The bulbs pictured show symptoms of Botrytis neck rot caused by Botrytis aclada and B. allii. When bulbs are cut, the symptoms resemble Fusarium bulb rot. The symptoms usually first appear in the neck area, and the disease advances through the bulb (photos 1 and 2). However, with Botrytis neck rot, gray mold as well as sclerotia can frequently be seen on the outer scales (photo 3), and sclerotia can develop in the bulb, as well (photo 4). Sclerotia are hardened masses of mycelia that can survive in plant debris and soil for several years. Infections with B. aclada or B. allii occur in the field during curing or at harvest, but symptoms don’t manifest themselves until the bulbs are in storage.
Both cultural and chemical management options are available to minimize neck rot incidence. Cultural management options include ensuring onions are mature at harvest, undercutting onions, and making sure necks are dry and well cured before topping. Late-season applications or excessive amounts of nitrogen should be avoided, and onions should be stored below 40 degrees Fahrenheit. Fungicide applications during the last four weeks prior to harvest have been most effective.
