Stop rot in salads! Crop rotation stalls carrot fungus

Carrots are a staple vegetable, found in most salads and filled rolls. The crisp carrots that we see every day only get to the table after a long battle against a bunch of pathogens.

Alternaria radicina is an ascomycete fungi that infect carrots. It can infect all parts of the carrot plant above and below ground. This fungus may cause severe damage in a crop, including damping-off of petiole, water-soaked lesions on seeds, dying back of crown and root, seed blight, reduction in size of the seeds and infection of seedlings.

Sunken black rots on carrot caused by black rot, Alternaria radicina ( CC BY-SA 3.0

Alternaria radicina is present in all parts of the world where carrots are grown and was first reported in Denmark in 1888. The fungus probably entered New Zealand in 1968-69 through infected carrot seed lots imported from other countries where the pathogen already existed. How do these pathogens affect our life and economy?

As the human population rise, so does the demand for food. The pathogen acts as a major problem for carrot growing farmers all over the world as it reduces the rate of carrot seed germination and infects both roots and foliage. This leads to a huge drop in their yield and affects the export industries. From a study conducted by Scott and Wenham in New Zealand, 22% of carrot seeds are affected by Alternaria radicina. How can we overcome these losses?

Spread of the pathogen can be controlled by practicing methods like plowing, removing unwanted and affected crops, application of fungicides, soil fumigation, cultivation of resistant varieties, crop rotation and sprinkle irrigation. How do these methods work?

Alternaria radicina affect areas in the world. (© OpenStreetMap contributors, © OpenMapTilesGBIF.)

Plowing reduces the intensity of the pathogen at initial stages of crop establishment but this method doesn’t show any long-term effect on final crop infestation. Fumigation, using metam sodium, can delay fungal production in the soil and so reduce infestation. However, the use of fungicides makes the crop harmful for human consumption.

Crop rotation is the practice of cultivating a series of dissimilar or non-host crops in the same area over various seasons. Crop rotation is one of the best methods to control the spread of pathogens as they can survive in the soil for more than 8 years without the presence of a host plant. Which crops can be grown in rotation with the carrot crop? How much time is needed between two carrot growing periods to eliminate the pathogen from the soil?

Plowing of the soil before planting the crop, Keith Weller, U.S. Department of Agriculture,, public domain (CC0)

In 2012, Hampton, from Lincoln University, and his colleagues studied the impact of crop rotation on the carrot pathogen Alternaria radicina. They conducted two experiments, a field survey experiment and a glasshouse experiment, both in the mid-Canterbury region of New Zealand.

The study measured the viability of the pathogen Alternaria radicina in New Zealand soil and identified an effective way to control the spread through soil medium. They also investigated whether the cultivation of crops, like wheat, barley, and ryegrass, had any impact on the pathogen population.

In the field survey experiment, they collected soil samples from 12 different fields where carrots had been cultivated between 1995 and 2006 and had been infected by the pathogen. They also collected the subsequent cropping history of those fields after carrot cultivation. Alternaria radicina was not detected in soils from all fields where carrot seed crops had been harvested between 1995 to 2000. However, the pathogen was present in soils from fields where a carrot seed crop had been harvested from 2001 to 2006. They concluded from their experiment that the pathogen population is higher in the field where carrots were cultivated recently.

Their other experiment was a greenhouse experiment where the silt loam soil was collected from mid-Canterbury fields where carrots had not previously been grown. Crops like wheat, barley, faba, ryegrass and pea were grown on the collected sample soils. Pathogens were added to half the plots. The samples were then left to grow for about four and a half months with all necessary support. Soils in which wheat, barley and faba bean were grown had a lower pathogen population, but those soils that had ryegrass and pea grown in them did not differ much from the pot kept as control.

From the studies it was concluded that the best method to control pathogen spread is by crop rotation as it reduces soil-borne infection of the pathogen when crops like wheat and barley are alternately grown with carrot seeds. Further investigation need to be conducted in full field conditions to confirm the effect of crop rotation and growing non-host crops to reduce the pathogen in carrot crops.

Liya Martin is a postgraduate student completing a Master of Pest Management. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.

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