"Mustard looks promising," Barbara said. "It doesn't appear to reduce the amount of Pythium violae fungus in the soil but it affects its ability to cause the disease."
He said research in the past four years had aimed to increase knowledge about the biology of the disease after attempts to use "genetic fingerprints" to measure P. violae levels in soils - and so predict disease risks - had failed. He said: "We've changed our understanding of the relationship between the fungus and cavity spot."
Barbara's research has shown that P. violae grows on the carrot root surface and spreads into the soil, but does not build up in the soil from year to year. "The soil population depends on moisture and temperature, not on the amount of inoculum before planting," he said.
In field experiments, P. violae levels in soil did not increase over four years, whether cropped continuously with carrot or cropped in rotations. One correlation that Barbara did find was that disease in overwintered carrots was related to P. violae levels in the root zone in August and September, so testing then could aid decision-making about lifting and storage.
He said early fungicide applications were the most critical for control while resistant varieties could often get some disease.
Liming before drilling does not control the fungus but modifies the environment and helps reduce its ability to cause disease.
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