Should I Compost Plants That Have Had Disease?

Composting diseased plant material is one of the most debated topics in home composting — with good reason. Done correctly, composting kills most fungal and bacterial pathogens reliably. Done incorrectly — in a cold, slow-composting system — it can spread disease through the garden when the compost is applied. The correct answer depends on the specific disease, the composting method, and how much risk you are willing to accept.

Low-risk diseases — safe to compost

Powdery mildew, downy mildew, rust, leaf spot, and other primarily foliar fungal diseases are generally low risk in a well-managed compost heap — they do not produce long-lived soilborne spores or resting structures. Plants affected by these diseases can be composted in a reasonably active heap without significant risk of spreading the disease back into the garden. Remove heavily affected material in autumn and compost through the winter rather than leaving infected debris in the bed where it can over-winter and reinfect in spring.

High-risk diseases — do not compost

Some diseases produce resting spores or structures that survive cold composting temperatures for years. Clubroot (Plasmodiophora brassicae) in brassicas produces resting spores that can survive in soil for 20 years — never compost infected material. Onion white rot (Sclerotium cepivorum) produces sclerotia that persist for decades — burn or bin infected onion material. Potato blight material can be composted in a hot heap, but not in a cold one. Verticillium wilt and other soil-borne vascular wilts should also not be cold-composted.

Alternatives to composting

For high-risk diseased material, the alternatives are: council green waste collection (commercial composting reaches temperatures that kill most pathogens); burning (where permitted); or bagging and putting in general waste. This is not waste — it is appropriate risk management that prevents a minor disease problem becoming a persistent soil contamination issue that affects crops for years.