Control of Frogeye Leaf Spot with Foliar Fungicides

Frogeye leaf spot’s (FLS) adverse effect on yield of susceptible soybean varieties is well-documented. Also, FLS resistance to the quinone outside inhibitor (QoI) or strobilurin group of fungicides (FRAC code 11) is an increasing problem in the Midsouth soybean sector. This has rendered this fungicide class mostly ineffective as a viable management tool for FLS in southern US soybean.

The pathogen responsible for FLS, Cercospora sojina, is known to overwinter in crop residue. Thus, the practice of burying plant residues by tillage has been promoted for decades to assist in the control of this disease in soybean. The major shift away from tillage–i.e., increasing use of no-till systems–has likely contributed to the increased prevalence and severity of FLS in soybean, and this has resulted in an increased reliance on foliar fungicides to protect yield when damaging populations of this pathogen are present.

From 2014-2016, a group of scientists conducted studies in West Tenn. to measure FLS severity and soybean yield under tilled and no-till cultivation with and without applications of six different fungicides applied at stages R3 and R5. Results from that study titled “Quantifying the effects of fungicides and tillage on Cercospora sojina severity and yield of soybean” by Mengistu et al. are reported in Plant Health Progress, Vol. 19, No. 3, p. 226. Major points from that article follow.

•   The objective of the study was to measure FLS severity and soybean yield under tilled and no-till cultivation with and without applications of foliar fungicides.

•   Soybean variety Asgrow 4832 that is susceptible to FLS was used in all 3 years of the study.

•   The recommended rate of six different fungicides labeled for FLS control was applied at the R3 and R5 growth stages each year. The product name, active ingredient, fungicide group name, and Fungicide Resistance Action Committee (FRAC) code are as follows.
 

•   Disease severity was recorded weekly from the first appearance of FLS to the last rating period. Maximum FLS severity was used to calculate disease control as [(untreated – treated)/untreated] x 100.

•   There was no tillage effect on maximum FLS severity or yield, which indicates that tillage to bury residue was not effective in controlling FLS in this study.

•   There was a significant fungicide effect on both FLS severity and yield as follows.

• Both Headline SC (FRAC code 11) and Bravo Weather Stik (FRAC code M5) provided poor FLS control (<30%) and yield protection every year compared to other fungicide treatments.
• Quadris TOP SBX (FRAC codes 3+11), Topsin (FRAC code 1), and Topguard (FRAC code 3) provided the best disease control (>70%) across the 3 years.
• Across the three years of the study, Quadris TOP SBX (FRAC codes 3+11) and Topsin (FRAC code 1) fungicides provided the consistently greatest yield protection. Yields resulting from these treatments were 15-20% greater than those from the untreated check treatments that ranged from 49-50.5 bu/acre. Thus, these results indicate that these two fungicides may be used to protect soybean yield from the QoI-resistant strains of Cercospora sojina.

Take-Home Message

•   Using Topsin (MBC group–FRAC code 1) alone for FLS control is a high-risk practice since the potential for resistance development to this group of fungicides is high (click here, Table 3). Thus, mixing Topsin with a fungicide from another group with a different mode of action [such as DMI (Group 3) with medium risk of resistance development] is recommended (click here, Table 2).

•   Control of FLS can be achieved by using selected fungicides with multiple modes of action.

•   Combination fungicide products are the most effective in controlling FLS and protecting yield.

•   Long-term use of fungicides with the same modes of action may result in selection for fungicide resistance.

•   To lower the risk of fungicide resistance development in the FLS pathogen, using effective fungicides should be coupled with selecting soybean varieties that have a significant level of resistance to the pathogen.

Composed by Larry G. Heatherly, Oct. 2018, larryheatherly@bellsouth.net