Agriculture Reference
In-Depth Information
dispersed and remain capable of initiating disease after being dispersed for several
kilometers. Van der Zaag (1956) concluded that in some cases they are dispersed at
least 11 km. Because sporangia are so sensitive to solar radiation, distance aerial
transport is very highly unlikely during periods of intense solar radiation. Thus, long
distance transport might occur during evening or night or under cloudy conditions.
The basic features of aerial transport have been described (Van Der Zaag, 1956;
Hirst and Stedman, 1960). It is generally regarded that sporangia are produced
during a wet period of some hours (typically overnight), are dispersed during
subsequent drying (in the morning) and then cause infection when the leaves are
again wetted (by dew or rain). However, rains during the day such as is typical of
some locations in the highland tropics can stimulate production and release of a new
crop of sporangia for dispersal later in the day.
Dispersal is comprised of three distinct processes: escape from the canopy,
transport through atmosphere, and landing on plant tissues (Aylor, 1986). Escape
from the canopy depends on the number of lesions, wind speed, change in relative
humidity or solar radiation, and location of lesions in the canopy. Sporangia are
typically produced overnight and then are released during change in relative
humidity the next morning (Hirst, 1958). The higher in the canopy the lesions are
located and the higher the wind speed, the greater the number of sporangia that can
escape from the canopy (Aylor
et al
., 2001). Wind speed of 1 to 2 m s can remove
considerable numbers of sporangia and also transport those propagules to distances
up to 20 km in less than 3 hours (Aylor
et al
., 2001), a time period during which
sporangia can still be viable in cloudy days.
Transport of sporangia has been described using a variety of models. Empirical
models such as the power law and the exponential model have been used for very
short-range dispersal (Paysour and Fry, 1983). Long-range transport of
P. infestans
is passive and can be modelled either by empirical or by physical models, i.e.
K
-Theory-based models, Gaussian plume (Spijkerboer
et al
., 2002) or Lagrangian
stochastic (Aylor
et al
., 2001). A more extensive discussion on the advantages and
disadvantages of the different modelling approaches is available from several
sources (McCartney and Fitt, 1985; Fitt and McCartney, 1986; McCartney and Fitt,
1987). Despite the availability of various models, and the knowledge that sporangia
can be transported long distances (Harrison, 1947) application of such models to
disease management has not yet occurred. Interestingly, the seemingly similar topic
of disease spread over space (Minogue and Fry, 1983a; Ferrandino, 1996a,b) has not
yet been linked to models of dispersal over longer ranges.
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1
17.3.3 Inoculation to germination
Sporangia germinate either by releasing zoospores or by producing a germ tube. The
mode of germination is heavily influenced by temperature. For most genotypes,
temperatures below 15°C cause only the production of zoospores. It appears that
when the sporangia are formed they are programmed to produce zoospores because
proteins that function in zoospores are present in the mature, uncleaved sporangium
(Hardham and Hyde, 1997; Judelson and Blanco, 2005). For some individuals, the
