Fungal fruiting structures (reproductive structures) are usually embedded in the dead interior. Frequently a “halo” of yellow or red color develops around the border. A halo indicates recently killed tissue that will eventually die.
dyzzypyxxy said:Housekeeping is the answer for preventing the spread of any fungal disease, Greg. i.e. exactly what you did - remove any leaves that are showing signs. Sometimes that is all you need to do.
But if it starts showing up more, a spray of baking soda and water 1/2tsp to a quart is a good preventative. You need to do it fairly often, especially after rain and overhead sprinkling since it will wash off.
Are you making sure to water in the mornings? Evening watering in summer is an open invitation to fungi to infect the leaves when they sit all night with water on them.