The first symptom observed is the splitting of the basal plate. Secondary growth of the affected bulb often occurs as one to several small bulbs protruding from the split basal plate.
Conditions for Disease Development
Uneven irrigation of onion fields increases the incidence of this disorder. Fields that are over-irrigated, allowed to dry completely and then over-irrigated again often have many split bulbs. This condition is more prevalent in areas of the field were stands are thin or uneven. These openings can provide an entrance for secondary microorganisms, which cause bulb decay. Bulb mites (Rhizoglyphus species) are frequently associated with bulb splitting. However, they have yet to be implicated as the causal agent.
Attention to seedbed prep, planting and the use of high quality seed will result in uniform stands, which will reduce this disorder. The maintenance of uniform irrigation and fertilization practices to prevent phases of rapid and slow growth of onion bulbs can reduce the incidence of this disease.