Spatial pattern of Verticillium wilt in commercial mint fields

Spatial patterns of mint plants with symptoms of Verticillium wilt caused by Verticillium dahliae were characterized in 10 commercial mint fields in Washington using several spatial analysis methods. The disease was assessed in 0.76-by-0.76-m quadrats (width of mint rows was 0.76 m) in randomly selected study sections varying in size from 5 to 76 in wide by 57 to 396 in long, The variance-to-mean ratio identified aggregation of diseased stems within quadrats even though probability distributions associated with cluster data did not fit the data well. Generally, there was more clustering within than across rows according to both doublets and runs analyses. Total number of wilt foci ranged from 5 to 170 per field, and mean size of foci ranged from 1 to 2.7 quadrats. In one field observed repeatedly, total foci increased from 24 to 104, and the mean size of foci increased from 1.0 to 1.3 quadrats in the same section of the field from one year to the next. Size of foci increased to 2.7 quadrats in a third year of sampling the same field. Mean focus size was larger within than across crop rows in 10 of 13 field-sampling occasions. The proximity index ranged from 0.88 to 1.00, indicating highly compacted disease foci. The statistical methods employed were useful in describing, quantifying, and visualizing spatial patterns of infected mint in commercial fields. Verticillium wilt spread during the life of the perennial mint crop. Inoculum for much of the secondary increase likely did not directly originate from micro-sclerotia present in soil before the crop was planted or from infected rhizomes that originally were planted.