Foliar fungicides as protective seed piece treatments for management of late blight of potatoes

Fungicides receiving Section 18 emergency exemptions for management of the foliar phase of late blight of potato were evaluated as protective treatmen ts for control of tuberborne inoculum of Phytophthora infestans in greenhou se and field studies. Three Section 18 products, Acrobat MZ (dimethomorph mancozeb), Curzate M-8 (cymoxanil + mancozeb), and Tattoo C (propamocarb h ydrochloride + chlorothalonil), when applied to the seed piece prior to ino culation with P. infestans, significantly (P less than or equal to 0.05) in creased sprout emergence, compared with the inoculated water control. Of th e three Section 18 products, Curzate M-8 was frequently the most effective. The increase in plant emergence with the protective seed piece treatment w as seen with cultivars that differ in susceptibility to late blight, two ge notypes (US-8 and US-11) of P. infestans, zoospore and sporangial inocula, and at different inoculation sites on the seed piece. In greenhouse studies , percent emergence with the protective seed piece treatments averaged 74% for cv. White Rose and 44% for cv. Shepody, compared with 9 and 8% for the inoculated water control of the respective cultivars. In field studies, whe n inoculum was placed either adjacent to or 4 cm away from the sprout, plan t emergence of the inoculated water control was 12 and 36%, respectively. A pplication of the fungicides to the seed piece prior to inoculation increas ed emergence by an average of 700 and 212% for the respective inoculation s ites. There was no evidence for translocation of the fungicides to the emer ging foliage in concentrations high enough to prevent foliar infection from airborne inoculum of P. infestans. Control of tuberborne inoculum of P. in festans with appropriate seed piece treatments will result in an increase i n plant emergence and improved crop uniformity.