Out, spot: how to combat blackspot on roses

If it’s your rose bush rather than your dog that you’re calling “spot,” then it’s time for action.

And getting rid of blackspot — the disease that’s marring your roses — need not mean dowsing the plant with chemicals.

Let’s first get to know the enemy: Diplocarpan rosae, a fungus feared by rosarians almost everywhere. Those black spots, if you look closely, have fringed edges and black pimples at their centers to distinguish them from other possible leaf-spotting diseases. 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.

Over the almost 200 years that the disease has been known, its causal fungus has paraded under about 25 different scientific names. Still, its life cycle is simple.

A year in the life of black spot disease

Blackspot disease spent the winter mostly in infected leaves that fell to the ground. Spring warmth and rain awakened the fungus to shoot spores up into the rosebush and infect young, unfolding leaves. Moisture was needed to get those spores moving, and then the leaves had to stay moist for a few hours before infection could set in. Another, lesser source of infection is infected areas wintering 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. As with the initial infection from fallen leaves, spores are released and get footholds only when moisture is present for enough time.

First steps at prevention

Blackspot needs moisture to take hold, so one way to control it is to plant rose bushes where they will dry off quickly from dew and rain: in full sunlight (which roses need for best flowering anyway), and away from walls or dense shrubs where air can stagnate. This also means pruning away enough stems that remaining ones can bathe in drying light and air. And, of course, wet the ground, not the leaves, when watering and avoid working among the bushes when they are wet.

We can also put roadblocks in blackspot’s life cycle. Gathering up and composting the leaves the bush drops in autumn can lessen the amount of disease inoculum 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. Besides mulch’s usual benefits, in this case it also acts as a barrier to keep that first batch of spores from the leaves.

As for those spores that come from infections on the stem, drastic pruning is a good way to deal with them.

Plan before planting

Roses vary in susceptibility to blackspot, and the easiest way to deal with it is to grow a plant that won’t get diseased in the first place.

Unfortunately, the most commonly grown roses, hybrid teas, are also generally the most susceptible to disease. Even among hybrid teas, though, there are varieties that resist blackspot, such as Tropicana, Mister Lincoln, Pink Peace, Carefree Beauty and Keepsake. Some grandiflora and floribunda types that resist blackspot include Queen Elizabeth, Sonia, Betty Prior and Bonica.

Don’t put too much stock in blackspot resistance, though, because there are a number of races of the blackspot fungus, so a variety may be resistant in one locale but not in another.

More reliable resistance is found among so-called species and shrub roses. Blackspot usually doesn’t cause problems with Father Hugo’s Rose, rugosa roses, and some of the newer varieties of shrub and landscape 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, a last resort

If you already have roses in the ground and don’t want to replace them, and they have a relatively good site, and you prune and mulch them, you could still be calling your rosebush Spot. Even then, before you reach for some highly toxic pesticide, try a more benign alternative: baking soda, that universal cure for everything from dirty dishpans to smelly refrigerators.

Mix a tablespoon and a half, along with either a few drops of dish detergent or two tablespoons of summer oil (also called horticultural oil) per gallon of water and spray weekly. For some gardeners, this spray makes Spot a dog’s name again.