The life cycle of septoria is not a precise science. Much has been made of certain elements, such as rain splash, in terms of explaining how certain components of weather impact on disease development. However, a recent piece of research on the subject of the full life cycle of Septoria tritici by Julie Smith of ADAS has helped unravel elements of epidemic development and how we can intervene in previously unknown ways.

The specific research was funded by BASF and focused on how epoxiconazole and Xemium impacted on disease control and development. While the results may be specific to these two actives, they point to aspects of septoria development that have not been known before.

Julie’s work can now be used to help lessen the pressure from septoria, both on the crop and the fungicide. This is especially important today as we enter a troubled time for our remaining single family of septoria active fungicides, the SDHIs.

By giving our fungicides a helping hand, we can help to secure a longer life span for the SDHIs and potentially reduce the cost of disease control in the process. And this is even before we begin to look at the contribution from more resistant varieties.

Making an epidemic

What makes a disease epidemic? The answer depends to some degree on the specific disease, but they are all impacted by the amount of inoculum that exists at the start of the season.

The first thing to know about septoria is that it has both a sexual stage, which is little researched, and an asexual stage, which is better understood and is associated with leaf lesions and pycnidia (see Figure 1). While the two phases are often looked at in isolation, they act in unison to produce the disease cycle.

The sexual ascospore stage is likely to be the main source of crop infection in the autumn. These small, light spores can become airborne and carry septoria from field to field, probably from county to county and possibly from country to country.

This ascospore cloud is generally thought of as a significant autumn event where earlier-sown crops are at higher risk of infection. But the production and dispersal of ascospores may well be a more significant feature for the full life cycle of the crop and the disease.

The asexual pycnidiospore

This is the phase of the disease that we see and know on the leaf. The more lesions and pycnidia present, the more spores are available to re-establish and propagate the disease.

This phase of the disease is impacted by heavy rainfall which helps to physically move septoria spores up the canopy via rain splash. Damp conditions then help the infection process to propel the epidemic.

Rain splash helps propel the pycnidiospores, which are exuded from the pycnidia in damp conditions (see Figure 2), further up the plant canopy. Wet leaves are hugely important for successful germination and infection, which must take place through the leaf-breathing stomata.

The germ tube of a germinating spore must penetrate the leaf surface through the stomata. If this does not happen before the energy reserves of the spore are used up, then infection will not occur.

Julie indicated that a significant percentage of spores fail to re-infect. But for the spores that do infect, the hyphae of the fungus then grow inside the leaf tissue and take their energy from the plant cells.

When a significant amount of hyphae develop in the leaf, the fungus is triggered to produce lesions and fruiting bodies, or pycnidia. Spores are produced in the pycnidia and when conditions are right, these ooze out and are dispersed by rain splash or leaves rubbing together in damp conditions (see Figure 1).

The infection, lesion and spore-production phases continue in cycles and the disease will progress if it is not contained either by genetic resistance or use of fungicides.

The time period needed from infection to the production of lesions – the latent period – is influenced by:

Nowadays, fungicides have less ability to halt a developing infection, so much of their activity is in preventing reinfection of the leaf by spores from previous infections.

Every time a fungicide does this, it delays the start of the next buildup and so slows the development of an epidemic. But eventually this defence will weaken and disease buildup will recommence unless another fungicide treatment is applied.

In this way, fungicide intervention can slow this process, delay the development of the disease and protect the leaves to help deliver full yield potential. But the work carried out by Julie Smith asked many more questions in terms of how individual fungicides interact with the development of septoria.

Intensity of infection

How do we measure septoria infection? At a basic level, it is the general cleanliness of a wheat crop. But researchers attempt to be more precise and measure the proportion of the area under specific lesions relative to the size of the leaf. But there is also the area surrounding the lesions which can be chlorotic – the yellow halo that surrounds an advancing lesion (see Figure 3).

Disease advance is generally measured using one or other of these definitions. But Julie went a lot further in her measurement of the developing disease. Rather than just measuring the lesion, she actually counted the number of pycnidia in the lesions and, by default, found the density of the pycnidia in the lesions. These measurements were conducted on untreated flagleaves and also on flagleaves that were treated at GS32 with either Xemium or epoxiconazole.

The results were both interesting and unprecedented. In the very early days of epoxiconazole, it was common to see lesions without pycnidia because of the high initial curative ability of this active.

This told us that the most active fungicides had the capacity to interfere with septoria development in the leaf post-infection. Plots treated with Xemium had fewer pycnidia than the untreated or the epoxiconazole treatment.

Then came the next measurement – the number of spores per pycnidia in each treatment. This showed a very significant reduction in the number of viable spores in the pycnidia of different treatments. Again, Xemium had a very significant benefit in terms of decreasing the number of viable spores in each pycnidium.

The infection process

A separate piece of work looked at the impact of the level of initial infection on septoria development. I commented earlier that the concentration of septoria hyphae within a leaf influenced how quickly that infection would break through to form a lesion. In other words, low infection pressure is likely to have a longer latent period than high pressure infections. This implies that anything we can do to reduce infection pressure could impact on the subsequent development of an epidemic.

To look at this theory, a trial was conducted on spring wheat in a predominantly grassland area. Spring wheat was chosen because the crop needed to be free of septoria at the time of treatment to test the impact of spore concentration at infection on the development of septoria.

In this case, pycnidiospores were applied artificially at different concentrations at flag leaf emerged stage and the plots were mist irrigated to promote infection.

The results showed that higher initial infection concentrations resulted in more septoria. The results also showed that lower initial infection resulted in less septoria infection later. So anything and everything we can do to reduce infection inoculum level will reduce the pressure in the crop.

Another experiment looked at the impact of the T1 treatment on septoria development. The results showed that the use of Adexar at T1 timing significantly reduced the overall disease pressure and septoria infection. It also showed that the higher rate of Adexar was even more successful and that Adexar at T1 followed by Ignite (Opus max) at 1.5l/ha at T2 had much less septoria on the flag leaf than where the Ignite (epoxiconazole) was applied at T1 and the Adexar at T2. Basically, actions which decrease the prevalence of the disease at T1 reduce subsequent pressure.

Julie said that other ADAS work had examined literature on septoria epidemiology. This concluded that the most significant factor in the development of resistance was the number of latent periods that occurred per year. So everything that slows disease development, decreases infection pressure, delays infection and lengthens the latent period helps to slow disease development and delays the onset of resistance.

The sexual stage

This is the second phase of the disease and it can co-exist with the asexual phase within the crop. Late in the growing season, septoria produces a different type of spore bearing mechanism called an ascus. This process allows a level of sexual recombination and produces much smaller spores called ascospores. These small spores can actually be taken up and moved around in wind currents.

Most of the research on septoria development has been done on the asexual phase because it is easier. But the asexual phase is very real and is thought to be a significant source of initial infection in the autumn as ascospores blow in the wind.

The production of ascospores appears to be influenced by temperature and moisture, with warmer temperatures likely to give higher release. For this reason, early planting is subject to higher infection in autumn, with late-sown crops likely to miss most of this initial autumn infection.

Because ascospores are a universal source of infection, it is difficult to work with this phase of the disease. However, Julie attempted to minimise this influence by conducting a trial in a very isolated grassland region in Wales. Wheat was grown in an isolated field and large plots were sprayed twice with either epoxiconazole, Xemium or were left unsprayed. In the following autumn, smaller plots of wheat were drilled by min-till into one-third of the previously treated plots, with winter barley planted in the majority of the field. The stubble from the previous crop was left close to the surface to provide a source of ascospores to infect the newly emerged crop.

In this isolated scenario, it was assumed that the more disease that was present on the previous crop plots the more ascospores would be produced to pressure the following crop.

As with pycnidiospores, the greater the concentration of ascospores that arrives on a crop leaf the higher is likely to be the disease pressure thereafter.

So, taking all of the previous findings on the effects of Opus and Xemium on the development of septoria within the asexual phase on the plant, it is reasonable to assume that there would be less disease coming from the stubbles where Xemium was used relative to Opus and that this should be lower than the untreated.

So what was the situation with regard to ascospore production?

Julie looked at ascospore production during November and December in these isolated plots. Using specific equipment to trap the airborne spores, she found that spore release was much lower where two sprays of Adaxar were used at T1 and T2 of the previous crop than where two sprays of Opus were used or indeed the untreated. The use of Adexar on the previous crop resulted in lower disease pressure on that crop and on the following crop due to reduced ascospore production. In a local sense, the less spores that come on to a crop the lower will be the subsequent septoria pressure.

In conclusion

Where Adexar was used in the crop, it decreased the area killed by septoria lesions, resulted in fewer pycnidia produced per lesion, massively reduced the number of pycnidiospores produced per pycnidia, prolonged the latent period of the disease within the plant leaf and reduced the amount of ascospores produced later in the season to re-infect the following crop.

Combined, all of the above help to reduce overall septoria pressure and this is one of the best mechanisms we have to delay the development of septoria epidemics and also reduce the pressure for resistance development to SDHI fungicides.

The research suggests that anything a farmer can do to reduce or get rid of stubble from the previous crop in the autumn has the potential to help reduce the inoculum pressure for the following crop. The development of an active soil biological system, which can quickly consume stubble and remaining straw, may also help reduce septoria inoculum pressure in the autumn. Later planting also reduces disease pressure by decreasing exposure to ascospore infection in the autumn.