| [Image: Anthracnose] | | [Image: table 1] | | [Image: table 2] | | [Image: table 3] | | [Image: table 4] | | | |
Each year diseases adversely affect soybean profitability. In 1999, for example, 4.37 million bushels were lost to disease in Louisiana. This was more than 17 percent of the total yield.
The predominant soybean diseases in the state are aerial blight, Cercospora blight/purple seed stain, pod and stem blight, and anthracnose.
These diseases, except aerial blight, can be found in all soybean-producing regions of the state. Aerial blight is favored by high humidity and is prevalent in the southern part of the state. Although aerial blight is potentially the most devastating soybean disease, an increased incidence in Cercospora blight has caused concern among Louisiana growers. Anthracnose and pod and stem blight are late-season diseases affecting the pods and can harm seed quality and yield when conditions favor their development.
Soybean diseases are managed using disease-resistant varieties and fungicides. The producer’s best option is resistant varieties, but fungicides are needed when disease pressure is severe or when resistant varieties are not available. Applications can cost $15 to $20 per acre and should be used only when necessary. These applications are likely to reap economic returns in locations with frequent afternoon and late-evening rains and high temperatures beginning mid-season.
Benlate was once the fungicide of choice, but this product has been removed from the market. This fungicide was used to manage Cercospora diseases and to suppress aerial blight. Topsin-M is similar to Benlate and has filled its niche. Quadris, a relatively new fungicide, is active on aerial blight, pod and stem blight, and anthracnose but is not as effective as Topsin-M against Cercospora. This difference in activity for these fungicides makes it critical to identify which diseases are present when selecting a product. Substantial information has been generated on Topsin-M; however, information on the activity and use of Quadris is not as extensive. The disease control spectrum of Quadris has been defined, but information on soybean seed quality and maturity is limited.
Northeast and Macon Ridge Tests
Quadris was evaluated in several LSU AgCenter tests conducted at the Ben Hur Research Station near Baton Rouge, the Macon Ridge Research Station near Winnsboro, the Northeast Research Station near St. Joseph and the Rice Research Station near Crowley. Diseases present and severity varied among years and locations. The predominant disease at Ben Hur and the Rice Research Station was aerial blight; pod diseases (anthracnose and pod and stem blight) and Cercospora blight were present at the Macon Ridge and Northeast stations.
Quadris was evaluated in soybean maturity groups IV or V during the 2000, 2001 and 2002 growing seasons. These maturity groups are the most popular in Louisiana and planted between mid April and mid June.
Quadris was applied at two rates, either 6 or 9 ounces per acre, and at two stages of development, either pod initiation (R3) only, seed initiation (R5) only or at both stages (R3 and R5). See accompanying tables.
Pod diseases were prevalent across years and tests; however, Cercospora blight was severe in 2002. Six or 9 ounces of Quadris applied at the pod completion or at both growth stages effectively reduced pod diseases compared to non-sprayed soybeans (Tables 1 and 2). Quadris was not as effective in reducing Cercospora epidemics. The percentage of green stems was higher and foliage was greener longer in plots receiving fungicide applications than in non-sprayed plots (Table 3). Seed quality was best in soybeans receiving sequential applications of Quadris at both growth stages. Yield response to fungicide applications varied across tests and years with yield increases over non-sprayed soybeans ranging from one to six bushels. These increases were associated more often with applications made at seed initiation or sequential applications at both growth stages.
Ben Hur Research Station Tests
In tests conducted at Ben Hur, Quadris was applied at 3, 6, 9 or 12 ounces per acre at both growth stages. Six ounces of Quadris applied at pod initiation and seed initiation provided a high level of aerial blight control, resulting in a 13 bushel per acre yield increase over non-sprayed treatments (Table 4). In addition, pod diseases were controlled, which resulted in a substantial increase in seed quality compared with the non-sprayed plots. Benlate was not effective in controlling aerial blight, and it follows that Topsin-M also will not be effective in controlling this disease. Consistent with results from tests conducted at the Macon Ridge and Northeast stations, higher percentages of green stems were observed in fungicide-treated plots. Similar results were obtained in tests conducted at the Rice Research Station where aerial blight was severe.
Quadris is effective for managing aerial blight, anthracnose, and pod and stem blight. An application during pod initiation was effective in controlling aerial blight; applications at seed initiation were most effective for controlling pod diseases. Therefore, to control both diseases effectively, applications at both pod initiation and seed initiation may be necessary. Quadris also provided suppressive activity against Cercospora blight and preserved seed quality in some tests. Senescence was delayed, however. This is the stage where leaves turn brown and fall off. This delay allowed more time for pod fill, resulting in higher yields.
A 6.2-ounce application of Quadris, the labeled rate, costs about $16 per acre, and a 9-ounce application costs about $23 per acre. To manage pod diseases, a 6.2-ounce application of Quadris at seed initiation could be economical when disease was severe. Therefore, applications should be considered when extended periods of rainfall occur during the reproductive growth stages of soybean development.
In South Louisiana, aerial blight is the most important yield-limiting disease, and an application of Quadris at 6.2 ounces per acre at pod initiation should provide economical control when this disease is severe in susceptible varieties. While the 9- or 12-ounce rates were effective, they were not always more economical than the 6-ounce rate. Furthermore, sequential applications of Quadris provided adequate disease control but were cost prohibitive. The LSU AgCenter is conducting research to further define rates and application timings.
(This article was published in the winter 2003 issue of Louisiana Agriculture.) |