INFLUENCE OF PLANT SPATIAL PATTERNS ON DISEASE DYNAMICS, PLANT COMPETITION AND GRAIN-YIELD IN GENETICALLY DIVERSE WHEAT POPULATIONS

Field experiments conducted at two sites across two winter wheat (Triticum aestivum L.) seasons tested the effects of cultivar mixtures and spatial planting patterns on foliar disease development and wheat yield. Two or three wheat cultivars were planted in pure stands, random mixtures, alternating rows, and alternating four-row swaths. Plots were inoculated with leaf rust (induced by Puccinia recondita Rob. ex Desm.) or stripe rust (induced by Puccinia striiformis West.). Fungicide-treated control plots consisted of a random mixture pattern (first season) or all patterns used in inoculated plots (second season). Random mixtures and alternating rows of cultivars reduced rust severity 15-82% compared with the mean severity in pure stands. Alternating swaths plots showed higher disease than the random mixture, but also yielded as high or higher than the random mixture. Disease control results are explained by the size of the continguous area occupied by a given cultivar, which is small for a random mixture, larger for alternating rows, and largest for alternating swaths. Mixtures reduced disease more effectively when this contiguous area was small than when it was large. In the absence of disease, the random mixture yielded similarly to or higher than alternating swaths. During the second season, yield in mixtures was dominated by one cultivar under conditions of disease pressure or close proximity (the random mixture), but yield was more evenly divided between the two cultivars in disease-free plots or in alternating swaths. Thus, planting pattern and disease interacted to influence yield by changing the competitive interactions among cultivars in the mixtures. The planting pattern used in cultivar mixtures may be important in determining disease control and yield results.