|LIN, BINBIN - University Of Tennessee|
|KELLY, HEATHER - University Of Tennessee|
|YU, HAO - University Of Tennessee|
Submitted to: Journal of Phytopathology
Publication Type: Abstract Only
Publication Acceptance Date: 6/15/2016
Publication Date: N/A
Technical Abstract: Frogeye leaf spot (FLS), caused by Cercospora sojina, is a yearly foliar disease of soybean in Tennessee and causes substantial economic losses if not properly managed. Quinone outside inhibitor (QoI) fungicides are often used to manage FLS, but C. sojina isolates have developed resistance to this class of fungicides. Currently, the majority of soybean fields contain both QoI resistant and sensitive isolates. Therefore a better understanding of isolates biology and aggressiveness is very important. In our competition studies, spore suspensions (˜1×105 Conidia/ml) prepared from QoI-resistant and -sensitive isolates were mixed in 1:0, 9:1, 1:1, 1:9. 1:99, and 0:1 ratios. When greenhouse soybean plants reached the V4 growth stage, three treatments were applied 24 hrs before inoculation at the label rate: no fungicide (water control), Quadris (azoxystrobin), and Quadris-Top (azoxystrobin plus difenoconazole). Each treatment was replicated 12 times using the highly FLS susceptible soybean cultivar, Blackhawk. Results of the greenhouse studies showed that: 1) Disease severity increased as the proportion of QoI-resistance in the inoculum increased, indicating that QoI-resistant isolates are more aggressive; 2) Quadris only inhibited FLS caused by QoI sensitive isolates and when the proportion of QoI resistance was less than or equal to 10%; 3) Quadris-Top fungicide provided more effective control of FLS by reducing symptoms and by delaying disease development. Fitness parameters of QoI-resistant and -sensitive isolates measured were (i) mycelial growth and sporulation capacity and (ii) conidial germination and growth inhibition (%) on media amended with azoxystrobin at 1µg/ml. As a group, no significant differences in the mean values of these fitness parameters were observed between resistant and sensitive phenotypes, except for sporulation capacity and growth inhibition. In addition, conidial germination assay results were compared to qPCR results to confirm the proportion of QoI resistant isolates recovered from inoculated plants. The resistant isolates dominated the population in all the mixed inoculation tests. These results indicate there are differences in QoI resistant and sensitive C. sojina isolates in-planta and will help influence effective disease management measures against FLS.