leaf spot on Marigolds?

Perki

I got some blackish spots which are brown in the middle on the top side of the leaves on my African marigolds ( french ones look OK ). i just need a bit of help identifying the problem please. 

Stupidly i convinced myself it would not freeze last night so left most of my plants  in the greenhouse with horticultural fleece over the top of them, so you probably already guess it froze   i was not impressed, 

Some pic here, not the best though but you can just make it out.

 

John mcleod

I am jealous. My seeds are just beginining to show true leaves. The French marigold is a staple annual I think. My granny always used to have them and I love the smell of them. I know this is not related to your problem but thought I would share

Perki

Hi john, i quite like marigolds myself easy to grow and germinate. It my first year growing African marigolds, from what i have found on the internet i think i might of found out what it could be.

African marigolds are not tolerant of low pH soils.Low pH causes iron and/or manganese toxicity, which exhibits itself as brown to black speckling of the foliage .

Could be that i am not really sure, but they need potting on anyway so i try and get my hands on some peat free compost tomorrow. 

I do have two other variety of marigold, a stripped one ( one in the background ) and some honeycomb i think. The honeycomb are nearly perfect  all nearly exactly the same height them little beauty's 

And before anyone asks yes all the plants are living in the house tonight there everywhere