All New Zealand onion cropping regions suffer losses due to onion white rot, Sclerotium cepivorum, a fungal pathogen that presents as fluffy white growth on the roots and base of the infected onions.

Symptoms of the disease seen above ground include stunted growth, wilted appearance and leaf yellowing. Numerous black sclerotia form on infected tissue and are the resting bodies of the disease. These hard masses of fungal threads enable white rot to remain dormant in soil for many years. This makes 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 the disease.

The most effective control is seen with applications of registered fungicides, but timing is important. The white rot degree days prediction model has been developed which measures soil temperature and calculates the cumulative degree days from planting. By using the model, fungicide application can be timed for 100-150 degree days from planting. Without intervention, the first infected plants can generally be found 250 degree days from planting.

A range of available crop protectants can be combined into an effective control programme. Consider the maximum number of applications allowed for the different chemical groups and their use patterns. Sercadis is a strong option for controlling white rot in onions having performed well in Fruitfed Supplies trials over recent years. A programme can easily be built around Sercadis, rotating with DMI and QoI options.

Also consider 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. This class of chemistry has a high risk of resistance build-up, hence the need to follow the SDHI resistance strategy.

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