Disease management update in cereals

Take home messages

• Don’t automatically farm the 2023 crop with 2022 disease management strategies.
• Following the very wet 2022 season, STB is widespread in the base of many wheat crops. However, unless it is wet during stem elongation, it is unlikely to spread up the canopy in lower rainfall regions such as the Mallee and Wimmera.
• One of the best fungicides for foliar disease is drier weather, particularly for wet weather stubble borne diseases such STB.
• Monitor conditions for disease, along with symptoms in the crop and the variety’s resistance rating, particularly during stem elongation when the top four leaves of the crop emerge. These leaves are termed the ‘money leaves’, as their activity is critically associated with the carbohydrate production for grain fill.
• The conditions during the stem elongation period GS31–GS39 (irrespective of wet or dry) will be crucial for determining whether or not fungicides will be economic for disease control.
• WPM tested across the Mallee and Wimmera in 2022 had a higher frequency of strobilurin resistant strains, relevant to Group 11 QoI fungicides such as azoxystrobin (for example, Amistar® Xtra, Radial®) and pyraclostrobin (for example, Opera®).
• When controlling foliar disease with two foliar fungicides in susceptible varieties, ensure that the interval between fungicides does not exceed four weeks, particularly if the stem elongation period (GS30–GS39) is wet.
• The number of fungicide applications over time is a key driver promoting the shift (the selection of more resistant strains) towards more resistant pathogen populations.

2022 vs 2023

2022 was one of the most conducive seasons for foliar diseases in at least 20 years, particularly for stripe rust caused by the pathogen Puccinia striiformis. In Mallee regions, susceptible cultivars gave large yield increases in response to the control of stripe rust. The 2022 season illustrated that control of diseases, such as stripe rust, is still very dependent on adopting the correct timings for foliar fungicides, particularly where we don’t have good genetic resistance to combat the disease. This was very apparent in a GRDC investment (DJPR2104-004RTX) conducted by Ag Vic, and more locally by Birchip Cropping Group (BCG), with grain yield losses of up to 50% in the most susceptible varieties (Table 1). However, we already know that 2023 is not providing the same kind of stripe rust pressure experienced in 2022, as at this stage, there have been fewer reports of stripe rust in wheat. In addition, the longer weather forecasts are predicting a warmer and drier second half to the season.

Table 1: Stripe rust severity (% leaf area affected), head infection and associated yield loss of six wheat varieties with and without disease at Nullawil (BCG), Victoria during 2022. (Table courtesy of Dr Hari Dadu, Agriculture Victoria.)

#Within column, means with one letter in common are not significant: ** = statistically significant at 5% Lsd. AHollaway and McLean (2022) Cereal Disease Guide 2022. Agriculture Victoria. BDate of assessment made and Zadoks growth stages Z51, Ear emergence; Z75, Milk development according to the Zadoks scale. C Max. disease = Maximum disease treatment; Min. disease = Minimum disease treatment. D Yield loss % for each variety was presented as percentage yield decrease vs the minimum disease treatment.

Septoria tritici blotch (STB) – are conditions conducive for this disease in low rainfall zones (LRZ)

The absence of stripe rust reports so far has been replaced with growers and advisers reporting widespread infection of STB in the base of their wheat crops. This stubble borne disease was encouraged by the very wet conditions that prevailed in 2022, and it is this higher inoculum loading on the stubble that has led to the more widespread nature of the disease this spring. The question however is ‘will this disease destroy the yield potential of 2023 wheat crops?’ The answer for Mallee and Wimmera growers is that this will depend on the weather conditions during stem elongation (GS30 start of stem elongation–GS39 flag leaf emergence). It is in this period of crop development, with frequent wet conditions, that this disease will be spread from the base of the crop to the upper leaves. If dry weather prevails in this period in 2023, it is unlikely that our losses to this disease will be anything as great as those experienced with stripe rust in 2022. In the high rainfall zone south of the Great Dividing Range, this disease will be far more problematic, since very often the best fungicide for STB is dry weather. If drier weather prevails in the period from GS30–GS39 when the top four leaves emerge, the impact of this disease in wheat will be minimal. In contrast, basal infection will spread up the crop canopy if the period of stem elongation is punctuated with regular rainfall events and very high humidity in the crop canopy.

One aspect of STB that is very important to understand is that the disease has a very long latent period, which means that the disease may have infected the crop several days prior, however the infection does not show in the upper leaves. The length of this latent period (time between the fungal spore landing on the leaf and symptoms being apparent to the naked eye can be as much 21–28 days in the spring. Another way to think about this long latent period is to understand that at flag leaf emergence (GS39), even with optimal conditions for STB, the top two leaves may appear clean to the naked eye even though they are infected.

To control both stripe rust and STB in susceptible wheats, the most effective foliar fungicide timings are 1st–2nd node (GS31/32) followed by a flag leaf fungicide. This will give protection to the top four leaves of the crop canopy. If the first fungicide is applied very early before GS31/32, ensure that the interval between the two applications does not exceed four weeks. Remember to ‘Mind the Gap’.

Wheat powdery mildew (WPM) – fungicide resistance update

Following WPM sampling conducted by Trengove Consulting in SA and FAR Australia in Victoria (GRDC investment TRE2204-001RTX), samples were collected and tested for the frequency of the G143A mutation, a mutation that renders the group 11 QoI’s ineffective (strobilurin fungicides such as azoxystrobin and pyraclostrobin). In general, there was a regional difference in the frequency of the G143A mutation, with the frequency increasing from west to east (Figure 1). The frequency was also noted to be increasing over time within regions, demonstrated in regions that have been sampled twice over time.

For the regions sampled in 2022, Vic had the highest mutation frequency, with a median of 68.2%, and it is interesting to note the presence of the G143A mutation in the Mallee region, especially given WPM is rarely identified as an issue in the Mallee. This indicates that the mutation is present in nearly all paddocks across the regions tested, and it would be expected that the use of fungicides containing a strobilurin will further select for WPM populations with this mutation. This means that using strobilurins for WPM control in the Mallee may result in poorer than expected results. With group 7 SDHIs not being particularly effective on WPM, it means we are more dependent on the group 3 DMIs (triazoles), which in Victorian trials are partially compromised themselves by reduced sensitivity (partial resistance). There are now new APVMA permits for three fungicides (quinoxyfen PER93197, metrafenone PER93198 and proquinazid PER93216) which could be used in severe WPM infections to control the disease in the regions where fungicide reduced sensitivity and resistance has been confirmed. However, it is important to note that whilst these new fungicides have a new mode of action and are effective on WPM, they are not broad-spectrum fungicides and therefore would still need to be used in mixture with other fungicides for control of diseases such as rusts or STB.

Figure 1. Frequency (%) of G143A mutation, conferring resistance to QoI (strobilurin, group 11) fungicides, for 145 samples from south-eastern Australia collected in spring, 2022.

What is the difference between resistance and reduced sensitivity?

The more widespread resistance in the WPM pathogen to strobilurin fungicides continues to develop in the eastern states as we have just described. Table 2 shows the latest situation with regards to pathogen resistance to our commonly used fungicides. When using this table, it is important to understand the various terms used to describe different levels of fungicide resistance.

Sensitive

When a fungal disease pathogen is termed sensitive to a fungicide, it is considered that the fungicide will control the disease at specified label rates.

Reduced sensitivity

Reduced sensitivity means that the fungicide does not work optimally to control the fungal pathogen but does not completely fail. In most cases, this is related to small changes in the pathogen at the target site where the fungicide binds to the fungus. It typically results in small reductions in product performance, which may not be noticeable at the field level. In some cases, for example with Group 3 DMIs against STB, growers may find that they need to use increased rates of the fungicide (as per the registered label) to obtain the previous level of control. Reduced sensitivity must be confirmed by laboratory analysis.

Resistant

Resistant means the fungicide fails to provide an acceptable level of control of the specified pathogen in the field at full label rates. Resistance must be confirmed with laboratory testing and be clearly linked with an unacceptable loss of disease control when using the fungicide in the field, at full label rates, under the correct application conditions.

Laboratory detection

Laboratory detection can detect mutations in the pathogen that effect fungicide performance before they are substantial enough to reduce efficacy in the field. In essence, laboratory detection of new mutations can give us an early warning system for problems that may affect us in the future as the proportion of the population carrying the mutation increases.

What is the current status of fungicide resistance and reduced sensitivity in Australia?

Over the last eight years, the Fungicide Resistance Group (FRG) of the Centre for Crop and Disease Management (CCDM at Curtin University) has been working with industry and other researchers to establish a fast and cost-effective monitoring system for fungicide resistance of common diseases of broadacre grain crops. Current cases of fungicide resistance and reduced sensitivity in Australian broadacre grain crops are outlined in Table 2.

Table 2: Fungicide resistance and reduced sensitivity cases identified in Australian broadacre cereal and canola crops (July 2023).

AFREN (Australian Fungicide Resistance Extension Network)

AFREN was established to develop and deliver fungicide resistance resources for grain growers and advisers across the country. It brings together regional plant pathologists, fungicide resistance experts and communications and extension specialists.

AFREN wants to equip growers with the knowledge and understanding that they need to reduce the emergence and manage the impacts of fungicide resistance in Australian grains crops.

As members of AFREN, the authors of this paper are keen to hear if you believe you are encountering reduced sensitivity or resistance in your broadacre crops.

Contact details

Nick Poole
FAR Australia
Shed 2/63 Holder Rd, Bannockburn VIC 3331
08 5266 1290
0499 888 066
nick.poole@faraustralia.com.au