to distinguish fungi
using morphological differences. To be effective, such methods need to combine
an extensive knowledge of fungal taxonomy with a practical understanding of
