Not just a storage problem

Dan Woolley
June 24, 2013
By Dan Woolley
Jun. 24, 2013 - University of Prince Edward Island student Michelle MacDonald has observed a new problem in the Island's carrot crop caused by a familiar fungus – Fusarium.

Crown rot is typically a storage disease of carrots. But MacDonald discovered an outbreak of the disease in the field while working as an Agriculture and Agri-Food Canada (AAFC) field researcher.

According to MacDonald, the fungus first started appearing in P.E.I. carrot plantings in 2011, resulting in rejection rates as high as 60 to 70 per cent of the crop at grading. While some carrots were salvaged by cutting them into smaller sizes, the majority were unmarketable as regular sized vegetables.

Crown rot traditionally appears after harvest, triggered by the Fusarium fungus, said MacDonald. Fusarium can overwinter in the soil and enters the carrot through cracks in the crown of the vegetable.

Initial studies identified F. acenaceum and F. oxysporum as the fusarium populations infecting the P.E.I. carrots. F. acenaceum was highly pathogenic to carrot tissue while F. oxysporum was less sensitive.

During her research, MacDonald tested two Fusarium pathogens: 10 isolates of F. avenaceum and four isolates of F. oxysporum. Each isolate was tested – in replicated trials – against five different fungicides: Bravo (chlorothalonil), Manzate (mancozeb), Polyram (metiram), Pristine (pyraclostrobin/boscalid) and Quadris Top (azoxystrobin/difenoconazole).

Nine different treatment applications were used (eight with fungicides and one control) during the trial and there was also monitoring for the occurrence in the carrot crop of Sclerotinia (white mold) and two leaf diseases – Alternaia leaf blight and Cercospora – MacDonald said.

While it was observed that all the treatments decreased Sclerotinia and provided foliar protection from Alternaia leaf blight and Cercospora, there were no treatments that significantly decreased crown rot in the field, she said.

In the lab, both Fusarium species proved insensitive to Mangate. Quadris Top had a big range of response in the lab, with greater sensitivity in F oxysporum. F avenaceum was insensitive in fungal response trials in the lab to Bravo; but F. oxysporum showed a little bit of sensitivity. Pristine and Polygren have yet to be tried for fungal response in the trials. MacDonald stated that in the lab, F avenaceum showed some sensitivity to Fludioxonil, although F. oxysporum did not. Both Fusarium funguses demonstrated a sensitivity to Thiabendizole in the lab trials.

Difenoconazole was predicted to be a successful treatment, but the lab results showed otherwise, said MacDonald.

Scholar demonstrated an effect on white mold, but MacDonald cautioned there are concerns with resistance.

She added that the crop can be scouted for crown rot but, after the infection appears, it is too late to spray. A preventative spraying program before infection appears would be the best management option.

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