That's common in Phals and is a response to strong light.
I recommend not using a fungicide unless you know you have a fungus, you know which fungus it is and you know the fungus is susceptible to the fungicide you plan to use. There are beneficial fungi growing around plants that keep pathogenic fungi in check. Fungicides kill the beneficial ones, too.
Different fungi are resistant to different fungicides, so you need to be certain it will work on the fungus you have. And plants growing on a windowsill rarely develop fungi unless the relative humidity is very high.
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