ONION DISEASES - The Epidemiology of Plant Diseases - page 508

further development of that disease. For example, in moist, warm soils it is mainly
temperature that determines infection of roots by and the ecological limits of
Pyrenochaeta terrestris and Fusarium oxysporum f.sp. cepae . In moist, cool soils,
moderate temperatures are critical for root infection by Sclerotium cepivorum and
Urocystis cepulae . Possibly the interaction of a greater number of epidemiological
factors is required for leaf infection of onions (ware and seed production plants)
by foliar pathogens. The temperature ranges for conidiation are clearly different for
pathogens of temperate and those of hot climates but in either situation the interaction
of temperature with long periods of leaf wetness and high relative humidities are
similar requirements and are overriding factors controlling spore production and leaf
infection, with decreasing relative humidities activating spore release of these
pathogens. These factors also apply in the infection of seed production onion plants
and some may apply to the transmission and spread of the diseases initiated by the
sowing of infected onion seeds.
The critical evaluation of epidemiological factors, and particularly of those
involved in foliar disease outbreaks, has resulted in the production of accurate
disease forecasting models, some of which have been used successfully in practice
to reduce fungicide applications in commercial crops of onions. However, many
such models have failed to consider availability of and/or concentration of inoculum
in assessment of disease risk (Phillon, 2003). Such information if applied (Phillon,
2003; Bugiani et al., 1995; Carisse, et al., 2003; Gilles et al., 2004) would likely
improve disease management (Carisse, et al., 2003), and if linked to newer, rapid
methods of inoculum capture and pathogen identification (Kennedy et al., 2000;
Wakeham et al., 2004) could produce faster, more accurate assessment of disease
risk and thereby may further reduce pesticide applications in field crops.
New molecular-based research has established genetic methods for fungal
identification (Nielsen et al., 2001; Nielsen and Yohalem, 2001; Yohalem et al.,
2003). Practical research is now required to investigate the respective pathogenicities
of B. aclada ( sensu Yohalem, et al., 2003) and of B. allii ( sensu Yohalem, et al.,
2003) and to establish their roles in the epidemiology of onion neck rot disease.
ACKNOWLEDGEMENTS
The author wishes to record his thanks to Miss Lindsey Peach, who redrew the
graphs from published information, and to Miss Jane Fellows who provided some of
the meteorological data. I am grateful to my wife for proof reading the manuscript.
The author is most grateful to Warwick HRI for access to library facilities at
Wellesbourne.
REFERENCES
Abawi, G.S. and Lorbeer, J.W. (1971a). Populations of Fusarium oxysporum f.sp. cepae in organic soils
in New York. Phytopathology, 61 , 1042-1048.
Abawi, G.S. and Lorbeer, J.W. (1971b). Pathological histology of four onion cultivars infected by
Fusarium oxysporum f.sp. cepae . Phytopathology, 61 , 1164-1169.
Abawi, G.S. and Lorbeer, J.W. (1972). Several aspects of the ecology and pathology of Fusarium
oxysporum f.sp. cepae . Phytopathology, 62 , 870-876.