I don't know if that's a disease or what, but in my experience, it will spread if you don't cut the affect leaves, make sure you cut it at least 2 mm beyond the affect part. Don't let it touch other stems, it will spread that way too.
BTW, Tonina can do very well in Flourite provided your water is soft, a lot of CO2, and A LOT of light!
Eheim pimp club #48
800L 100% Flourite
80L plant/shrimps nursery tank inconnected with the main tank
Total 600W HQI, 2 X 5200K 2 X 6500K
Eheim 2028 X 2 2080 X 1
Dupla Substrate Heater
Dupla S external reactor
Aquamedic 2hp & Teco R15 400W chillers
Sega pH Controller
Light period: 6 hours on each bulb across 12 hours
Water Parameters: pH 6.4, KH 2, GH 1, NO3 40, PO4 1 TDS 250