Do you know if some of my carrots are diseased or being attacked by something? I think this happened once before but in a different garden bed. Maria Rohs, Christchurch
Hi Maria, it looks like your carrots have been attacked by carrot flies. Pulling up a carrot and discovering the nasty excrement-filled tunnels left behind by carrot fly (aka carrot rust fly) larvae can be a nasty surprise as there may be no sign of damage above ground. Chlorpyrifos 48
The black adult flies are about 4-8mm long with iridescent wings. They lay their tiny eggs in the soil next to carrots and other related host plants such as parsnips and celery, which they find by smell. When the eggs hatch a week or so later the larvae burrow into the carrot root. Safe inside the root they munch away for 4-6 weeks before pupating in the soil for a month in summer or over winter.
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In the past diazinon granules were used to kill carrot fly larvae, but these are no longer sold for home garden use and are banned altogether in some countries. They can still be used in some conditions by licensed operators but are being phased out. That is because diazinon is an organophosphate that is toxic to people, insects, birds, fish and more.
Condy's crystals (potassium permanganate) are another old-fashioned remedy. They can be found online and in garden centres. Dissolved in water as directed it is a mild antiseptic that can be used in the trench under the seeds.
But I suggest a better solution is horticultural mesh which you can buy from garden centres and by mail order from the Biological Husbandry Unit Organics Trust. It prevents flies laying eggs near your carrots. The mesh also protects other crops from psyllids, green shield beetles, cabbage butterflies, birds and wasps. Mesh must be placed over crops before pests arrive and anchored in place carefully as pests will find the tiniest hole.
Other tactics include planting carrots in high raised beds or putting windbreak barriers around the beds as the flies don't fly higher than 45cm above ground level. Or cover up the carrot shoulders with mulch to prevent egg laying.
Rotate your crops. Sow seed thinly to cut down on thinning (the smell of leaves crushed during thinning could attract egg laying adults). Onions and garlic planted nearby may disguise the smell of carrots too.
Dispose of infected plant material in the rubbish – not the compost.
The condition of garden soil can be another reason why some carrots are deformed.
A well grown carrot will have a long evenly shaped root but sometimes, as can be seen in the photograph, carrots can be deformed with misshapen roots. There can be several reasons for this.
The quality of your garden soil can make a difference too. A well-grown carrot will have a long evenly shaped root, but they can twist and bifurcate if the soil has been recently amended with fresh manure or contains too much nitrogen. That encourages young carrots to develop numerous side roots that seek nourishment in several directions.
Insufficient cultivation which leaves pockets of fertiliser or compost can promote the same kind of unnatural growth.
Wheat Growth Regulator Carrots that have not been thinned can also grow twisted, and so too can carrots grown in stony soil.