Use Cobra to suppress white mold

Last year, Cobra was effective in reducing white mold infection under high disease pressure when the chemical was applied without surfactant at the beginning of flowering. However, application of Cobra this late in the season may affect soybean yields if disease pressure is light. In 1998, white mold pressure was light in Iowa, which provided us with different conditions under which to study white mold control measures.

In a regional project supported by checkoff dollars, we carried out experiments on a commercial scale at two locations, Humboldt and Rudd, in collaboration with growers. Manure was applied to fields in Rudd. Strip plots for each treatment were at least 1 acre. At both locations, three soybean varieties were used: Asgrow 2242 (susceptible variety), BSR101 (moderately susceptible variety), and Asgrow 1901 (tolerant variety).

Our results (Table 1) showed that in a season with light disease pressure, the use of Cobra would not reduce yields when applied without surfactant. The difference in yield among treatments was small and could be due to experimental error.

When disease pressure is low, application of Cobra without surfactant at the R1 growth stage can reduce disease without yield penalty.

Our data, together with that gathered by other plant pathologists, show at least two mechanisms for control:

  1. induction of plant defense compounds to reduce soybean infection, and
  2. reduction in the density of the soybean canopy, which results in less production of white mold mushrooms in an infested field.

Our data also indicate that when disease is light (<30 percent), yield loss from white mold is not consistent. Notice that in one field, the Asgrow 1901 had 30 percent infection, which also was observed in 1997.

It is important to note an inconsistency in yield response with studies by other universities when Cobra is applied with surfactant. Yield reduction sometimes occurs when Cobra is applied with surfactants in seasons of low disease pressure. This reduction may be because applications with surfactants at the R1 growth stage often result in more foliar damage than without surfactant and, therefore, may lower yields, especially in fields that have low yield potential.

Chemicals in the same family as Cobra, such as Flexstar and Blazer, have been tested by other universities and found suppressive to white mold. We also tested Actguard in small plots (10 ft by 20 ft) at Humboldt. Actguard suppressed white mold on Asgrow 2242 significantly. The rate of application will be further studied.

Table 1. Results of spray strips using Cobra without surfactant to suppress white mold.

Location & row space Variety Time of spray Infected plants (%) Yield (bu/acre)
Rudd Farm 1, 30 in.

1 acre/strip, 2 replicates
Asgrow 2242 No spray 30 51.0
Asgrow 2242 V5 (June 22) 25 51.4
Asgrow 2242 R1 (July 8) 15 52.4
Asgrow 1901 No spray 1 52.1
Asgrow 1901 V5 (June 22) 0 53.2
Asgrow 1901 R1 (July 8) 1 52.6
BSR 101 No spray 9 54.4
BSR 101 V5 (June 22) 5 53.9
BSR 101 R1 (July 8) 3 53.9
Humboldt, 15 in.

1 acre/strip, 2 replicates
Asgrow 2242 No spray 23 42.3
Asgrow 2242 R1 5 46.3
Asgrow 1901 No spray 0.5 49.0
Asgrow 1901 R1 0.5 48.5
BSR 101 No spray 4 49.7
BSR 101 R1 0.1 47.4
Rudd, Farm 2, 30 in.

3.3 acre/strip
Asgrow 1901 No spray 30 58.0
Asgrow 1901 R1 5 58.5
Rudd, Farm 3, 30 in.

7 acre/strip
Latham 660 No spray 40 61.2
Latham 660 R1 10 67

This article originally appeared on page 196 of the IC-480(25) -- December 7, 1998 issue.

Updated 12/06/1998 - 1:00pm