Nature waters its garden early in the morning. Think dew. There are a number of 'reasons' for this, but here is the elevator lesson.
There is less heat at dawn, so evaporation is lower and plant activity is higher (with stomates open and transpiration not compromised by high heat) so more of the water is used as the gardener intends.
There will be a higher level of UV light before the light decreases again. This reduces the ability of moulds and mildews to take hold in a garden. Imagine if you took a shower at dawn. The bathroom would be all warm and moist, wouldn't it?
If you shut the bathroom door and left the room still and dark, you would have mildew pretty soon.
But if you could open windows and let lots of light and air in, the problem would be greatly reduced. The same thing happens in your garden. If you water early in the morning, the sun has a chance to get a healthy dose of yeast-killing UV light into the foliage to reduce infection. If you water in the evening, the moisture sits on the plants all night, in the dark, giving infection a greater chance. The short story: water your garden early in the morning.
As always, if you have any questions, shoot me an email (clayton [at] greenowlmowing [dot] com [dot] au
Enjoy your gardens.