Controlling Neck Rot in Onions

October 2006
Botrytis is a pain in the neck for onion growers. It causes progressive rotting of the neck of onions, which may take 3 4 months to become apparent, by which time the crop is probably in an export market. At first it is seen as a watery decay at the top of the neck and as it moves down the scales become soft and translucent and the neck sinks.

Onions are a valuable crop with an export value of $70 million. Last season there were 4300 ha planted, 156,000 tonnes produced and most of it exported to the UK, Belgium and Japan. It is the second largest vegetable export crop by volume and No.3 in terms of export earnings. Consequently control of neck rot is important.

Growers pay twice for neck rot through the cost of measures currently used to control it and losses in the marketplace.

The disease is caused mainly by Botrytis allii. Overseas work has shown that seed can be a source of infection, and that even low levels of infection can provide an initial inoculum and spread the disease to surrounding plants.

Another susceptible stage is after emergence of the seedling flagleaf, which bends over and dries off. Spraying with fungicide is often carried out at that point. It is also important about a month before harvest when the tops are bending down and there is a certain amount of wounding. Spraying reduces the likelihood of infection during curing.

Main sources of infection are the seed, and infected weeds and bulb debris from the previous crop. Wet, humid weather can also play a part, particularly around harvest.

Research at Crop & Food

Evaluating methods for testing seed for Botrytis contamination and determining the most accurate.

Obtaining seed samples and testing them using that method. It was found that five of nine lines of seed had low levels of infection.

The infected seed lines were followed up to see where they had been planted. The crop was followed into storage to determine whether there was any correlation between the level on the seed and the incidence of neck rot. Crops were examined at harvest because a high incidence can be detected then, and the weather at harvest was also monitored to see whether that had an effect. This work is ongoing and the intention is to examine around 25 seed lines in total.

Growers are being interviewed to see whether their practices correlate with infection or lack of it. Spray diary information what is sprayed when is being gathered, and examined closely especially where growers have unusually high Botrytis problems. Note that the problem with assessing neck rot is that the symptoms are not normally apparent until well into storage.

Infected onions from around the country were collected and Botrytis isolated from them 62 isolates in all.

Carbendazim is a fungicide that may be used up to six times per year, starting with seed treatment, two applications at flagleaf stage, and two more before harvest. It may also be used after e.g. hailstorms. Overseas evidence suggests that Carbendazim is losing its effectiveness, so 50 isolates were treated with this product. One third of them were found to be resistant to it. (NB. Funding for this experiment came from the industry, not SFF.) The problem for growers is knowing whether or not the botrytis they have is resistant, and testing the seed before it is planted would be expensive.

The take-home messages for growers:

The key points coming out of the study are the importance of disease-free seed, crop rotation, and maintaining the health of the onions during growth and harvest.

Use certified, disease-free, treated seed

Select a quick maturing cultivar so that neck tissues dry before storage

Select a light, well drained, well prepared fertile seedbed

Avoid heavy soils, heavy seeding rates leading to overcrowding and poor air circulation

Minimum of two year crop rotation i.e. maximum of two consecutive crops in one paddock, and if there is any problems in the first year then change paddocks

Use fertilisers sparingly on the basis of soil tests. Aim for steady vigorous plant growth rather than soft luxuriant growth

Do not irrigate excessively and especially not when tops are drying

At harvest, follow practices that help plants fully dry down at the end of the season so that the tops are mature

Ensure neck tissues are dry before topping

Avoid injury during harvest and storage

Eradicate weeds, remove unharvested plants and destroy affected plant debris after harvest so that there is less chance of infection of the next crop

Ideal storage is at 0-1 deg. C at 65-75% humidity.

A healthy onion with a well-cured neck is seldom affected by neck rot during storage.