Agriculture Reference
In-Depth Information
losses of 60-75%; losses as high as 100% were reported for salad onions, where
complete crops may be rejected because of mildew and frequent reductions were
caused in seed production onion crops through collapse of flowering stalks and poor
germination of harvested seeds (Gilles
et al.,
2004).
The pathogen causes pale leaf lesions on onions on which sporangiophores
bearing infective sporangia are produced which give a greyish-violet colour to the
lesions. Decaying onion debris may release oospores into the soil to act as a further
source of inoculum (Dixon, 1981; Viranyi, 1988).
The main infection source in seed production fields may be infected mother
bulbs.
Factors that are important in the epidemiology of the disease include
temperature, light, RH and air-flow.
(b) Epidemiology
Sporangia produced on leaf lesions are mostly released in the morning, as leaves dry
and humidity is reduced, and not during the hours of darkness when air is saturated
with moisture. Greatest discharge occurs when leaves are vibrated while exposed to
brief periods of red-infrared radiation under reducing humidity (RH less than 59%)
(Leach
et al.,
1982). Spores survive under a wide range of weather conditions during
the day but are frequently killed at night by alternating wetness and dryness,
especially when these cycles are associated with low dew deposition (Hildebrand
and Sutton, 1984a).
Production, sporulation and germination of released sporangia and penetration of
leaf tissues are favoured by temperatures of 10 to 13
o
C, high relative humidities
(RH) and prolonged periods of leaf wetness (Viranyi, 1988). Sporangia germinate
only in free water on dull days.
The infection cycle is characterised by long latent periods (about 9 to 16 days)
and short periods (about 1 to 2 days) when the pathogen sporulates (Hildebrand and
Sutton, 1982). Factors that contribute to its variability include temperature and
inoculum dose (Hildebrand and Sutton, 1984b).
The disease is controlled by the routine applications of fungicides and it was
suggested by de Weille (1975) that it might be possible to reduce their number
and/or the periodicity by developing a bioclimatological modelling approach to
disease forecasting as part of an integrated strategy for disease control.
The first model forecasting downy mildew was given the acronym DOWNCAST
and it utilised information for predicting sporulation and infection of onions by
Peronospora destructor
under field conditions in Canada (Jesperson and Sutton,
1987). A sporulation-infection period was predicted when conditions were conducive
to sporulation, spore dispersal, spore survival and infection. These criteria, derived
from earlier work (Hildebrand and Sutton, 1982; 1984a,b,c) established quantitative
and temporal relationships of temperature, rain, high humidity, rate of dew deposition
and its duration with the infection cycle of the pathogen (Jesperson and Sutton,
1987). The method correctly predicted sporulation incidence on 111 of 119 nights in
two growing seasons (Jesperson and Sutton, 1987). Since then, a number of new
