I just came back from vacation and one of my coneflowers, which had been growing beautifully, was black and totally dead. Also, one of my coreopsis has the same symptoms...black and dying. There are 2 plants in between them so I'm not sure if it can be the same thing spreading or just bad luck. Does this sound like a spreading disease or just 2 isolated events? All other plants seem to be doing fine. Thanks!

It sounds like you may have a root rot or stem rot disease. Root rot and Stem rot diseases are very common and are generally caused by fungi, although there are some bacterial rot diseases as well. There are many different types of fungi that can cause these diseases. Some are more common under cool, wet conditions; others are more common during hot, humid weather. And it is almost guaranteed that rots will be common in wet, hot conditions. The disease symptoms will vary somewhat according to the particular fungal vector that is attacking your plant. Rather than get specific, here are some generalizations.

Symptoms of fungal infections include stunting and wilting. Leaves can be smaller than normal and either yellow or brown, or a combination. Stems may whither or wilt individually at the soil level or an entire plant will be affected. Depending on the plant, the decline of the plant can be gradual or rapid.

Root rot and stem rot disease vectors will attack mature plants as well as seedlings. These fungi are naturally occurring soil borne organisms. They might already be in your garden or they may get introduced into your garden via potting mix from the garden store.

To control these diseases, your best defense is a good offense. Choose cultivars that are disease resistant. Plant in well drained soils; if your soil doesn’t tend to drain well, add compost to improve the drainage. There is some evidence that adding an active compost can increase the populations of good microbes that will out compete and reduce populations of disease causing or bad microbes.

Be mindful to not over water your plants: allow the soil surface to dry out between waterings so that stems or roots are not continually moist.

Mulching techniques need to be monitored as well. Don’t over mulch; 2-4 inches of mulch is plenty to be a good thing, more than that and it can become too much of a good thing. Do not pile mulch up around plant stems (or tree trunks); this will also contribute to too much moisture.

Space plants properly, to allow for good air movement.

Remove and destroy affected plants to prevent the spread of the disease. Apply fungicides to prevent disease spread. Baking soda is a natural fungicide. Mix 3 Tablespoons of baking soda in a gallon of water and apply spray. You can also check at your local garden center or hardware store for sulfur spray, which prevents germination of fungal spores; lime-sulfur spray, which destroys newly germinated spores; and copper sulfate based fungicides that inhibit growth and germination of spores.

Soil borne fungi can be controlled by solarizing your garden soil. Cover the infected area with clear or black plastic for a few weeks. This allows the sun to heat up and destroy the fungi.