I got a fairly big mint plant at the nursery and planted it in a huge container outside. It gets at least 6 hours full sun and I water often. I also cut a piece and planted it in a pot inside to see if it will grow roots. Both plants gets these brown spots on their leaves until finally the whole leave is brown and dies. It happens almost on a leave by leave basis. There's almost nothing left of the indoor one now. Does anyone know why this could be?
It is a possibility that the cause is a fungus (Diplocarpon rosae or another fungus) and to treat this you could use commercial fungicides or neem oil or perhaps water and milk solution , if it isn't too wide spread. The fungicides may not be a good idea if you have the mint to eat...if it is only for looking and/or smelling, then it will not play a health role. I would try and collect all the leaves and separate them. Then burn or throw them away. You do not want to spread or keep the spores in the soil near your plants. The worse case scenario is that it is too late. In this case you will need to isolate and burn or destroy the entire plants. I have had things like this happen and it is almost always with having given my plants too much water. I am not entirely sure if that one thing caused it all, but moisture and fungi do go hand in hand. Good luck with this.
Oh no that's a bummer. I like putting mint leaves in my coffee but i wouldn't want to do that with fungi obviously! It does make sense though, because there would just appear a random little brown spot in the middle of the leave. How do you use the water and milk solution? On the leaves themselves?
Yes spray directly onto the leaves. I believe that the solution is 1 part milk and 10 parts water. You may want to google this to see what the current dilution is. I learned 1:10, but that was years ago. I used the milk/water solution for mildew on courgette leaves. The black fungus is a different fungus...but still a fungus. It may be worth a try. Good luck with this. Mint in tea is good, so I can see that you would want a safe solution to the problem. Addendum: If this whole process goes pear-shaped and you loose your plants. I would not give up. I would buy another plant and keep on trying; however, I would not place the new plant in the same containers that you are using with these sick plants because the spores from this fungus are invisible and could re-infect your new plants. Sometimes you buy a plant and it is already infected--'tis difficult to combat this.
