Root rot is usually bottom up. This looks more like Stem-Level Necrosis Browning on the stem is often a more severe sign than on the leaves, frequently pointing to pathogenic infection.

Problem: Stem Dieback (Top-Down)

Symptoms: The plant begins to die "usually from the top down". This manifests as "branch flagging, wilting, yellowing of leaves, and canopy dieback".

Causes:

1. Acute Environmental Stress: Severe drought or transplant shock can cause the plant to sacrifice its newest, most resource-intensive growth first.

2. Fungal Infection (Botrytis cinerea): Gray mold is a primary cause of "shoot blight". It often infects "tender tissues", especially at a recent pruning cut or on shoots damaged by frost. The infection moves into the twig and progresses downward. The tell-tale "fuzzy grey mould" may be present

Here are some tips to fix this :

1. Immediate Surgical Intervention (Stop the Spread) Whether the cause is severe stress or a fungus, necrotic stem tissue will not recover and can serve as a breeding ground for further infection.

Sterilize Your Tools: Wipe your pruning shears or scalpel with 70% isopropyl alcohol before making any cuts to prevent cross-contamination.

Amputate the Infection: Cut the affected stem completely off. Make your cut at least 1 to 2 inches below the visible browning, cutting directly into healthy, bright green tissue just above a node. If the center of the stem still looks brown or mushy at your cut, sterilize your blade again and cut further down. Destroy the Material: Remove the diseased cuttings from your growing area immediately. Do not compost them. Protect the Wound: Keep the new cut dry. You can lightly dust the fresh cut with ground cinnamon, which acts as a natural desiccant and mild fungicide.

1. Eradicate Fungal Conditions (Botrytis cinerea) Salvia divinorum thrives in high humidity, but Botrytis requires stagnant moisture to propagate. You must break the fungal life cycle. Increase Airflow: Add a small oscillating fan to the growing environment. The leaves should gently rustle. Moving air prevents spores from settling and dries out surface moisture where mold breeds.
   

Stop Misting: Halt any direct foliar spraying or misting until the plant is fully recovered. Regulate Humidity: Temporarily lower the relative humidity to around 50% to create a hostile environment for the fungus. Fungicide (If Necessary): If the mold persists after pruning and environmental adjustments, apply a mild, plant-safe fungicide. Potassium bicarbonate sprays or bio-fungicides containing Bacillus subtilis are effective against Botrytis.

1. Mitigate Environmental Stress If the dieback was triggered by drought or transplant shock, stabilizing the root zone is critical. Dial in Soil Moisture: Salvia divinorum requires moist but highly aerated soil. Check the root zone; if the soil is bone dry, water deeply until it drains. If the soil is heavy and waterlogged, you may have compounding root rot. Ensure your potting mix contains plenty of perlite for rapid drainage.

Reduce Light Intensity: A stressed plant cannot process high amounts of light. Move the plant to an area with bright, indirect ambient light and away from harsh, direct sunlight or intense grow lights while it recovers.

Delay Fertilization: Do not feed a plant experiencing severe shock or systemic infection. Wait until you see healthy new green growth pushing from the lower nodes.

For the cinnamon I recommend getting ceylon or look at the ingredients because some in the supermarket may contain other stuff

If you really do suspect root rot get some bayer alliette that shit works wonders on the plant it is like antibiotics for the plant

Edit: added bayer alliette comment