RELATIONSHIP BETWEEN VERTICILLIUM-ALBO-ATRUM INOCULUM DENSITY AND POTATO YIELD

Population studies with Verticillium albo-atrum (microsclerotial form) have shown that a significant negative correlation exists between inoculum density in field soils determined before planting and yield of potatoes in some areas of Colorado but not in others. In Southern Weld and Morgan Counties where potatoes are planted early in sandy soils, reduced yield in Norgold Russet and Norchip cultivars occurred as inoculum densities increased. In Northern Weld County where potatoes are planted later in heavier soils, no such relationship was found. The significant relationship between inoculum density and reduced yield was associated with growing areas where conditions, particularly air temperature, appear to be favorable for verticillium wilt development when plants begin to mature. A minimum number of Verticillium propagules was required to cause significant yield reductions in total, marketable and U.S. No. 1 grade categories. Below this minimum number, no significant yield reductions occurred. In Southern Weld and Morgan Counties, this minimum number was found to lie between 17.5 and 23 propagules per gram of soil. Controlled greenhouse studies showed that as air temperature decreased from 29.4 C (85 F) to 23.9 C (75 F) more inoculum was required to cause significant disease expression and to shorten plant life. This study has shown that in some areas soil inoculum assays prior to planting may provide a basis for selecting fields which are safe for planting potatoes to avoid severe losses from verticillium wilt. Furthermore, they may help in identifying fields where preventive control measures may be required for satisfactory potato production. © 1979 Springer.