Septoria leaf blotch (septoria tritici) is the most damaging foliar disease of UK wheat, causing significant yield loss in most years.

With the first case of the disease’s resistance to SDHI fungicides recently confirmed in Ireland, and wet weather cycles creating perfect conditions for the disease to thrive and spread, growers need to adopt an early and comprehensive crop protection programme in order to maintain adequate disease control.

1. Understand the life cycle

Infection usually occurs when airborne spores land on the leaves of the developing plant. The fungus grows within the leaf but the leaf exhibits no symptoms during a latent period of 15-18 days.

In the UK, our warm, wet springs and humid summers provide the perfect growth conditions for the disease, with rain splash translocation helping the disease to spread between leaves.

Septoria can remain dormant on crop debris, trash, grass hosts and volunteers over winter, and re-infect plants with the arrival of warmer weather.

Early-sown (September) crops are typically infected quickly by septoria, thus giving the disease more time to develop in favourable autumn conditions.

0249_Septoria_Disease-Lifecycle-Diagram_v2

2. Recognise the symptoms

Older leaves show symptoms of disease in the autumn: brown, oval-shaped leaf lesions, which contain easily recognised black dots, known as pycnidia, are clear indications of the disease.

Several thousand spores can be produced within one lesion, but just one spore can generate a single lesion. Lesions will join together to cover large areas of the leaf, turning it brown.

The position of the inoculum in the canopy is important: lesions that are adjacent to a newly emerged leaf allow the spores to transfer very readily, giving a higher risk of infection. Lesions that are 30-40cm below emerging leaves are a lower risk, especially if dry conditions prevail.

3. Know the impact on potential yield

Septoria-tritici-risk-levels-resized2

In a high-pressure season, septoria infections can impact wheat yields by 30% to 50%, with areas of higher rainfall seeing the biggest yield losses.

1% of disease coverage on the flag leaf will give a 1% yield penalty as a result of a reduction of the plants capability to photosynthesis energy and cellular production.

The economic value of septoria yield losses is calculated by the University of Exeter to be between £89m and £180m each year, with the final figure depending on the severity of infection.

In 2014, when septoria levels were very high, the disease affected 99% of UK wheat crops.

While it has always been the biggest threat to UK cereal crops, septoria has come into sharper focus as the fungicides available for its control have started to lose some of their efficacy against the disease. In particular, the loss of curative – or kickback – activity has put protection at the centre of control strategies.

4. Adopt integrated control methods

Airborne ascospores of the disease are always present from the previous harvest, so rotations and cultivations will do little to control the disease or limit its spread.

Effective management therefore requires a package of chemical control measures as well as a series of cultural approaches: selecting disease resistant varieties, later drilling date and improved crop husbandry.

Pushing the drilling date back by three weeks or so into October is advised, as this will reduce the crop’s exposure to inoculum, without having too significant an effect on yield. To ensure successful establishment, greater attention must be given to seedbed conditions when using a later drilling date.

Fortunately, varieties with good resistance are starting to appear. While most of the winter wheat varieties on the AHDB Recommended List have scores between 4 and 6, there are now a few varieties with a rating of 7.

These can be used strategically reduce the farm’s exposure to the disease in wet years, although they are still likely to need spraying.

Despite the arrival of improved varieties, fungicide use remains essential, with the timing of application essential in order to protect wheat leaves so that infections are unable to really take hold.

Given the decline in curative activity from azoles, it is no longer possible to rescue a disease situation that has got out of control. For this fungicide programmes must offer protection from the disease.

For most growers, a four spray programme will be effective, although there may be a need for an additional spray in very difficult seasons.

Spray Timings
T0 Protects leaf 4 at GS 30 (2-4 weeks before the T1 spray)
T1 Protects leaf 3 at GS 31-32 
T2 Protects leaf 1 (flag leaf) at GS 39 
T3 Protects the ear at GS 61-65 
N.B. In some cases, a T1.5 spray may be required to protect leaf 2

Optimum-Spray-Timing-Diagram-resized

5. Know your armoury

The septoria fungicide toolbox is limited to three main groups of chemistry: azoles, SDHIs and multi-sites.

Although other fungicides – such as morpholines – can have some effect on the disease, they are not active enough to be used as a key component of a septoria strategy. Strobilurins are ineffective against septoria due to widespread resistance, but still have a place for other diseases.

While azoles still provide some benefit, they are no longer fully effective as curative products: trials have shown that the preventative effect of azoles has reduced by 20% over the last 10 years, while their curative efficacy has fallen by 60% or more.

Similarly, SDHIs are less effective as curatives, although they are still useful as preventatives – but only if applied early enough. They do however need to be used in moderation in order to protect against increasing levels of disease resistance.

This leaves multi-site fungicides as a vital tool in septoria management: actives such as folpet are protectant in action and have no known resistance. They are also active on various sites within the pathogen, unlike the other single site fungicide options, so remain at a very low risk of resistance.

Single Site Vs Multi Site

Stacking and sequencing of azoles is recommended so that different strains of septoria are controlled and not exposed to repeated applications of the same chemistry. In addition, SDHI chemistry should be employed in programmes, according to the latest guidelines on their use.

Azoles

Azoles belong to a group of fungicides known as demethylation inhibitors (DMIs). The two strongest products for septoria control are prothioconazole and epoxiconazole. These have been the backbone of most septoria control programmes for a number of years, and despite their drop in performance and loss of eradicant activity in recent years, remain an important component.

Others, including metconazole and tebuconazole, still have a place in spray programmes. The azoles are considered to be at medium risk of resistance, according to the Fungicide Resistance Action Committee.

SDHIs

SDHIs – or succinate dehydrogenase inhibitors – are a newer introduction. They include active ingredients such as bixafen, isopyrazam, penthiopyrad and fluxapyroxad.

Although some are sold as straights, they are usually co-formulated with a triazole component.

Despite the first signs of reduced sensitivity appearing in 2015, they are highly effective against septoria and offer broad spectrum control of other foliar diseases, and are mainly protectant in their activity. The SDHIs are classed as being of medium to high risk of resistance.

Multi-sites

Multi-sites are fungicides that affect a number of different metabolic sites within a pathogen. They include actives such as folpet, chlorothalonil and mancozeb; most of which have been on the market for a considerable length of time and are used on a range of crops where they remain as effective as they were when first introduced.

Multi-sites are protectant materials, with a very low risk of resistance, making them suitable for resistance management strategies.

The main septoria fungicide groups

 Azoles  SDHIs  Multi-sites
 Declining efficacy  High efficacy  Moderate efficacy
 Partial resistance Septoria currently well controlled  Low resistance risk
 Need to protect for future Single site mode of action  Regulatory threats
 Reduced curative activity Need to protect for future  
 At risk from regulation    

6. Build your fungicide programmes

Building a robust fungicide programme, consisting of multi-site, SDHI and azole chemistries, is the key to managing septoria effectively.

Early control, using a protectant rather than an eradicant, is essential to limiting the disease’s impact.

This ‘prevention is better than cure’ approach means that a greater emphasis must be given to earlier spray timings so that leaves are protected as they emerge and not exposed unnecessarily to disease inoculum.

A multi-site, such as folpet, should be the first port of call for early T0 and T1 sprays as it will not only provide effective protection, but will also extend the existing SDHI armoury by preventing resistance.

Fungicide-Strategies-resized

In practice, folpet should be applied at T0 and T1 at 1-1.5l/ha, depending on the partner product and disease risk situation. At T2, folpet (as Phoenix) has been shown not to impact on triazole uptake, making it a better choice than other multi-sites at this key spray timing.

Multi-site fungicides should be used as follows:

  • T0 (early spring): before stem extension to reduce infection and its spread to developing leaves
  • T1 (GS31-32): to eradicate disease and provide protection to emerging leaves.
  • T2 (GS37-39): to protect leaves from secondary infection

(NB: check labels to prevent loss of activity of co-mix fungicides)

SDHI use should be limited to two applications per crop, and should also be used in conjunction with at least one other fungicide from an alternative mode of action. This will not only ensure better crop protection, but will also reduce disease resistance to SDHIs.

As the season develops, the spray programme can be adjusted according to the weather and the disease pressure. The use of a T0 spray gives some leeway if the subsequent T1 treatment is delayed by poor weather or heavy workloads.

The spray window for effective disease control on a particular leaf layer is relatively narrow. Spraying too early could mean that the leaf is not fully emerged, while too late will mean that the disease may have had time to become established.

The optimum spray timing is when a leaf has just fully emerged. In wheat, the upper three leaves appear at approximately 10-14 day intervals – with the exact period depending on temperature. The optimum spray timings for each of the upper three leaves are therefore 10-14 days apart.

Cereal Growth Diagram

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