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Title: Detection and prediction of post harvest carrot diseases

Author
item HERMANSEN, A. - Bioforsk
item NAERSTAD, R. - Bioforsk
item Wanner, Leslie
item KLEMSDAL, S. - Bioforsk

Submitted to: European Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/2011
Publication Date: 11/18/2011
Citation: Hermansen, A., Naerstad, R., Wanner, L.A., Klemsdal, S. 2011. Detection and prediction of post harvest carrot diseases. European Journal of Plant Pathology. 133:211-228.

Interpretive Summary: Carrot is the most important vegetable crop in Norway with a market value of NOK 675 million in 2010. Diseases that develop during low temperature storage of carrots are responsible for 20 to 50% total crop losses. We developed specific methods for identifying the two most important fungal pathogens of carrots during storage, Mycocentrospora acerina, the cause of liquorice rot, and Fibularhizoctonia carotae, the cause of crater rot. We examined the relationship between the two diseases and the presence of DNA from the two pathogens in carrots and in carrot field soil samples at various times in the growing season. The amount of pathogen DNA was well-correlated with the incidence of liquorice rot after 5-6 months’ storage, but poorly correlated with the incidence of crater rot. Best results were obtained from soil adhering to carrots during the last part of the growing season or at harvest, while soil samples taken at planting or after harvest did not correlate with either disease. The DNA methods we developed for use in soil and on carrots give a good prognosis for predicting development of liquorice rot in storage, while further work on sampling strategies is necessary for accurate prognosis of crater rot.

Technical Abstract: Carrot is the most important outdoor vegetable crop in Norway with a market value of NOK 675 million in 2010. Because demand for carrots is year-round but the growing season is short, the crop is typically stored after harvest for as long as 6 months. Diseases that develop during low temperature storage of carrots are responsible for 20 to 50% total crop losses. To determine the relationship between disease incidence and presence of pathogen DNA in carrots and in soil samples from carrot fields at various times in the growing season, we have developed specific PCR methods for identifying the two most important fungal pathogens of carrots during storage, Mycocentrospora acerina, the cause of liquorice rot, and Fibularhizoctonia carotae, the cause of crater rot. We applied the PCR assays to carrot samples taken in early fall, at harvest, and in soil shaken from harvested carrots, as well as to soil samples taken in summer, fall and in spring from field plots at 5 farms in different counties and 3 growing seasons in Norway. Soil samples were also taken at different sampling densities. PCR detection of M. acerina, and F. carotae using either standard or quantitative PCR was rapid and sensitive, allowing routine detection of less than 10 pg of M. acerina or F. carotae DNA in the presence of large excesses of plant or soil DNA. PCR results were well-correlated with the incidence of liquorice rot after 5-6 months’ storage, but poorly correlated with the incidence of crater rot. Use of quantitative PCR did not substantially improve the results. The best correlation between PCR results and disease incidence was from soil adhering to the roots during the last part of the growing season or at harvest, and larger numbers of field soil samples improved results. PCR data from soil samples taken in the spring or after harvest did not show significant correlation with incidence of crater rot or liquorice rot. The low incidence of crater rot in all of the experiments may be related to uneven distribution of this pathogen in soil. The PCR methods we have developed for use in soil and on carrots give a good prognosis for development of liquorice rot in storage, while further work on sampling strategies is necessary for accurate prognosis of crater rot.