With low levels of stem rust found in Yitpi wheat paddocks in the southern Mallee this week, farmers are contemplating whether a fungicide application is a feasible option at this stage of the growing season.
Reports so far suggest that the stem rust is appearing low down in the crop canopy and with most Mallee crops already at, or close to, the grain fill stage it has been suggested that a fungicide application may not necessarily deliver any real benefits.
However, stem rust is capable of inflicting significant yield losses particularly if there has been a build-up of inoculum prior to sowing, susceptible varieties have been sown and conditions are conducive (hot and humid below the crop canopy).
With this in mind, and weather conditions not always easy to predict, the decision to spray or not can be a tricky one.
Initially growers are encouraged to walk their crops and, if they do discover disease, to talk to their agronomist.
What to look for
Stem rust is characterised by reddish-brown, powdery, oblong pustules that mainly occur on the leaf sheaths (the leaf that surrounds the stem) and stem. The pustules have a characteristic torn margin.
Sometimes stem rust can also be found on the plant’s leaves; identifiable because the postules will appear on both the top and underneath of the leaf. That is, the stem rust elongated postule butsts out of the other side of the leaf in exactly the same spot. Stem rust spores are much darker than leaf rust spores, which are light brown and don’t have torn margins. As the plant matures, the pustules produce black spores known as teliospores.
Given the rain event which occurred last Saturday (October 6), if stem rust infection is present, Yitpi wheat should be showing obvious pustules in 7-10 days (Monday/Tuesday next week).
Plant pathologists recommend growers spend at least 15 minutes walking through their susceptible wheat crops (every four to seven days if disease has been confirmed in the area). If stem rust is detected, walk through the crop in a ‘W’ pattern and collect ten stems from ten random locations (total 100) to determine the percentage of stem rust infection.
Spraying considerations
There are a number of factors that need to be given due consideration when deciding whether to spray crops with fungicide.
- Rainfall – Stem rust is generally more prevalent in a wetter environment. The Bureau of Meteorology has forecast 1-5mm of rain in the southern Mallee this week, which will be followed by dry weather for the next seven days. The spring outlook is average to slightly drier.
- Temperature – Warm (15-30°C) and humid conditions are optimum for stem rust development and spread. The current forecast is for mild weather but it is expected to be hot next Monday (October 15). The spring temperature outlook is average to slightly warmer.
- Variety disease rating – The poorer the variety’s disease rating the greater the infection risk. Yitpi is susceptible (S) to stem rust so the risk of infection is high. Varieties with some resistance (a MS rating) will have a lower risk of infection.
- Level of infection – The lower the infection is in the crop the lower the risk. If infection is higher up the stem (eg the peduncle – the part of the stem immediately below the head), the consequences are likely to be greater.
- Growth stage – Stem rust is most severe in susceptible varieties when it begins to develop in the crop before flowering. Most Mallee Yitpi crops are now at the flowering to early grain fill stage, however in the Wimmera crops are mostly at late booting to half head emergence. The Department of Primary Industries recommends spraying if more than five per cent of stems are infected in susceptible crops that are at ear emergence to mid dough stage. According to DPI, a late, low level occurrence of stem rust after mid-dough will have little impact on yield.
- Costs – Fungicide application is relatively cheap with the product (Folicur @ 290 ml/ha) about $4/ha and contractor costs about $5/ha. However crop loss from the application process should also be calculated and considered. Depending on the growth stage, growers can expect to lose about three per cent of their crop if using a self-propelled sprayer and over five per cent if the spray machine has wider tyres.
- Yield potential – Risks and spraying costs need to be weighed against how well the crop is expected to perform. On average current Mallee crops are expected to yield two to three tonnes/ha, while Wimmera crops are likely to average between three and four tonnes/ha.
- Wheat price – Knowing what a crop is likely to make can be helpful when making decisions about fungicide applications. Currently APW is selling for approximately $270/t (as of October 9).
Research
Last year BCG undertook trials when stem rust became apparent in late October/early November and no yield advantage was achieved through fungicide application (BCG 2011 Season Research Results, pp 115). In this trial 40-95 per cent of control of stem rust was achieved depending on product type and rate. All fungicide treatments resulted in the control of stem rust, with the highest rates of active ingredient giving the best control. At the high rates tested, there was no difference between Amistar Xtra, Folicur, Opera and Prosaro, all of which gave 85-95 per cent control of stem rust.
From this research the recommendation was, “If growing stem rust-susceptible varieties such as Yitpi be vigilant. Check crops regularly and apply Folicur at 290ml/ha if the disease is observed at late head emergence or before.”
For more information about foliar fungicides for stem rust control can be found here.
Fungicide spraying
Regardless of what ‘research tells us’ for many growers spraying will be the right option, sometimes simply for ‘peace of mind’. If you do decide to spray, be aware of withholding periods, no spray buffer zones and the weather! If rust is low down in the canopy, keep water rates up and speeds down to achieve the best penetration and spray under cooler, overcast conditions. Use course droplets and choose nozzles that complement your selected product but remember, when it comes to coverage, water is King!
