One of the most common questions I'm asked is how to deal with pests like powdery mildew on beans and peas, black spot or roses and other similar fungal infections.
My favourite way to both treat and prevent these infections is to use a milk spray. Just dilute 1 part full cream milk with 10 parts water and spray liberally over infected areas (or over susceptible plants). It's best done first thing in the morning - if you do it when the sun is at it's hottest, the leaves may burn and if you do it in the evening, it's not as effective because the moisture perpetuates the problem (fungi love moist conditions).
I recommend using it once a month as a preventative if you know your plant is prone to such infections - I have 3 roses that were already in my front garden when we moved in that are frequently affected by black spot because of where and how they're planted and I've seen a massive improvement with this spray. If you happen to get an infection, use it once a week (or even more often if you like) until the infection is completely cleared up and then for a further 2 weeks to ensure it doesn't reoccur. If you live in a wet area or a tropical area, you may like to use it more often when treating and preventing fungal infections due to the increased likelihood of them occurring - especially in the tropical wet season.
If you have a really bad infection (perhaps because you delayed treating it for too long) you can up th anti with a copper based fungicide. This will clear up the infection and then you can switch back to milk as a maintenance spray.
This is a simple, effective and cheap way of helping your plants and it's safe to use around beneficial insects, pets and wildlife. It's also a great way of using up milk that is close to or past its use by date.