RELATIONSHIP BETWEEN FLAG LEAF SYMPTOMS CAUSED BY XANTHOMONAS-TRANSLUCENS PV. TRANSLUCENS AND SUBSEQUENT SEED TRANSMISSION IN WHEAT

The relationship between foliar disease symptoms on parent plants, see d contamination by the causal bacterium (Xanthomonas translucens pv. t ranslucens), and subsequent development of bacterial leaf streak in wh eat was studied in microplots and in the laboratory to determine the r ole of seed transmission in disease epidemiology. Microplot experiment s were carried out during the 1994-95 and 1995-96 growing seasons usin g seed harvested in Baton Rouge, Louisiana, in 1994 and 1995, respecti vely. Treatments were seed lots from plants with differing levels of b acterial leaf streak severity on the flag leaves of the parent tillers . X. translucens pv. translucens was detected in 1 to 20% of seed from susceptible cultivars Florida 304 and Savannah collected from plants with leaf streak symptoms. Correlations between seed contamination and disease on plants that developed from this seed were detected only wh en seed came from parent tillers that expressed flag leaf disease seve rity greater than or equal to 15 to 20% in 1994-95 and greater than or equal to 30 to 35% in 1995-96. However, symptoms of bacterial leaf st reak on plants that developed from these seed were evident on only les s than or equal to 3% of plants. Results suggest a possible threshold level for bacterial leaf streak on flag leaves that is necessary befor e X. translucens pv. translucens can be detected in seed. Seedling eme rgence in microplots correlated negatively with leaf streak severity o n parent tiller flag leaves. Artificial infestation of seed with X. tr anslucens pv. translucens also reduced seed germination, but this was more evident in Savannah than in Florida 304.