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 t ritici-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 (pi nk/red smudge and black point) were measured after harvest. In most ca ses, there was no significant correlation between the incidence and se verity of symptoms in all Ptr-discolored kernels. The severity of kern el discoloration increased more than incidence when the incubation per iod was increased from 18 h to 36 h. The interaction of genotype by tr eatment was significant for percent incidence and severity, resulting in different rankings of genotypes at 18 h and 36 h. Some genotypes ha d high infection levels after either incubation period whereas others had low infection levels at 18 h but high levels at 36 h. Ptr infectio ns were low only in DT675 when the incubation period was 36 h. Genotyp es were ranked in a similar manner at different incubation temperature s. Temperature of incubation had a greater effect than period of incub ation on black point, which developed least at 10 degrees C. These res ults suggest that the reaction of durum wheat genotypes to infection o f kernels by Ptr might be most accurately assessed when inoculated spi kes are exposed to high humidity for a prolonged period of time. Expos ure to high temperatures would make it easier to obtain differential b lack point development among genotypes. However, under field condition s, kernel infection by Ptr would be most easily identified at low temp eratures, which would allow for a greater proportion of red smudge rel ative to block point symptoms.