J. from Minneapolis Mn writes
I realized prevention is best -- but it's too late now. I've already thinned the plants which may have been part of the problem.
Liquid Copper Fungicide can be used to treat Septoria Leaf Spot but ideally should also be applied at the first sign of the disease. Applying in the fall would not 100% prevent it in the spring.
Unfortunately, once the disease has established, it is difficult to control. Encouraging air movement to reduce leaf wetness, removing infected leaves, even if just a few, will help, and then a general purpose garden fungicide may also help reduce the spread of the disease. These are preventative fungicide treatments and will not cure the disease but will help keep it from spreading.
Answer last updated on: 08/27/2018