Resistance mechanisms

There are two closely similar rusts of maize. These are Pucciniapolysora and Puccinia sorghi. The latter is climate-insensitive and occurs wherever maize is grown. When the tropical rust first arrived, the maizes of tropical Africa had the minimum level of horizontal resistance to P. polysora, and the maximum level of horizontal resistance to P. sorghi. Some 10-15 maize generations later, they had the maximum level of horizontal resistance to both rusts. There were no obvious resistance mechanisms, and no detectable differences between the susceptible and resistant maizes, other than their reaction to tropical rust.

We have no idea what the resistance mechanisms to P. polysora may be. We know that they are polygenically controlled, and that they confer durable resistance. They are also quite distinct from the resistance mechanisms to P. sorghi. That is all we need to know. When breeding crops for horizontal resistance, forget about looking for resistance mechanisms. Just breed holistically.

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