Powdery mildew, cereal cells and sustainable crop production

Sustainable cereal cropping needs reliable disease control and various strategies exploiting host resistance have been proposed. Specific (R gene) resistance is unreliable because cereal mildews evolve virulence rapidly. Combining R genes within a cv. might decrease the risk, but, more simply, epidemics can be suppressed by growing cv. mixtures/multilines containing different R genes. An alternative is to breed for durable resistance. Apart from the (exceptional?) case of barley mlo resistance, durable resistance is likely to be under polygenic control and hard to manipulate. Nevertheless, independent resistance mechanisms can impede pathogenesis at different stages. Even if each mechanism has a minor effect, their combination may confer effective resistance. This conclusion arises from studies that generally use a single inoculation and precisely controlled conditions. But this is an over-simplistic approach to understanding natural complexities. For instance, in fields where leaves are challenged repeatedly, outcome of an initial attack drastically affects subsequent cellular responses through locally induced resistance or susceptibility. New evidence shows such effects can be instigated within 30 minutes of inoculation, probably being mediated via conidial extracellular material. Very recently we have also found dramatic effects of mildew attack on stomatal behaviour. In all barley lines examined, stomatal conductance was reduced during attempted fungal penetration in the light. In suscepts, stomata subsequently shut during dark periods but persistently failed to open fully in light, whereas in mlo barley normal stomatal response was largely restored by 36 hours. In contrast, where R gene resistance caused epidermal cell HR, nearby stomata locked permanently open, even in darkness, thus these apparently disease-free plants were severely compromised. Such complexities must be considered in designing strategies for exploiting resistance in sustainable production