Agriculture Reference
In-Depth Information
host canopy show that increased levels of nitrogen application might reduce disease
development because higher canopy density could lead to the obstruction of spore-
carrying droplets (Lovell
et al.,
1997). Therefore, the rainfall-canopy structure
interaction should be considered in detail to improve disease forecasting. Along
these
lines, the use of overhead irrigation influences both growth of field beans and
development of
Ascochyta fabae
; canopy height and LAI on an unprotected irrigated
plot were approximately twice the values of those on the reference plot with no
irrigation (Huber, 1992). Disease incidence on the irrigated plot was at least twice
the value of the non-irrigated plot. In this case, disease development depends on
dispersal and infection, the latter being influenced by water persistence. In contrast
to
Septoria
spp. in winter wheat, the increase in LAI of the field bean crop favours
Ascochyta
disease development. One can probably assume that dispersal from the
bottom leaves decreases as density increases but that frequent overhead irrigation (or
rain) leads to substantial and higher infection efficiency, due to prolonged and
recurrent wetness. Thus, depending on the disease/environment combination,
reduced inoculum movement by rain-splash could limit the epidemic development;
but in other situations reduced dispersal is compensated by increased infection or
sporulation. Because of the multi-faceted
effects of rain (or irrigation) on disease,
simple systems such as those developed to forecast or predict septoria tritici blotch
on winter wheat would likely be sufficient to assess the risk of disease (Hansen
et al.,
1994). Otherwise splash dispersal is only one process among other epidemio-
logical processes and hypotheses must be made on how to incorporate it in the full
epidemic cycle.
These examples show that splash dispersal events have not been used as field
indicators of disease progress and outbreaks to the same extent as infection periods
based on wetness and temperature. It is likely that simple forecasting methods
applied to pathosystems for splash-dispersed pathogen diseases should be further
investigated using rain variables (amount of rainfall, number of days with rainfall
greater than a minimal threshold, etc.); field experiments should include some
characterisation of rainfall and/or rain-splash. Examples where rain variables have
been successfully related to disease development include leather rot and anthracnose
of strawberry (Reynolds
et al.
, 1988; Madden
et al.
, 1993). On the other hand, pea
anthracnose (caused by
Ascochyta pinodes
) is a disease whose epidemiological
components are mostly known (Roger and Tivoli, 1996) to start modelling the host
disease pathosystem; however, the dispersal of conidia by rain-splash is still
a poorly characterised component whose quantitative influences on inoculum
movement and disease progress require further research.
16.7 CONCLUDING REMARKS
The presence, persistence, and movement of liquid surface water greatly affect the
epidemiology of plant diseases. Surface wetness is crucial to the infection process of
many fungal pathogens and can be modelled using energy balance concepts (Huber
and Gillespie, 1992). Rain-splash is second in importance to wind as an agent for
inoculum dispersal. However, this essentially small-scale physical process controls
