Collar rot
A soil-borne fungal rot at the base of the stem, where it meets the soil, that causes plants to wilt and collapse.
Also known as: Stem rot, Foot rot, Crown rot
Symptoms and identification
The stem at soil level turns brown, soft or dry and shrunken, sometimes with white fungal threads and small seed-like bodies. Leaves yellow and wilt, and the plant eventually falls over. On trees, the bark at the base cracks and oozes gum.
Causes
Fungi such as Sclerotium and Phytophthora attack stems kept wet at the base. Heavy, poorly drained soil, deep planting, mulch piled against stems and wounds from tools all increase the risk.
Organic treatment
Remove badly affected plants with surrounding soil. For trees, scrape away the rotten bark, let the area dry, and paint it with Bordeaux paste. Drench the soil with a Trichoderma product.
Prevention
Plant at the same depth as in the nursery pot, keep mulch a few centimetres away from stems, and improve drainage. Avoid injuring stems when weeding.
Chemical treatment (use with care)
Drench the base with a fungicide labelled for collar or crown rot, following the label rate.