There are few problems with herbs that people encounter more often then there home grown basil leaves turn black.
Yellowing leaves are often a slow and predictable symptom of overwatering, whereas when basil leaves turn black, it tends to be much faster moving.
To undertsand why this happens, we ned to understand the underlying plant pathology so we know what to do next.
My article lists the exact physiological causes of blackening leaves from my own experience and tells you how to save your basil…
My Triage Guide for Black Basil Leaves
Those of you in a rush, here’s the quick guide to diagnosing black basil.
| Prune affected leaves with sanitised shears; switch to bottom-watering. | Speed of Onset | Likely Root Cause | Immediate Action |
| Whole upper canopy turns uniform matte black/grey. | Overnight | Cold Shock | Move indoors to a warm room; do not prune immediately. |
| Angular, water-soaked black spots with yellow borders. | 3 to 5 Days | Bacterial Leaf Spot | Prune affected leaves with sanitized shears; switch to bottom-watering. |
| Deep black streaks climbing the main stem; leaves wilting while soil is wet. | 1 to 2 Weeks | Fusarium Wilt | Destructive fungal infection. Discard plant and soil entirely. |
1. The Overnight Blackout: A Lesson in Cold Shock
When I first started growing herbs, I made the classic mistake that we nearly all make. It was April, and there was a run of gorgeous warm afternoons, so I moved my home-grown basil (which started on my windowsill) onto the patio table outdoors. It was fine for the first night but then there was a sudden drop in temperature (still above freezing), and it actually dipped to 4°C (39°F)
The following morning, the top 2 thirds of the basil had a sort of bruised/scorched look, and the leaves looked wilted.
The Cellular Science of Cold Injury
Basil is native to tropical Southeast Asia, and its cell walls are full of water, and it has little resistance to the cold. When the temperature drops 10°C (50°F) the internal moisture in the leaves expands and rapidly crystallises.
As the morning begins and warms the basil back up, the crystals melt, and the delicate cell membrane is damaged. The fluid in the leaves spills out and exposes the plant tissue to oxygen in the atmosphere, causing cellular oxidation, turning the leaf black in hours.
How to Rescue it… When this happens, what you need to do is move the basil’s pit indoors, ideally to a stable 21°C (70°F), keep the soil slightly damp and wait 5 days or so. This gives you the time to see where the living tissue of the basil ends and the black tissue begins.
After this, prune the blackened sections back to just above the lowest green node (the node is just where two leaves emerge).
If you don’t wait five days and allow the basil to get back to a stable temperature and conditions the plant can die back.
I have done this several times and revived a blackened basil only for it to grow abndant new leaves.
2. The Kitchen Windowsill Experiment: The Danger of Overhead Misting
Many indoor plant and house plant growers like to mist their indoor plants to keep them fresh. However, you should not do this for basil. To demonstrate this, I ran a little 14 day experiment compacting to nearly identical, healthy store-bought basil pots, which were split from the same crop.
- Plant A was The Control: I placed it on a warm window sill as watered it exclusively from the bottom by placing it in a shallow saucer of water for 20 minutes (to allow the soil to draw up water through the drainage holes in the base), and the leaves stayed completely dry.
- Plant B My The Test Subject… I placed it on the same windowsill and misted the leaves twice a day to simulate high humidity.
It was day 5, and plant B developed small, translucent, water-soaked spots, and by day 9, the spots increased into dark, angular black spots.
The Verdict: Pseudomonas cichorii
The experiment showed me how quickly bacterial leaf spot can occur when the leaves are consistently damp.
The bacteria that are responsible are Pseudomonas cichorii, and it floats through the air and sits dormant on many surfaces.
When water from the misting sits on the basil leaf for more than several hours, it creates the perfect conditions for the bacteria to navigate directly to the stomata (which are the breathing pores of the leaves).
When the bacteria get through the stomata, the bacteria increase and destroy the leaf tissue from the inside out, which creates the black spots.
How to Stop the Spread of Black Spots
If your basil has black spots (either from overhead watering or misting) then this is what you need to do…
1.Isolate and Sanitize is Step 1.
The first thing you need to do is move the infected basil pot away from other herbs. You then need to disinfect your pruners with a cloth soaked in any kind of sanitizer. Do this between each snip to prevent transferring bacteria from infected material to otherwise healthy basil.
2.Careful Pruning:Step 2.
Snip every leaf that has black spots and, ideally, I place the leaves into a plastic bag and put them in the home bin. Don’t put them in the compost heap, as the bacteria can live in the compost for years.
3.Improve Airflow and adjust your Watering:Step 3.
I would ban the spray bottle completely and switch to bottom watering as I do with my basil to ensure that the leaves are dry. If you grow indoors, you can set up a little USB desk fan nearby to improve airflow, but if its ouside, it should be okay.
3. The Vascular Stem-Scrape Test
In my opinion, the most dangerous cause of basil leaves turning black is Fusarium Wilt (Fusarium oxysporum), which is a fungal pathogen that enters the root system and clogs the basil’s xylem system, which is responsible for transporting water around the plant.
Most basil growers misdiagnose this because it causes the basil to wilt heavily, and it just looks like it requires watering, but if you water a basil that has Fusarium, it makes the problem worse.
How to Diagnose
For us to differentiate between a basil that is just dehydrated and a basil that is dying from Fusarium wilt, you can do a simple vascular stem-scrape.
Take a sharp thumbnail or pruners and gently scrape away the outer green skin of the main stem around 2 inches above the soil line.
- If the leaf tissue underneath is white, and light green, then you can be assured that the plant’s water transport is healthy. It is likely just underwatered or potentially overwatered.
- If the tissue under is dark brown, the fungus has infected the vascular system, preventing the basil from drawing up water.
The Truth… There is no cure for Fusarium Wilt and once it has infected the plant, I would just throw the basil away and even throw the soil away. The best move is to look for specific varities of basil that are resistant which is often marked with F or FR on the packet.