A lethal stem rust has spread to southern Africa's wheat crop. The fungus, identified in Uganda in 1999 and called Ug99, is a new contender in the long hot war between plant breeders and plant pests. Stem rust is an old enemy, but until Ug99 turned up, plant breeders had thought they were in the ascendant. The spread of yet another destructive element – along with drought, flood, locusts, windstorm and rising fuel costs – in the challenges that face the African farmer is a reminder of several things.

One is that in a world in which 800 million people are chronically undernourished and more than 2 billion live on $2 a day, anything that reduces the food supply has potential for tragedy. A second is that agricultural science is a battle that can never be won outright. Any evolutionary biologist would have predicted the arrival of a new pathogen – and any evolutionary biologist would also predict that somewhere in the plant world there must already be genes resistant to the latest devastating pest. These genes must be identified, then spliced or bred into appropriate varieties and distributed to the blighted areas. All of which takes time, money, manpower and relentless scholarship.

But the stem rust is a reminder of two more unforgiving facts of life. One is that as human population levels continue to rise, the farmland available to feed each individual on the planet continues to fall. Sooner or later, there could be a crisis of the kind predicted by Thomas Malthus more than 200 years ago. The reason there has been no Malthusian crisis so far is that as the population doubled, agricultural science tripled crop yields. Ominously, although yields are still increasing, the rate of increase has for three decades been slowing down. Improvements will require investment not just in crop research, but in plant science as a whole. Researchers must understand not just the ideal conditions for experimental wheat, but the natural ecosystems in which rusts, blight, mildew and other pests flourish; they need to understand not just the molecular biology of rice but the evolutionary origins of all the grasses, and the mechanisms that produce genes for drought tolerance, or pest resistance, or high yield and so on.

The other fact of life is that money grows, so to speak, on trees. Almost everything that humans eat, drink, wear, burn or take as medicine is ultimately the gift of the vegetable world, along with the oxygen we breathe. So the new pathogen in Africa is a reminder that we need to do more than invest in aid budgets and crop science: we must learn much more about the intricate natural world around us. That means spending money on very basic research: at the grass roots, you might say.