Why Did My Onions Rot?

Onions are usually a pretty easy crop to grow. They can be grown from seed or from sets. Sets are one year old onion bulbs that look similar to a tulip bulb. They are planted in early spring and harvested in July and August. What we didn’t expect this season was to lose half of our onions to rot.

Why Did My Onions Rot?

They looked fabulous when they were harvested with long green stems and healthy bulbs. I laid the onions on plant trays and placed them in our gazebo to cure. Curing is done in a dry area for a few weeks. Two weeks into the curing, I noticed the leaves were yellowing but also had some grey streaks on them. I decided to cut off the dead leaves and remove the roots of the onions.

Why Did My Onions Rot?

The first flat was fine. Upon starting the second tray of onions I noticed that as I cut the stem back the inside of the stem was brown and moist. It should be a green colour or white if it’s cured. This was not a good sign. I kept cutting the stem back towards the neck to see how far the brown had travelled.

Why Did My Onions Rot?

Upon looking closer at the onions there was also the presence of white spots at the base of the onion. On some bulbs my thumb went right into the onion. The mushy onions went right into the trash.

So what caused this rot on the onion crop? Our summer started out very cool and wet and June had more rain than normal. The rain was too much for the onions  and the perfect environment for fungal diseases to take hold.  Onions start to mature underground in June and prefer a drier soil.

What I’ve determined by how the disease has spread is that the onions suffered from botrytis neck rot. Even though the plants look fine when harvested botrytis can show up during the curing process. Since more than one of us had this issue with our onions at the community garden its probably due to our wet weather in June and high humidity.

I also know there were some issues getting the onion sets to grow so was the seed infected to begin with? Late season rain, which we had a lot of, overhead irrigation, cool temperatures or too much nitrogen fertilizer late in the season are all risks to an onion crop.

Going forward I’ve recommended that we rotate the onion crop to another raised bed or not grow them at all. Unfortunately the botrytis allii spores stay in the soil so it’s best not to plant in the infected area for three years. On a good note we were able to salvage half the onions and they will be used up right away. Onions can be chopped and frozen for later use in cooking if they don’t look like they will store well.



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