Effect of adjuvant and spray volume on mancozeb residue on potato and onion leaves and on Phythophthora infestans in potato and Peronospora destructor in onion

As part of the vegIMPACT project a laboratory test in the Netherlands was done with potato (Solanum tuberosum) var. Bintje and onion (Allium cepa) var. Sturon. In both crops mancozeb was sprayed with and without the adjuvants Bond and Indostick. In both crops these three treatments were applied with high and low spray volumes. After spraying, rain was simulated (0.5, 3 or 6 hours after spraying and a control without rain) to test the sticker properties. After five days, plant leaves were analysed on mancozeb residue. Five days after spraying the potato plants were inoculated with Phytophthora infestans and onions with Peronospora destructor. Subsequently, disease symptoms were observed and visually rated as percentage of infected leaves. In potato lower mancozeb residue levels were measured at high spray volumes. Under dry conditions the effect of adjuvant on mancozeb residue in potato was limited. In onion the spreader characteristics of the adjuvants resulted in higher mancozeb residue concentrations than without adjuvant. The impact of rain simulation was much bigger than the impact of adjuvant on mancozeb residue and late blight incidence. Therefore, it is not recommended to spray when rain is expected within the same day as spraying. With rain simulation in both onion and potato more mancozeb residue was present when using the adjuvant Bond than when spraying mancozeb without an adjuvant. However, also with Bond after rain simulation high amounts of mancozeb were washed off. Mancozeb with Bond in potato resulted in less late blight incidence than with spraying mancozeb without adjuvants. Overall the adjuvant Bond performed better than Indostick in terms of mancozeb retention on both potato and onion leaves . Indostick had good spreading properties that under dry conditions resulted in higher mancozeb residue levels in onion. Under rainy conditions the effect of Indostick on mancozeb residues was not or only slightly better than mancozeb without adjuvant.