Root, bulb, leafy and brassica vegetable pest report – 28 August 2012

The thresholds for emergence of third generation onion maggot flies and third generation cabbage maggot have been reached in Wellington.

The presence of carrot leaf blights have been reported since mid-July in the Holland Marsh, perhaps seen earlier in other areas. Warm, humid conditions favour the development of both Cercospora and Alternaria leaf blights. For example, Alternaria infection is favoured by warm temperatures (16-25C) and prolonged leaf wetness. Cercospora infection requires a minimum of 12 hours of leaf wetness at 20-30C. Rain, irrigation, high humidity and/or dew promote ‘humid’ conditions in the carrot canopy. Obviously as the canopy grows larger, less air movement is occurring within the canopy also creating a ‘humid’ environment.

Fungicides can be used to manage these diseases. Usually the first fungicide application is timed when 25% of randomly sampled mid-age leaves have one or more leaf blight lesions (i.e. 13 out of 50 leaves, 25 out of 100 leaves). Spray intervals of 7-10 days, depending on the fungicides applied, after the initial fungicide application can help manage these diseases, especially after the leaves have closed over the rows. The intervals can sometimes be extended if the weather in unfavourable for blight development. Please see Pub. 838 Vegetable Crop Protection Guide for more information on fungicides registered for this use.

Here are the degree days accumulated from April 1st until August 28th, 2012.

(These values are provided as guidelines only. Local environments, soil type, field history, etc. may influence activity of these pests.)

*Data provided by Muck Crops Research Station, University of Guelph.

Degree day model information for pests