Jay Capasso, UF/IFAS Columbia County Extension. Keith Wynn, UF/IFAS Hamilton County Extension. Dr. Nicholas Dufault, UF/IFAS Plant Pathology Department.

Introduction

Sulfur is one of the earliest known pesticides. The ancient Greeks found the application of sulfur to prevent wheat smut and rust diseases in wheat. Lime sulfur and sulfur dioxide were also used as insecticides. Sulfur is one of the earliest fungicides to be used in peanut production for the control of foliar diseases such as early and late leaf spot.

Today, sulfur is still used as a preventative fungicide, meaning it must be applied prior to fungal disease development. Sulfur is sold in various pesticide formulations including dust, liquid, dry flowable, and wettable powder. Oil based sprays and sulfur should not be mixed or applied within short succession of one another as the combination can be phytotoxic.

The use of sulfur in peanut production declined in the early 1970s with the introduction of new fungicides such as benomyl and chlorothalonil. Recently, growers have begun adding micronized sulfur into their spray program. Researchers have found micronized sulfur to improve leaf spot control compared to control treatments when applied alone on a bi-weekly basis. However, it has not been found to perform as well as traditional fungicides when there is no resistance. Micronized sulfur is generally mixed with fungicides of the 3, 7, and 11 FRAC groups with the goal of improving leaf spot control and slowing the development of resistance against current fungicide products. Micronized sulfur provides growers with an economical option for leaf spot control. Generally, micronized sulfur is applied in 2 or 3 separate applications early to mid-season. An application rate of about 5 lbs. per acre is recommended. Agitation is required to keep micronized sulfur in solution in the spray tank.

Micronized Sulfur On-Farm Trial Results from Hamilton and Columbia County

During the 2021 peanut season, on-farm trials were conducted in Hamilton and Columbia County. An application rate of 3 lbs. of micronized sulfur per acre was added as a tank mix to 2 applications of the fungicide spray program. Yield results were recorded from the Hamilton County trial. Only leaf spot ratings were recorded from the Columbia County trial. Treatments containing micronized sulfur tended to have lower defoliation results than the other treatments in both trials. However, this reduction did not lead to a significant yield increase in the Hamilton County trial as seen by comparing treatments 3 and 4. In fact, all treatment yields were not significantly different from each other in the Hamilton County trial despite treatment 1, Provost® Silver added to sprays 4 and 6, having a numerically higher yield. Disease onset also occurred late in the Hamilton County trial beginning around the end of July. It is important to note that all the micronized sulfur applications were completed on July 20^th, before disease onset.

Figure 1: Fungicide spray programs by treatment for the Hamilton County trial. Planted on 5/10/2021. Peanut variety used was TifNV-HiOL. DAP = Days after planting.

Table 1: Percent defoliation and leaf spot ratings¹ recorded on September 16^th. Foliar disease onset in this trial occurred in late July to early August.

Table 2: Leaf spot ratings¹ were collected twice towards the end of the season on August 13^th and September 13^th from the Columbia County on-farm trial. Planted on 04/26/2021. Peanut variety used was Florida-07. Two spray program treatments were compared which differed only by micronized sulfur being mixed into the 3rd and 4th fungicide spray. Leaf spot ratings were recorded from 4 representative locations in each treatment area.

¹ The guidelines below were followed when recording leaf spot ratings from the Florida 1 to 10 intensity scale for leaf spots and rust: 1 = No disease, 2 = Very few lesions (none on the upper canopy), 3= Few lesions (very few on upper canopy), 4 = Some lesions with more on upper canopy than rank of 3 and slight defoliation noticeable, 5 = Lesions noticeable on upper canopy with noticeable defoliation, 6 = Lesions numerous and very evident on upper canopy with significant defoliation (50%+), 7 = Lesions numerous on upper canopy with much defoliation (75%+), 8 = Upper canopy covered with lesions with high defoliation (90%+), 9 = Very few leaves remaining and those covered with lesions (some plants completely defoliated), 10 = Plants are dead.

The commercial products listed are possible recommendations to help manage plant diseases in peanut production. This blog post is intended for educational purposes. It is not the intention of the authors to provide a complete list of product recommendations or endorse a particular product or brand.

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