Sclerotinia (white mould) on sunflowers

The symptoms

Sclerotinia takes three forms on the sunflower. Basal stalk rot starts at the stem base: the plant wilts suddenly and topples, with a soft brown patch and white fluff just above the soil. Mid-stalk rot appears where a leaf joins: a watery, rotting lesion that girdles the stem. Head rot attacks the flower head, which turns brown, limp and fibrous and breaks apart. In every case hard black sclerotia form — rice-grain to pea-sized — in or on the affected parts.

Why it is so serious

Sclerotinia is the leading disease of sunflower cropping worldwide. According to CABI, a heavy attack of white mould can cause tens of percent of yield loss; in epidemic years, local losses above 50% have been documented in North American and European growing regions among others. WUR plant pathology calls the fungus notorious because it has a very wide host range (more than 400 plant species, including oilseed rape, beans and lettuce), so soil infestation is sustained through many crops.

What makes the disease so awkward is its life cycle. Sclerotia in the soil germinate in two ways: they can directly infect the roots and stem base (basal rot), or they form tiny mushrooms (apothecia) that shoot spores into the air. Those spores land on dead or damaged flower parts and from there cause head rot. So a field that flowers apparently healthy can still develop head rot within a week of a damp spell. Spotting it early therefore matters: walk your plants during and after flowering and check regularly for the first soft, watery patches, especially at leaf axils and the back of the head.

Management, ranked

1. Rotation of at least four years

The cornerstone. Because sclerotia survive in the soil for years (according to CABI, often five years or more in practice), a wide rotation is the only way to bring inoculum pressure down. Rotate with non-host grasses or cereals; avoid other susceptible crops such as oilseed rape, beans and lettuce in the intervening years.

Cost/effort: no direct cost, but it needs planning. In a small garden it means: do not sow in the same spot for at least four seasons.

2. Resistant or tolerant hybrids

Breeders have selected for tolerance to stalk and head rot for decades. Full resistance does not exist, but tolerant hybrids noticeably slow the attack.

Cost/effort: comparable to ordinary seed; just a deliberate choice at purchase. Always combine with rotation — tolerance alone is not enough.

3. Avoid dense, wet canopies

The fungus germinates and spreads best in a damp, poorly ventilated microclimate. An open canopy that dries quickly gives it less chance.

Cost/effort: free to low. Keep wider spacing, water at the root and in the morning, and avoid excess nitrogen, which makes soft, dense foliage — see watering and feeding.

Prevention checklist

• Rotate for at least four years; avoid other host crops (oilseed rape, beans, lettuce) in between.
• Choose tolerant hybrids where available.
• Keep generous spacing for an open, fast-drying canopy.
• Water at the root and in the morning; avoid prolonged leaf wetness.
• Moderate nitrogen; avoid an over-dense canopy.
• Remove and destroy affected plants at once; do not compost them.

Not sure it is Sclerotinia or something else? Compare the symptoms on the overview page. Wilt without rot and without fluff is usually plain drought stress; a white bloom on the leaf without stem rot is more likely mildew. For general growing tips and soil-disease prevention, see the growing page.

The message is strict but honest: there is no cure for Sclerotinia on the plant itself, only prevention and stopping a fresh infection. In a garden the impact is luckily limited as long as you do not sow in the same spot year after year. So rotate the position, remove diseased plants at once, and keep the canopy open and dry — then white mould stays an incident rather than a recurring problem.

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