Rotted Carrots: Not A Pretty Picture

Scleretinic Rot of Carrots

Sclerotinia Rot of Carrots

Sclerotinia rot of carrot (SRC), caused by Sclerotinia sclerotiorum, is one of the most economically important diseases of carrots. Losses to this disease can occur from pre-harvest epidemics, originating in the field, or from post-harvest outbreaks caused by infected carrots entering storage. Post-harvest outbreaks are particularly devastating, with storage losses of some commercial growers reported as high as 50% in Canada.

“Typically, in Prince Edward Island (PEI), where about 300 ha of carrots are grown, losses in storage due to SRC occur in most years. In 2006, losses due to SRC cost carrot growers in PEI approximately $500,000. In 2007, losses in storage of approximately 25% (valued at $350,000) occurred, and yet, 2007 was not considered a year with high disease pressure. After witnessing the serious disease pressure and crop losses occurring at the local vegetable co-op first hand, our research teams initiated a program to examine potential post-harvest control options for SRC in storage.

Based on the results of our studies, fludioxonil (Scholar) was found to be an excellent tool for managing SRC in storage. …In 2008 and again in 2009, Scholar 50WP (a product currently registered in Canada for post-harvest control of some fruit diseases) received emergency registration in Canada for post-harvest use by carrot growers for control of SRC in storage. Feedback from growers in PEI indicated that the post-harvest use of fludioxonil in 2008 prevented the type of losses due to SRC experienced in previous years.”

Peters, R. D., et al.
Affiliation: Agriculture and Agri-Food Canada.
Title: Post-harvest Application of Fludioxonil for Control of Sclerotinia Rot (Sclerotinia sclerotiorum) of Carrots in Storage.
Source: Carrot Country. Winter 2009. Pgs. 12-15.

Fungal Spots on Carrots are not Acceptable to Consumers

Carrot Cavity Spot

Carrot Cavity Spot

Cavity spot is a common disease of carrots grown in California. The disease results in a small fungal spot on the carrot, which can be easily trimmed away by consumers. However, this “cosmetic” defect is not acceptable to consumers. If growers do not control cavity spot in their fields by using fungicides, their carrots would have to be sorted manually to remove the damaged ones. The sorting costs are prohibitively high; fields with high levels of cavity spot are simply not harvested…… because of consumer standards.

“Out of the 62,000 acres of carrots grown in California, cavity spot is one of the top three problems that farmers need to worry about. Unlike other vegetable diseases and pests, cavity spot doesn’t cause direct yield losses. It does, however, cause cosmetic damage, which in the world of carrots, is equivalent to yield loss.
Cavity spot is a chronic problem and even growers with the best managed fields know that,” says Jim Farrar, a professor of plant pathology in the Department of Plant Science at California State University, Fresno.
Farrar, who has studied cavity spot for the past several years, along with UC Davis researchers Joe Nunez and Mike Davis, says that most growers can accept some level of cavity spot damage in their fields. But fields with even 5 percent cavity spot damage can cause a real problem.
“The cost of hand-culling carrots in the packing shed can be more than the value of whatever carrots there are to save,” Farrar says, “I’ve seen fields where growers have totally had to walk away from entire crops.”
To deal with cavity spot, most growers apply fungicides three to four times throughout the carrot growing season.”

Author: Lieberman, L.
Affiliation: Writer, Carrot Country.
Title: New Materials and Techniques for Treating Carrot Cavity Spot.
Source: Carrot Country. 2012. Fall:13-14.

Carrots are Unharvestable Without Fungicide Use

Unharvestable Carrots

Harvesting Carrots

Alternaria leaf blight is the most common foliar disease of carrot. Under optimal conditions, severe foliar epidemics rapidly develop, leading to loss of foliage and reduced yields. Alternaria also indirectly reduces yields by interfering with mechanical carrot harvests. Leaves weakened by blight break off when gripped by a mechanical harvester and the roots are left behind in the ground.

“Because high humidity and frequent rainfall or irrigation is common during the growing season, yield-threatening foliar blights are a recurring problem for carrots. …Michigan growers harvest carrots mechanically and weakened foliage can disrupt harvest due to carrot tops breaking off during lifting. In situations where foliar disease is severe and not controlled, the tops may be compromised to the extent that the crop cannot be harvested. Therefore, fungicides currently play a critical role in the management of foliar diseases.”

Author: Hausbeck, M.K.
Affiliation: Michigan State University, Department of Plant Pathology
Title: Carrot Disease Update
Source: Carrot Country. 2012. Summer:6-8.