Present-day Mars is a frozen desert, colder and more arid than Antarctica, and scientists are fairly sure it’s been that way for at least the last 3 billion years. That makes a vast network of water-carved valleys on the flanks of an impact crater called Lyot — which formed somewhere between 1.5 billion and 3 billion years ago — something of a Martian mystery. It’s not clear where the water came from.

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The plant disease fire blight, caused by the bacterium Erwinia amylovora, is dreaded by fruit growers. It affects apple and pear trees and other plants in the rosacea family, and if a tree becomes affected it usually has to be cleared and burned.

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