From back cover Signet paperback December 1971:

planet in turmoil!

The Solar System had long ago been colonized by an Earth suffering from a dwindling food supply and a millionfold increase in population. The colonies were her very lifeblood. Without the daily flow of products from them, Earth would experience mass starvation and chaos within weeks. Suddenly and unexpectedly, reports of fatal food poisoning, traceable to Martian produce, began to reach the ruling Council of Science. Each new case was treated with intense secrecy for it the people of Earth learned the cause of these deaths, a worldwide panic would surely ensue.

To David Starr, Space Ranger, these deadly incidents formed a terrifying pattern -- they were clearly part of a clever and brutal scheme by an alien conspiracy to cripple Earth's economic life and topple its government!