Fungus on onion

Mar 01, 2018 • 3:48 AM UTC

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Microorganisms

TCIS Outreach

We are a group of students, volunteers and staff working with TIFR Hyderabad's Science Education and Outreach program: http://www.tifrh.res.in/~outreach/

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Onions in India often grow a black powdery fungus. Here’s a folscope-look at this fungus and the cells that it grows on.

White onion with fungus 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.

Inner wet layer with fungus 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.

Inner wet layer without fungus This one has two layers with a 3D effect. Seems like only the lower nuclei got stained.

Two inner layers The next time we got a red onion. From the outside it had yellowish patches but inside, sure enough was the fungus.

Red patchy onion

Outer skin peeled to show the fungus We looked at this dry fungus-covered skin, without staining.

Fungus on dry skin of onion 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.

Inner wet layer without fungus At the edge of the dried skin the fungus was clearer to see.

Conidiophore on the edge of the dry onion skin 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.

Spores eject out under 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.
K. Ashalatha
Jayashree Ramadas