Of major concern are bacterial rots thought to be caused mainly by Burkholderia gladioli pv. alliicola (BGA) - particularly in heat-treated red Rijnsburger-type onions, neck rot caused primarily by Botrytis allii and Fusarium basal rot caused by Fusarium oxysporum fsp cepae.
One possibility is that necessary heat treatment of some cultivars may lead to increased disease risk. Project FV 392 was a first step to address these issues by determining the incidence of the major bacterial and fungal onion pathogens thought to be associated with sets of different types and establish whether there is a relationship between disease incidence in sets and subsequent problems in the harvested bulb onion crop.
The results showed that the main cause of rots in stored onion bulbs grown from sets was the bacteria BGA and the fungus Fusarium oxysporum. The neck rot pathogen, Botrytis aclada, was not detected.
There was an indication that the primary source of BGA may be sets and that heat treatment may predispose them to or exacerbate infection/disease in the harvested bulbs. Bulk testing of sets for BGA may give an indication of the risk of disease in the harvested crop.
The primary source of Fusarium in these trials appeared to be the field sites themselves, with a different Fusarium "type" responsible for disease at each site.
The final grower summary for FV 392 is available on the HDC website at www.hdc.org.uk.
Horticultural Development Company
For details on all HDC activity, visit www.hdc.org.uk