The cottony rot disease is caused by the fungal pathogen, Sclerotinia sclerotiorum and is becoming a serious economic disease in carrots.
The key characteristic of S. sclerotiorum is the production of black resting structures (sclerotia) and white, fluffy fungal growth on the surface of the infected tissues.
It is also known as watery soft rot, sclerotinia soft rot or crown rot.
The disease is favoured by high soil moisture and a dense canopy. In warm and moist soils within a crop canopy, the sclerotia germinate and produce apothecia, which produce millions of microscopic ascospores that get disseminated over long distances with wind and rains. When the ascospores land on wet leaves they cause infection. Foliage infection is often followed by crown infection.
Most of the sclerotia germinate directly producing mycelial structures which can infect the crop residue, carrot leaves and crowns.
Under favourable conditions the leaves may turn brown and die-off and damage to foliage tops prevents mechanical harvest of such carrots.
A few weeks after infection black sclerotial bodies develop on old infections. These infected carrots may often not show any significant symptoms by the time of harvest and may be missed during grading for long term storage.
In storage the disease development starts from infections initiated in the field, from un-cleaned pallet boxes or storage walls. High humidity in the storage allows the white mycelium of the pathogen to grow rapidly and infect carrots over large areas. During storage many of the infected carrots develop sclerotial bodies, and are diagnostic.
Infected carrots are prone susceptible to secondary infections e.g. soft rot bacteria and lead to greater losses.
SIGNS & SYMPTOMS
Disease attack can occur at any stage of growth, and extensive root decay may occur before symptoms of wilt and collapse appear on the upper part of the crop.
Infection is often accompanied by a characteristic cottony, white mycelium that appears on the surface of the infected tissues. On or inside the white mycelium appear black, round-to-irregular-shaped structures called sclerotia, which are survival structures of the fungus.
In the storage, the white mycelium of the pathogen grows rapidly and infects carrots over large areas.
The fungi Pythium or Rhizoctonia may also produce cottony growth, but do not produce sclerotial bodies in the mycelium. Cottony soft rot is usually soft and watery compared to Rhizoctonia rot, which tends to be firm and dry. Bacterial soft rot tends to be slimy and smelly and is often secondary to cottony soft and other fungal rots.
Systemic fungicides are recommended for an effective control of cottony soft rot disease. These include the following;
- GEARLOCK TURBO 250WP
- MILLIONAIRE 690WDG
- TRINITY GOLD 452WP
- PYRAMID 700WP
- COLONIZER 440WP
- CHANCETYL ELITE 800WDG
- DOMAIN 250EC
- GREEN COP 500WP
- Crop rotations, i.e. grow non-susceptible crops e.g. onions and cereals for about 2 years before planting susceptible crops like carrots, beans, crucifers etc.
- Plant resistant/tolerant cultivars.
- Use disease free planting materials.
- Clean and disinfect storage bins.
- Ensure correct storage conditions, i.e., a temperature near freezing and humidity of 85- 95%.
- Improve air movement within the crop canopy to prevent conditions favourable for disease development, e.g. by effective weed control, increasing row-row spacing, growing on ridges or raised beds.
- Maintain field hygiene/sanitation.
- Incorporate all foliar sprays with INTEGRA 3ml/20l; a sticker, spreader and penetrant which improves the efficacy of the fungicides.
- To prevent resistance build up by the pathogen, alternate various classes of fungicides through the crop season.
- Ensure timely disease control and maintain a proper crop nutrition.