Have you ever wondered why your fodder beet crop, that looked green and healthy before Christmas, turns yellow in January and February? Or why your brassica crops start to turn slightly purple on the leaf edges in the late summer or early autumn?
Quite often farmers will turn straight to the fertiliser spreader to correct these perceived nutrient deficiencies. While that may be the correct decision, it would be prudent to take a leaf tissue test to determine the cause. In most cases, providing sufficient nutrients were applied early in the crop’s life cycle, the yellowing in fodder beet or purple/yellow leaf margins in brassicas is more likely to be a virus spread by aphid vectors especially Myzus persicae or green peach aphid.
Several viruses cause leaf distortion, plant dwarfing and yellowing or reddening of lower leaves on fodder beet, brassicas and potatoes.1 Plant’s show initial symptoms 12 to 35 days after inoculation by the aphids. Yellowing or purpling with brassicas develops in older and middle-aged leaves as a mild, chlorotic spotting of interveinal areas near the leaf tips.² As the virus progresses, the discoloration intensifies. With fodder beet, the infected leaves develop a golden yellow coloration and become brittle. Ultimately, the entire leaf will become yellow and is often
attacked by secondary fungi, such as Alternaria, that will destroy the leaf.¹
The main virus present in New Zealand is Beet Western Yellow Virus (BWYV) which is found in weed species, hedgerows and volunteer crop plants and is difficult to control due to its wide host
range and persistent aphid transmission.³ Aphids will overwinter at these sites as eggs before hatching in the spring to produce stem mothers that give birth to female aphids. These aphids suck up the virus from their winter site and then land in newly sown crops to feed where they can be found on the underside of the leaves.
During this feeding, the aphid will wash out its stylet with saliva and transmit the virus.³ Transmission is persistent with the aphid vectors able to transmit the virus for up to 50 days and only need to feed from a plant for five minutes to transmit the disease.³
The best defences against BWYV
» Insecticide seed treatments.
» Later drilling to avoid aphid flights.
» Avoiding host plant cover crops between drillings such as beet, radish, peas, potatoes and wild or weed crucifers.
» Adequate rotations.
» Crop hygiene with the removal of bolters, weed beet and a good herbicide strategy.
» Timely insecticide applications using alternating modes of action to help prevent resistance developing.
» Use of Integrated Pest Management, with lady birds and lacewings a good way of controlling aphids.
To discuss controlling viruses in brassica and fodder beet crops, contact your local PGG Wrightson Technical Field Representative.