CROP MONITOR

Downy Mildew (Peronospora viciae) in Spring Beans

Summary of Web based forecasting scheme 2005

Reports on the development of downy mildew in spring bean crops were received from seven sites in 2005. Weather records from stations as close as possible to each site were updated at the same time to provide a comparison between 2005 and the 30 year average for rainfall and air temperature within a 10 Km radius circle of the crop location. The comparison with long term weather factors was used to indicate when the risk from mildew was higher or lower than that usually experienced by the location. As accumulated rainfall and the rolling seven day average temperature fell either below or above the long term average at the same date.

At the monitoring sites, mildew developed to a significant extent at Crowland (Quattro), Newborough (Compass) and Wicken (Nile). A lower incidence of disease was observed at the Marden (Syncro) and Thorley (Quattro) sites. Too few reports were received from the Shrewsbury and Berkshire crops for disease risk to be judged accurately.

The weather at the Crowland and Newborough sites was drier than the long term average throughout the observation period but it was also cooler than expected as the beans approached flowering. Two significant rainfall events occurred in late May and early June just as flowering started which raised the likelihood of infection according to the forecast scheme. In these cases the long term expected rainfall total compared to the current total would have been a poorer guide to mildew infection risk than tracking sharp increases in rainfall. These locations were close together and shared the same weather station (Wittering) so the slightly lower incidence of mildew at Newborough is likely to have been due to the planting of Compass, a more resistant cultivar.

At the sites where mildew failed to develop to a severe extent (less than 10% plants infected) the rainfall pattern did not show such large events. A short wet period at Thorley coincided with a warmer than average spell, otherwise these locations were characterised by long dry periods from mid May to early June. The provisional forecasting scheme marked these locations as low risk based on the dry conditions, particularly at Marden.

The crop at Wicken suffered the most severe mildew attack with almost 60% of plants infected. The accumulated rainfall was closer to the long term average for the locality than any of the other sites and was also much cooler than normal for three weeks in May. The disease was well established before drier spells occurred in late May and early June.

The weather records and disease assessments provided valuable data to help validate the warning scheme developed during earlier work. Although it was accurate with its general advice at the observation sites in 2005 it will remain difficult to say how much cooler and/or wetter observed conditions should be before infection becomes much more likely at a field scale. The new results from this first year of validation will help to determine if particular rain events just before flowering are consistently significant for mildew infection.

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