Yes, a mulch will help control Septoria leaf spot on tomatoes, but there are other things that will help more. A fabric weed barrier is good because it lets in water but does help contain the disease in the soil. Because it does not prevent all weeds from growing, add another mulch on top, such as grass clippings or wood chips. Black plastic, even with tiny holes in it, does not let enough water through, so we do not recommend plastic as a mulch unless drip irrigation is installed under the plastic.

Septoria leaf spot is caused by a fungus that overwinters in your garden in dead plant debris and perennial weeds related to tomatoes (e.g., nightshade). It infects the plant through airborne transmission or by splashing up from soil onto the new plants during rainfall or overhead watering.

The primary ways to prevent Septoria leaf spot are:
1. Clean up all dead plant debris and remove perennial weeds in the fall. If you have not done so, clean them up and burn them or put them out with the landfill trash before tilling. If you put them on the compost pile the disease will come right back the next year because the fungus can be carried by the wind.
2. Rotate your crops. Don't plant your tomatoes where you planted tomatoes or related crops last year. The fungus may still be in the soil. The fungus can live in the soil for three years or more.
3. Look for Septoria leaf spot-resistant tomato varieties. If there are no further stresses on the plant such as drought, intense heat, flooding, etc., these varieties should do well.

After the tomatoes have been planted, use these methods:
1. Water in the mornings at the base of the plant through drip irrigation. Overhead watering will splash up material from the soil onto the plant. Watering at night leaves the plant wet overnight and very inviting to fungal diseases.
2. Stake the plants so they have minimal contact with the ground.
3. Mulch the plants to prevent splashing.