Late Blight Control - Date and Product Strategy Must be Correct!

Krautfäulebekämpfung - Termin und Mittelstrategie muss stimmen! - Deutsche Version

Summary

When does the late blight come? For the potato grower, this question arises anew in every year. It is conspicuous that it always then comes to early outbreaks of an epidemic, when the soil was strongly waterlogged for a longer period of time because of intensive precipitations from May till the middle of June. The first symptoms are then mostly appearing in form of infestations of the stems or of the tops of the foliage. For the avoidance of greater yield losses, an extensive use of fungicides is inevitable in most of the years.

Fundamental items about fungicide use

In case of an optimal spraying-technique, contact preparations form a dense coating on the leaf surface hindering penetration of fungal spores into the plant tissue. Therefore the preparations have an exclusively prophylactic effect and can only prevent from an infection if the first spraying and joining sprayings are carried out according to schedule. That is, treatment must be carried out before infestation begins. Spraying coating has to be renewed in shorter time intervals (about seven days) in periods rich in precipitations or in case of a lot of new growth, in longer lasting time intervals (about 10 days), in phases of dryness or in case of a standstill of growth.

Fungicides having a local-systemic or a systemic effect are taken up by stem and leaf in a short time (one to two hours). Therefore these preparations are less sensitive to subsequent rain and allow spraying at intervals from 10 to 14 days. Besides, these products show also an effect, though fungus has already penetrated into plant tissue (prophylactic and curative effect). But it has to be taken into account, that an infection can only be stopped in the early phase (maximum 48 hours after its beginning). Furthermore infections dating back longer or even a visible infestation cannot be killed off any more by these preparations.

Local-systemic fungicides, especially the systemic ones, have also a more secure effect, if applicated before infection: in local-systemic fungicides a contact product is additionally present. Applicated before infection the contact product gains full effect.

Carry out treatments in the morning hours

At which time of the day should the fungicide treatments be carried out? This question is frequently asked by practisioneers. For this, the following is carrying weight: Systemic active substances must actively be taken up by the plant. Suffering from heat or drought stress potato plants are hardly able to do this. In periods of fine weather, the fungicides must therefore be applied in the early morning hours. In principle, this period has to be regarded as being optimal for contact fungicides, too. However in the case of preparations with exclusive contact fungicide effect - they have to remain on the leaf surface - the early evening hours have also to be considered as treatment time. Around noon or in the afternoon, the risk of wind drift is especially high (caused by air moving also from the soil surface upwards). For this reason, no fungicide sprayings should be carried out at this time. Figure 1 gives an overview about the range of products.

Figure 1: Fungicides for Late Blight Control

Infected tubers are the starting point for the epidemic

The late blight (brown rot) is caused by the fungus Phytophthora infestans. During winter the pathogen survives in infested potato tubers. These are representing the starting point for the epidemic in spring.
Earlier studies carried out in cooperation with the chair of phytopathology of the TU Munich-Weihenstephan showed, that in former times the diseased tubers rotted in field stacks or in badly air-conditioned potato cellars were picked out during seeds preparation in spring. In modern potato stocks with temperature control, these tubers survive the winter largely unhurt and are planted in spring. Particularly under humid soil conditions a severe primary infestation comes into beeing. If the soils are not able to be driven on in the early growing phase (May to the beginning of June) because of abundant precipitations (for example in the year 2000 in the region Weser-Ems or in the area of Munich), a massive initial infestation has to be expected also on the stem. The consequence is that nowadays in case of humid spring weather the primary infection pressure is far stronger than in times, when field stack was the standard of potato storing. This phenonomen also explains the more frequent occurrence of symptoms on the stem in comparison to former times.

Which fungicide for the first spraying?

For the first spraying, the use of a systemic product - preferably "Ridomil Gold MZ" - has been proven advantageous. Systemic fungicides penetrate into the stem and thus reduce the latent (not visible) infestation level. The best results are obtained, if a period of time of 10 till 14 days is between the appearance of the first late blight symptoms and the fungicide spraying. The infection pressure can approximately be estimated by the aid of forecasting procedures. If a small disease pressure is forecasted, pure contact fungicides are sufficient for the first treatment.

Which fungicides for the following treatments?

Here also the choice of preparation is orientating towards weather. But moreover the current infestation occurrence has to be taken into account. In case of a small disease pressure (e. g. dry weather periods), contact fungicides can be used without risk. In case of changeable weather and of occurrences of stem infestation, local-systemic or systemic fungicides will be advantageous. If it is not sure that the crops stands can be driven on, the user of these preparations is essentially more flexible because of the longer spraying intervals. If a sporulating infestation occurs, so-called stopping-sprayings have to be advised. In this situation, the mixture of "Curzate M" (2.5 kg/ha) and "Shirlan" (0.3 - 0.4 l/ha) into one tank filling has stood up to the test. The late blight fungicide "Tanos" (0.7 kg/ha) is also appropriate in this situation as a mixing partner with "Shirlan" according to our experience. Because of the resistance situation, we advise to use the preparation "Ridomil Gold MZ" no longer in case of a visible late blight infestation. For the same reason, the use of this product must be limited to one or maximum two treatments, otherwise considerably reduced effects can occur. An overview about the optimal positioning of the fungicides in dependency on the infection pressure and the potato growth gives Figure 2.

Figure 2: Dates of Use for the Late Blight Fungicides

Further details to the systemic and the contact late blight fungicides are demonstrated in Figure 3 and Figure 4. Figure 5 referes to the risk of resistance.

Figure 3: Comparsion of Systemic Late Blight Fungicides

Figure 4: Comparsion of Contact Late Blight Fungicides

Figure 5: Resistance management with potato fungicides

Final treatment

For the final treatment, "Shirlan" and "Ranman" comes into consideration. This product shows a spore-killing effect. By this, the risk of tuber infections can be reduced. But additionally a period of time of at least three weeks must lay between the dying of the potato foliage and the harvest. Phytophthora spores beeing washed off from the potato plants, that is to say, remain, infectious so long in the soil. The skin strength, too, increases considerably in this period. Because of that, the risk of tuber wounding during potato-lifting works is essentially smaller. Pathogens causing tuber rots (Phytophthora infestans, Fusarium spp., Erwinia ssp. among others) have thus less possibilities of penetration.

The warning service signals the optimal treatment date

A successful control of late blight in potatoes stands and falls by the first spraying on schedule. In addition, the used fungicide must suit the infestation events. In order to give the farmers and advisors more security of decision in fungicide strategy and in fixing the date of controlling measures, the plant protection services of the federal countries offer a federal-republic-wide warning and extension service for Phytophthora. Surveys among farmers and the number of calls on our informations in the Internet prove a high demand for this information offer.

Onto what is the warning service based on?

Core of the late blight information service is the current evaluation of the epidemics according to weather conditions by means of the computer program "SIMPHYT". Firstly, the model prognosticates the start of spraying. In the course of this, the emergence date, the susceptibility of the cultivars and especially critical growing conditions are included into the prognosis. The recommendation for the beginning of the spraying is calculated by the simulation model with a lead time of about 10 days. By this, sufficient time is remaining for the farmer under normal weather conditions to carry out the first fungicide spraying.

From the assessment of the weather with regard to the infection pressure, the spraying intervals for the following treatments and an optimized fungicide choice as well can be derived. For this, the agrometeorological data of the last 14 days are considered. This period of time is roughly corresponding to a complete development cycle of the fungus on the leaf from setting the infection on over the latent phase (the infestation is not yet visible) and the infectious phase (spores are produced) to the dying of the parasitized leaf tissue. Beside the infection pressure, the rate of new infections (occupation) of late blight on the individual days is a further important factor, which is computed by the model. If the weather conditions are over a longer period of time so unfavourable for the fungus, that new infections were not possible, the fungus dies in the foliage. Under these conditions, the fungicide sprayings can be interrupted till positive rates of re-infection are computed again.

In parallel to this, employees of the plant protection services organize an areas-covering valuation of the infestation (monitoring) on plots of practisioneers representative for the region. For instance, more than 50 plots in Bavaria alone are controlled and evaluated by the Ämter für Landwirtschaft und Ernährung (Authorities for Agriculture and Food). The results of valuation serve primarily for controlling of the infestation predictions drawn up by the computer. By involving these inquiries into the warning and extension service, a high degree of decision security is ensured. The experiences collected since 1997 in extension and practice as well as in exact experimentation in Bavaria confirm the high accuracy.

Where do I get the informations?

On the web-site of the Bayerische Landesanstalt für Landwirtschaft (Bavarian State Research Center for Agriculture) ( www.lfl.bayern.de), own tables of results are made out for each regional administrative district. For the other federal countries, the instructions can be called up under the Internet address "www.phytophthora.de".

In Bavaria the unions of community organizations for cropping are offering a faxing service in co-operation with the Ämter für Landwirtschaft und Ernährung (Authorities for Agriculture and Food) on the level of the region as administrative district. Beside the tables of results, comments on late blight situation as well as fungicide recommendations specified per region are also given. The faxing service is open to each member of the union of community organizations for cropping for a small contribution to the costs. Registrations are accepted by the responsible office of the union for cropping. Additionally, corresponding information are provided on the level of the rural district by the answering machines of the Authorities (for Agriculture and Food). Local particularities are standing in the foreground of this information offer. In the other federal countries too, comparable information offers are being at the disposal. Information about that is given by the regional plant protection services.

An overall view in a table with a short text part for questions in late blight control down for solving can be found weekly in the agricultural press, too.

Mai 2009
Dr. Michael Zellner, Steffen Wagner, Bernhard Weber, Johann Hofbauer
Bayerische Landesanstalt für Landwirtschaft
Institut für Pflanzenschutz
Tel.: 08161/71-5651 • Fax: 08161/71-5735