Fighting a fungus on tomato plants. Need help to what it is and what to use
Thank you for your question. Initially from your photo the problem could be a number of issues such as over watering, under watering, excess or under fertilization but on further research it appears that you have the two common leaf spot diseases of tomatoes: early blight and septoria.
These diseases thrive in wet and humid conditions which we have been experiencing in Ohio! It only effects plants in the Nightshade family of which tomatoes are a member.
This fungus thrives on plant material and can over winter on them. It is spread by wind, splashing when watering, use of unsanitized garden equipment. You may be able to treat it with an appropriate fungicide listed for this fungus; however you should rotate you tomato plants in a new location every two to three years, water early in the morning at the base of the plants to avoid splashing on the plants, provide ample distance between your plants to provide good air circulation, and sanitize tools, cages, or stakes used with you tomatoes. If you decide to utilize a fungicide treatment be sure to read and follow the label directions! chlorothalonil, maneb, macozeb, or a copper-based fungicide, such as Bordeaux mixture, copper hydroxide, copper sulfate, or copper oxychloride sulfate are fungicides available. Follow harvest restrictions listed on the pesticide label.
I have listed several links below that will go into a more detailed explanation of you apparent problem(s). Also you may want to contact your County OSU Extension Service for additional suggestions. I have also listed them below.
Again thank you for allowing us to be of assistance to you in your gardening concerns.
Hamilton County OSU ExtensionOffice
4210 Dane Ave
Cincinnati, Ohio 45223