Potatoes in Europe

The history of blight (Phytophthora infestans) of potato (Solanum tuberosum) in Europe falls conveniently into periods of approximately forty years, starting in 1845. This history reveals a remarkable fluctuation in the levels of horizontal resistance.

Before 1845: The edible potato evolved in South America, and the blight fungus evolved in Mexico. When the two were brought together, first in New York, and then in Europe, this was a new-encounter disease, which revealed a serious crop vulnerability (see 3.9). The horizontal resistance of the cultivated potatoes was minimal and, in the absence of fungicides, there was a near-total loss of crop.

1845-1885: The first blight epidemics were extremely severe and this was the first historical occurrence of a devastating plant disease. The resulting famines gave rise to the phrase 'The Hungry Forties', and they were responsible for the birth of the science of plant pathology. Most of the potato clones in Europe had minimal levels of horizontal resistance and they became extinct. Only the somewhat less susceptible clones survived the blight and, as these slightly more resistant clones began to predominate, the severity of the epidemics declined.

Potato breeding was common because propagation from true seed was an effective method of obtaining virus-free plants. At that time, the cause of virus diseases was unknown and their effect was called 'decline'. After the appearance of blight, there was an extreme selection pressure for horizontal resistance during breeding, because the blight killed all susceptible seedlings. Obviously, only resistant seedlings could survive, and horizontal resistance accumulated by transgressive segregation. It must be appreciated also that there were no vertical resistance genes in this potato material. Vertical resistance genes were found only decades later in Mexican wild material.

For most of this forty-year period, potatoes were a crop of major economic importance, and they were cultivated without any use of fungicides. The level of horizontal resistance was obviously substantial.

1885-1925: In 1882, the discovery of Bordeaux mixture had two major effects on potato blight. Routine spraying with this fungicide provided an effective control of blight and it both improved commercial potato yields, and reduced tuber rot in the store. Secondly, potato breeding became much easier, because all the seedlings could be protected with this fungicide.

However, the levels of horizontal resistance in new cultivars began to decline because of negative selection pressure during this process of easy breeding. The mechanism of this negative selection pressure was that susceptible seedlings were in the majority. Their susceptibility could not be observed because of the fungicide. Susceptible seedlings then tended to be selected more frequently because of their other attributes. This is the vertifolia effect (see 5.54).

Many famous potato cultivars were produced during this period and they are still being cultivated with expensive fungicidal protection. Their blight susceptibility was first revealed during World War I, when Germany was critically short of copper, which was essential for the manufacture of armaments. Germany was unable to spray her potato crops, and food shortages were a major factor leading to her defeat. These shortages were aggravated by the shortage of nitrogenous fertilisers caused by the demand for explosives.

1925-1965: First Germany, and then Britain, Holland, the United States, and other countries decided that potato blight had military significance. Blight resistance breeding was started in all these countries, and it involved the most modern techniques of that time. These techniques used vertical resistance only. The levels of horizontal resistance continued to decline due to the vertifolia effect (see 5.54). The vertifolia effect is a decline in the level of horizontal resistance that occurs during breeding for vertical resistance. Its cause is closely similar to the decline of horizontal resistance that occurs when the screening population is protected with insecticides or fungicides. The level of parasitism, and hence the level of horizontal resistance, cannot be observed, and there is negative selection pressure for this resistance.

Vertical resistance to blight is too short-lived to be of agricultural value. A vertically resistant potato cultivar usually has a commercial life of 3-5 years before its resistance is matched by the blight. Eight years are needed to breed a new potato cultivar. By the end of this forty-year period, breeding for vertical resistance blight was being abandoned. But, except in Mexico (see 7.20.5), Kenya (see 7.20.6), and Scotland (see 7.20.7), there was no breeding for horizontal resistance either.

1965-Present: During the 1960s, the futility of vertical resistance, followed by the discovery of the systemic fungicide metalaxyl, led to an almost complete abandonment of blight resistance breeding. New cultivars were selected on the basis of yield and other qualities only. Horizontal resistance continued to decline during the breeding of new cultivars, which were still being protected by fungicides during the breeding process. This decline in horizontal resistance has now continued for more than a century. We have regressed close to minimal levels of horizontal resistance, in which potato cultivation without fungicides is likely to lead to a total loss of crop. This situation has been further aggravated by the appearance of metalaxyl-resistant strains of blight.

The second mating type (A2) of the blight fungus was recently taken from Mexico and distributed all over the Northern Hemisphere, mainly in certified seed tubers. It is perhaps too early to assess the final effects of this development. The two mating types of the blight fungus are both hermaphrodite but self-sterile, and only one mating type (called A1) was taken from Mexico to North America and Europe in the 1840s. Consequently, no functional oospores occurred, and the only initial inoculum was in the relatively rare diseased tubers that had survived the winter. With both mating types present, it is likely that there will be a greatly increased initial inoculum in the form of functional oospores. The blight epidemics may then be very much more severe. New strains of the blight fungus will appear far more quickly, and the effectiveness of both vertical resistances and unstable new fungicides will fail correspondingly quickly. Furthermore, higher levels of horizontal resistance will be required to control the disease.

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