Study looks at cause of carrot browning

Crops may need closer monitoring after research finds browning associated with CMD viruses.

Carrot crops may need closer monitoring, following the results of an Horticultural Development Council (HDC) project suggesting internal browning in carrots is associated with viruses from the carrot motley dwarf complex (CMD).

The study concluded that internal browning did not appear to be associated with parsnip yellow fleck virus (PYFV) - a semi-persistent virus carried by aphids and against which most current aphid control measures in carrots are aimed.

Instead it found that internal browning appeared to be associated with the presence of CMD viruses - most commonly in combinations including carrot mottle virus (CMoV).

Researchers were prompted to carry out the project, named 'Symptomatic survey of virus complexes', after growers began to experience significant crop losses from the condition.

Carrot crop advisers found that the condition was evident in regional virus "hot spots" and appeared in addition to the text book foliar symptoms and root stunting.

The HDC project therefore aimed to determine the extent of the problem and which viruses were associated with it.

Root crops consultant Howard Hinds said the project's finding was significant because of the persistent nature of the CMD viruses.

He said: "Aphids carrying them (CMD viruses) remain infectious throughout their lives. So aphid control programmes - to control both CMD and PYFV - will have to be modified. The period of vigilance will need to be extended, with more monitoring of aphid activity for integrated pest management-based decision-making."

TRIALLING PRODUCT EFFECTIVENESS - Achieving more robust control of aphids

Anticipating carrot motley dwarf complex (CMD) as the culprit, root crop consultant Howard Hinds last year tried a new approach to achieve more robust control of aphids.

During the early season period of peak aphid activity - early May to late June - he used two sprays of the systemic anti-feedant Bayer CropScience Biscaya (thiacloprid), which had recently gained a specific off-label approval for carrots. These were alternated with Dovetail (pirimicarb + lambda-cyhalothrin) or a mix of pirimicarb + deltamethrin.

From the usual mid-season 'crash' in aphid numbers in mid-July, Pyrethroid sprays targeted carrot root fly activity up to mid-September and provided incidental control of any remaining aphids. Thereafter, Hinds monitored closely for aphid resurgence.

He said: "Carrot willow aphid activity can pick up again in a warm September and continue to the end of October, creating a late season CMD risk period."

When late spikes in aphid activity were detected he resumed control using pirimicarb. "It significantly reduced the incidence of root necrosis symptoms and returned marketable yields to what they should be," he says.

This year, he plans to deploy the two Biscaya treatments at the first or second and third or fourth spray timings.

Biscaya now has a full label approval for control of carrot-willow aphid in carrot and parsnip crops.

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