Downy Mildew Control in Spring Beans
Introduction
Downy mildew first became a problem in spring-sown beans in 1988, when infected crops were mainly confined to the main production area in the eastern half of the country. Late-drilled crops appeared to be the worst affected. Troy was the main cultivar affected; others, including Victor, showed severe infection, whilst newer cultivars, such as Maya and Quattro, are also moderately susceptible. Maris Bead appears to remain relatively free of the disease. However, disease/yield interactions have not been quantified for bean downy mildew. Fungicides are applied prophylactically and, in the majority of cases, are not justified. This increases the costs of production and adds unnecessarily to the pesticide burden on the environment.
A three-year study on the control of downy mildew of spring beans (Vicia fabae) caused by the fungus Peronospora viciae was undertaken to determine the requirement for disease control in susceptible cultivars of spring beans. The optimum timing for the application of fungicide in relation to disease development and growth stage was established by investigating the relationship of disease development and severity with local meteorological conditions.
Results
Disease / Yield Loss Relationships
With the exception of the Lincolnshire trial 2002, treatment programmes that included a spray at early flowering plus 14 days resulted in a significant yield response where significant levels of disease occurred. Delaying the start of a spray programme beyond this timing, or stopping treatments prior to this growth stage, resulted in loss of disease control and a declining yield response.
Yield responses for the Lincolnshire 2002 trial suggest that a two-spray programme, with applications both before and after early flowering plus 14, would be require to achieve significant yield increases. In two trials (Lincolnshire 2000 and Yorkshire 2001) where no significant yield response was recorded, low levels of downy mildew infection were seen early in crop development. This initial infection failed to develop and this indicates the need to be able to predict the development of epidemics within a crop environment.
Forecasting
For practical monitoring of crops, it would be possible to incorporate a 'trigger event' if localised. and preferably hourly. weather information were available. Otherwise, a less sophisticated warning period - comprising a maximum temperature during extended periods of damp conditions - may serve equally well. So, to avoid spray applications when there is little risk of diseased spread, a high risk situation has been defined as follows: conditions below 10°C when crops have been wet for more than 12 hours, combined with an observation that disease is obvious in the crop seven days before the onset of flowering. Low risk would be higher temperature when the weather had been dry during the same period and disease was absent. Although disease may be present at higher levels in the crop when the crop is in flower, it is considered that no benefit will accrue from spraying at this time.
The financial support of PGRO is gratefully acknowledged.