STUDY ON YIELD LOSS DUE TO XANTHOMONAS-CAMPESTRIS PV UNDULOSA IN WHEAT UNDER HIGH RAINFALL TEMPERATE CONDITIONS

Yield losses due to bacterial leaf streak caused by Xanthomonas campes tris pv. undulosa in wheat were evaluated using a method based on asse ssing infection in single tillers in a high rainfall temperature envir onment in Mexico. Trials conducted over 3 years showed that yield was significantly reduced in relation to the percentage area of flag leaf damaged at early milk-dough stage. The 1000-grain-weight was affected every year. The number of kernels per spike was affected during 2 year s. Data over 3 years showed that, on average, losses below 5% can be e xpected when the percent flag leaf area diseased is below 10%. However , up to 20% yield reduction can be anticipated, on average, if 50% of the flag leaf is diseased. Yield loss is a linear function of the perc ent flag leaf area diseased and a small leaf area damaged has already an effect on yield. A formula is proposed to calculate at Zadoks' grow th stage 73-83 the expected yield losses based on disease severity and on field incidence. The aim is to provide a tool for better quantific ation of yield losses, particularly, in high rainfall temperature loca tions where the disease is sporadic and yield losses difficult to appr aise. The study showed that significant losses due to X. c. pv. undulo sa can be expected in wheat.