Growing Streptocarpus

[Fuerte Rav]

I grew Streptocarpus for years in the UK and was sorry to leave them behind. I was delighted to see some for sale in our supermarket style garden centre a couple of weeks ago and treated myself to three. The problem with that garden centre is that the plants they sell are not always suited to our climate and/or they all get treated the same while on sale. Orchids, fruit trees, herbs, African Violets: they all get soaked, and I mean soaked, from above by a hose every morning. I've learnt to buy within a day or two of the plants arriving! At least the whole place is completely shaded and their watering methods mean the humidity is always high.
So, the Streps looked a bit sad when I got them home but recovered within 48 hours. We then had very high temps, day and night, for 4 or 5 days and very low humidity. Today when I checked them I found some patches of something nasty on the leaves of all of them. Not all leaves thank goodness. I trimmed the leaves to get rid of all the nasty stuff but I wondered if anyone knows what it is/what caused it. Since I got them home I've had them in the same area as my phals, so shaded natural light, warm/hot (28-48C) 24 hrs, good air movement and I water when the pot gets quite light in weight. I never had these kind of temps in the UK so I'm not sure if it's just too hot for them.
Any thoughts welcomed!