Tan spot of wheat, caused by Pyrenophora tritici-repentis, has become important in Morocco in recent years. Surveys conducted during the last decade revealed that the disease affected durum wheat more severely than bread wheat. Yield losses from field experiments, using current moderately susceptible cultivars, were calculated at 12-28% under moderate infestation levels. Number of grains per spike and 1000-kernel weight were generally not affected. Comparisons of field inoculation methods showed that spraying was more effective than mulching with infected straw under humid conditions prevailing in northern Morocco, while mulching was more reliable under dry conditions. Several laboratory and greenhouse studies were conducted to refine screening and evaluation methodologies. Lesion size on the upper leaves best showed the variation in tan spot expression related to genotype. Disease severity on both upper and lower leaves was found to be a useful indicator. According to several race identification studies, virulence of different tan spot isolates varied significantly; however, there was little physiological specialization. Screening for disease resistance showed large variation in genotype response, with moderately to highly resistant accessions identified. Resistance was found to be linked to maturity date and plant height. Breeding for resistance to tan spot using the available tall, late maturing, resistant parents in crosses with adapted material, coupled with selection under semi- arid conditions, was not effective due to the linkage of resistance with undesired traits. The current breeding approach is to search for materials with flexible maturity date and plant height that could produce acceptable levels of tan spot resistance.

English

9806|AGRIS 9802

Jose Juan Caballero

CIMMYT Publications Collection