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Title:Evaluation of race population distribution, fungicide sensitivity, and fungicide control of Exserohilum turcicum, the causal agent of northern leaf blight of corn
Author(s):Weems, Japheth Drew
Director of Research:Bradley, Carl A
Doctoral Committee Chair(s):Bradley, Carl A
Doctoral Committee Member(s):Bohn, Martin; Babadoost, Mohammad; Miller, Andrew; Villamil, Maria
Department / Program:Crop Sciences
Discipline:Crop Sciences
Degree Granting Institution:University of Illinois at Urbana-Champaign
fungicide resistance
Demethylation inhibitor
Quinone outside inhibitor
northern leaf blight
northern corn leaf blight
qualitative resistance
Ht genes
Abstract:Northern leaf blight (NLB) of corn, caused by Exserohilum turcicum, is a yield reducing foliar disease common across the north central U.S. Previous race population distribution studies identified five physiological races present in the U.S., prior to 1995. For this study, 156 E. turcicum isolates were screened on corn differential lines containing Ht1, Ht2, Ht3, Htm1, and Htn1 resistance genes. Isolates were collected from fields in Illinois, Indiana, Iowa, Minnesota, North Carolina, Ohio, and Wisconsin, including: 143 isolates collected between 2007 and 2014; and 13 isolates collected between 1979 and 1985. Twenty different physiological races were observed based on the resistance response of the differential corn lines. Exserohilum turcicum race 0, 1, 1mn were the most prevalent races, comprising 21%, 27%, and 13% of the 156 isolates, respectively. Race populations were diverse within states and years. Virulence to multiple Ht resistance genes within individual isolates was observed in 47% of those tested, with 3% of the isolates conferring virulence to all Ht resistance genes. Virulence to the Ht1, Ht2, Ht3, Htm1, and Htn1 resistance genes was present in 64%, 20%, 18%, 32%, and 27% of the E. turcicum isolates, respectively. Virulence to Ht resistance genes was fairly evenly distributed across states in isolates collected after 2008. Ht2, Ht3, Htm1, and Htn1 virulence decreased after 2010. Variations in race population diversity are difficult to explain without knowing the level of selection pressure present in fields, and information regarding Ht resistance gene deployment in commercial varieties is not publicly available. While virulence was observed for all Ht resistance genes, qualitative Ht resistance genes could be used in conjunction with quantitative resistance to increase NLB control. Demethylation inhibitor (DMI) fungicides have been labeled for corn use since the early 1990s; in this dissertation a study was conducted to measure E. turcicum baseline sensitivity to DMI fungicides and monitor shifts in sensitivity over years. Metconazole, propiconazole, and prothioconazole are DMI fungicides commonly used to control NLB. Monitoring for shifts in DMI sensitivity in E. turcicum populations is important for making management decisions and maintaining fungicide efficacy. Sensitivity to metconazole, propiconazole, and prothioconazole was determined for E. turcicum isolates collected prior to DMI use on corn (baseline group) and E. turcicum isolates collected in 2009, 2010, 2011, 2012, and 2014. An in vitro mycelial growth assay was used to determine the effective fungicide concentration at which 50% of the fungal growth was inhibited (EC50) for each isolate-fungicide combination. Baseline EC50 value lsmeans for metconazole, propiconazole, and prothioconazole were 0.032 µg/ml, 0.060 µg/ml, and 0.254 µg/ml, respectively. When lsmeans of EC50 values for 2009, 2010, 2011, 2012, 2013, and 2014 E. turcicum isolates were compared to the lsmean of the baseline E. turcicum EC50 values, no significant (P < 0.05) shift towards reduced sensitivity was observed in metconazole, propiconazole, or prothioconazole. Three isolates had EC50 values significantly higher (P < 0.05) than the least sensitive baseline isolate for metconazole, and one isolate had an EC50 value significantly higher (P < 0.05) than the least sensitive baseline isolate for propiconazole. These isolates will require further evaluation to determine if they demonstrate reduced field sensitivity. Small but statistically significant (P < 0.05) positive correlations were found between metconazole and propiconazole (r = 0..3269), as well as metconazole and prothioconazole (r = 0.0.0295) but not between propiconazole and prothioconazole. Positive correlations between metconazole and the other fungicides suggest the potential for cross-resistance between these DMI fungicides. To date, no loss of NLB control has been observed with the use of metconazole, propiconazole, and prothioconazole in the field. Fungicides containing quinone outside inhibitor (QoI) and demethylation inhibitor (DMI) active ingredients alone or in combination are frequently applied to control NLB. Field trials were conducted in Illinois at DeKalb, Monmouth, and Urbana in 2012 and Dixon Springs and Urbana in 2013 to evaluate NLB control of DMI, QoI, and QoI + DMI fungicides applied at the solo label rates and the reduced rates present in QoI + DMI premixed fungicides. A moderately susceptible field corn hybrid (Pioneer 33W84) was planted at all site locations across years. Trials were inoculated at the 4-leaf growth stage and fungicides were applied at silk emergence. The mean NLB percent leaf infection for the ear leaf, leaf above the ear, and below the ear and the plot NLB percent severity were evaluated at corn reproductive stages R1, R2, R3, R4 and R5. Stalk rot severity, plant maturity, and yield data were collected. Sweet corn trials were conducted in Urbana in 2012 and 2013 using the same methods. NLB leaf and plot severities were evaluated at reproductive stages R1, R2, and R3 and mean ear weight was calculated at harvest. In the greenhouse, trials were conducted to evaluate NLB percent leaf severity on plants inoculated with E. turcicum up to seven days before and after fungicide application with azoxystrobin, propiconazole, prothioconazole, or pyraclostobin. In field trials with low disease severity, no significant differences in treatments were observed for NLB severity ratings, stalk rot severity, plant maturity, or yield. In field corn trials with moderate disease severity, label rates of metconazole and azoxystrobin + propiconazole significantly (P ≤ 0.05) reduced NLB leaf and plot disease severity compared to the non-treated control across reproductive stages. DMI fungicides at high rates and QoI + DMI premixes offered greater NLB control than other treatments. Fungicide treatments did not significantly affect stalk rot, plant maturity, or yield in field trials with moderate disease severity. In sweet corn trials, metconazole, propiconazole, and azoxystrobin + propiconazole significantly (P ≤ 0.05) reduced plot disease severity compared to the non-treated control at R2 and R3. DMI fungicides controlled NLB better than other treatments when compared by fungicide chemical group and rates. Ear weight was not significantly affected by treatments in sweet corn trials. In greenhouse trials, all fungicides significantly (P ≤ 0.05) reduced disease severity when applied 3 days, 1 day, and 3 hours before inoculation and 3 days and 7 days after inoculation. QoI and DMI fungicides can control NLB when applied prior and post infection; however, products containing DMI fungicides offered better NLB control in the field.
Issue Date:2016-04-04
Rights Information:Copyright 2016 Japheth Weems
Date Available in IDEALS:2016-07-07
Date Deposited:2016-05

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