The headline conclusion of an HDC project is that internal browning in carrots is associated with viruses from the Carrot Motley Dwarf complex (CMD).
Over recent seasons carrot crop advisors have experienced regional virus hot spots with damaging consequences. In addition to the textbook foliar symptoms and root stunting they have been finding roots with internal and external necrosis leading to significant yield loss. HDC project FV 382 ‘Symptomatic survey of virus complexes’ was initiated to determine the extent of the problem and which viruses are associated with it.
Current aphid control measures in carrots are aimed primarily against Parsnip yellow fleck virus(PYFV),which is a semi-persistent virus lost by the aphid between moults. However, the HDC survey found that internal browning does not appear to be associated with PYFV but with the presence of CMD viruses, most commonly in combinations including Carrot mottle virus (CMoV).
Root crops consultant Howard Hinds says this is significant because the CMD viruses are persistent. “Aphids carrying them remain infectious throughout their lives. So to control both CMD and PYFV, aphid control programmes will have to be modified and the period of vigilance will need to be extended along with more monitoring of aphid activity for IPM-based decision-making.”
Anticipating CMD as the culprit, last year Howard tried out a new approach aiming 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 Biscaya (thiacloprid) which had
recently gained a SOLA for carrots. These were alternated with Dovetail (pirimicarb + lambdacyhalothrin) or a mix of pirimicarb + deltamethrin.
From the usual mid season ‘crash’ in aphid numbers in mid July, pyrethroid sprays targeted at carrot root fly activity up to mid-September provided incidental control of any remaining aphids. Thereafter he monitored closely for aphid resurgence.
Focus on CMD to control carrot root necrosis “Carrot-willow aphid activity can pick up again in a warm September, albeit with lower numbers, and continue to the end of October creating a late season CMD infection risk period.” When late spikes in aphid activity were detected he resumed control using pirimicarb.
Summing up last year’s results Howard says this new approach worked well. “It significantly reduced the incidence of root necrosis symptoms and returned marketable yields to what they should be.” This year he plans to deploy the two Biscaya treatments at the first or second and third or fourth spray timings for greatest impact on peak aphid activity.” Biscaya now has a full label approval for control of carrot-willow aphid in carrot and parsnip crops.