Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Plant Pathology and Microbiology

First Advisor

C. A. Martinson


Approximately 30% of US corn seed is produced in Iowa. Conservation tillage practices expose seed corn fields early to overwintering inoculum of foliar diseases in the Midwest, e.g., gray leaf spot (GLS), thus increasing the likelihood of severe disease epidemics. Information is lacking on yield losses in seed corn caused by foliar pathogens and the economics of management via fungicide applications;Field experiments relying on natural infection were conducted during 1990-1992 in commercial seed production fields to quantify the relationship between fungal foliar disease severity at different crop growth stages and yield of saleable seed, and to assess the need of fungicide use. Chlorothalonil, propiconazole, and copper thallate applied at different times and frequencies generated a range of disease severities and provided information on fungicide efficacy. Mancozeb was included in 1992 as an additional commercial control;Disease dominance varied, although the same diseases were always found. In all instances, quantifiable disease levels appeared two to four weeks after mid-silk and became severe later in the season. Kernel weight and size were the primary yield traits affected by disease. Gray leaf spot accounted for losses of 7.7% and 25.1% seed in two trials, respectively. Critical point regression models were significant only when regressing yield on disease at denting stage, and explained only 36% of the variation of yield. Yield losses from common rust (CR) in five trials ranged from 7.7% to 18.9%. Critical point models of yield on CR severity at later growth stages, i.e., mid to full denting, explained better the variation than models based on disease assessed at earlier stages. A late epidemic of Northern leaf spot (NLS) caused losses of 5.9%;Chlorothalonil sprays initiated at 16-leaf stage usually controlled all diseases and yielded better than similar programs initiated at earlier or later stages; two applications were almost as effective as three or four-spray programs in reducing yield losses. At equivalent frequencies and timing, mancozeb rendered better control of CR and NLS than chlorothalonil, although yields were not different. Likewise, propiconazole was slightly less effective than chlorothalonil.



Digital Repository @ Iowa State University,

Copyright Owner

Jose Mauricio Rivera Canales



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118 pages