Evaluating onion neck rot seed treatments

Neck rot causes significant losses in stored bulb onions. It is mainly caused by two very similar species of Botrytis: B. allii and B. aclada. The disease is seedborne, but symptoms are not apparent in the field and only develop in store.

As a seedborne disease, the most effective means of control is to use 'clean' seed free from pathogens. More easily said than done. Traditionally most conventional seed has been treated with fungicides to control seedborne inoculum. Most recently, thiram and/or fludioxonil (Maxim 480 FS) have been used.

We tend to think that seed treatments 'eradicate' the disease from the seed, but this is generally not the case in practice. In earlier AHDB-funded research on onion neck rot seed infections, we showed that the pathogens could still be present, even on fungicide-treated seed, and that fungicide treatments are not completely effective. Combined with the loss of thiram, there is now an urgent need to identify alternative treatments that will kill the pathogen on the seed or reduce transmission from seed to seedling or both.

As part of SCEPTREplus, we've recently started work to evaluate potential seed treatments for onion neck rot caused by B. allii/aclada. We will be evaluating chemical, physical, and biological treatments using a range of approaches. These include:

1. Evaluation of chemical fungicides in agar plate tests

These tell us how effective the fungicides are at limiting the growth of the pathogen. Whilst this approach gives us a quantitative and cost-effective indication of the relative potential of different fungicide products, it is not suitable for physical or biological treatments.

2. Evaluation of chemical and physical treatments in seed tests

This gives us a direct estimate of how effective the treatments are at eliminating or reducing pathogen levels on the seeds. This allows us to get an indication of the relative potential of physical and disinfectant type products and some fungicides. It is not suitable for the evaluation of biological treatments.

3. Evaluation in seed-to-seedling transmission tests

We consider this the gold standard for evaluating seed treatments and is the only way to evaluate biological treatments effectively. However, it is more expensive than the other two approaches. With this test, we sow the treated seed and then determine what proportion of the emerging plants are infected. If you can reduce the amount to seed-to-seedling transmission, you reduce the starting levels of disease in the field.

We will be doing this in a glasshouse. While it is tempting to do this in the field, our experience is that secondary spread can be a real issue for airborne diseases. Disease will tend to spread from plots with higher levels of disease (ineffective treatments) to plots with lower levels of disease (effective treatments). This means that all plots tend to end up with similar levels of disease depending on the season, masking the effects of the seed treatments. This doesn't mean that field evaluation is not possible, but it does make the experiments even more expensive and difficult to evaluate.

The plate tests are underway and have already shown differences between products.