The correct strategy against late blight in potatoes

late blight on potato leaf

late blight on potato leaf

Losses of crop and quality are especially high if late blight appears before or with the row closure respectively as so called stem-late blight.

Therefore the main objective for those working in this field must be to prevent infestations in the potato field as long as possible. Indeed, the weather on which is beyond our control plays a decisive role at the start and the progress of the infection. Nevertheless the potato farmer can take a series of measures to keep infection low.

Experienced farmers know that the scheduling of the first fungicide application and the appropriate product decides about the success of controlling late blight. Depending on the amount of rainfall from May to the middle of June, the first spraying can be necessary before row closure of the potatoes as well as considerably later. Spraying fungicides should begin 7 to 12 days before late blight appears in the plot. Beyond that it is essential for the economic efficiency of the treatment that the choice of the product is adjusted to the infection pressure. Warning services and inspection of the crop are essential tools for an up to date scheduling of fungicide applications.

Why is the soil moisture relevant for late blight?

Development of Late Blight Primary InfectionsZoombild vorhanden

Figure 1: Development of Late Blight Primary Infections

The late blight pathogen (Phytophtera infestans) hibernates in latently (not visibly) infected potato tubers. When these potatoes start growing in spring the fungus expands into the stem. Extensive research activities at the Bavarian State Research Centre of Agriculture (LfL) in recent years have shown that with high soil moisture another way of infection is possible (fig. 1). Spores develop on the diseased tuber and are distributed via soil water. This is how neighbouring plants or stems of the same plant can be infected. They will grow up diseased as well and typical Phytophthora nests develop. In both cases the plants show an infestation of the stem very early (initial infestation). This is why in years with dry weather in spring a considerably lower initial infestation and with that a lower impact of infections occurs than in humid years. But also the soil type indirectly affects the progress of the disease. Light soils drain water faster and the danger of an early and heavy infestation is considerably lower than on loamy soils. Sites with a high ground-water level and areas with high rainfall are especially endangered by a very early and massive infestation of the stems.

Wind and rain splashes provide the distribution of the Phytophthora spores formed on the infected stem. But in the end the weather (optimal: at least 4 hours of damp leaves and 15 to 20 degree Celsius) decides on how heavy the infestation of the leaves will be (secondary infection). In addition further establishment of the disease in the crop is affected by the growth rate of the potato tops, which depends on the variety and the N-fertilizer expense. Lush potato tops create a favourable microclimate in the crop and therefore present a higher risk (figure 1).

Eliminate the source of infection!

What can those working in this field do? Farmers, who have treated their seed potatoes with Cuprozin progress against bacterial soft rot (Erwinia) with the ULV-method (Mantis implement) can count on a side effect against an initial infection (stem infection) of Phytophthora. This is an important step to delay the occurence of late blight in the potato crop as shown in current research work at the LfL. Beyond that the removal of potato volunteers and waste heaps is a reasonable phytosanitary measure. It is very important to start with the spraying against late blight at the right moment. This should be about one week before the visible initial infestation in the plot. Treatments carried out too early cause expenses without achieving an effect against the disease. Treatments carried out too late often do not have a sufficient effect. The potato blight forecasting, offered by the official Plant Protection Service, is an important tool to determine the optimal moment for the first spraying. This information is available at in Bavaria and at for most of the other German states. However it is not enough to rely on the calculated forecast exclusively. Other factors – such as local particularities in the weather or variations of the soil moisture - caused by a high groundwater level for example – must also be considered. Therefore checking the own potato crops for primary herds and early infections still is a must and should be done once or twice a week starting three weeks after emergence of the potatoes and depending on the infection pressure (forecasting model).

Every fungicide against late blight has strengths and weaknesses. Therefore it is important to choose the most effective product depending on the current conditions. Because late blight epidemics are different every year (as shown above) the choice of suitable products must also be reconsidered and adjusted. For this reason it is essential to know the strengths and weaknesses of the fungicides. In addition the experienced farmer will consider late blight pressure and growth phase when choosing which product to use. The following sections can be distinguished.

Start of spraying:

As already mentioned above aim of the first fungicide treatment is to reduce the growth of the fungus from the tuber through the stem of the potato. On heavy soils spraying should basically start with systemic products (Infinito, Rival Duo Pack, Zorvec Endavia). These products spread into the stem tissue and seize the fungus when it grows upwards. If the risk from infected seed potatoes persists (for example continuing high soil moisture) it is absolutely necessary to use a systemic product for the second treatment as well. Until now Infinito does not form resistances and since the product has a low risk of losing its effectiveness it can be applied several times. To begin sprayings with a contact product is only recommended on light soils along with a dry spring.

Main growth phase:

At this stage potato tops grow so quickly that only a few days after the treatment unprotected leaf-area has considerably increased. With a high pressure of late blight the risk of infections on leaves that are not sufficiently protected is particularly high. Under these circumstances it is recommended to use localsystemic or partsystemic fungicides respectively. These products penetrate into leaves and can also protect new leaves to a certain degree and there is an effect also if the fungus has already penetrated the tissue (protective and curative effect). But it has to be considered that these products can only stop the disease within 48 hours of infection even with the full application rate. Infections that date back longer or visible infestations can not be stopped.

Cymoxanil containing products have the best curative effect (stop effect) according to our experience. For this reason these fungicides qualify especially for stop sprayings (sporulating late blight existing) then however use a tank mixture with the full application rate of fluazinam-containing fungicides. If weather conditions continue to be favourable for the disease sprayings have to be repeated after four to six days. The disadvantage of cymoxanil-containing fungicides is the shorter effective period of two to three days compared to other part systemic products. If one does not want to face an unpleasant surprise during the period of high late blight pressure this should be considered.

As soon as the leave growth has come to a stop, from the flowering period on, also fluazinam-containing fungicides or Ranman Top achieve good effects at high to medium infection pressure.

In phases of low late blight pressure, independent of the development stage of the potatoes, late blight protection can be sufficiently assured by cost saving contact fungicide based on the active substances metiram. These fungicide also re-spread on the leaves with dew and can therefore protect new leaves to a certain extent. Unlike Shirlan and Ranman, these contact products also have a good effect against Alternaria. This is important with late maturing and/or Alternaria susceptible varieties (Kuras) during weather periods in which high temperatures and rain frequently alternate.

Maturation and closing treatment:

As soon as potato tops stop growing and lower leaves turn yellow, protecting tubers against late blight is important. If there is visible late blight infestation it is recommended to apply fungicides with a spore-killing effect (fluazinam-containing fungicides, Ranman Top) mainly. Even without an infestation there is a benefit from a last spraying with one of these two fungicides.

Phytophthora spores are produced as long as green potato tops exist. Through wind, dew- and raindrops they reach the soil and there they remain infectious for about three weeks. The greatest risk of tuber infection (late blight) occurs during lifting. Reason: for a successful infection spores have to come into direct contact with tubers and surface injuries facilitate late blight to enter into tubers. To provide the best possible protection against late blight during lifting it is reasonable to combine the closing treatment with a chemical potato tops killing, about three weeks before the lifting.

Not too little water

It is important to wet plants completely. Many farmers underestimate the influence of the water volume rate on control results. According to our experiments 400 l water/ha are necessary to achieve the maximum performance of a fungicide. Only in feeble foliaged crops (until row closure and after beginning of maturation) one can manage with 300 l/ha. However with strong foliage formation (main growth phase and/or varieties with strongly developed foliage) 500 l/ha are recommended.

Carry out the treatments in the morning hour

At which time of the day should fungicide treatments be carried out? This is a frequently asked question. The following applies: systemic active substances have to be absorbed by the plant actively. Potato plants that suffer from heat or drought stress are not able to do this. Therefore in periods of fine weather fungicides must be applied during the early morning hours. It is advantageous if the plants are wet from dew. The morning is also optimal for applying contact products. As contact fungicides stay on the leaf surface the treatment is also possible in the early evening hours. At noon and in the afternoon the risk of wind drift is especially high because of increased air movement (also from the earth’s surface upwards). For this reason no treatments should be carried out in this time. Further details about late blight fungicides and important terms of application are enclosed in the pdf-document Fungicides for Late Blight Control.

Asses risk of damage potential of early blight

In contrast to late blight early blight caused by the fungal parasite Alternaria solani or Alternaria alternate only play an inferior role. Trial results show that in our climate conditions only varieties, which store starch late during growing season (for example Kuras) or some late varieties for processing, along with heat and drought stress can be affected. From experience it is known, that primarily sprinkle irrigation plots are infested with early blight if during late growing season there is not enough soil moisture available.

On plots with food potatoes and potatoes for processing, where a mechanical or chemical defoliation is put into effect in order to ensure the quality and to facilitate lifting, even a strong appearance of the weakness parasite Alternaria can not affect the potato crop yield to an important extent. This is why even using strobilurin-containing fungicides (Ortiva, Signum), does not provide a crucial yield increase although assimilation of plants is extended. An additional use of these fungicides only makes sense economically if starch content is expected to turn out too low otherwise. It should also be noted that fungicides have only shown an effect against Alternaria solani. In our studies results of Alternaria alternata control are considerably weaker. Which of the two Alternaria varieties appears depends on weather conditions and therefore varies from year to year.

However, in general Phytophtora and early blight can not be considered independently. Many of the approved products against late blight have a sufficient side effect on Alternaria (see pdf-documents Comparsion of Contact and Systemic Late Blight Fungicides). Based on our long-time test results, if these products are applied almost exclusively, it is not necessary to add special products in general. If the spraying scheme, as used for late maturing varieties, is composed of products without sufficient side effects on Alternaria (Banjo Forte, Carial Flex, Infinito, Ranman Top, Revus or fluazinam-containing fungicides) and if during the growing season heat and drought stress can be expected then a spraying mixture of Phytophthora fungicide plus Signum or Ortiva can be efficient. The best effectiveness is achieved if the additive is used in the second and fourth (under difficult terms possibly again for the fifth or sixth) late blight spraying. Signum is registered for maximal four and Ortiva for maximal three treatments. It exists a widespread resistance by Alternaria Species against by fungicide Ortiva and Signum therefore it should be applied only once.

Management of fungicide resistance in potatoes

Phytothora infestans belongs to the pathogens that can turn resistant (unsusceptible) to fungicides quickly. The more frequent products with the same mode of action are used, the greater the risk is of them developing a resistance. That is why one should change to another group of active substances after two applications – especially if the applied fungicide is classified as having a medium or high risk of forming a resistance.

IIn pdf-document Management of fungicide resistance in potatoes those products which contain the same active substance or whose active substances have the same mechanism (cross resistance) are marked in red. In contrast fungicides that belong to different groups of resistance and that can be used successively in the spraying scheme are marked in green. For reasons of resistance prevention products with the same mode of action must not amount to more than 50 percent in the spraying scheme! There does not exist a risk of resistance with the classical contact fungicides (Metiram, Copper).