From Clemson Plant Pathologist Tony Keinath.
Stored carrots can rot if they are infected by one of several different fungi in the field. Among the culprits is Sclerotinia, typically known as white mold on beans, Sclerotinia stem rot and watery soft rot on brassicas, or timber rot on tomato. On carrot, the same disease is called cottony rot, shown in the photo below.
As the disease common name suggests, affected carrots look cottony, due to abundant growth of the Sclerotinia white mold. Rotted carrots feel slippery and slimy. The black nuggets are sclerotia, hardened masses of white mold growth that survive heat, cold, and dry conditions. Sclerotia in soil allow Sclerotinia to survive between crops.
White mold diseases develop when weather or crops are continuously moist and cool. Disease starts in two ways: from sclerotia in soil or airborne spores. Sclerotia germinate like seeds or other fungal survival structures and infect stems and crowns at the soil line. Sclerotia within a couple inches of the soil surface produce tiny mushrooms after a cold spell. Airborne spores from the mushrooms are spread by wind and infect above-ground parts of susceptible crops, even those planted in noninfested soil.
Note that the diseased carrots shown above were harvested in early December, before the Christmas cold snap, so the rot did not develop from sclerotia that were triggered to germinate then. That cold period, however, probably triggered germination and may be leading to Sclerotinia stem rot on overwintered brassicas, such as my kale here at CREC.
Management of white mold is challenging. Although several conventional fungicides are registered, there are limits on the number of applications. It’s difficult to predict when white mold will develop, so it’s difficult to know when to spray, or the favorable period is so long it’s not economical to spray the entire time.
Contans from Sipcam Agro is the one effective biofungicide. The active ingredient is a dried formulation of a fungus that grows on sclerotia and kills them. The product should be applied to fallow soil several months before planting a susceptible crop. Tilling soil after application will disrupt the growth of the beneficial fungus. Note also that the product should be applied while temperatures are between 50 and 80°F. This means if you are having or have had problems with Sclerotinia diseases, it’s time to order Contans and apply it soon.
Trimming lower senescing leaves to open the carrot canopy and thin the foliage by 20% was tested in Ontario. Fewer Sclerotinia mushrooms developed on sclerotia in trimmed rows than in non-trimmed rows. It is unknown if this reduction in spores would help reduce cottony rot, because no cottony rot developed in either of the two trials.
In general, keep stored produce cool and as dry as possible, without dehydrating it, to prevent postharvest Sclerotinia diseases.
For a more lighthearted look at white mold, see this recent article.