It says that growers should establish how much STB is in the crop. If the old leaves have gone brown and are covered with small black spots (pycnidia) there is a high disease level. If the older leaves are dying off and are yellow with few pycnidia there is a low disease level.

Once Septoria tritici blotch (STB) is established in the base of the crop the key driver for disease spread is wet weather. For an infection to occur spores move by rain splash or wet leaves rubbing against each other in the wind. At Chertsey the rainfall for September to date is below average at 27 mm but it has been very humid. It is considered that 20 hours with a relative humidity over 85% is a sufficient period for infection to occur. The infection pressure increases in severity as the period of high humidity extends to 48 hours and longer. In Canterbury over the last week there have been two periods of 40 hours plus with the relative humidity over 85%. Therefore, the risk of STB progressing up the plant from wet weather was moderate to high in late September in Canterbury. Weather can be variable over short distances so use local data when evaluating disease risk.

If there is none or a low level of STB visible in the base of the crop, the T1 fungicide could be triazole alone. If the level of STB is higher combined with the weather conditions that have been conducive for STB infection in the last 7 days, consider more fungicide at the T1 timing. For example, if growing STB moderately resistant cultivars such as Starfire and Wakanui consider the fungicide options from “More susceptible scenarios with higher disease pressure” in the latest edition of Cropping Strategies – Cereal Disease Management as follows.

Where disease pressure is high as a result of wet weather, early sowing or susceptible cultivars, mix the triazole (Proline or Opus) with SDHI fungicide e.g. Seguris Flexi (isopyrazam) (0.8 – 1.0 L/ha) or use the pre-formulated mixtures of SDHI and triazole such as Aviator 0.7- 1.0 L/ha (prothioconazole and bixafen) or Adexar 1.0 - 1.25 L/ha (epoxiconazole and fluxapyroxad). Since the dose rate of the triazole in these pre-formulated mixtures is less than a full label rate for the triazole alone, (Aviator at 0.7 L/ha contains the equivalent of 0.4 Proline and Adexar at 1.0 L/ha contains approximately 0.5 Opus) consider topping up the triazole component in order to better compliment the SDHI (e.g. add Proline at 0.2 L/ha to the lower label rate of Aviator or add Opus at 0.4 – 0.5L/ha to the Adexar depending on whether 1.0 or 1.25L/ha are used).

Source: FAR