Effect of environmental variables on the development of kernel discoloration by Pyrenophora tritici-repentis in durum wheat

Spikes of six durum wheat genotypes were inoculated with Pyrenophora tritici-repentis (Ptr) at the milk stage and incubated at high humidity for 18 h or 36 h at constant temperature (20 degrees C), or for 22 h at 10 degrees C, 20 degrees C, or 30 degrees C. The percent incidence and severity (> 50% of kernel surface area discolored) of symptoms (pink/red smudge and black point) were measured after harvest. In most cases, there was no significant correlation between the incidence and severity of symptoms in all Ptr-discolored kernels. The severity of kernel discoloration increased more than incidence when the incubation period was increased from 18 h to 36 h. The interaction of genotype by treatment was significant for percent incidence and severity, resulting in different rankings of genotypes at 18 h and 36 h. Some genotypes had high infection levels after either incubation period whereas others had low infection levels at 18 h but high levels at 36 h. Ptr infections were low only in DT675 when the incubation period was 36 h. Genotypes were ranked in a similar manner at different incubation temperatures. Temperature of incubation had a greater effect than period of incubation on black point, which developed least at 10 degrees C. These results suggest that the reaction of durum wheat genotypes to infection of kernels by Ptr might be most accurately assessed when inoculated spikes are exposed to high humidity for a prolonged period of time. Exposure to high temperatures would make it easier to obtain differential black point development among genotypes. However, under field conditions, kernel infection by Ptr would be most easily identified at low temperatures, which would allow for a greater proportion of red smudge relative to block point symptoms.