Onions in India often grow a black powdery fungus. Here’s a folscope-look at this fungus and the cells that it grows on.
First try – we discarded the dry papery outer peels and looked at the inner wet peel of the onion which was spotted with fungus. We stained it with methylene blue.
It seems like there are nuclei in only some of the cells, hyphae of the fungus are winding their way through the cells and there are black spores. Later trying to figure out the criss-crossing pattern of cells we thought perhaps the peel had got folded on itself.
Further inside the onion the layers looked free of fungus and we saw nice cells with clear nuclei.
This one has two layers with a 3D effect. Seems like only the lower nuclei got stained.
The next time we got a red onion. From the outside it had yellowish patches but inside, sure enough was the fungus.
We looked at this dry fungus-covered skin, without staining.
Here is an inner wet layer without fungus. This time we used a different, aqueous methylene blue which had undissolved bits that we washed off. It did not work so well. We assume that there are nuclei here but they did not get stained.
At the edge of the dried skin the fungus was clearer to see.
This one is called a “conidiophore”. The hypha (filament) is translucent and segmented. At its tip are the “conidia” or a bunch of spores. Our best guess from searching on the web is that this is Aspergillus niger, which colonises various parts of over a hundred-odd food plants, from apples to maize.
We put a drop of water on one conidiophore and its globular tip burst! Here it is, releasing its conidia into the water.
Here’s a video of what Aspergillus niger can do to its host onion over just 22 days.
In Indian kitchens we deal with A. niger on a daily basis. We chuck out the infected layers of onion or wash off the powdery fungus. But this close contact with the fungus got us a bit tickly in the nose and throat. Though generally not dangerous, A. niger does contain toxins which could affect you in a variety of ways. We must take care if we do this with school students, especially in low resource environments where gloves and masks are not available.