New Mexico provides around 30% of the bulk demand for onion in the United States during the months of June and July (U.S. Department of Agriculture, 2012). Abundant sunshine, high temperatures, and a relatively dry climate during summer months make New Mexico an ideal environment for onion bulb production. However, sporadic periods of rain at the time of maturity aggravate bulb decay because of various diseases (Corgan, 2002).
Black mold, caused by Aspergillus niger, is a frequent onion disease in the southwestern United States (Wall and Corgan, 1994). Every year, this disease causes yield and quality losses in the field and during storage (Wall and Corgan, 1994). Bulbs with black mold exhibit shallow lesions on dry outer scales and streaks of mycelial growth and black-colored conidia under dry outer scales (Ko et al., 2002a; Royal Botanic Gardens and Domain Trust, 2012). Eventually, the entire bulb surface turns black, shrivels, and loses quality (Royal Botanic Gardens and Domain Trust, 2012).
In nature, A. niger can be saprophytic and can reside as an endophyte until infection occurs and symptoms become apparent at harvest or later during storage (Lorbeer et al., 2002). The fungus survives in soil on decaying organic matter and infects onion bulbs via wounds (University of California, 2012). Black mold is generally a postharvest disease but can be seen on mature bulbs in fields before harvest. Onions are harvested when they are physiologically mature at 80% tops down (Wall and Corgan, 1994).
In New Mexico, onion harvesting often coincides with rainfall and higher temperatures, conducive for development of black mold. Disease incidence can vary from 11% to 50% for short-day cultivars. No resistance to black mold has been described, however (Ko et al., 2002a). This variation for disease incidence among onion cultivars indicates cultivars can be identified for lower disease incidence (Ko et al., 2002a, 2002b). New Mexico State University (NMSU) onion cultivars have not been studied for black mold resistance. The objective of this study was to screen autumn-sown NMSU onion cultivars for resistance to A. niger.
ArdizziM.C.P.AschkarG.A.PellejeroG.GilM.I.2008Effect of the storage conditions and some intrinsical properties of the bulbs on the Aspergillus niger incidence in onion (Allium cepa L.), in the lower valley of the Black River, ArgentinaRevista Pilquen Seccion Agromomia10114
CorganJ.N.2002Contributions of the NMSU breeding program to the New Mexico onion industry. New Mexico State Univ. Coop. Ext. Serv. Circ. 577
CorganJ.N.WallM.M.CramerC.S.SammisT.W.LewisB.SchroederJ.2000Bulb onion culture and management. New Mexico Coop. Ext. Serv. Circ. 563
KamalN.2007Effect of harvesting stage on onion bulb quality and screening onion germplasm for resistance to black mold. New Mexico State Univ. Las Cruces MS Thesis
KoS.S.HuangJ.W.WangJ.F.ShanmugasundaramS.ChangW.N.2002aEvaluation of onion cultivars for resistance to Aspergillus niger, the causal agent to black moldJ. Amer. Soc. Hort. Sci.127697702
KoS.S.ChangW.N.WangJ.F.CherngS.J.ShanmugasundaramS.2002bStorage variability among short day onion cultivars under high temperature and high relative humidity, and its relationship with disease incidence and bulb characteristicsJ. Amer. Soc. Hort. Sci.127848854
LorbeerJ.W.TuffleyJ.W.RansomV.E.SnoverK.L.2002The nature of Aspergillus niger as an endophytic fungus in onion plants and subsequent development of black mold. Proc. 2002 Natl. Allium Res. Conf. 11–14 Dec. Pasco WA. p. 43–45
Royal Botanic Gardens and Domain Trust2012Aspergillus. 26 June 2012. <http://www.rbgsyd.nsw.gov.au/science/plant_pathology_research/Soilborne_plant_diseases/Vietnam_template3/Aspergillus>
SumnerD.R.1995Diseases of bulbs caused by fungi—Black mold p. 26–27. In: H.F. Schwartz and S.K. Mohan (eds.). Compendium of onion and garlic diseases. APS Press St. Paul MN
University of California2012UC IPM: UC management guidelines for black mold on onion and garlic. 26 June 2012. <http://www.ipm.ucdavis.edu/PMG/r584100411.html>
U.S. Department of Agriculture2012Vegetables 2011 summary. January 2012. Vg 1-2 (10). U.S. Dept. Agr. Natl. Agr. Stat. Serv. U.S. Govt. Printing Office Washington DC