(17) Bulb-to-Bulb Transmission of Botrytis allii in Dry Bulb Onion Production

Abstract

Botrytis neck rot, caused by Botrytis allii, is a major storage problem in dry bulb onions. This disease has been responsible for losses exceeding 50%, resulting in financial devastation to growers. Botrytis infections likely occur around the last month onions are in the field prior to harvest and before moving into storage. Earlier studies indicated that the spread of this fungus did not occur in storage. More recent studies suggest that bulb-to-bulb transmission in storage is possible, especially when bulbs are handled roughly during harvest. `Vaquero' dry bulb onions were planted in 2003 and 2004, and in 2005 the cultivar Renegade was used. All bulbs were produced using standard commercial practices. Bulbs were hand-lifted at harvest to reduce the amount of mechanical injury, then cured for 2 weeks. To simulate impact bruising, a 1/4-lb weight was dropped through a 2-ft PVC pipe, striking a healthy bulb on the outer scale. Shatter bruising occurred by dropping healthy bulbs down a 7-ft column containing seven offset immobile metal bars. To evaluate the interaction of inoculum load with bruise injury on disease spread, healthy bulbs were surface inoculated with botrytis and incubated until visible sporulation. Twelve infected bulbs were added to onion samples immediately after bruising. The treatments were then stored under ambient conditions. In 2004 and 2005, shatter bruising caused significant increases in botrytis infections to the inside and outside of the bulb over impact bruising, and impact bruising caused significantly more infections than the nonbruised check treatments. Adding botrytis infected bulbs increased infections, but only when injury of either type was present. Shatter bruising had the most significant effect on total botrytis infection.