What is this strawberry plant disease

1) it's possible the "white fungus" was mildew - depending on the weather, if conditions are warm and dry, some plant varieties are more susceptible to powdery mildew.  If conditions have been very wet for a while, then there are various other moulds that relish damp conditions.  All moulds thrive in crowded conditions which you already mention is the case - you need to get air circulating round plants so you really need to thin out those plants.  In the long run, this would be better as fruit generally are more susceptible to botrytis moulds in damp conditions which can spread like wild fire across the crop.

2) I see nibble marks on the leaves so something, maybe snails, have been munching.  Reddish marks are typical of dying plant matter - could be damage due to aphids, age (if this is an older leaf, then nothing unusual, but if it's new vegetation, then you need to do some pest control such as slug and whatever you use to tackle bugs)

from what I've read about leaf spot, the treatment seems to be improved air circulation  and removal of diseased leaves to reduce infection

ok. I've read up on leaf spot and it looks exactly the same. Most online reviews recommend good circulation and spaced out plants, so I think a thinning out is in order. However, with the infected plants, many fungacides have been recommended online. I think I will buy some generic fungicide and try that, if that doesn't work then I'll go for the more specific online suggestions.

Thanks.

Oh yes of course. When I first try the generic home base bought fungicide that shouldn't be a problem, but if I go to the more specific fungicides, then I will make sure that there safe and can be applied to fruiting plants.