Monday, June 14, 2021

Early Season Disease Management

With the crop moving beyond seedling stage, it is time to think about common fungal pathogens.

Preseason decisions, such as varietal selection and in-furrow applications, have been made, and peanuts are in the ground starting to grow. Decisions for management of tomato spotted wilt and nematodes are mostly done. Now it is time to think about early season disease management options for common fungal pathogens — peanut rust, leaf spot diseases and white mold/stem rot, and in preparation for next year, Aspergillus and Rhizoctonia.

Peanut Rust

rust on peanut leaf
This picture shows peanut rust pustules on the underside of a leaf.

We saw a reemergence of peanut rust during the 2020 season, which may have this disease on many producers’ minds in 2021. Peanut rust can be a devastating if left unchecked, but luckily many of our leaf spot fungicides also manage this disease. Typically, we do not need to concern ourselves with rust early in the season, as this disease is generally spread from more tropical climates where volunteer hosts are abundant.

This means it takes some time for rust to appear in our peanut fields. This was the case last year when it began to be identified in late August and early September.

The primary concern with peanut rust early in the season is in terms of scouting for this disease and monitoring the crop for any early introductions. More information about peanut rust can be found in the article, “The What, When and How of Florida’s Peanut Rust Issue” on the Panhandle Ag News website at http://nwdistrict.ifas.ufl.edu/phag/.

Leaf Spot Diseases

Peanut leaf spot diseases are an annual problem for producers. These leaf spot diseases are caused by two different pathogens, both of which tend to show up every year in varying amounts. Typically, we see more early leaf spot in the Suwannee Valley and more late leaf spot in the Florida Panhandle. However, it is not uncommon to have different leaf spot pathogens in neighboring fields. This means we need to have a good mixture of fungicide products in our programs, as our generic products, such as tebuconazole, azoxystrobin and pyraclostrobin, all vary in how they manage each pathogen.

Comparison of early and late leaf spot.
Know the difference between late and early leaf spot symptoms, which are caused by two different pathogens.

The most often asked question related to peanut leaf spot disease management is, “When do I pull the trigger and apply my first spray?” There is no simple answer to this question. Leaf spot disease onset can occur anytime between 25 and 90 days after planting. However, if we are looking to delay our first fungicide application, then we must focus on reducing our risk for leaf spot disease as much as possible.

Peanut Rx is an excellent tool to provide insights related to risk reduction. Typically, leaf spot disease quantity, as well as onset, depends on variety, crop rotation, in-furrow treatments and the timely application of fungicides. Having a resistant variety in a well-rotated field, three or more years away from peanut and applying either Velum or Thimet in-furrow will likely delay disease onset. Each of these factors alone will have an impact on peanut leaf spot, but it is always better to integrate these techniques, as their impact can change from season to season.

If these techniques are implemented, then we can consider delaying the first fungicide application until about 40 DAP or a little later. Growers are encouraged to consult with an Extension agent and ag-chemical representatives to better understand how and when to delay a leaf spot disease management program.

White Mold/Stem Rot

White mold on peanut seed
This shows early season white mold/stem rot developing on a peanut seed away from the plants.

The impact of stem rot on Florida peanut production is variable across the state, but when this disease is severe, it can have a tremendous impact on yields. Stem rot was apparent in the 2020 peanut season, partly because of the warm conditions experienced periodically throughout the season. Fields with a history of stem rot or ones in a reduced rotation should consider an early banded application of the fungicide Proline before 60 DAP. This is especially true in peanuts planted before the third week in May.

Scouting is also critical for effectively managing this disease. As University of Georgia Extension plant pathologist Bob Kemerait says, many peanuts stay in the ground 150 days, meaning more attention needs to be given to this disease besides protecting the crop at the traditional 60 and 90 DAP. This includes early season scouting and management, especially in fields with a history of stem rot.

Aspergillus And Rhizoctonia Crown Rot

Aspergillis crown rot spores on peanut seedling
Aspergillus crown rot on peanut is recognized by the black spores produced by the fungus.

Unfortunately, there are no economically viable management options for crown rot diseases once the furrow has been closed. However, early season monitoring of these pathogens is critical so that it can impact your management decisions for 2022. Not all crown rot pathogens are created equal, and there are a few more options available for the management of Rhizoctonia spp. than Aspergillus niger. This means the proper identification of the pathogen is needed. It is recommended that growers contact UF/IFAS Extension or a diagnostic lab when a crown rot issue is present. PG

Article by Nick Dufault, University of Florida Extension pathologist, Ethan Carter, regional crop IPM agent, De Broughton, regional specialized agent and Ian Small, UF/IFAS plant pathologist.

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