The efficacy data needed for product registration doesn’t necessarily answer all the questions about the performance of a new active ingredient. CPM gets an insight on a series of trials which set out to test the boundaries of Revysol.
The pressure exerted by 100M septoria spores/ml is akin to a nuclear strike to the plant.
By Lucy de la Pasture
The unique chemistry of Revysol (mefentrifluconazole) gives the new azole some key attributes that should translate into superior performance in the field. Tasked with finding out whether it would live up to expectation, ADAS senior research scientist Dr Julie Smith set out to test Revysol’s capabilities.
“There are five main attributes that BASF believe Revysol has because of its chemistry. These are superior rainfastness and rapid uptake of the fungicide, contributing to strong curative activity; long lasting performance in the field; maintenance of efficacy under high disease pressure; effectiveness against DMI-adapted (insensitive) strains of septoria; and a benefit on resistant varieties,” explains Julie.
To put Revysol through its paces she designed a series of field trials to tests these probable attributes, using it as a straight rather than the co-formulated product, Revystar XE (mefentriflucoazole+ fluxapyroxad), which is the one that will be available to growers.
“To look at Revysol’s rainfastness, we set up a trial where we were able to irrigate overhead the plots and compare the performance of different fungicides, with and without a period of ‘rainfall’ after application. To mimic a shower of rain we irrigated for a 15-minute period, 15 minutes after spraying,” she explains.
“We allowed disease to develop then assessed septoria levels on the flag leaf for each fungicide in the irrigated and unirrigated plots and found that Revysol was the most rainfast, with only a very small reduction in control when it was irrigated 15 mins after application.”
Revysol is also believed to have good curative activity and to assess whether this is truly the case, Julie set up a field experiment to compare it with other azoles. The field plots were inoculated with field isolates of Zymoseptoria tritici on 17 May 2019 and fungicide applications were made at either one, five, 10 or 15-day intervals after inoculation, followed by disease assessments at GS65 and GS73.
“This trial is a good test because as well as natural background infection in the field, there’s a precise infection event. The results show septoria control from Revysol was consistently highest of all treatments, for all application dates, on both leaf one and leaf two. For leaf two (the leaf that was fully expanded at time of inoculation) Revysol gave 30% higher efficacy than the next best azole, prothioconazole, for 1-12 days after inoculation, indicating 12 days of curative activity,” she explains.
Julie highlights that Revysol is providing more than 80% control of septoria in a shifted population, even when it’s applied a week after infection. In a stronger test, when Revysol was applied 14 days after inoculation, the new active is still giving more than 50% control.
“On leaf three, there’s also evidence of strong curative activity. Revysol has more than twice the efficacy of the older azoles for all fungicide applications over the 12-day period after leaf two was inoculated,” she adds.
“In practical terms this means that you would get the same level of control from Revysol applied 14 days after infection as you would with a well-timed prothioconazole application.”
Given the slide in efficacy of the current azoles, the ability Revysol has to control insensitive strains of septoria is of key interest to growers and agronomists, highlights Julie. To see how the efficacy of Revysol compares, a field trial was conducted using a range of different septoria strains.
“We looked at Revysol’s ability to control DMI-adapted (azole insensitive) strains of septoria by setting up a trial with a number of different inoculation treatments. We included a control where we allowed septoria to develop from natural field infestation. For the other treatments, increasing septoria pressure was achieved by inoculating with local field isolates (Hereford), applied when the flag leaf was fully expanded, at either 1M spores/ml (106), 10M spores/ml (107) or 100M spores/ml (108), and DMI-adapted septoria was applied at 10M spores/ml.
“To put these treatments into perspective, the pressure exerted by the 10M spores/ml level of inoculation is extreme and 100M spores/ml is akin to a nuclear strike to the plant,” describes Julie.
Fungicides were applied five days post-inoculation using Revysol (1.5 l/ha), Proline (0.72 l/ha), and compared with an untreated control. Septoria spores can germinate within 12h of hitting the leaf surface so waiti8ng for five days before applying the fungicide is a robust test, she assures. The progress of septoria was monitored and Revysol demonstrated its ability to protect the crop under all disease pressures, including the shifted strains.
“I still gasp when I see 80% of a flag leaf covered with septoria, but these are the disease levels that we achieved with some of of our inoculation treatments. But there’s very little difference between the control from prothioconazole and the untreated. Under the highest inoculated pressure most fungicide products would break but Revysol still performed,” she comments.
“The levels of control from Revysol when under pressure from the DMI-adapted strains was good, so this work supports the claim that it will currently control insensitive strains,” she adds.
The adoption of more septoria-resistant varieties is part of an integrated approach to controlling the disease, but is there still a benefit from applying Revysol?
Julie and her colleague Chloe Morgan investigated this by setting up a ‘domino’ experiment using a range of varieties at different ends of the resistance scale – Santiago (AHDB Recommended List rating for septoria 4.3). Hardwicke (5.8) and Graham (6.9).
“We established three disease pressures, which were natural infestation; natural plus inoculation with field isolates; and natural plus inoculation with DMI-adapted strains. Fungicide treatments were an untreated control, Proline (0.72 l/ha), Revysol at full-rate (1.5 l/ha) and at half-rate (0.75 l/ha).
“We found that there was a benefit to applying Revysol irrespective of the wheat variety, although the disease control and yield benefit this was biggest in the more septoria susceptible variety,” she adds.
“The important thing is that we don’t rely solely on fungicides or varietal resistance, because doing so is likely to result in a more rapid breakdown of either. We need to integrate our disease management approaches and use actives to protect resistance genes and vice versa, to prolong the performance and longevity of both our chemical and genetic resources.”
Performing as an azole used to
To investigate the long-lasting performance of Revysol, Julie set out to replicate some of the work that led to the basis of the spray timings we currently aim at to provide the best septoria control –
GS32 (to protect leaf three) and GS39 (to protect leaf one), with the combined timings protecting leaf two. This original work is summarised in an iconic graph published within AHDB’s wheat disease management Guide.
“We set up a ‘wave’ experiment on five sites over two seasons (2017-2018). Fungicides were applied either five, 10 or 15 days before and after the optimum T2 timing (GS39) and we were able to analyse the results and identify the level of control from Revysol and Proline either side of the optimum timing. This indicates the fungicide’s flexibility at different spray timings, providing the ‘spray window’,” she explains.
“The results show the decline in standard azole performance since the original work was done, with the levels of disease control from prothioconazole application falling well short of the ‘curve’. For Revysol, the data shows its performance is pretty much what we’d have expected from an azole before their performance was eroded by the evolution of a less sensitive septoria pathogen population.”
While the level of disease on the main leaves that contribute to yield is a useful assessment, looking at the Healthy Area Duration (HAD) for each fungicide treatment provides a measure of both canopy size and its duration. This data can help explain yield effects, highlights Julie, and is also the most objective and unbiased way of assessing the performance of a product because it’s not just a snapshot in time.
Analysis of yield across all five sites demonstrated a yield response for the Revysol treatments of just over 1t/ha above prothioconazole at GS39, maintaining an advantage of nearly 0.5t/ha even at the GS39 plus 15 days timing.
“A cross-site analysis of HAD showed that Revysol treatment produced a much higher HAD than prothioconazole at all the treatment timings, which helps explain the higher yields in the Revysol treatments,” she explains.
The range of spray timings either side of the optimum timing at GS39 produces a bell-shaped curve for the yield data. “The shape of the Revysol curve shows there is a spray window either side of the optimum timing, further supporting the curative and protectant claims for the new azole.”
The latent challenge
One of the most difficult decisions to make in the field is the best approach to take at T1 and T2. “There’s always a debate about whether there’s an opportunity to cut back on chemistry, but the big unknown is how much latent infection is already present in the leaves you’re trying to protect,” says Julie.
To get more insight into this, Julie selected leaves with little or no disease visible when fungicide was applied. When the fungicides were applied at GS39 plus five days, septoria was visibly present on the flag leaves at a level <1% but DNA analysis of the septoria present inside the leaves painted a very different picture.
“The untreated had the highest amount of septoria DNA, as expected, and prothioconazole treated had significantly more septoria DNA than the Revysol-treated leaves. The level of septoria within leaves was tracked using DNA analysis on a weekly basis to monitor its progressions and even in the most curative treatment, GS39 plus 15 days, the control from Revysol was still good,” says Julie.
Additional data from Cura Crop research, carried out for BASF in 2017, supports the results of the ADAS work, says Steve Dennis, BASF’s head of business development in the UK.
“When a septoria spore lands on a leaf, it germinates and hyphae enter the leaf through its stoma, producing a mycelium within. The fungus gradually invades the tissues but it’s not until cell death occurs that visual symptoms on the leaf become visible. So by the time the pycnidia can be seen, the damage has already been done within the leaf itself,” he explains.
The aim of the Cura Crop research was to bust the myth that no visible infection means no disease, even in a lower disease year.
“Leaf two is generally out for some time without any fungicide protection, so for the Cura Crop work we asked growers for a visual assessment of septoria on leaf two just before the T2 timing and 60% of them thought their crops were free of disease.”
“We collected leaves one and two, to represent the top of the plant, and leaves three and four which received a fungicide at T1. Latent disease levels could then be clearly detected by DNA analysis. At the T2 timing, the vast majority (75-90%) of lower leaves (L3 & L4) were infected and 50-65% of upper leaves (L1 & L2) were also infected.
“Even though it’s not intended to apply a T2 fungicide as a curative, in most cases there is latent septoria present in the upper leaves and infection on the lower leaves at the time of application,” he comments.
“Leaf two is the forgotten leaf and we often forget how much we’re really expecting T2 fungicides to deal with,” he adds.
In the ADAS work Julie looked at the effect of Revysol on spore production. “Because septoria is polycyclic, what happens on one leaf layer will influence disease on the other leaf layers, so inhibiting septoria spore production is an important function of a fungicide.
“We induced spore release and quantified the number of spores in untreated, prothioconazole and Revysol-treated plots. There were significantly fewer spores released where Revysol was applied, resulting in a lower inoculum load to initiate the next cycle of disease which will decrease the epidemic in the upper canopy,” she explains.
Under a tsunami of disease
ADAS also looked at the efficacy of Revysol under differing disease pressures using a tsunami experiment at a site in Hereford, explains Julie.
“We wanted to set up different disease levels in a single randomised trial so we designed a series of treatments to manipulate disease pressure, ranging from very low septoria (<3%) to very high (>75%). Each level of disease was then subjected to fungicide treatment with either Proline or Revysol. We erected rain-out shelters to mimic a dry season and low disease pressure and irrigated plots to increase disease pressure trough natural routes. To increase septoria pressure further, we irrigated and inoculated with a mixture of field isolates and insensitive strains. The control was natural field conditions.
Source: ADAS, 2019
“The results show an increasing separation in the performance of Revysol and prothioconazole as disease pressure increases. Our programme of work validates the claims made by BASF about the efficacy of Revysol in the field. We tested Revysol using the most robust methodology available and were not able to break it.
“Revysol consistently out-performed competitors irrespective of disease pressure, pathogen insensitivity or wheat variety. Despite the tremendous challenges that we currently face BASF have re-set the clock in terms of azole efficacy against septoria leaf blotch and that achievement should not be underestimated,” she concludes.
