Carrot crops in some areas could be at greater risk than usual from sclerotinia infection reckons ADAS plant pathologist Dr Peter Gladders. Its effective control is vital because the industry has no surplus production capacity to compensate for the loss of yield the disease can cause.
During April and May this year the weather in the north and west wasn't conducive for the germination of sclerotia (these produce spore-bearing apothecia), he explains. So larger numbers than last year remain intact, and could germinate during the early stages of carrot growth resulting in significant infection of the crop.
But in areas where the risk of sclerotinia is lower Alternaria and powdery mildew could be a problem, maintains Peter Gladders. So all three diseases must be controlled to avoid yield loss and maintain the healthy foliage that's particularly important for growers using top lifting harvesters. In badly infected crops large patches of foliage can be killed by sclerotinia, leading to root rotting in the field if September rainfall is heavy.
"Not many years ago the carrot industry had ten to twenty per cent spare crop but not now," he says. "Every bit of crop counts and so growers cannot afford any yield loss from disease."
These are all good reasons for using a preventative spray programme based on fungicides like Signum (boscalid + pyraclostrobin), which recently received PSD approval for use on carrots. It showed strong activity against sclerotinia when used in programmes in our field experiment last year and is "probably the most effective product" for sclerotinia control, claims Peter Gladders.
"Last year we were seeing sclerotinia leaf symptoms in carrots by mid-August which meant that infection was occurring quite a bit earlier in the crop's development," he recalls. "So for good control you've got to start spraying early, in late-June or early-July, before closure of the canopy and while you can get good spray penetration."
During rapid crop growth he recommends a spray interval of two to three weeks bearing in mind that fungicide cover needs to be extended into September.
BASF trials also show that Signum will control Alternaria and powdery mildew. The recommended dose for Sclerotinia is 1.0 kg/ha and 0.75 kg/ha for Alternaria and powdery mildew.
