RESEARCH BRIEF: Influence of Cultural Practices and Fungicides on Frogeye Leaf Spot and Cercospora Blight Diseases of Soybeans

Clayton A. Hollier

Frogeye leaf spot (Cercospora sojina) and Cercospora blight (Cercospora kikuchii, C. flagellaris, C. sigesbeckiae) are two important diseases of soybeans in Louisiana. Left unchecked, both can induce serious quantity and quality yield losses on susceptible varieties. Frogeye leaf spot is managed, to some degree, by genetic resistance but more commonly by fungicide use. On the other hand, Cercospora blight is not easily managed by any approach.

Before the common use of fungicides, cultural practices, in particular destroying crop residue by plowing it under to bury it, were used to reduce inoculum for the following crop. This study used both crop residue destruction and in-season fungicide application to determine how they influence subsequent disease development of frogeye leaf spot and Cercospora blight.

A comparison of no-till, minimum till and deep tillage, with and without an azoxystrobin fungicide, using four replications per year, was performed for three successive years in southwest Louisiana. Data on the severity of frogeye leafspot and Cercospora blight were collected at each reproductive growth stage. Results are in Tables 1 and 2.

The percentage of disease severity of frogeye leaf spot depended on the treatments. Table 1 shows that tillage in any form reduced subsequent disease development, with minimum tillage reducing frogeye leaf spot by 18 percent over no-till and deep tillage reducing frogeye leaf spot development by 45 percent over no-till. Using a fungicide as an in-season management tool provided additional reductions over the use of tillage alone. The combination of minimum tillage plus fungicide use during the season reduced frogeye leaf spot development by 41 percent versus 18 percent by tillage alone. Deep tillage combined with the fungicide reduced frogeye leaf spot development 57 percent versus 45 percent by deep tillage alone. Fungicide use without tillage reduced frogeye leaf spot development by 35 percent.

Table 2 shows that regardless of tillage practice alone or in combination with fungicide use, Cercospora blight development was not affected.

Diseases reduce soybean yields annually, and all disease management practices should be considered for use to reduce inoculum for subsequent crops as part of a complete plant disease management system. In recent years, deep plowing has fallen out of favor for several reasons. Soil conservation efforts to reduce erosion have influenced producers to drop the practice. Additionally, fuel costs for deep plowing have risen, reducing profits, and effective fungicides are available. Nevertheless, deep tillage is a viable option for the producer who wants to reduce inoculum while reducing the pressure on fungicide use, thus reducing the chances of fungicide resistance. This study demonstrates the effectiveness of plowing and fungicide use in a plant disease management system for frogeye leaf spot.

Clayton A. Hollier is a professor emeritus in the Department of Plant Pathology and Crop Physiology.

(This article appears in the summer 2018 issue of Louisiana Agriculture.)

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Table 1. Percentage disease severity of frogeye leaf spot with no-till, minimum till and deep tillage, with and without a fungicide, 2015-2017.

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Table 2. Percentage disease severity of Cercospora blight with no-till, minimum till and deep tillage, with and without a fungicide, 2015-2017.

9/17/2018 4:15:00 PM
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