The primary causal agents of root rots are fungi and fungus-like organisms from the genera Fusarium, Pythium, and Rhizoctonia. Of particular importance is Fusarium basal rot, caused by F. oxysporum f. sp. cepae, as well as pink root rot, caused by Phoma terrestris. The infection attacks the plant under field conditions and can also develop during bulb storage.
In the case of Fusarium wilt, yellowing and tip-dieback of leaves, softening of the basal plate, root necrosis, and the appearance of characteristic mycelial growth are observed. Pink root rot is characterized by the roots acquiring a specific pink hue, followed by their desiccation, which leads to the plant being easily pulled from the soil due to the degradation of the root system. Early stages of seedling infection are often classified as damping-off.
The identification of universally accepted races or specific resistance genes to these pathogens in onions has not yet been described. Breeding efforts are conducted by evaluating lines under natural infection pressure or through marker-assisted selection, and in variety characteristics, resistance to Fusarium is indicated as a complex trait.
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Root rots.