Sero-X: Total propagules per gram 24-25
Exposure to the Verticillium dahliae over the whole cotton season.
Efficacy of Sero-X Application Intervals in Cotton
Innovate Ag Pty Ltd, with NSW Department of Primary Industries and Regional Development. Field efficacy report supporting a label change submission. Trial 3, Application Optimisation Programme.
Abstract
Sero-X at 0.5 L ha⁻¹ was tested at 14-, 21-, and 28-day intervals versus current label and untreated control at site LT-1, Lower Namoi Valley. Viable Verticillium dahliae propagules were quantified by dry-plate (PPG) at PrePlant (Oct 2024) and PostIncorp (Jun 2025), yielding paired change scores (n = 33 per treatment).
Untreated control increased by 4.16 PPG. The 14-day interval declined by 0.05; the 21-day interval rose by only 0.11. The 28-day interval and current label rate were intermediate. Treatment effect was significant (ANOVA F(4,160) = 4.08, p = 0.0035, η²p = 0.093), robust to Welch and Kruskal-Wallis, and stronger on log-ratio (p = 0.0003, η²p = 0.122). Games-Howell vs control: 14-day −4.21 PPG (g = 0.98, p = 0.001); 21-day −4.05 PPG (g = 0.88, p = 0.005). Proposed reduced-input options: 14- and 21-day intervals at 0.5 L ha⁻¹.
Introduction
Verticillium wilt (V. dahliae) is a major soil-borne disease in Australian cotton. The pathogen persists as melanised microsclerotia; viable inoculum levels drive next-season disease pressure. Inputs that suppress in-season increases in viable propagules reduce future pressure and extend field productivity.
Sero-X is registered at 3 × 2 L ha⁻¹ through the season. This trial evaluates whether reduced rates and modified intervals can match or improve efficacy against viable inoculum. Objective: compare 14-, 21-, and 28-day intervals at 0.5 L ha⁻¹ versus current label and untreated control, using the change in PPG from PrePlant to PostIncorp as the endpoint. Dry-plate PPG is the accepted benchmark; efficacy is expressed as the difference in seasonal PPG change relative to the control.
Trial Design and Treatments
Site & Design
LT-1 (AFF Field 41), Lower Namoi Valley; randomized complete block, five treatments × three plots; 0.96 ha per plot; 33 samples per treatment (10 single-plant + 1 bulk per plot).
Intervals @ 0.5 L ha⁻¹
- T1 (SX_14): every 14 days; 6 in-crop + 1.0 at defoliation; total 4.0 L ha⁻¹.
- T2 (SX_21): every 21 days; 4 in-crop + 1.0 at defoliation; total 3.0 L ha⁻¹.
Comparators
- T3 (Label): 2 L ha⁻¹ at early/mid flowering and defoliation; total 6.0 L ha⁻¹.
- T4 (SX_28): every 28 days; 3 in-crop + 1.0 at defoliation; total 2.5 L ha⁻¹.
- T5 (UTC): untreated control.
Applications spanned early flowering to defoliation (Jan–Apr 2025); fixed 1.0 L ha⁻¹ top-up at defoliation for all interval treatments.
Assay, Sampling, and Change Metrics
PPG Assay
Dry-plate: five replicate plates of 0.2 g dry soil; 23 °C, 14 days; colonies counted at 20×. PPG = mean count × dilution. Detection ~1 propagule g⁻¹; zeros indicate below detection. Zeros retained; ln(x + 1) used for log-ratio.
Sampling Dates
PrePlant (Oct 2024), Mid season (Dec 2024), Post Pick (Apr 2025), PostIncorp (Jun 2025). Efficacy endpoint: change between PrePlant and PostIncorp.
Change Scores
- Absolute: Δ = PostIncorp − PrePlant (PPG).
- Log-ratio: ln(PostIncorp + 1) − ln(PrePlant + 1).
Descriptive Results
Control Trend
UTC increased from 1.52 to 5.68 PPG; Δ = +4.16.
Short Intervals
SX_14: 3.15 → 3.11; Δ = −0.05. SX_21: 4.00 → 4.11; Δ = +0.11.
Intermediates
Label: Δ = +2.45. SX_28: Δ = +1.60. Log-ratio means followed the same order.
PPG levels were low overall, but treatment effects on seasonal change were pronounced, with the 14- and 21-day intervals holding PPG near baseline.
Omnibus Treatment Effects
Variances were homogeneous (Levene p = 0.70). Normality held for SX_14, SX_21, UTC; violations for Label and SX_28 motivated robustness checks. Treatment effects were significant across ANOVA, Welch, and Kruskal-Wallis. On the log-ratio scale, effects strengthened (η²p = 0.122) with Welch p = 0.0001 and Kruskal-Wallis p = 0.0004.
Pairwise Contrasts vs Control
SX_14
Δ difference −4.21 PPG [−6.26, −2.16]; Hedges’ g = 0.98; p = 0.001; log-ratio p < 0.001.
SX_21
Δ difference −4.05 PPG [−6.25, −1.86]; Hedges’ g = 0.88; p = 0.005; log-ratio p = 0.001.
SX_28
Δ difference −2.57 PPG [−5.17, +0.03]; g = 0.47; p = 0.311; log-ratio p = 0.147.
Label
Δ difference −1.71 PPG [−4.18, +0.75]; g = 0.33; p = 0.654; log-ratio p = 0.693.
Discussion: Mechanism and Practicality
All treatments showed seasonal increases in viable propagules, consistent with senescing host tissue, microsclerotia formation, and stubble return. The key question is the magnitude of increase versus the untreated control. The 14- and 21-day intervals held the seasonal rise to less than one PPG, while the control accrued over four, yielding large effects (g = 0.98 and 0.88).
Mechanistically, efficacy requires presence during microsclerotia formation/germination; shorter intervals maintain effective dose continuity. The 21-day interval matched 14-day efficacy with fewer applications. On cost-benefit grounds, 21-day totals 3.0 L ha⁻¹ (half the 6.0 L ha⁻¹ label), while 14-day totals 4.0 L ha⁻¹. Both delivered equivalent or superior suppression at reduced seasonal loading.
Conclusion and Label Proposal
Proven Suppression
Sero-X 0.5 L ha⁻¹ at 14- or 21-day intervals (with 1.0 L ha⁻¹ at defoliation) significantly suppressed seasonal PPG increases relative to control (Hedges’ g ≥ 0.88).
Reduced Loading
21-day: 3.0 L ha⁻¹ total (half of 6.0 L ha⁻¹ label). 14-day: 4.0 L ha⁻¹. Both are reduced-input options.
Primary Option
Propose 21-day interval as primary reduced-input label option: fewer applications, equivalent efficacy at half the current loading.
Sero-X: Long term trial timeline