You can compost leaves with leaf spot only if you run a true hot pile. A cool backyard pile will not kill the spores. The bugs live on through winter and come back in spring with your finished compost. Only a hot pile that hits 60°C (140°F) for several days can clean the leaves enough for reuse.
I learned this lesson the rough way one fall. I tossed all my Septoria sick tomato leaves into my cool backyard pile. I figured the worms would do the work for me. I spread the finished compost the next spring on a fresh tomato bed. Within a few weeks, Septoria was back in force. The compost was the spore source.
Compost infected leaves in the wrong pile and you spread the bug across your whole yard. Spores stick to the leaf bits even as they break down. Your finished compost looks fine and smells fine. But it carries live spores back to your beds. You end up planting the bug into clean soil.
Disposing diseased leaves the safe way starts with knowing your pile. Most home piles run cool. They sit at 20 to 30°C (68 to 86°F) most of the year. That heat is not enough to kill most fungal spores at all. If your pile is on the cool side, bag the sick leaves for the trash instead.
Hot compost temperature is what makes the difference for sick leaves. You need a pile that hits 60°C (140°F) for at least three days in a row. Some pathogens need a full week at that heat to die off. A good thermometer is your key tool here. Stick it deep in the pile and check daily.
To kill plant pathogens compost must reach the right size and mix. A pile under three feet by three feet will not hold heat well. Mix your greens and browns at a 1 to 2 ratio by volume. Add water so the pile feels like a wrung out sponge. Turn the pile every week to keep the heat up.
Gunasinghe et al. and UMN both find that spores stay alive on leaves for up to 9 months in normal outdoor conditions. That covers a full winter in most climates. Your cool fall pile gives those spores a warm, moist home all winter long. Spring rolls around and they wake up ready to spread.
If you cannot run a hot pile, bag the sick leaves and put them out with the trash. Do not burn them where smoke can drift into your yard. Do not bury them where they may meet plant roots later. Bagged trash is the cleanest path for a small home garden setup.
For larger yards, a true hot pile pays off in a big way. You get clean compost back for your beds. You skip the trash haul each fall. You close the loop on your yard waste. But you must commit to the weekly turning and the daily temp check during the hot phase.
So can you compost leaves with leaf spot? Yes, but only with a real hot pile. Cool piles fail this job and spread the bug. When in doubt, bag and trash. Your spring beds will thank you with clean new growth instead of a fresh round of spots.
Read the full article: Leaf Spot Disease: Complete Guide
