In mid-winter, I pruned my standard roses back harshly. Black-spot can be a problem on these roses. The pruning is such that they have hardly any leaves. The remaining leaves do not have any black-spot.
Should I spray now to remove any remaining fungus on the plants? Will this keep black-spot from the new foilage that will grow as spring starts? Or will the fungus spread through the air to the new foilage in any case?
Blackspot is a difficult disease to control, as I know from bitter experience. I see that you are in Australia, where it will soon be spring, so here is what I suggest:
Remove all dead leaves from around your rose bushes, as the disease spores overwinter on them and, in spring, as soon as it rains, are splashed or blown up up onto the new foliage. Dispose of the dead leaves in a sealed bag.
Rake the area and put down a 3 or 4" mulch, but keep it away from the stems, to avoid giving them collar rot;
Spray the new leaves with both a contact and a systemic fungicide as soon as they begin to emerge, and ring the changes on the type of fungicides you use, to prevent the fungus from developing immunity - see *Richmond Rose Society advice below.
Whenever you water, try to avoid wetting the leaves and stems, as blackspot spores are water-borne, and this will minimize the spread of infection
Promptly remove any infected leaves and prune any stems with lesions
Feed regularly with a good rose fertilizer. Roses are heavy feeders and fertilizing at regular intervals will help yours to recover from the severe pruning.
Unfortunately, although you should be able to bring the disease under control, you're unlikely to eliminate it completely, as spores will be blown in from elsewhere on wind-borne rain.
*Use a systemic fungicide: These are compounds that prevent the germinating black spot spore from taking hold on the leaf. While systemics do not kill the spores, they do stop the fungus dead in its tracks by interfering with its metabolism. The fungus can’t digest its food and soon dies. Popular systemic fungicides include Banner Max and Compass. Mixing a contact and systemic fungicide in the same sprayer works well. By killing off spores and preventing the growth of any existing fungus, black spot is quickly eradicated. Although using both a systemic and a contact fungicide is the key to controlling black spot, don’t make the mistake of using the same systemic and contact fungicide each time you spray. Just as bacteria have become increasingly immune to antibiotics, the black spot fungus is becoming resistant to some of the compounds used to control it. To ensure that an immunity does not develop in your garden, switch the fungicides you use each week. Here is a list of some of the more common systemic and contact fungicides that control black spot.