Decayed layer in sliced onion

The Detroit News
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Sometimes you don’t find out about a garden problem until long after it happened.

The other day as I was preparing a meal, I cut open one of my onions only to discover a spoiled area inside.

The outside of the bulb looked perfectly good but there was a layer inside that had rotted while the rest of the onion was fine. Anyone who has done much cooking at all has occasionally come across this in store-bought onions but can it happen in homegrown onions, too.

Farmers call this disorder “slippery skin,” so named because the inner part of the onion bulb can be easily slipped out by squeezing the bulb near its base.

This problem is caused by a fungus or bacterium entering the onion bulb through the neck of the plant. Generally only one layer of onion is affected making it impossible to diagnose until the onion is cut open.

The outside of the bulb looked perfectly good but there was a layer inside that had rotted while the rest of the onion was fine.

The infection most often occurs just before or right at harvest time. If onions are left a little too long in the soil after they become mature, the pathogen can more easily enter the bulb through the withering leaves and neck. Rainy weather can exacerbate the problem. On-time harvest will help prevent that type of infection route.

Mechanical damage during harvest caused by tools or rough handling can also create a route for infection.

A portion of an onion stem, usually a couple of inches long, is always left on to dry, which prevents infectious organisms from entering the bulbs. Cutting the stems too short at harvest time can contribute to slippery skin.

A less common but similar disorder called internal dry scale also can be the culprit. In this case it is caused by excessive heat during the growing season.

Temperatures over 90 degrees fahrenheit will cause onions to dramatically slow their rate of growth. During this period onions take up much less water, so much less that newly developing leaves can die. Later when growth starts back up, the new leaves simply grow over the dead ones.

If the gardener or farmer continues to irrigate at the same rate during the heatwave, the dead leaves can become water soaked and the fungus can enter the bulb like that. Slowly, over time,  the layer decomposes until the entire bulb becomes rotten. Eventually secondary colonies of yeast take hold and cause odors.

We don’t see these problems that often in store-bought onions because onions are inspected very carefully before going to market. If even a tiny percentage of onions in a particular load is suspected of having symptoms, the entire load will be rejected and thrown out, causing a huge economic loss to the farmer.

In my case, I probably left some onions in one bed too long before harvesting them. Fortunately I’ve only seen one bad onion so far. The rest still look good, but I won’t know for sure until later.

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