Science into practice - how crop rotations affect carrot cavity spot

Cavity spot remains a significant disease for carrot growers and it is hoped that a better understanding of it may lead to finding novel solutions for its control. Genetic fingerprint tests developed in Norway have now been adapted to detect the fungi that cause cavity spot (mostly Pythium violae in the UK) in soil and carrot tissue.

Further work at Warwick HRI has led to a test that can identify and measure the quantities of fungi present enabling researchers to find out much more about what influences the disease in the field.

HDC studentship project CP 46 is being funded to examine how crops grown before carrot in the rotation influence disease, the effects of companion planting, whether other crops or weeds influence P. violae and whether soil characteristics can affect survival time for disease spores. The project is ongoing, with the final report due next year, but interesting field trial results have already emerged.

Rotations have so far proved ineffective in suppressing cavity spot disease, but disease levels were low and the third season results are awaited. However, companion crops reduced disease by up to 80 per cent - Phacelia, leek and grass appear the most effective, but their direct competition reduced carrot yield. Competition was overcome by establishing companion plants six weeks after the carrot crop, but then there was no control of cavity spot.

For field plants, there was clear detection of P. violae in root tissue of wheat, barley, beetroot, Tagetes and black nightshade. So weeds and non-carrot crops may act as hosts to cavity spot and perpetuate it in the soil, but it does not build up in the soil in the way most soil-borne pathogens do.

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