Saturday, May 25, 2024

Starting Leaf Spot Management

The first leaf spot fungicide application depends on several factors. 

⋅ BY DAN ANCO ⋅
CLEMSON EXTENSION PEANUT SPECIALIST

A question that comes up often during production meetings in South Carolina is when should a farmer consider starting a fungicide spray at 30 days after planting compared to 45 days? The answer depends on several production factors, including field history and how wet a year it is. 

Applying an in-furrow fungicide that brings with it leaf spot protection, such as Velum (plus imidacloprid) or Thimet, for example, will provide some protection. However, even then I consider those in-furrow applications as helping to manage overall levels of leaf spot. It helps slow leaf spot development and lower total levels on the back end of the growing season. 

Favoring 30-Day Applications

Factors that favor a fungicide application at 30 days after planting include whether the field itself, or a nearby field, has had challenges with leaf spot within the last two years. An easy example of this is when a field is split and rotated in sections. While there is localized rotation occurring, the sections are near enough to each other where leaf spot inoculum could spread across them quickly. 

This is essentially the situation when volunteers are near the field being planted to peanut as well. Volunteers often show leaf spot lesions the earliest in a year and serve as a source of spreading those spores to start new infections. In some years, we have seen this happen as early as the end of May. Volunteer management goes a long way in helping to reduce survival of fungal diseases like leaf spot, or nematodes, which helps to make populations easier to manage in following years.

Other situations generally benefiting from fungicide coverage nearer 30 DAP include Virginia types planted at the end of May or into June.

Favoring An Extended Start

Risk Factors For Late Leaf Spot:

• Short rotations (less than two years out of peanuts).

• Highly susceptible variety (e.g., Virginia types,
TUFRunner 511, Georgia 13M, Spain).

• Late planting (May 26 and later).

• Poor control of volunteer peanuts in rotational crops or nearby fields.

• Poor end-of-season control of late leaf spot in an adjacent, upwind field the previous year.

• Starting fungicide programs any later than 45 DAP; better early than late.

• Extending spray intervals beyond 15 days.

• Repeated, frequent periods of leaf wetness: excessive rain, frequent irrigation.

• Rain off immediately after application – wait 24 hours to irrigate.

• Consecutive use of fungicides with the same mode of action (except chlorothalonil); products like strobilurins (Abound, Headline) and Topsin must be tank-mixed (with chlorothalonil) to reduce the risk of resistance development or control failure.

Field history greatly affects late leaf spot risk because leaf spot spores persist on peanut residue in the soil. Fortunately, other row crops and weeds are not hosts for late leaf spot. Fields should be rotated out of peanut for a minimum of two years to reduce late leaf spot pressure and longer is better (three or more years out of peanut). Adjacent fields that had poor late leaf spot control at the end of the previous season can also be a source of significant infection, especially if upwind. 

Application of an in-furrow product with some leaf spot protection (e.g., Velum or Thimet), plus a lack of significant field history of leaf spot offers some flexibility in delaying the first leaf spot spray beyond 30 DAP, but under high-pressure environments, it is still beneficial to retain the 30-day application timing. 

Extended Intervals In-Season

Chlorothalonil is the foundation of peanut leaf spot control programs because it has multiple modes of action to reduce the risk of developing leaf spot resistance. Multiple sequential chlorothalonil applications have been used for more than 40 years without resistance development. Alternating or tank-mixing chlorothalonil with other products can delay development of resistance towards those alternative compounds. Chlorothalonil in the last spray can also help to prevent resistant leaf spot strains from overwintering and causing infection in the following year.

In the past few years, there has been interest in obtaining extended control for leaf spot diseases following a fungicide application that includes Miravis. However, this is best reserved for situations where overall disease pressure is more moderate to lower in intensity. 

Depending on the year, our research trials have reported greater leaf spot disease control efficacy and corresponding pod yield from treatments where a 15-day application (with Bravo in between) schedule was maintained compared to when Miravis + Elatus or Miravis + tebuconazole were applied according to a 21-day schedule. This was the case in 2021. 

Integrate Multiple Strategies And Fungicides

In 2021 and 2022, we had encouraging results with two applications of Miravis + Provost Silver, both at full rates, on a 28-day interval. Overall, the collective risk of our leaf spot management program can be reduced if, when integrating extended intervals, we limit this to one extended interval in an overall program. Data suggests the combination of Miravis + Provost Silver would be a more robust combination along these lines. 

The use of Peanut Rx is a great way to visualize the collective integration of multiple production practices, alongside which we can plan our in-season fungicide management strategy. PG

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