Vegetable brassicas disease control update (Sept 2011)
Disease pressure on vegetable brassica crops in Lincolnshire has been low so far this season. It seems that last winter’s freeze did a good job of breaking the green bridge for white blister and, although rainfall through July and August has been average, periods of leaf wetness have been too short for the spotting diseases to get going. 
With the arrival of autumn, longer cooler nights and morning dews in conjunction with more changeable weather could soon start to provide the periods of leaf wetness that will favour development of the spotting diseases – ring spot, Alternaria, Phoma and light leaf spot (LLS).
In addition to ring spot, which is often the number one disease threat, LLS certainly warrants extra vigilance this autumn. Oilseed rape (OSR) is a major source of LLS inoculum spreading to vegetable brassicas and the spring 2011 CropMonitor survey recorded LLS infection in 84% of English crops, confirming that it should no longer be perceived as a ‘northern disease’.
Incorporating Nativo 75WG (tebuconazole + trifloxystrobin) and Rudis (prothioconazole) treatments into fungicide programmes will provide robust broad-spectrum control of all four spotting diseases, with the complimentary curative and protectant activity needed under sustained disease pressure.
Rudis is the only DMI fungicide to deliver strobilurin type physiological effects, so alternating it with Nativo 75WG, which contains trifloxystrobin, will also maximise the boost to crop quality and yield.
These effects have been demonstrated consistently in trials. Last year with Brussels sprouts we looked into the impact of both fungicides on button quality at harvest and subsequent shelf life.
The trial, in Lincolnshire on the variety Dominator, compared programmes of three sprays with the third and final one applied on 17th September. When button quality was assessed at harvest on 16th December, both the Rudis and Nativo 75WG programmes had restricted disease blemishing to just 5% of buttons. Untreated areas averaged 22% blemished buttons with symptoms mostly related to ring spot.
Rudis Untreated Nativo 75WG
As can be seen from the pictures, the difference in quality between treated and untreated was dramatic and clearly illustrates how these fungicides help not just to achieve excellent disease control, but also deliver the quality of marketable produce required by the most demanding of retailers.
