19 April 2001
Sclerotinia tipped for 10-year high

By Andrew Swallow

GROWERS are being urged to be vigilant following warnings that sclerotinia infection levels this year could be the highest since 1991.

Consulants ADAS say farmers must assess the risk from the soilborne fungus on individual sites and watch the weather from the start of flowering.

“Last year we observed twice the level of sclerotinia than for the previous two or three years,” says plant pathologist Peter Gladders.

Crops will be exposed to a high level of inoculum this year, and if it coincides with damp conditions that make petals stick to stems, could lead to serious levels.

Each infected plant loses about half its yield potential, so control only becomes economic if over 10% of plants are infected.

However, there is an argument for control at 3-5% of plants infected to prevent a long-term build up of the sclerotinia or resting bodies in the soil, he says.

But no technique has yet been developed for farm use that can reliably predict infection.

So a growers best guide to fungicide decisions is history of the disease on the farm and weather at flowering, says Dr Gladders.

If the farm has had crops with 20% or more of plants infected in the past then growers must be on their guard.

“There is a growing pool of farms in this category,” he warns. Such farms have high levels of sclerotinia in the soil.

In moist conditions, as soil temperatures rise above 10C, these germinate and release spores which settle on the petals of flowers.

If there is sufficient moisture in the canopy to make these spore-carrying petals stick to stems when they fall then infection can occur.

“Look two to three days ahead to see if rain or showers are forecast and try to pre-empt infection,” said Dr Gladders.

“If in doubt it is better to go early rather than late because the fungicides have very little kickback activity.”