This winter two questions on soybean white mold have been frequently asked by growers and agronomists in agribusiness: (1) Can white mold fungus infect soybean seed and spread with the seed? and (2) What are the latest findings on the use of chemicals, especially Cobra, to control white mold by foliar spray? Plant pathologists in the north central region have been working on the answers to these questions for the past two years. Although final answers are yet to come, significant progress has been made. In this article I will address the two questions by using the latest research results.
Internally infected seeds. In this month's issue of Plant Disease, an international journal of plant pathology, plant pathologists at Iowa State University (ISU) reported that internally infected soybean seeds can produce sclerotia in soils under controlled laboratory conditions. In one study, seed from a field that had 70 percent disease severity was collected and sorted into three classes: (1) normal quality seed that included moderate or good seed; (2) poor quality seed (shriveled, and/or whitish seed); and (3) seed of regular size with visible mycelial mats (white mold or downy mildew) on the seed coat. These seeds were 2, 44, and 6 percent infected, respectively. When planted in soils, sclerotia were not found from seeds that had germinated, regardless of emergence. Sclerotia were mainly found from seeds of poor quality with an average of 12 percent of such seeds producing sclerotia. Frequency of sclerotia production in infected seeds of normal size was 0.4 percent, and no sclerotia were produced from seeds with mycelial mats. The finding that internally infected seed can produce sclerotia in soil has been confirmed by plant pathologists at the University of Illinois and was reported in a recent white mold research meeting.
Because the seed experiments were conducted under controlled conditions, the results only raised the possibility of dissemination of white mold fungus from infected seeds. Field experiments are needed to determine the risk under natural conditions. Nevertheless, the laboratory results indicated that the risk of white mold spread by infected seeds may not be as great as once thought because mainly poor quality seeds produce sclerotia and these seeds can be cleaned mechanically. Seed scientists at ISU are studying methods to remove these infected seeds with engineering methods.
The data further indicate that if we want to save seeds from a white mold-infested field, it is important to remove not only sclerotia but also infected seeds, mainly the poor quality seeds. A potential method is to treated infected seeds with fungicides containing TBZ. Last year, ISU plant pathologists treated white mold-infected seeds with fungicides containing TBZ and the results were promising. Other information on white mold-infected seed can be found in various articles in the 1997 ICM newsletters.
Foliar sprays. Postemergence application of Cobra has been reported to supress white mold by growers for years. Cobra is currently labeled only for weed control and a label change for disease suppression is pending. In a check-off dollar-funded project led by the University of Wisconsin, plant pathologists and agronomists in the north central states are working on suppressing white mold by using herbicides, Cobra, and other chemicals in the same family as Cobra. No general recommendations have been made.
Available data show that Cobra, if applied at the R1 growth stage (just blooming), can consistently reduce white mold infestation. Yield response, however, can be variable, especially if disease was low or absent when the chemical was applied during the R1 stage. Some studies show that Cobra may induce soybean resistance to white mold. If application is done too early before flowering, the suppression effects may decrease to a low level by the time white mold attacks soybean. If application is late in the flowering stage, yield reduction may be associated with plant burn when disease is light or absent. Data are limited on whether or not the use of a surfactant (Cobra label has surfactant) influences control.
Below: Yield map of field 1 (see photo of field 1, below).
||Suppression of white mold by Cobra in field 1. Left, sprayed and right, not sprayed..
||Suppression of white mold by Cobra in field 2. Left, sprayed and right, not sprayed..
Experiments by an innovative grower in northeastern Iowa clearly show that if applied correctly, Cobra can reduce white mold and increase yields at a commercial scale. Photos 1 and 2 are taken from two white mold-infested fields that were treated with Cobra on July 4 at 7 oz/acre without surfactant. Soybeans in the two fields were drill-planted in mid-May. The percentage of plants killed by white mold in treated (left) and nontreated (right) areas was 20 percent and 80 percent, respectively, for field 1 (Photo 1) and 7 percent and 67 percent, respectively, for field 2 (Photo 2). Significant increase in yield is evident by the yield map for field 1. In field 2, consistent yield increase (6-9 bu/acre increase with an average yield of about 50 bu/acre) also was reflected by a yield map (not shown).
Importantly, the yield map in the lower right corner shows differences in yield between a tolerant variety and a susceptible variety in an adjacent field not treated with Cobra. The tolerant variety out yielded the susceptible one. The data clearly show that use of white mold-tolerant cultivars can achieve equal or superior effects to that of chemical controls. Data from Wisconsin also show no yield increase when Cobra was applied to white mold-tolerant varieties. Variety selection should be the first line of defense in combat of white mold.
This article originally appeared on pages 13-15 of the IC-480 (2) -- February 16, 1998 issue.