OUT, SPOT: HOW TO COMBAT BLACK SPOT ON ROSESIf it is your rose bush rather than your dog that you are calling "spot", then it is time for action. And getting rid of black spot - the deseas that is marring your roses - need not mean downsing the plant with chemicals. Let first get to know the enemy Diplocarpan rosae, a fungus feared by rosarians almost everywhere . Those black spot have have fringed edges and black pimples at their centres to distinguish them from other possible leaf-spotting deseas. Infected leaves soon yellow, then drop, meaning less energy for the bush which in turn means fewer flowers and sometimes the death of the plant.
Black spot deseas spent the winter mostly in infected leaves that fell to the ground. Spring warmth and rain awakened the fungus to shoot the spores up into the rosbush and infect young , unfoldong leaves. Moistures was needed to get those spores moving and then leaves had to stay moist for a few hours before infection could set in. Another lesser source of infection is infected areas winterring on young canes. Once spores get up into the bush in spring, infection can continue through the summer as spores hopscotch from leaf to leaf.
First steps at preventionBlack spot needs moisture to take hold, so one way to control it is to plant rose bushes where they will dry iff quckly from dew and rain: in full sunlight which roses need for best flowering and away from walls or dense shrubs where air can stagnate. This also means pruning away enough stems thet remaining ones can bathe in drying light and air. And, of course, wet the ground, not the leaves. When watering and avoid working among bushes when they are wet.
We can also put roadblocks in black spot's life cycle. Gathering up and composting the leaves the bush drops in autumn can lessen the amount of desesas in colum the following spring. Even better is to mulch the ground sometime between late autumn and late winter, each year leaving old mulch in place as you pile on new. As for those spores that come from infections on the sten, drastic prining is a good way to deal with them.
Plan before plantingRoses vary in suspectibility to black spot and the easiest way to deal with it is to grow a palnt that won't get deseased in the first place. unfortunately, the most commonly grown roses, hybrid teas, are also generally the most susceptible to deseas. Even among hybrid teas, though, there are varieties that resist black spot. Such as Tropicana, Mister Lincoln, Pink Peace, Carefree Beauty and Keepsake. Some grandiflora and floribunda types that resist black spot include Queen Elizabeth, Sonia, Betty Prior and Bonica. More reliable resistance is found among so-called species and shrub roses. Black spot usually doesn't cause problems with Father Hugo's Rose, rigosa roses and some of the newer varieties of shrub and land scape roses such as some of the David Austin roses (especially the variety The Mayflower), the Knock-Out and the Canadian Explorer series of roses, and varieties of Buck roses.
Spraying as a last resortIf you already have roses in the ground and don't want to replace them and they have relatively good site, and you prune and mulch them, you could still be calling your resebush "spot". Even then before you reach for sime hughly toxic pesticide, try a more benign alternative: baking soda, that universal cure for everything from dirty dishpans to smelly refrigrators. Mix a tablespoon and a half, along with either a few drops of dish detergent or two tablespoon of summer oil. Also called horticultural oil, per gallon of water and spray weekly.