Violet Root Rot (Rhizoctonia Crocorum): Carrot Disease & Prevention Techniques

Violet Root Rot And Its Effect On Carrot Crops

A carrot suffering from violet root rot (Rhizoctonia Crocorum)Violet Root Rot is a fungal disease caused by a variation of Rhizoctonia Crocorum, which severely affects carrot crops the length and breadth of New Zealand. This is most commonly seen in the Ohakune region, where it continues to threaten the viability of carrot crops and their growers.

The disease itself often starts out as a superficial purplish rot, which can also be seen in the carrot’s root threads once the carrot has been lifted from the ground. 

From here, the disease often progresses through increasingly severe stages before rotting the carrot completely.

Violet Root Rot tends to favour older carrot crops, mostly occurring within waterlogged, acidic soils in warmer conditions that are conducive to the cultivation and exacerbation of the disease. 

The disease itself tends to be spread as a contaminant on various seeds, plant matter, water, tools and other farming equipment. Surviving in the soil as Mycelium or Sclerotia, it also lives on in susceptible crops and a variety of weed hosts. 

Symptoms & Signs Violet Root Rot May Be Affecting Your Carrot Crops

Signs of violet root rotThe external signs of Violet Root Rot affecting Carrot crops are limited. Which means it often appears far too late for any preventative action (such as with soil fumigation) to be taken other than the removal and destruction of the affected crops.

Violet Root Rot often affects patches – rather than the entire field or crop – where it can initially be identified in a stunting, yellowing and wilting of the above-ground foliage of the Carrot crops. Eventually, these blacken and die off completely. 

Below ground? The signs of Violet Root Rot are far more apparent. The fungus that causes this disease – H Purpureum – produces a thick, violet coloured mycelial mat which covers the underground portions of the crop. It’s within this mat that the small, dark Sclerotia live. From here, they lead to an infection within the carrot crops themselves.

While Violet Root Rot doesn’t actually penetrate too deeply into the carrot itself, it’s the damage caused by this soil based disease that often leaves crops susceptible to other diseases that can enter via these wounds. Leading to secondary diseases and rots – such as wet rot – which are often responsible for finally killing the affected crop.

Images via WSU Vegetable Pathology Program & Saint-Jean-sur-Richelieu 

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