Agriculture Reference
In-Depth Information
The early stages of many diseases are inconspicuous and it is not feasible to make a
rapid visual assessment until the level of disease is sufficiently high, which may be so
late in the life-cycle that timing may become critical. In this case, alternative methods
may have to be adopted such as the detection of airborne spores by the use of traps or
the collection of samples for laboratory examination by other methods.
Symptoms that are observed are the accumulated result of numerous changes at
the cellular level generally as a result of damage to the host tissues. Sometimes the
most obvious indication that a plant is diseased is the appearance of the vegetative or
fruiting bodies of the pathogen. The latter are often termed the 'signs' of infection.
In some diseased plants, the only symptoms or signs are internal and so
assessment
requires dissection, thus destroying the host.
Another difficulty is that few symptoms are so completely distinctive that they
can be used by themselves to distinguish the presence of a particular pathogen
unequivocally on every occasion. In most cases, the identification of a pathogen
relies on the presence of a characteristic syndrome or specific combination of
symptoms, some of which might not appear very distinctive. This raises the
difficulty that more than one pathogen might be present. Since plants are covered in
epiphytic fungi and bacteria, it can sometimes be surprisingly difficult to be sure that
the microorganism that is most obviously present on or in plant material is actually
the pathogen responsible for those symptoms that are observed. In many cases a
vigorously growing saprophyte is far more noticeable than a more deep-seated
pathogen. For this reason, unless an organism is familiar, it may be necessary to
satisfy Koch's postulates by testing its pathogenicity. Even in a routine investigation
the identity of pathogen should be confirmed. In many cases, this may only require
the use of a hand lens but frequently some microscopy or even isolation in the
laboratory may be needed.
For the majority of fungi, an extra stage involving microscopy is not such a
significant problem as it would be for bacteria; for viruses such an examination
would be still more complicated. For this reason, in the latter cases, the recognition
of clear symptoms in field assessments is more important than for most fungi.
Fortunately, there is a profusion of well illustrated literature in the areas of
symptomology and fungal morphology to aid identification of the pathogens of
major crops but this help is less readily available for minor or exotic hosts.
Even though their use is so widespread and generally acceptable, the use of
symptoms in practice is beset with a few other problems. Their main drawback is
that both pathogens and a number of other causes - such as insects and other pests,
as well as abiotic agents - result in damage that originates due to very similar
cellular or physiological injuries. Partly in order to avoid such confusion, the types
of symptom that occur have been classified into several major groups and sub-
divisions. For example, a number of pathogens disrupt the growth of their hosts.
Some pathogens stunt the host, causing symptoms such as dwarfing, atrophy,
suppression of growth or differentiation. Others cause excessive elongation
(etiolation). If the meristematic activity of the host is affected, the result may be
abnormal cell enlargement causing deformations such as curling, or excessive cell
division causing proliferations such as galls, tumours and callus. The life of storage
organs can be shortened. The behaviour of other organs, such as petioles, may be
