I’m in northern ontario (zone 2 to 3). Our spring, May 20th, was 2 inches of snow, cold and wet. Azalias, Rhododendrons, and Roses are in good shape though. Two weeks ago, I brought in two new Clematis from Walmart, other clematis started withering, she’d flowers, leaves. Cause or/and cure please?
I am not quite sure if the withering clematis you describe in your question, are older ones in your garden, if plants were struggling in the store, or if you purchased plants, brought them home and now they’re showing symptoms. You also do not tell us if you planted the newly purchased plants or if they are still in pots. In any case, you are concerned that the symptoms these plants exhibit might be due to clematis wilt.
Symptoms of clematis wilt and environmental stress can be very similar. It could well be that the withering plants, had just not been properly cared for while in transport or at the store. Lack of watering and/or too much sun exposure, while sitting on shelves waiting for customers, can cause trauma, shock or injury and take a serious toll on young plants.
On the other hand clematis wilt is indeed a serious fungal disease usually introduced to a garden with infected, but still healthy looking nursery plants. The only way to avoid introducing it to your garden is to purchase your plants from a reliable source committed to the meticulous growing practices that help avoid this disease in their stock.
Clematis wilt does not affect all clematis alike. It is predominantly a disease of Large Flowering Clematis. Small flowering clematis and cultivars derived from Clematis Viticella Group are resistant to the disease.
If your clematis does belong to the Large Flowering Group, look for a sudden wilting or blackening, first of the growing tips then of entire shoots. This is caused by the fungal infection blocking the water conducting tissue of the plant. Moist conditions, like you experienced this spring, are needed in order for young stems to be infected, but symptoms may appear only some time later. You can further find out if your clematis has the disease by slicing an affected stem lengthwise. Its inside will show blackened areas about 10 cm long. Most often these blackened areas can be found at or just below soil level.
There is no cure for clematis wilt and the fungus once in the soil is very hard to eliminate, but the disease does not necessarily kill clematis. Regularly remove diseased stems, be vigilant about removing all plant debris from the soil, and make sure your clematis is properly planted and well cared for. If you plan to replace a diseased plant find a resistant cultivar, or choose another site in your garden for a Large Flowering cultivar.
You might find the information about clematis care helpful, see link below.
For further reading on clematis wilt, I recommend this link.