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Fungicide-resistant late blight pathogens are spreading. Keeping the potatoes healthy becomes a challenge. What strategies are suitable for this growing season.
Fungicide-resistant late blight pathogens are spreading. Keeping potatoes healthy becomes a challenge. What strategies are suitable for this growing season.
Our authors: Dr. Luitpold Scheid and Steffen Wagner, Institute for Plant Protection, Bavarian State Institute for Agriculture (LfL)
Readily understood
Controlling late blight is becoming more difficult due to resistances. Double resistances are now also present. Resistant strains require suitable tank mixtures and a consistent rotation of active ingredients. Late blight incidence depends heavily on the weather. Adjust spray intervals according to the disease pressure. Alternaria control is only meaningful for starch, industrial, and late table potatoes. Even here, rotation of active ingredients is important.
What resistances are there?
Changing active ingredients and mechanisms of action
Choosing the right time to spray
The appropriate spray sequence
Choosing the right intervals
Recommendations for follow-up treatments
Effectively stopping infection
What else to consider?
How to deal with Alternaria?
Controlling late blight has always been a challenge. However, with the spread of fungicide-resistant strains, the fight against late blight now becomes a Herculean task. Only those who keep track and draw the right conclusions will successfully combat the fungus.
What resistances are there?
Due to the lack of new active ingredients, resistances are nothing new. It is important to distinguish between a gradual decrease in effectiveness (quantitative resistance) or a complete loss of effect (qualitative resistance) of certain active ingredients.
Currently, there is the "OSBPI resistance". It only affects the active ingredient Oxathiapiprolin (product: Zorvec) in Germany. When using the product Zorvec Entecta (Oxathiapiprolin + Amisulbrom) on OSBPI-resistant isolates, the late blight effect comes exclusively from the active ingredient Amisulbrom (Leimay, Gachinko), as Oxathiapiprolin shows no effect here.
The "CAA resistance" affects all active ingredients from the group of carboxylic acid amides. These include Mandipropamid (Revus (top), Carial Flex, Pergovia-Pack), Valifenalate (Voyager), and Dimethomorph (Banjo Forte, Presidium; the shelf life of both products ends on May 20, 2025).
In the case of CAA resistance, the situation is more complicated than with OSBPI/Zorvec resistance. In this resistance, it makes a difference whether it is heterozygous or homozygous.
In the case of homozygous resistance, the situation is comparable to Zorvec resistance. The homozygous resistant pathogens are not detected by means such as Revus or Revus Top. If, for example, Carial Flex (active ingredients Mandipropamid, Cymoxanil) is used instead, the effect is solely due to the second active ingredient, namely Cymoxanil.
If the CAA resistance is heterozygous, the effectiveness of the CAA active ingredients Mandipropamid or Valifenalate is still given in principle, but not to the full extent.
Although resistances occur with varying frequency in different regions, they are found throughout Germany. Therefore, no matter where you grow your potatoes, always take all possible measures to prevent further resistance spread. There will be no new active ingredients for the foreseeable future - rather, there is a concern that more active ingredients will be lost.
Changing active ingredients and mechanisms of action
First and foremost is the "mixing of late blight active ingredients" with different mechanisms of action, the so-called MoA (modes of action). In the case of OSBPI resistance, this was taken into account with the approval of Zorvec Entecta, as the product contains the active ingredients Oxathiapiprolin and Amisulbrom. Unfortunately, the application rate of Amisulbrom was reduced. Therefore, Curzate 60 WG is a useful additional partner - as recommended by the manufacturer; especially since, due to the resistance situation, it can happen that the active ingredient Oxathiapiprolin has no effect.
In the case of CAA resistance, the approval holders do not implement the mixing of active ingredients with different mechanisms of action in some cases, but in other cases, they are very successful.
Product Voyager stands out positively in this respect, as it contains the full application rate of Shirlan. The composition of the Pergovia-Pack is similar to the situation with Zorvec Entecta, as here the active ingredient Amisulbrom is significantly underdosed. More than questionable is the solo marketing of Revus and Revus Top. Although Revus Top contains Mandipropamid and Difenoconazole, which is only used for Alternaria control. Therefore, Revus Top is essentially a solo product against late blight.
Equally important are the correct "spray sequences". Therefore, do not use products with identical mechanisms of action (MoA) directly one after the other. Overview 2 shows the MoA of the active ingredients.
For example, after Voyager (MoA H5 and C5), Carial Flex (MoA H5 and U) should not be used. Because in this case, an active ingredient of the same FRAC group H5 would be used directly one after the other. However, after the application of Voyager, a Ranman Top (MoA C4) could be applied, followed by the above-mentioned Carial Flex (MoA H5, U). This way, the principle of active ingredient rotation is consistently implemented.
However, it gets even more complicated. Today, we are not only dealing with late blight pathogens that have either an OSBPI resistance or a CAA resistance. There are now numerous isolates that have double resistances. This means that these pathogens are not detected by the Zorvec active ingredient Oxathiapiprolin or a CAA active ingredient (e.g. Revus). Such resistances are also promoted by spray sequences in which Zorvec immediately follows a CAA active ingredient (Carial Flex, Pergovia-Pack, Revus (Top), Voyager) and vice versa. Therefore, avoid these types of spray sequences at all costs. Also, do not be misled by the change in MoA or FRAC code, as there are isolates with double resistances.
Choosing the right time to spray
To hit the right time to spray correctly, we can rely on forecasting models, such as ISIP, which calculate the start of treatment based on weather data. However, the models reach their limits in certain weather conditions. These include heavy rainfall that saturates the potato ridges. Therefore, observe the weather closely from the emergence of the potatoes.
With high rainfall after the emergence, it usually only takes ten days for the first stem infection to occur. To prevent this as much as possible, start spraying after rainy weather. Start the first treatment about seven days before the expected onset of infection. Such a date is close to the infection process and promises high efficacy.
If heavy rain after the emergence of the potatoes forces you to start spraying, the size of the plants is irrelevant. Even 5 to 10 cm high potato plants can show massive stem infection! However, the potatoes should be large enough to absorb enough active ingredient.
The appropriate spray sequence
Stem late blight cannot be completely prevented. However, by acting quickly and using the right products, it can be significantly reduced. Normally, systemic fungicides like Infinito or Zorvec Entecta (see Overview 1) help with this.
For example, it is useful to start with Infinito (1.6 l/ha), followed by a tank mixture of Zorvec Entecta + Curzate 60 WG (0.25 l/ha + 0.2 kg/ha) and a further application of Infinito (1.6 l/ha). With such a spray sequence, the necessary rotation of active ingredients between applications would be ensured, and the goal of fungicide mixtures with different mechanisms of action would be achieved.
However, it should be noted that the effectiveness of the mixture of Zorvec Entecta plus Curzate 60 WG on sites with Zorvec resistances may be based solely on the active ingredients Amisulbrom and Cymoxanil. Therefore, a mixture of Reboot + Ranman Top (0.45 l/ha + 0.5 l/ha) would be an alternative option that avoids a selection advantage for resistant pathogens. Instead, the tank mixture of Zorvec Entecta + Curzate 60 WG could also be applied during the main growth phase.
Choosing the right intervals
The key to the success of follow-up treatments is the correct spray interval. This depends crucially on the infection pressure and the weather. With very high late blight pressure, the spray interval should be a maximum of one week; if it is very low, the interval between treatments can be extended to just over two weeks. High late blight pressure is known to occur in warm, humid weather. If you do not want to rely solely on experience and intuition, you can use forecasting models like Simphyt 3 from ISIP. The latter calculates, for example, the infection pressure.
Consider in addition to the late blight pressure when choosing the spray interval other factors such as plant growth, variety susceptibility, and rainfall. So, shorten the spray intervals until the beginning of flowering, as the crops are not sufficiently protected in the case of rapid plant growth. Conversely, you can wait if there is no new leaf growth or no rain has fallen since the last spray date. Similar considerations apply to less susceptible varieties.
Recommendations for follow-up treatments
For follow-up treatments, we have Propamocarb-containing products at our disposal. These systemic fungicides are characterized by good curative effects and can be easily used up to flowering.
Until three years ago, products like Revus (Top), Pergovia, Carial Flex, Voyager, and Zorvec were the foundation of all late blight control. However, due to resistance development, their frequency of use should be reduced. Therefore, Zorvec Entecta and the CAA fungicides should only be used twice in a season. Also, only spray these products in rotation with fungicides that have a different mechanism of action.
For follow-up treatments, we also have Cymoxanil-containing fungicides like Curzate 60 WG, Cymbal Flow, Grecale, or Reboot. They have a very good curative effect, but their duration of action is limited. Therefore, mixtures of Cymoxanil-containing agents and pure contact agents (Shirlan, Ranman Top, Gachinko/Leimay) are useful to enhance their preventive effect. In periods of low late blight pressure, these contact fungicides can also be applied alone.
Effectively stopping infection
If there is already an infection, the resistant products Revus (Top), Pergovia, Carial Flex, Voyager, and Zorvec should be avoided. Sporulating infections are best treated with so-called stop-sprays.
For this, a tank mixture of, for example, 0.2 kg/ha Curzate 60 WG + 0.4 l/ha Terminus or Carneol is suitable. If the desired success is not achieved, it is recommended to repeat the treatment about three to four days later. For this, 1.6 l/ha Infinito + 0.5 l/ha Ranman Top, for example, are suitable.
If the stop-spray is only done late in the vegetation, a slightly cheaper mixture of 0.5 l/ha Ranman Top + 0.4 l/ha Shirlan is suitable as a follow-up. These contact agents are also very suitable for final sprays as they contribute to protection against tuber infections.
What else to consider?
Overall, the range of active ingredients for late blight agents has narrowed compared to the previous year.
The shelf life of Polyram WG, Versilus, and Zorvec Endavia ended in late 2024. The shelf life of Banjo Forte, Presidium, and Orvego ends on May 20 this year. New on the market and available in 2025 for the first time is Revus Pro. It contains the active ingredients Propamocarb and Mandipropamid and is therefore also affected by CAA resistance.
What to do against Alternaria?
In addition to late blight, Alternaria can also cause significant yield losses. This mainly affects starch and industrial potatoes, but also late table potatoes. Therefore, Alternaria should only be controlled for these varieties. Generally, the best way to control Alternaria is to use an Alternaria-effective fungicide six to eight weeks after emergence (or when initial infection occurs on the middle leaf layers).
The optimal follow-up date also depends solely on the weather. Under ideal Alternaria infection conditions, the spray interval should be a maximum of twelve days - if treatment is sensible due to late harvest. Since Alternaria measures should be carried out up to about three weeks before vine death, multiple treatments may be necessary.
Also, with Alternaria, ensure that no further resistances develop. Therefore, use the carboxamide-containing Propulse (Fluopyram + Prothioconazole) a maximum of twice per season - even if it shows the best Alternaria effect. Propulse should always be used in rotation with another azole.
Belanty (Mefentrifluconazole) and, for example, the active ingredient Difenoconazole, which is contained in Revus Top and Narita XL, offer themselves as alternatives. Narita XL is the successor product to Narita and contains 500 g/l Difenoconazole. This doubles the active ingredient concentration compared to Narita, so the maximum application rate of Narita XL is 0.25 l/ha. The product is approved for four applications and does not seem to quite achieve the efficacy of Belanty.
Even with Alternaria, active ingredient rotation is important. Since Revus Top/Narita XL, Belanty, and Propulse all contain an azole, the switch between mechanisms of action - unlike with late blight - is not possible. Therefore, never use the same active ingredient twice in a row. If multiple sprays are needed, a possible spray sequence could look like this: Ortiva - Belanty - Propulse - Revus Top/Narita XL - Propulse - Belanty.
