Sydney, Newcastle and other coastal cities have a combination of warm days, cool nights and high humidity. Consequently we have a harder time than inland growers.
In other words: if you grow roses in Sydney, prepare to get fungal diseases on them.
Well fed roses grown for blooming and performance are often more prone to disease than those left to their own devices. Why? For a start, fertiliser prompts sappy new growth and the decay of older growth near the base of a bush. Mildew usually attacks the new growth, while the oldest leaves are the easiest prey for black spot. Secondly, sap-sucking insects, unable to penetrate old woody stems, love young growth and can spread disease spores as they migrate.
The most novice gardener can tell black spot, because it's very visible on the leaves of the bush. The fungus stops food production in the leaf, which eventually dies and falls off the plant.
It's pretty contagious, and the best cure is to attack it completely when you see it. The first and most important step is to remove infected leaves from the bush and the ground underneath. Put them in the bin. Then spray fortnightly with either Triforine, Myclobutanil, Chloronil or Mancozeb Plus. Mancozeb looks and smells disgusting, but it works the best on severe infections. (It also repelled possums for me!)
Mildew is one of Sydney's biggest problems in Spring and Autumn, when the nights are cool and moist. A white powder fungus covers new growth, distorting the foliage of healthy plants and interfering with food production. Here's a photo of an early infection spreading on leaves:
Ladybirds love feeding on the stuff, but they're not going to solve the problem unless you have hundreds of them. Fortnightly sprays of Triforine or Myclobutanil are one answer. An organic alternative is a weekly spray of 1 litre of milk per 5 litres of water - it definitely works but you'll need to spray all of the foliage including the underside of the leaves. A pest oil solution smothers mildew the same way. Sulphur-based sprays and rose dusts (lime sulphur, mancozeb, wettable sulphur etc) work very well but be prepared for an offensive smell for a few weeks if you use them!
Don't remove infected foliage, just treat it. Also, do your watering early in the day so that the foliage can be as dry as possible by nightfall.
Not common in Sydney at all. It's another fungus that makes leaves look like they are literally rusting away. Triforine and Myclobutanil are supposed to fix it.
Stems can sometimes become infected, eg. after cutting a rose for the vase or removing a dead head. The best prevention is using clean secateurs (dip them in methylated spirits before use).
Infections will spread down a stem to its base. The whole cane will die if left untreated. here is an example:
When you see the black infection, cut off the stem a couple of centimetres below the lowest point of infection.