Interacting effects of interrupted humid periods and light on infection of wheat leaves by Mycosphaerella graminicola (Septoria tritici)

Septoria tritici conidia require a long period at 100%, humidity to infect wheat plants successfully. When periods of 100% relative humidity were interrupted by up to 48 h at 75% relative humidity, infection by S. tritici on winter wheat cvs Longbow and Avalon was only slightly reduced. Breaks at 50% relative humidity had larger effects, but still allowed infection to occur. Infection was reduced more by a break in humidity which began in the light, but previous and subsequent light regimes strongly influenced the final outcome in all cases. When a wet period was interrupted twice, the two dry breaks interacted strongly under some circumstances; in these cases the probability of infection was much greater than expected from multiplying the effects of the single breaks. Under other conditions two breaks had more nearly multiplicative effects, or interacted to reduce the probability of infection below that expected from the individual breaks. There was evidence that light reduced the probability of successful infection earls in the infection process, but stimulated it later. The data were consistent with infection occurring faster on cv. Longbow, but otherwise being affected in qualitatively similar ways on both cultivars. The complexity of the effects and their interactions suggests that it will not be possible to find a simple set of infection conditions to apply in the field. The data rule out a number of simple mathematical models of the infection process.