For healthy potatoes - Bayer CropScience

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Birds-eye view of a trials field: the untreated plots can be clearly distinguished from the treated ones. A glance at the untreated plots shows the devastating effect of Phytophthora infestans on potato plants. After infection, the further development of the fungus is predominantly determined by climatic conditions. Sporangium formation requires high humidity, and has a temperature optimum in the range 18°-23°C. The sporangia are dispersed by wind. If a sporangium lands on a potato plant, 6-12 zoospores can be released. This process can only occur in water droplets on the plant surface. The zoospores quickly germinate to form a germ-tube, which penetrates into the plant’s tissues. Under optimal conditions, a successful infection can usually occur within 2 hours. The infection process can take place on either side of the leaf. The incubation period, i.e. the period between penetration of the host and the first appearance of lesions, lasts 2-3 days. The infection cycle continues when the pathogen sporulates. Sporangia are released into dew or raindrops and can enter the upper soil layers when this water runs off the leaf. Here, they release their zoospores, which are able to penetrate into very young tubers through the epidermis. In older tubers, in contrast, the pathogen can only penetrate into the tuber’s starchy interior through open lenticels, via stomata, or through the eye. The pathogen has a further opportunity to infect tubers during the harvesting process, during which tubers come into contact with infected foliage or with earth contaminated with sporangia. Inoculum in the form of sporangia can remain viable in soil for some time (ca. 30 days). Even small injuries to tubers are sufficient to allow infection. Symptoms Because potato leaves tend to develop various brown spots during the vegetative period, it is important that advisors and farmers are able to identify late blight symptoms clearly. Primary infections are seen on the stem and the petioles (leaf-stalks). These plant parts become brown, and eventually nearly black, and the associated leaves die off. Surface growth of the fungus is rarely seen on stems. Initial symptoms of leaf infection are small, yellowish to dark-green spots. Infection usually first occurs on leaf margins and leaf tips, because this is where water droplets are retained longest. Under favourable environmental conditions, the spots quickly enlarge and become darkbrown to black. A whitish-grey zone of downy fungal growth can be seen on the underside of the leaf at the border between infected and healthy tissue. This is particularly obvious to the eye during periods of high air humidity, in the early morning (dew period) or after rainfall. The white zone is the unmistakable sign of late blight infection. Infected tubers have more-or-less large, irregular and slightly sunken, blue-grey spots on their surfaces. Below the areas showing these symptoms, large parts of the starchy tissues are discoloured rustybrown; there is no sharp distinction between the rusty-brown areas and healthy tuber tissue. 12 COURIER 2/06
Life cycle of Phytophthora infestans Sporulation Mycelia of two mating types meet Sporangia dispersal Direct Germination (High temperature) Oospore formation Sexual Reproduction Mycelial growth in leaves & tubers Sporangial germination Zoospore formation Oospore germination Indirect Germination (Low temperature) Sporangial germination Cyst germination Zoospore mobility Population studies Zoospore encystment The late blight pathogen Phytophthora infestans originated in the central highlands of Mexico. The spread of the pathogen to the rest of the world occurred in two major waves of migration. The first migration occurred about 160 years ago. The fungus was inadvertently transported across the Atlantic on ships, after which it established itself quite rapidly throughout Europe, Asia and Africa. For a long time, populations in Central Europe remained very uniform: only mating type A1 was present in Europe and Asia. Any adaptive changes by the pathogen that occurred would have been through mutation or mitotic crossing-over. Exchange of genes through sexual recombination would not have been possible, because only one mating type (A1) was present (sexual recombination depends on two mating-types {A1 und A2} being present). This remained the situation until about 1975. The pathogen’s second great wave of migration occurred in the mid-nineteenseventies. In 1976 and 1977, large numbers of Mexican ware potatoes were imported into Europe: among them were tubers with latent infections, including new (to Europe) 2/06 COURIER 13
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For healthy potatoes - Bayer CropScience

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