Stem Necrosis

Do not cut anything, this will just cause it to spread. Do not cover it with anything, this will cause it to get even worse due to the extra humidity under any type of cover. Otherwise, what Chucky324 wrote is exactly what I would recommend too. Treatment with sulfur will kill it and the problem will cease to exist. There are many types of "wettable" sulfur available at any garden centre and all do the same job of killing fungus. Definitely get onto it ASAP, the risk of it spreading, especially to any flowers, is very high. This fungus is the famous and common, botrytis, effective control is a must!! The question though is "why has this happened?" Too much water, too often with low air flow = prime fungus habitat. Work on those things to prevent further outbreaks. Hope this helps, Organoman.

Yeah you definitely have some stem rot going on there. I would scrape all the black soft stem off then spray it with some 3% hydrogen peroxide. Then get an oscillating fan blowing under your plants to keep more airflow moving

Hi! First, put a "nap" over the soil (if you still have CDs cut it in half wish a little disc grinder an put the two parts around the trunk) and spray a solution of Hydrogen Peroxide on the rotten spot. You gonna start to see some little bubbles, let act a bit. Then, use a sterile scalpel and cut out the black parts. Try to be as steady as possible, cutting strait but not removing too much. Spray some H₂O₂ again and made a plaster with tree wound plaster or super glue.

I do not like to see such heavy training so close to the soil. Hard bends can split and open the skin of the plant making is more suseptible to diseases. Higher up is safer but close to the soil like that is a potential problem. You can even see bits of soil splashed up and sticking to the stem close to the damage.

You can sprinkle a little into your soil and water in as well...It is antifungal

Try this.. Mix some cinnamon in water and make a thin paste and brush it onto the affected area. It may help, may not., but it won't hurt anything to try...

White mold, also known as Sclerotinia stem rot is almost impossible to get rid of. If this is what it is. I'd get rid of it and dispose of used soil. Do not re-use the soil.

Try and keep it dry, I'd dab it with some peroxide to kill off any topical fungus if its harbouring any. It might be in the system and to late. Good Luck!