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 a second mating type (A2) entered Europe from Mexico, allowing the fungus to start reproducing sexually (A1 x A2) as well. Oospores (which develop via sexual reproduction) are now found in virtually all Western and Northern European countries. Early infection caused by oospores occurs mainly in lighter soils and areas with very short crop rotations.
Oospores are a considerable threat
Laboratory tests and field studies have shown that the sexually-reproducing populations can be considerably more aggressive than their asexual predecessors. 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. Often, newly-collected isolates also survive much better on the tuber, as they tend to damage the tuber less severely: the tuber rots less rapidly and consequently is less easy to sort out on the basis of external symptoms.
The new population also has a considerably shorter latent period, meaning 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 possible now, compared with one generation before. It is thus important to have shorter 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.
More action against Phytophthora necessary
Keeping Phytophthora infestans under control will require more vigilance and action towards prevention and eradication than before. Prevention always begins with the eleminitation of sources of early infection, for example by covering cull piles early on, and by removing volunteer plants. Moreover, early pockets of Phytophthora infection must be suppressed as rapidly as possible. A second factor that can curb the disease is extended crop rotation. Research has shown that oospores can survive in the soil for 3 to 4 years. If a second crop of potatoes is grown within this period (or if volunteer plants are allowed to grow on the same plot of land), early infection can develop from oospores.
Choice of variety can also help keeping Phytophthora in check. Infection occurs considerably less rapidly in highly-resistant varieties. With regard to crop protection, the spraying strategy must be tightened up where mixed populations are present, in order to counter the increased risk of infection. Among other things, this might mean starting spraying earlier, with shorter intervals between applications, so that the pathogen has less chance to develop. In addition, where there is high disease pressure, more attention will have to be paid to combining different active substance (modes of action).
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