Potatoesâ¦ - Bayer CropScience

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Phytophthora infestans is becoming increasingly aggressive The fungal pathogen Phytophthora infestans is increasing its hold on potato crops in Europe. In recent years, a wide range of new pathotypes has emerged through generative (sexual) reproduction, allowing the disease to become more aggressive and more damaging. This is clear from data collected by Euroblight, the European knowledge network for Phytophthora infestans. Potato researcher and Phytophthora specialist Dr. Ir. Huub Schepers of the PPO Research Centre in Lelystad (NL) outlines the development of the disease and indicates how it can be controlled. At the beginning of the 1970s, only one mating type (A1) of Phytophthora infestans was present in Europe. This meant that the pathogen could only reproduce asexually (vegetatively) at that time. The life-cycle was therefore quite stable and predictable, so it was relatively easy to control the disease effectively. This situation changed when, halfway through the 1970s, a second mating type (A2) entered Europe from Mexico, allowing the fungus to start reproducing sexually (A1 x A2) as well. Initially, type A2 was found locally, mainly in the Netherlands and Scandinavia: but by now, in 2008, both 18 COURIER 1/08
“Phytophthora infestans will continue to adapt more rapidly in the coming years. We must therefore outsmart the pathogen and attack it in all possible ways.” Huub Schepers mating types are endemic throughout Europe. Oospores (which develop via sexual reproduction) are now found in virtually all Western and Northern European countries. In the Netherlands, 60 to 70 % of all potato-growing areas have mixed populations of A1 and A2, according to long-term surveys by the PPO Research Centre. Early infection caused by oospores occurs mainly in lighter soils and areas with very short rotations. This applies particularly to the North-East of the Netherlands, where starch potato cultivation is concentrated. In the period 1999 – 2005, about 32 percent of all early infections in that region appeared to be caused by oospores. Oospores are a considerable threat Sexual reproduction via oospores is a serious threat for potato cultivation, because it provides for greater variation in the pathogen population. Laboratory tests and field studies have shown that the sexuallyreproducing populations can be considerably more aggressive than their asexual predecessors. Research has shown that the new, mixed populations adapt more rapidly to temperatures that are normally unfavourable for Phytophthora; the pathogen used to be inviable at temperatures below 10°C and above 25°C, but this is not the case for the new variants. Newly-collected isolates are also often able to survive better on the tuber, as they tend to damage the tuber less severely: the tuber rots less rapidly, and is consequently less easy to sort out on the basis of external symptoms. The new population also has a considerably shorter latent period, meaning that the life-cycle of Phytophthora infestans can be completed more rapidly than before. This, in turn, allows the fungus to adapt more rapidly through sexual reproduction. Under favourable conditions, two generations a week are now possible, compared with one generation before. It is thus important to have short intervals in the spraying programme in areas where oospores are being formed. Further threats presented by the new, mixed populations include the fact that they often have high spore production rates, and can break through varietal genetic resistance more easily. In addition, they have a broader range of host plants compared with the ‘old’ Phytophthora: for example, it is known that the new mixed populations can also infect green night- Phytophthora infestans – symptoms If “Phytophthora-weather“ prevails, potato tubers can be infected as soon as they are formed in the middle of the season. Above-ground parts of the plant must be adequately protected right from the start, in order to prevent inoculum from being washed down into the soil. Severe infection of foliage leads to significant yield losses. 1/08 COURIER 19
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Potatoesâ¦ - Bayer CropScience