Hi Xuan, Sorry, I can't help with your problem, but heard somewhere that you used to live here in Walnut. I've been here for 30 years. Linda

Hi Linda. Yes, I used to live in Walnut Creek, San Francisco East Bay for a brief period of time.

I'm glad to know you.

Xuan

Your plant looks to me like it is wilting and more pale than vinca should be. Is the pot being overheated and boiling the roots? Vinca grown here is in mostly shade and cool soil. As your mix has no nutrient value I presume you are feeding regularly with enough nitrogen to darken the green of the foliage. I don't know the cause of the blackening stem, so will be interested when it is explained. Al

Hi, Xuan!
I'm sorry about the blighted stems. Here's something I found while searching....

"Symptoms and Causes
According to the Alabama Agricultural Experiment Station and Clemson University Extension, there are several diseases and fungus that can cause vincas to wilt, including phytophthora stem blight and root rot, pythium root rot, rhizoctonia stem and root rot, gray mold and the tomato spotted wilt virus. Symptoms of these diseases and fungus include dark colored lesions on the stems and branches, yellowing leaves with stunted growth, spots on the foliage, gray mold on the plant or black concentric rings on the leaves.

Many of these diseases and fungus are prevalent in soil that has poor drainage or gardens that have been watered incorrectly.

Prevention and Treatment
Select healthy vinca to plant in the garden. Enrich the garden's soil with well-rotted manure or compost before planting to help increase the soil's moisture-retention. Do not plant the vinca deeply and space 8 to 12 inches apart to allow for air circulation. Vincas need about 1 inch of water each week; if rainfall is insufficient, avoid overhead irrigation systems and water deeply in the early morning to allow the plants time to dry before nightfall.

Remove and destroy any affected plants. Treat the remaining plants with a fungicide; contact your local extension office to determine which fungicide is needed."

Josh

Thank you Calistoga.

Very helpful information, Josh. I really appreciate it.

Xuan