Jan Nechwatal & Michael Zellner Bavarian State Research Center for Agriculture Institute for Plant Protection Strategies to reduce primary Phytophthora infections in conventional and organic potato production Jan Nechwatal & Michael Zellner Introduction: Late blight caused by Phytophthora infestans is one of the most devastating diseases of potato. Basically, there are two major pathways of how the pathogen will infect a given potato field to start an epidemic: I) infection will start from outside the field as sporangial inoculum originating from diseased volunteer plants, infected refuse tubers, or infected neighbouring stands, and blown into the field by wind; II) infection will start from within the field, with infected seed tubers being the source of inoculum. This will eventually lead to early primary Phytophthora stem infections (Fig. 1) from which, via sporulation and secondary leaf infections (Fig. 2), disease spots will develop that soon affect the whole field. Figure 1 Figure 2 Background: Previous PCR based studies have shown that an average of 10% of the seed tubers of various origin are latently infected with P. infestans and as such, potentially carry the inoculum into the field. Even if only 1% of these produce plants with diseased sprouts, this would eventually result in approx. 40 disease spots per hectare. Aim: This study has investigated ways to reduce the extent of primary stem infections in conventional and organic potato production by fungicide applications on the foliage or on the tuber. In both conventional and organic farming, a timely and effective control of primary Phytophthora infections is crucial for any further disease management efforts and for the prevention of yield and quality losses. Methods: Field tests with foliage or tuber treatments were carried out using a ‘double setting’ technique of potato seed tubers, with one healthy tuber and one artificially infected tuber planted on the same position in the field. The artificially infected tuber served as a source of inoculum for the neighbouring healthy tuber, thus facilitating the development of early stem infections. In case of the tuber treatment tests, the healthy tuber received the fungicide seed dressing, while the infected tuber remained untreated. Components of a management strategy: Foliage treatment against growth of the pathogen inside the plant Foliage treatment against inoculum deposition on the tubers at harvest Tuber treatment against pathogen propagules on the tuber surface Results (conventional farming) In a field test on the prevention of stem infection by foliar treatments all three systemic fungicides caused a significant reduction in stem blight incidence as compared to the control. The reduction achieved by the use of a non-systemic fungicide was not significant (Fig. 3). In a field test on the prevention of stem infection by tuber treatments, stem infection could be significantly reduced by the two contact or locally acting fungicides used as seed dressing. The systemic fungicide could not reduce the incidence of stem infection, just as the seed dressing active against Rhizoctonia (Pencycuron) (Fig. 4). Figure 3 Figure 4 Results (organic farming) When applied under field conditions, apart from Cu hydroxide, other organic products that provided good foliage protection in in vitro tests were not effective against leaf infections. Some more promising Cu-free products (see example in Fig. 5) have not yet been tested in the field and are included in field trials in 2013. In a field tests on the influence of several alternative seed dressings on stem blight (application in autumn or in spring) incidence of stem blight was low, due to relatively dry weather conditions in spring. Therefore, no data on the effect of seed dressing on stem infections are available. However, when considering secondary leaf infection data, an effect of all treatments on the degree of leaf necrosis could be observed in one stand (Fig. 6), and an effect of the spring copper treatment in the other. control water garlic product Figure 5 Figure 6 Discussion: In conventional farming, primary stem infections can be effectively prevented by an early foliar application of a systemic fungicide. These fungicides are taken up into the plant tissue, and thus can reduce the growth of the pathogen from within the plant. To prevent the initiation of an epidemic, the first application ideally takes place approx. 1-2 weeks ahead of the first visible symptoms to reach optimum efficiency. In organic potato production, foliar treatments are rather considered a method to postpone the onset of an epidemic and to minimise the extent of leaf infections. This will also minimise the deposition of sporangial inoculum on the soil surface and on the potato crop, and as such will reduce the extent of latent infection in the harvested tubers – an important prerequisite for the production of disease free seed tuber material. Seed treatments might be an additional way to reduce incidence of primary infections in both conventional and organic potato farming. They are meant to both reduce the risk of infection of the developing sprouts and to affect sporulation on the infected tuber in the soil and thus, to protect neighbouring tubers. Parts of this study were funded by the German Federal Office of Agriculture and Food within the Federal Programme for Organic and Sustainable Farming. Contact: Dr. Jan Nechwatal, Dr. Michael Zellner, Bavarian State Research Center for Agriculture, Institute for Plant Protection IPS3c, Lange Point 10, 85354 Freising, Germany. Contact: michael.zellner@lfl.bayern.de