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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, seed contamination by the causal bacterium (Xanthomonas translucens pv. translucens), and subsequent development of bacterial leaf streak in wheat was studied in microplots and in the laboratory to determine the role of seed transmission in disease epidemiology. Microplot experiments were carried out during the 1994-95 and 1995-96 growing seasons using seed harvested in Baton Rouge, Louisiana, in 1994 and 1995, respectively. Treatments were seed lots from plants with differing levels of bacterial 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 when seed came from parent tillers that expressed flag leaf disease severity ≥15 to 20% in 1994-95 and ≥30 to 35% in 1995-96. However, symptoms of bacterial leaf streak on plants that developed from these seed were evident on only ≤3% of plants. Results suggest a possible threshold level for bacterial leaf streak on flag leaves that is necessary before X. translucens pv. translucens can be detected in seed. Seedling emergence in microplots correlated negatively with leaf streak severity on parent tiller flag leaves. Artificial infestation of seed with X. translucens pv. translucens also reduced seed germination, but this was more evident in Savannah than in Florida 304.