| Historically, frogeye leaf spot (FLS; caused by Cercospora sojina) of soybean has been observed more frequently in the southern U.S. than the north central U.S. However, in recent years, FLS field observations have been on the increase in the north central U.S., including Illinois. To better understand the survival rate of C. sojina in Illinois, a field study was conducted at three locations: Monmouth (west-central Illinois), Urbana (east-central Illinois), and Dixon Springs (southeastern Illinois). At each location, soybean leaves affected by FLS were placed at depths of 0, 10, and 20 cm and retrieved after 12, 19, and 24 months. To determine the viability of C. sojina in the collected leaves, a greenhouse bioassay was developed. Survival of C. sojina declined with time equally at all three locations through 19 months. After 24 months, C. sojina from leaves collected from Monmouth and Urbana was no longer viable, but the fungus was still active in leaves collected from Dixon Springs. Depth of leaf placement had no effect on survival of C. sojina. These results suggest that planting a non-host crop for two years in central Illinois will reduce the level of C. sojina inoculum to a negligible amount; however, soybean farmers in southern Illinois may need a longer rotation for FLS management.;Another topic addressed in this dissertation was the monitoring of Quinone outside inhibitor (QoI) fungicide resistance in C. sojina. QoI fungicides have been effective in managing frogeye leaf spot, but the risk of selecting C. sojina strains with resistance to this class of fungicides is considered high. A QoI fungicide resistance monitoring program was initiated, in which sensitivities to azoxystrobin, pyraclostrobin, and trifloxystrobin were determined in C. sojina isolates collected prior to QoI fungicide use on soybean (baseline population) and C. sojina isolates collected from soybean fields in 2007, 2008, and 2009. For the baseline population, the mean effective fungicide concentration at which 50% of the conidial germination was inhibited (EC50) was determined to be 0.01287, 0.00028, and 0.00116 µg/ml for azoxystrobin, pyraclostrobin, and trifloxystrobin, respectively. When mean EC50 levels of 2007, 2008, and 2009 C. sojina isolates were compared to baseline C. sojina EC50 levels, a small but statistically significant (P ≤ 0.05) shift towards less sensitivity was observed for trifloxystrobin in 2009. In 2010, QoI fungicide resistant isolates were found at two locations in Illinois, one location in Kentucky, and two locations in Tennessee. QoI fungicide sensitivity levels of the resistant isolates were over 200-fold higher than baseline isolates using petri dish assays. A greenhouse trial was conducted with a QoI-resistant C. sojina isolate from Tennessee and a QoI-sensitive baseline isolate. FLS caused by the QoI-resistant isolate was not significantly (P ≤ 0.05) reduced with QoI fungicides compared to a water control, but FLS caused by the QoI-sensitive isolate was significantly reduced with QoI fungicides compared to a water control. Several fungicides in the demethylation inhibitor (DMI) group and the methyl benzimidazole carbamate (MBC) fungicide, thiophanate methyl significantly reduced FLS caused by the QoI-resistant or QoI-sensitive isolate compared to their respective water controls. These results indicate that C. sojina isolates resistant to QoI fungicides are present in Illinois, Kentucky, and Tennessee, and that FLS caused by QoI-resistant isolates may be managed with DMI or MBC fungicides. To develop the best management tactics for control of FLS caused by QoI resistant C. sojina and the best fungicide resistance management tactics, a better understanding of how QoI resistant C. sojina isolates compare to QoI sensitive isolates in their biology and their aggressiveness in causing FLS on different soybean cultivars is needed. Results from a laboratory study indicated that no differences in mycelial morphology, number of spores produced after 5 days, and radial growth after 6 or 12 days were observed between QoI resistant and sensitive C. sojina isolates. Results from a greenhouse study indicated that on a FLS susceptible cultivar (‘Blackhawk’), QoI resistant C. sojina isolates caused significantly ( P ≤ 0.05) greater disease severity than QoI sensitive isolates 7 to 8 days after inoculation, but no differences in severity were observed after 9 days. On a FLS resistant cultivar with the Rcs3 gene for resistance (‘Davis’), QoI resistant C. sojina isolates caused significantly greater disease severity than QoI sensitive isolates 8 to 14 days after inoculation. In general, these comparisons between QoI resistant and sensitive C. sojina isolates indicate that they are similar in growth and sporulation, but the QoI resistant isolates were slightly more aggressive in causing greater FLS severity on soybean. |
