If not, I'd trim any sign of rot around the stem area and replant it. These nuisances feed on bud tips, flowers and leaves and then, left unchecked, burrow down inside the dahlia stem. 1. This deadly disease causes stems to become mushy and rotted. Viral diseases. This causes the plants to wilt. Stems that are affected will be wet and subject to rot. You didn't mention what area of the country you're in but I'm wondering if you've had a lot of rain followed by excessive heat. Choose a location – either indoors or out – that won’t dip below freezing, has good air circulation, and indirect sunlight. Bacterial wilt - this makes the stem droop. 1. Viral diseases – Viruses are often transmitted by thrips, which burrow deep into the stems and buds. Could be rot (fungal disease), but I suspect thrips. Typically, bacteria enters plants through a wound or natural opening. To treat borer attacks, spray BT (Bacillus thuringiensis) on the dahlia foliage when the worms are feeding. A dahlia succumbing to bacterial stem rot. If it were mine I'd dig down to see if the tuber was rotted. Welcome to the dahlia forum! Bacterial Diseases. Bacteriosis – if the stem is brown and soft, this can be the cause. Below is a list of some of the problems that might be encountered while taking dahlia cuttings. Stem rot – Dahlias planted in poorly drained, soggy soil are highly susceptible to stem rot. 2. However, now that the problem has been identified, it’s time to work on a solution. If a dahlia gets infected during the growing season, overwintered tubers can carry that bacteria on to the next year. Root and stem rot is not an uncommon problem, nor is it the end of the world. Treating, controlling, and preventing root rot can be an arduous process, but anything worthwhile takes a bit of blood, sweat, and tears. This helps to cure them and to prevent rot during storage. Symptom: Cuttings turn black and rot at the soil line. Viral diseases; Mosaic Cucumber mosaic virus: Dahlia mosaic virus Tomato spotted wilt virus: Ringspot Impatiens necrotic spot virus: References. Look for small holes in the underside of the bud; thrips chew in and eat the inside of the bud killing the flower and not allowing it to open (what you see as rot is a symptom and not the problem). Dahlia diseases and what to look for. Stem and tuber rot Rhizoctonia solani Thanatephorus cucumeris [teleomorph] Cottony stem rot Sclerotinia sclerotiorum: Vascular wilt Fusarium oxysporum Verticillium albo-atrum. Bacterial diseases are difficult for a home gardener to diagnose. The infection causes pith to blacken and rot with an … Most dahlia varieties should form roots within 10-14 days when the proper environmental conditions are provided. After your tubers have dried, cut back the rest of the dahlia stem with pruners or a sharp knife. It might re-sprout.