Trey Price, Boyd Padgett, Myra Purvis, Dustin Ezell, Dustin Harrell, James Leonards, Fred Collins, Laura Lee, Jeremy Hebert and Jimmy Meaux

In areas of Louisiana where rice and soybean rotations are common, aerial blight of soybean (Figure 1) is a major problem. The fungal pathogen Rhizoctonia solani also causes sheath blight of rice (Figure 2). Since the late 1990s, strobilurin fungicides, which are known as Qol fungicides, have been used extensively in rice and soybean production to combat many foliar diseases. During the 2010s, field failures using these materials to manage aerial blight and sheath blight began to occur, particularly in the southwest portion of the state where 70%-75% of Louisiana rice is produced.

The concept of fungicide resistance is very similar to that of insect and weed resistance. One caveat is that fungicide resistance is not as easily noticed. Working with microscopic organisms, often the pathogens responsible for disease development are not as obvious as insect and weed pests. Resistant mutants naturally exist in the population, and repeated applications of fungicides eliminate sensitive individuals, leaving resistant strains. Resistant individuals then become dominant in the pathogen population. Most of the time, laboratory research in the form of poison plate assays or molecular testing is used to determine if fungicide resistance is occurring. Another way to detect fungicide resistance is with small-plot replicated field efficacy trials. If the products used to work and they no longer do, there likely is a resistance issue.

The aerial blight and sheath blight pathogen can survive in the soil for many years by producing sclerotia (Figure 3), which are small, hard masses that are the size of mustard seeds to purple hull peas and are white when fresh and brown when mature. Farmers need to keep in mind that any operation that moves infested soil is also moving R. solani to new areas, thereby making sanitation a key management consideration. In the case of aerial blight in soybean, rain splashing transports the pathogen in contact with plants, and flooding floats sclerotia directly in contact with rice. High humidity, frequent rainfall events and warm temperatures fuel epidemics. There are rice varieties somewhat resistant to sheath blight; however, it is unknown if there are current soybean varieties that have any resistance to aerial blight. It is also unknown if QoI-resistant strains incur a fitness cost with the mutation. For example, resistant strains would have lower growth and reproduction rates than sensitive ones.

With limited crop rotation options and long-term survival potential of the pathogen in the soil, the only current management option for aerial blight of soybean is application of effective fungicides. QoI resistance in R. solani has not yet been documented on LSU AgCenter research stations. Therefore, out of necessity, research to fight this problem has been conducted on farms where resistance has been documented or suspected. Over the past six growing seasons, multiple Louisiana soybean farmers have allowed researchers to conduct small-plot replicated research trials that are 0.25 to 0.5 acres in total area in their fields with the aim of identifying effective fungicide options for aerial blight management. Research trials were focused on identifying experimental and commercially available fungicides effective on QoI-resistant R. solani.

Conducting research on farms is more challenging than it is on research stations. When beginning this project, researchers would have to apply treatments with a hand boom and harvest plots by hand, making it highly labor intensive. More recently, with funding obtained from outside sources, researchers were able to purchase a self-propelled multi-boom transportable plot sprayer that could be used to spray trials more efficiently with minimal impact to growers’ fields. Once soybeans reached the appropriate growth stage, which was usually R3 to R5, or when aerial blight was first observed, which was sometimes as early as R2, researchers would travel to the farm and apply the fungicide treatments. Initial disease ratings were recorded along with two to three disease ratings throughout the season to monitor disease development over time. At the end of the season, we would estimate yields or coordinate with H. Rouse Caffey Rice Research Station personnel to harvest plots with a specialized combine. Parish agents greatly facilitated coordination with farmers and were a big help in executing trials.

Results from the on-farm experiments confirmed that QoI materials are no longer a viable option for managing aerial blight in some areas. Further, results from multiple experiments over locations and years implicate several succinate dehydrogenase inhibitor (SDHI) fungicides as viable alternatives for QoI-resistant aerial blight management. Properly timed applications of SDHI fungicides at appropriate water volumes resulted in significantly less aerial blight damage and preserved yields (Table 1).

Although helpful, the identification of alternative fungicides for aerial blight management is a relatively short-term solution. Fungicides with specific modes of action historically have been short-lived in their usefulness, and it is only a matter of time before fungal pathogens develop resistance. Future research should continue to focus on experimental fungicides and commercial fungicide use patterns as well as identification and development of soybean varieties resistant to aerial blight.

Acknowledgments: Louisiana soybean farmers for allowing research on their farms and the Louisiana Soybean and Grain Research and Promotion Board for financial support

Trey Price is an associate professor based at the Macon Ridge Research Station, Winnsboro, Louisiana; Boyd Padgett is a professor based at the Dean Lee Research and Extension Center, Alexandria, Louisiana; Myra Purvis and Dustin Ezell are research associates at the Macon Ridge Research Station; Dustin Harrell is a professor and research coordinator at the H. Rouse Caffey Rice Research Station, Crowley, Louisiana; James Leonards is a research associate at the H. Rouse Caffey Rice Research Station; Fred Collins and Laura Lee are research associates at the Dean Lee Research and Extension Center; Jeremy Hebert is an extension agent in Acadia Parish; and Jimmy Meaux is an extension agent in Calcasieu Parish.

(This article appears in the summer 2021 issue of Louisiana Agriculture.)

Table 1. Effective fungicides for management of QoI-resistant aerial blight in soybean.

Figure 1. Aerial blight of soybean.

Figure 2. Sheath blight of rice.

Figure 3. Sclerotia (survival structures) of Rhizoctonia solani, causal agent of soybean aerial blight.