The A Mating Type of Blight

For about a century, scientists were baffled by the fact that the blight fungus apparently had no sexual reproduction. The problem was solved by Jorge Galindo, a famous Mexican scientist.

Galindo showed that the blight fungus has two mating types. Each mating type is hermaphrodite (i.e., it has both sexes) but it is self-sterile. This means that both mating types must be present if the blight fungus is to reproduce sexually. Sexual reproduction has two advantages for the fungus. First, it produces great variation in the fungal population and, second, it produces very tough 'oospores' that can survive outside the host during a winter, a drought, or an otherwise rough environment, without being killed. If only one mating type is present, the fungus can only reproduce vegetatively, by means of very fragile, asexual spores. It can then produce variants by mutation only, and it can survive a winter only inside its host. That is, it can survive only in potato tubers.

When the blight fungus was taken from Mexico to New York, and then to Europe, it was taken as one mating type only, now known as A1. For about 150 years the entire population of the blight fungus in the temperate regions consisted of this one mating type only. But, recently, the second mating type, discovered by Jorge Galindo, and now known as A2, was accidentally taken from Mexico to Europe. Before its presence was detected, it had been spread all over the northern hemisphere in certified seed potatoes.

The presence of A2 means that the blight fungus can now form oospores in huge numbers. This has several effects on the blight epidemics.

> First, the greatly increased variability in the fungus means that vertical resistances will break down even more quickly, and that they are even less useful than before. And modern synthetic fungicides, such as glyphosate, will also break down more quickly.

> Second, when only A1 was present, the blight could survive a winter only in potato tubers. Blighted tubers were relatively rare, and this meant that the initial inoculum was small, and the epidemics required more time to develop fully. The disease was known as 'late blight' for this reason. But, with huge numbers of oospores in the soil, the initial inoculum becomes very large, and the epidemics start earlier, and they are more serious. This means that higher levels of horizontal resistance will now be necessary if the blight is to be controlled by breeding.

> Third, when only A1 was present, tomatoes could get blight only from diseased potatoes, and this made it 'late blight' on tomatoes also. But tomatoes can now get blight directly from oospores, and they get it much earlier. The disease is now much more severe on tomatoes, and some plant breeding clubs might care to breed tomatoes for comprehensive horizontal resistance, in parallel with their potato breeding.

> Fourth, areas that are still free of the A2 mating type should be very careful not to import it. It is certain to reach all such areas eventually, but any delay will be useful, until such time as local cultivars with adequate resistance to blight have been produced.

There are two advantages obtained from the presence of the second mating type. First, a dangerous crop vulnerability has been realised and, once overcome, it will be permanently eliminated. Second, the inactivation of vertical resistances to blight is so rapid that the one-pathotype technique is no longer necessary during potato breeding.

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