Monday, May 27, 2024

Disease Notes

Use past problems and multiple management tools to combat pathogens in your fields.

⋅ BY NICHOLAS DUFAULT ⋅
UNIVERSITY OF FLORIDA ASSOCIATE PROFESSOR OF PLANT PATHOLOGY

A review of past disease problems that affected the peanut crop offers a good look at management considerations for this season. Three prevalent diseases, southern stem rot/white mold, peanut leaf spot and rust, cause much of the damage to peanuts in our state.

Stem rot pathogen mycelium on peanut pods.
Late leaf spot lesions and stroma on the underside of a peanut leaflet.
Spray burn lesions on peanut leaves.
An example of peanut rust pustules.

Additional Tips From Bob Kemerait, University of Georgia Extension Plant Pathologist Peanut leaf spot diseases, especially late leaf spot, were problematic in 2023 but not as severe as in 2021. Leaf spot diseases remain a critical threat to profitability. Factors that increase the threat to leaf spot include environmental conditions, such as multiple rain events that are favorable for the development and spread of disease and also affect a grower’s ability to make timely fungicide applications, and short crop rotations. Combinations of these factors put tremendous pressure on some fungicide programs. Management Tip: To prevent losses to leaf spot, especially late leaf spot, it is imperative to: a) stay on a timely, proven program; b) select fungicides or mixtures of fungicides based upon threat of disease in the field; c) continue appropriate management programs through the end of the season. Because of the increased threat from late leaf spot in recent years, some fungicide programs may have changed. Carefully consider the choice of product and timing for application throughout the season to minimize losses to disease. White mold was generally more severe in 2023 than in 2022, possibly because of the hotter conditions. Also, with peanuts “staying in the ground” for nearly 160 days, more attention must be given to protecting the crop from white mold even after the traditional “four-block, 60-to-104-days-after-planting” window ends. PGSouthern Stem Rot/White Mold

Southern stem rot, also called white mold in the lower Southeastern peanut production areas, is a disease that received a lot of attention in 2023. The environment of hot and wet followed by periods of hot and dry led to peanut yield losses in Florida, especially in dryland production. In some situations where varieties with resistance to the pathogen were planted and high-quality fungicide programs were used, significant damage from this disease was still present.

What can be done in 2024 to better manage this disease? First, consider adding an extra year or two to your crop rotation in fields that had significant yield losses despite using a resistant variety with a stem rot-focused fungicide program. If land availability for an adequate rotation is limited, consider adding an early stem rot-focused fungicide spray to your program and finding a way to apply the fungicide product to the crown, such as spraying at night or using a higher gallons-per acre water volume  at 75 days after planting.

Remember, fungicides are great disease management tools, but they have limitations when it comes to disease control.

Leaf Spots

There are two types of leaf spot diseases that affect peanuts: early leaf spot and late leaf spot. This is important because our peanut varieties and fungicides can respond differently to these two diseases, and it is especially important when using fungicides with Fungicide Resistance Action Committee Group 11 (QoI) fungicide products. Resistance to this fungicide class has been detected in pathogens that cause both diseases; however, complete resistance to QoI fungicides has only been detected in the early leaf spot pathogen. Thus, it is still possible to see adequate disease management of late leaf spot with this fungicide class.

It is often difficult to tell the difference between these diseases and abiotic problems in the field, but accurate diagnosis is possible with a microscope and/or hand lens. To avoid fungicide management problems with fungicides, it is recommended that a leaf spot-resistant variety is planted, as it can reduce the impact of a fungicide failure.

Rust

Peanut rust showed up late in the 2023 season but quickly spread in fields where it was present. Late-season infections do not always lead to yield losses in crops harvested within a timely manner; however, a five-to-seven-day delay in harvest can cause significant damage to a peanut crop. A late-season tropical storm would delay timely fungicide applications that can slow this disease down.

Should rust become an issue in 2024, most leaf spot products work well for managing this disease, but adding in FRAC group 11 modes of action fungicides can improve management of this disease.

Three additional diseases of interest to consider when developing your management program in peanuts are cylindrocladium black rot, Tomato Spotted Wilt Virus and Aspergillus Crown Rot.

Use Multiple Management Tools

It is important to identify the diseases you have each year, as this will help you prevent disease problems from carrying over into the next season. Diseases can be a substantial problem when the right environment is present even if we use resistant varieties and a good fungicide program. In areas where those two techniques fail, it is critical to consider adding a third management tool, such as crop rotation or delayed planting. For more information on peanut disease management, please visit the UF/IFAS Florida Peanut Team website or contact your local county agent. PG

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