General
Black spots on cold-stored carrots are a major problem and can lead to significant postharvest losses Several different fungal pathogens cause similar symptoms, which are collectively known as black spot diseases. When carrots are stored during winter, diseases on the carrot roots can continue to develop during storage. Alternaria radicina, A. dauci and Rhexocercosporidium carotae (syn. Acrothecium carotae) have been identified as the most prevalent black spot pathogens in Northwestern Europe.
Life cycle and appearance of Root rot of carrots
The fungus of R. carotae has delicate, simple morphological features hence is not easy to place it at the generic level. Årsvoll was the first to identify and describe the fungus in Norway, in 1965.
An infection can occur through the soil and the air. The fungus develops at temperatures ranging from -3°C to 25°C, with an optimum around 18°C. Lesions are dark brown to black, moist, mostly circular spots 0.5 -1.2 cm diameter that extend only to 2 mm in the root, up to 20-25 per carrot root. They occur haphazardly. The fungus sporulates on root, leaf, and petiole lesions.
Continuous light is best for in vitro sporulation. Spore germination in vitro requires free water. The pH for growth, sporulation, and spore germination ranges from 3.4 to 7.1 (optimum 4.2–4.7). This affinity for acidic conditions may explain the concentration of this disease in the rather acidic soils (of southwestern Norway). Spore germination was inhibited in nonsterile soil but not in similar soil that had been steam sterilized. Spore germination in nonsterile soil was proportional to the proximity to carrot rootlets. Conidia on leaflets produced a germ tube, an appressorium and an infection peg into an epidermal cell, and then intracellular and intercellular mycelium. Conidia buried in nonsterile soil remained viable at 0°C and 3°C for 12 months, at 6°C for 6 months, and at 12°C for 2 months, indicating a capacity to overwinter in cold regions.
Inoculum can survive in perennial (umbelliferous) weeds or alternate hosts and transfer to annual crops. As far as is known, Daucus carota (carrot) is the only host plant of the fungus.
Damage symptoms
Rhexocercosporidium carotae is a slow-growing fungus that can infect both roots and foliage. The infection starts with small black spots over the entire surface of the carrot. These spots gradually change into larger dark brown to black lesions of various shapes. The lesions can merge and eventually cover the entire surface of the carrot. The affected areas are firm, shallow, and clearly visible. Under certain conditions, an olive-green mycelium can develop on the surface.
Initially, the disease forms circular to irregular spots on the leaves that vary in colour from grey to almost black. The spots appear to be sharply defined, but under a magnifying glass it appears that the infected area radiates into the healthy tissue. The spots can increase in number and size until they merge, after which whole sections of the leaf will die off. The fungus appears to prefer older tissue.
In the field, the disease is difficult to distinguish from both Cercospora and Alternaria dauci. Cercospora prefers younger leaves. A. dauci starts mainly along the leaf edges. Rhexocercosporidium infection can cause loss of seedlings. The subsurface parts turn dark brown to black, while the cotyledons remain green. During humid spells the surviving plants exhibit by necrotic leaf spots
Damage symptoms
How to prevent Root rot of carrots
Studies have shown that 3several key factors influencing the occurrence of blemishes and the presence of R. carotae in carrot roots:
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Mechanical Damages: Pathogenic fungi often enter through wounds formed during harvest. Pre-storage treatments to harden the periderm and heal wounds may help control infections, but preventing contamination is more effective.
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Topping Efficiency: Mechanical harvesting always causes some damage to carrot roots, which can be infected by R. carotae. Pre-storage treatments to harden the periderm and heal wounds may help control infections. Additionally a strong foliage and uniform stand allow better functioning of mechanical harvesters, leading to less mechanical damage of the carrots.
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Temperature: R. carotae develops at temperatures ranging from -3°C to 25°C, with an optimum around 18°C; temperature affects both host defense processes and fungal pathogenesis.
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Calcium Content: Higher calcium content of the roots reduces disease severity by strengthening cell walls and aiding in early signal transduction in host-pathogen interactions.
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Presence of Umbelliferous Plants: Inoculum can survive in perennial (umbelliferous) weeds or alternate hosts and transfer to annual crops.