Spot blotch (Bipolaris sorokiniana) is of increasing concern in developing countries. It mainly occurs in warm, humid wheat growing environments (ME5) in Latin America, Africa, the Indian Subcontinent, Southeast Asia, and China. More recently, spot blotch has also expanded into the cooler, traditional irrigated rice-wheat production areas (ME1), which may be due to pathogen adaptation, changes in varietal spectrum, reduction in tillage practices, a broader host-range, or climate modifications. In the hot and warm wheat production regions (ME5), wheat researchers consider spot blotch the number one economic pathogen. Total affected area is estimated at about 25 million ha. The lack of resistant, adapted cultivars is an urgent problem. Sources of spot blotch resistance are: Latin America (e.g., BH1146, CNT1), China (e.g., Shanghai #4, Suzhoe #8, and Yangmai #6), and wild relatives of wheat or alien species (e.g., Aegilops squarrosa, Thinopyrum curvifolium). One or six genes have been shown to be involved in resistance. No immunity in wheat is known. Low heritabilities, environmental components, and unknown variability in the pathogen may make breeding for resistance difficult. At CIMMYT, directed crosses are made between high yielding germplasm, adapted cultivars from target countries, and distinct sources of resistance. In Mexico, early generations are shuttled between Toluca (central highlands) and Cd. Obregon (coastal plains). Segregating germplasm is selected for resistance to diseases other than spot blotch, such as leaf and stem rusts, foliar blights, head scab, and for agronomic type, potential yield, and relative adaptive traits. ClMMYT's main testing site for spot blotch within Mexico is Poza Rica (21ºN, 60 masl). Due to high temperatures and relative humidity, spot blotch is naturally prevalent. In the F4-F7 populations, selected materials are grown in Poza Rica. Lines showing slow disease development, a "good finish", and plump healthy grain are selected. These are grown in yield trials under irrigated conditions at Cd. Obregon under late planting conditions to also allow selection for heat tolerance. Yield is the final deciding factor, plus grain appearance. The top entries are distributed internationally for multilocation testing in the Warm Areas Wheat Screening Nursery to about 50 cooperators around the world. Resistant, widely adapted lines are once again used in crosses at CIMMYT, thus building 011 what was achieved.

English

9806|AGRIS 9802|anterior|R97-98PROCE|FINAL9798

Jose Juan Caballero

CIMMYT Publications Collection