Minimising postharvest losses of carrots

Diseases

Sclerotinia rot

Sclerotinia rot is caused by the fungus Sclerotinia sclerotiorum which can occur both in the field and in storage. The major sign of the disease is the presence of white fungal growth and small black grains (sclerotes), which look similar to mice droppings. Roots become covered with white cottony fungal growth – see Figure 1.

Figure 1 Carrot infected with Sclerotinia

Control

To avoid problems with sclerotinia rot at the postharvest stage, control the disease in the field.

Other fungi such as grey mould (Botrytis cineria), crater rot (Rhizoctonia species) and black rot (Alternaria radicina) have been reported to cause postharvest losses in carrots, but these are usually prevented by using good postharvest handling and storage.

Bacterial soft rot

Bacterial soft rot is caused by the bacterium Pectobacterium carotovorum (formerly Erwinia carotovora). Affected carrots develop soft lesions (Figure 2) and eventually become a mushy and slimy mass of tissue.

Stored carrots can become affected on any part of the root and, in severe cases, an unpleasant odour is present. The tips of carrots are often affected.

Figure 2 Bacterial soft rot infected carrot

The bacteria are found principally in soil and the disease becomes apparent under conditions of high soil temperature and moisture. In Western Australia, bacterial soft rot is seasonal — it is most severe under warm, wet conditions (summer and autumn) and is usually not a problem in cooler weather.

The carrot tissue, once decayed, may become infected by secondary organisms. The presence of these secondary organisms makes accurate diagnosis by pathologists difficult.

Control

• Chlorination is only recommended if carrots break down, because the disease occurs only under certain conditions. Management practices in the field and weather conditions probably predispose carrots to bacterial soft rot.
• A final rinse or spray with chlorinated water (50 to 200mg/L available chlorine and water pH 7.0 to 7.6) after washing, at entry to the grading line, will reduce bacteria.
• Use sodium or calcium hypochlorite as the source of chlorine.
• Chlorination should take place after dirt and debris are removed from the carrot root. Chlorine quickly loses effectiveness when soil, leaves or diseased roots are present in the water, so check chlorine concentration and pH often.
• Washing water should be changed as often as practical.
• There are limitations to chlorination. Chlorination acts as a preventative measure only; if there is already disease, decay or injury in the field, chlorine will help limit spread of soft rot in storage.
• Do not leave harvested carrots in bulk bins in the heat, because respirational heat in the middle of the bins will encourage disease development.
• Avoid harvesting during the heat of the day. High temperatures accelerate the rate at which disease organisms grow.
• Remove field heat from carrots by cooling as quickly as possible. Soft rot usually occurs when carrots have not been cooled adequately or cooling has been delayed.
• Minimise bruising and mechanical damage that allow entry of soft rot bacteria.

Black root rot

Black root rot is seen as blackening on the surface of the carrot. This blackening is often circular in outline, is superficial and has a sooty appearance. Either Thielaviopsis basicola (Chalara elegans) and/or Chalaropsis thielavioides cause black root rot (see Figure 3).

The fungi that cause black root rot have a wide host range and occur in soil. Carrots are likely to be contaminated with these fungi in the field before harvest.

Figure 3 Black root rot

After harvest, these fungi infect carrots through wounds or abrasions. They develop most rapidly on carrots stored at 25^oC and high humidity. Black root rot often develops in pre-packed carrots kept at room temperature and when moisture is retained within the plastic bag.

Losses caused by black root rot occur during periods of hot weather when there are problems with rapid cooling of harvested carrots and maintaining the cool chain. Losses can be minimised by rapid cooling of harvested carrots and storage at 0^°C.

Minimise mechanical damage during harvesting and in the packing shed and maintain packing shed hygiene.