Influence of adjuvants on the rainfastness of mancozeb in potato

Late blight is a very destructive disease in potato in Indonesia. Indonesian framers frequently use Mancozeb-based fungicides to protect the potato plants against late blight. In the rainy season, adjuvants are added to enhance ‘sticking’ of fungicides on the leaves and to improve the distribution of the spray droplets over the leaves. Many adjuvants use the suffix –STICK- in their name, suggesting that they improve the rainfastness, but until now no trial results were available to underpin these claims.

Researchers, Huub Schepers and Herman de Putter of Wageningen University & Research, study the influence of adjuvants and spray volume on residue deposits of mancozeb and biological efficacy in potato and shallot. The rainfastness was tested using potted potato and onion plants. The best conditions for infection and disease expression after artificial inoculation, were created by using facilities of the Wageningen University & Research location in Lelystad, like a spray cabin, artificial rain installation and climate rooms.

Schepers and De Putter presented their results at the 11th International Symposium of Adjuvants (ISAA 2016) in Monterey, California. Adjuvant, rain and spray volume had a clear impact on both mancozeb retention and rainfastness in potato, and consequently on the efficacy of strategies to control late blight.

Under dry conditions (pre-rain) the use of adjuvants in sprays applied to potato did not result in significantly higher mancozeb residues (retention) or less P.infestans incidence. Except at 750 L/ha with the addition of Bond® a significantly higher retention was observed.

Under wet conditions (post-rain) even six hours after spraying a significant amount of mancozeb was washed-off from the leaves. The adjuvant Bond® produced much higher residues of mancozeb, and increased rainfastness. This resulted in better control of P. infestans. The adjuvant Indostick® did result in higher mancozeb residues in potato compared to Tridex® without an adjuvant, though this effect was not statistically significant.

The detailed results of this study are published in the Proceedings of the ISAA 2016 Symposium and/or can be provided by Huub Schepers.