Onions' reaction to illness

I have observed something about the onions in my pantry. Occasionally, one or two of the onions will develop what appears to be layers of black mildew sandwiched between the onion layers. Invariably, these onions are found to be breaking dormancy, putting out leaves in the pantry. Other onions, that are still dormant, are sound, with no mildew. It seems to be a pretty quick reaction: the last onion that I found putting out leaves appeared at first glance to be sound, but then I found one patch of mildew, one layer in. So it would appear that the onions react to getting mildew by starting to grow – presumably, in an effort to get in one last reproductive attempt before they die; or perhaps, active growth is necessary for mounting an immune response. Could such an effort succeed, I wonder?

Anyway, we are so seldom able to see plants reacting, I find this a fascinating observation.


Or could the mildew be a result of increased moisture in the area of the shoots? Interesting observation, either way!

Could it be the other way? Growing leaves makes bulbs more receptive to illness?

The black storage rot of onions will probably be Aspergillus niger. Moisture availability during storage will favour its growth, and inreased moisture will also favour sprouting of the onion. This mold is also one of many known to produce gibberellin. Many fungi produce secondary metabolites that are plant growth hormones like gibberellin, auxins etc. How that evolved is the interesting question for which I don’t have an answer.

Observations · iNaturalist


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