Black-eyed Susan or Black-Leaf Susan?

Goldsturm black-eyed Susan (Rudbeckia fulgida sullivantii ‘Goldsturm’), which you may also know as Goldsturm coneflower or just yellow coneflower, is one of the most popular perennials in modern gardens. With its masses of yellow daisies, each with a black central cone, it brightens up our plantations from late July to late September, attracts bees, birds and butterflies, makes an excellent cut flower, and is very hardy (zone 3). In fact, many gardeners consider it to be the perfect perennial!

However, in recent years, a “new” disease seems to be spreading into Goldsturm plantings. It infests the leaves, starting as a few round or angular purple-brown spots on the lower leaves, but soon spreading to the upper ones that sometimes turn almost entirely black before summer’s end. What is this disease and what can be done to stop it?

Rudbeckia Leaf Spot

There are actually several diseases that can cause spotting on rudbeckia leaves, but the most common is rudbeckia leaf spot or rudbeckia septoria (Septoria rudbeckiae), a fungus. The disease first starts to appear in June or early July as a few small dark spots, usually hidden from view on lower leaves. But August usually brings heavy dews and since the disease spreads on moist foliage, it really comes to the fore. The number of black spots increases and the spots enlarge and start to coalesce, often leading to a leaf that is more black than green.

Septoria spores are spread by water droplets. When it rains or when you water and a drop of water hits a contaminated leaf, it can splash over to a neighboring leaf or plant. And wind can carry contaminated water droplets several meters away.

What to Do?

Normally rudbeckia leaf spot is primarily a disease of senescence (aging), hitting leaves late in the season when their work collecting solar energy is largely done, so it doesn’t much interfere with the growth or survival of the plant. In addition, by that time, the plants are so covered in flowers that the blackening leaves may be visible to no one but the gardener. If the flowers are beautiful and continue to attract rave reviews, the easiest thing to do is to… do nothing! Simply apply the 15 pace rule: if you can’t see the problem at 15 paces, it’s not worth treating.

Prevention Next Year

To reduce the impact of the disease in the years to come, here are a few simple things you can do:

1. Buy healthy plants and certainly avoid plants with dark leaf spots. This disease is almost always originally caused by purchasing already contaminated nursery stock;
2. Make sure the plant grows in a well-ventilated location where the foliage will dry out quickly after rain or dew.
3. Fertilize moderately so the plant is more robust and thus more resistant to disease. Don’t overapply nitrogen, that will simply make things worse;
4. Avoid coneflower monocultures. They allow the disease to travel easily from one plant to the next. Instead learn to mix rudbeckias with other plants that don’t carry the disease. Note that rudbeckia septoria only attacks rudbeckias and won’t infest your other plants, so you can mix your rudbeckias with sedums, nepetas, agastaches, ornamental grasses, and just about anything else. Even echinaceas (Echinacea cvs), although closely related to rudbeckias, can’t catch rudbeckia septoria… although sadly they do have their own form of septoria (S. lepachydis);
5. If possible, water without moistening the foliage, perhaps by using soaker hose;
6. Cut and destroy infested rudbeckia stems and leaves at the end of the season (after they’ve stopped blooming) to reduce the quantity of overwintering spores.

Of course, some gardeners accept no leaf damage whatsoever and will do anything so their plants will be perfect. If you’re in this category, you can try spraying the leaves preemptively with fungicide (a copper fungicide would work well) starting early in July and continuing into August. On the other hand, fungicides will not eliminate the symptoms already present on the leaves. At best they simply help prevent the disease from spreading any further.

And if you’re really at your wit’s end, yank out your rudbeckias and plant something else: life is too short to waste bemoaning a minor plant disease.

And there you go! You don’t necessarily have to intervene when your rudbeckia leaves start to blacken, but at least now you know why it happened!