EVALUATION OF BREAD WHEAT GERMPLASM FOR RESISTANCE TO BACTERIAL STREAK

Bacterial streak caused by Xanthomonas campestris pv. translucens (J.J .&R.) Dye (Xct) reduces grain yield of wheat (Triticum aestivum L. em. Thell) by as much as 40%. Resistant cultivars offer the best control in the absence of effective bactericides. However, few resistant genot ypes have been identified for use in breeding programs. Five thousand bread wheat accessions from the USDA National Small Grains Collection were inoculated with Xct and evaluated for resistance to bacterial str eak in the held during 1992. On the basis of agronomic characteristics and bacterial streak severity, a total of 428 accessions was selected for continued testing. These accessions were inoculated with Xct and evaluated in replicated hill plots in the field during 1993 and 1994. Accessions were also evaluated for water-soaking (WS) reaction to Xct in growth chambers using a seedling syringe inoculation method. Averag e bacterial streak severity (BSS) during 1993 and 1994 was negatively correlated with heading day (HD) (r = -0.55; P < 0.001). To reduce the maturity effect, accessions with HD > 106 Here dropped and the remain ing accessions were grouped into the HD classes: less than or equal to 84, 85 to 89, 90 to 94, 95 to 99, and greater than or equal to 100. S election was practiced within a HD class based on the following criter ia: (i) BSS less than or equal to 10% in each year, (ii) if HD 94, the n maximum average BSS less than or equal to 10%, (iii) if HD 95-99, th en maximum average BSS less than or equal to 9%, and (iv) if HD greate r than or equal to 100, then maximum average BSS less than or equal to 7%. Twenty-six resistant accessions are recommended for the southern USA. Two controls, Terral 101 and GA 22 triticale (X. triticosecale Wi ttmack), also met these criteria with 6% average BSS. BSS and WS were not positively correlated, suggesting that the WS reaction to Xct from the syringe inoculation test may have limited usefulness. Field scree ning was effective in identifying resistance, but reaction of genotype s was confounded by relative maturity and genotype x environment (GxE) interactions.