Timing critical for white rot control in onions

Onion white rot, Sclerotium cepivorum, can cause significant losses in onion crops around the country. This fungal pathogen causes fluffy white growth on the roots and base of infected onions.

Above ground symptoms include stunted growth, wilted appearance and leaf yellowing. Numerous black sclerotia form on infected tissue. These hard masses of fungal threads are the resting bodies of the disease which enable it to remain dormant in soil for many years and make effective control very difficult. Crop rotations of two to three years provide no control and even longer rotations will not fully rid a paddock of white rot.

Fungicide applications offer the most effective control of white rot and timing is important. The white rot degree days prediction model has been developed which uses soil temperature readings to calculate the cumulative degree days from planting. Using the model, growers can time fungicide application for 100-150 degree days from planting. Without intervention, the first infected plants can generally be found 250 degree days from planting.

An effective control programme can be created using a range of available crop protectants. Ensure you consider the maximum number of applications permitted for different chemical groups and their use patterns. Having performed well in Fruitfed Supplies trials over recent years, Sercadis is a strong option to use as the basis of a programme, rotating with DMI and QoI fungicides.

Consider also factors such as seed coatings and the Maximum Residue Levels (MRLs) specified for any export markets. Confirm the MRLs with your exporter. Seed coatings containing SDHI fungicides count as one of the three applications allowed in an onion crop. Note a maximum of two applications of the same SDHI active per crop.

For more information regarding onion white rot control options or white rot degree days, speak to your Fruitfed Supplies Technical Horticultural Representative.