where are you .. big city name ... what are current night temps.. how much rain have you had in the last month.. is your lawn otherwise sodden all the time .. do you irrigate the lawn in early evening.. and it goes into dark wet???

is night humidity ridiculously high ...

and how about pix of the actual lawn ...

i relate mildews to mid to late summer nights.. when it balmy and stagnant out there ... and that just seems so weird in april .. but again.. i dont know where you are ...

the more facts you provide.. the better answers you will get ...

ken

Im in the north west of England on the coast. Not much rain over the past couple of weeks so yes I’ve been watering it at night. it’s been going dark and the lawn will be wet.

cold of an evening with very little humidity. the lawn gets a lot of sunlight during the day. This is not a shaded lawn.

First, never water a lawn at night. It sets up the ideal conditions for the development of fungal issues. Water early in the day - even at dawn - so the grass has a full day to dry before nightfall. And unless a new lawn or very recently overseeded, I can't imagine a lawn anywhere in the UK requiring watering in April! My climate is very similar to much of the UK although possibly drier, specially through the summer months, but it is unusual for an established lawn here to ever require watering before midJune.

The good news is the most common lawn fungal issues will correct themselves through good cultural controls rather than by treatment with a fungicide. Keep mowing high, back off the watering and when you do water, do so as early in the day as possible....not at night.