By Travis Faske, Extension Plant Pathologist
Frogeye leaf spot (FLS) continues to be one of the most important foliar diseases in Arkansas and this year is no exception despite the warmer cropping season. Fungicide-resistant FLS has been detected in almost all soybean producing counties; thus, producers are utilizing triazole fungicides to manage these strains of FLS. Some triazole fungicides can cause phytotoxicity on soybean that looks very similar to the initial symptoms of sudden death syndrome (SDS), which is being reported in several fields north of I-40. This is a brief guide to differentiate triazole phytotoxicity from SDS.
- Typically, phytotoxicity appears 14 to 21 days after a fungicide application.
- Symptoms: Interveinal chlorosis (Fig 1.) and necrosis (Fig. 2) on soybean leaves.
- Newly emerging leaves have no phytotoxicity that result in a layer of symptomatic leaves within the canopy.
- Symptoms usually occur field-wide or follow a spray pattern.
- Leaves remain on petioles.
- Vascular tissue and pith remains white below the crown.
- Some triazole fungicides (tebuconazole and prothioconazole) are more commonly associated with triazole phytotoxicity than others; however, phytotoxicity depends on fungicide rate, cultivar, ambient temperature when fungicides applied, and premix/tank mix with other fungicides, insecticides, or adjuvants, which complicates the predictability of triazole phytotoxicity based on fungicide alone.
- No evidence of yield loss.
Sudden death syndrome:
- Typically appears at late reproductive stages (R5) in fields with a history of the disease.
- Symptoms: Interveinal chlorosis and necrosis (Fig 3.) that occur throughout the canopy.
- Rarely do symptoms expand over the entire field but often appear as irregular shaped spots that expand over years.
- Symptoms often appear near the manifold of furrow irrigated fields. It is suspected that cool, saturated soils compromise root growth allowing for fungal invasion.
- SDS is often associated with nematodes; Soybean cyst nematode and root-knot nematode compromise the roots system allowing for fungal invasion.
- Symptoms are due to a toxin produced by the fungus (Fusarium virguliforme) that remains in roots and lower stem.
- Pith tissue below crown remains white, while vascular tissue is often light brown.
- Leaves detach leaving leafless-petioles (classic “nail-in-the-coffin” symptom, Fig. 4).
- Yield loss is highest when symptoms are severe at early reproductive stages of growth.
- Use of moderately resistant cultivars is the most economical practice to manage SDS.
- Currently, few control practices, but some new seed treatments may slow early season infection of the fungus.
Diagnosis often requires the use of more than two of these points as initial foliar symptoms are similar for triazole phytotoxicity and SDS. See earlier post on this blog for more detailed management practices of SDS.