Are mancozeb formulations rainfast?

The broad-spectrum fungicide mancozeb is the go-to option for apple-scab control. But what happens to mancozeb when it rains? By Engela Duvenage.

“Some mancozeb products on the market claim in their advertising to be more rainfast than others,” says Prof. Adéle McLeod of the Department of Plant Pathology at Stellenbosch University. She has been overseeing research on this topic.

In one study, master’s student Cornelius Rossouw tested different mancozeb formulations — Dithane M-45 800 WP NT, Ventum 800 WP, and Ventum 800 WP combined with the sticker-spreader adjuvant Nu-Film P.

There were no significant differences in rainfastness among the two formulations at rain intensity of 5 mm per hour and rain volumes of 1–15 mm. In all three mancozeb treatments, nearly a third of the spray residue was lost after just 1 mm of rain.

“This percentage did not really differ between 1 mm, 5 mm or 10 mm of rain,” says McLeod. “That’s because the products redistribute so well across leaves, to provide a protective layer against spores.” This finding differs from those of a 2007 German study, which reported a much higher residue loss after 5 mm than after 1 mm of rain.

A follow-up study by another master’s student, Phillip Rebel, assessed the quantity of mancozeb needed to control the apple-scab fungus.

Rebel tested how much mancozeb is required to control apple scab on leaves using a laboratory-based apple-seedling model system. He also developed spray-deposition benchmarks for orchards using a mancozeb and apple-scab model.

His results showed that even when very low concentrations were used, mancozeb was still able to control apple scab since no visible lesions developed on leaves. This was even achieved at a dose that was 85% lower than the registered dose for mancozeb in South Africa.

“The study showed that very little mancozeb is required to control the pathogen,” says McLeod. But she stresses that aspects such as the influence of rain or sun on the longevity of the product were not tested in Rebel’s study, and results should be applied with caution. Similar tests under orchard conditions are needed before producers make any changes to their spraying regimes.

“Therefore, it will still be best to keep to the guidelines provided per product, since applications must always be made according to label recommendations,” advises McLeod. “Reapplication, however, following less than 10 mm of rain is not required due to the good rainfastness of mancozeb.”

Bonus: European Union mancozeb developments

Following the European Union — EU — decision to withdraw all authorisations of mancozeb in the EU, the non-renewal regulation was published in December 2020, with a related phase-out period regarding use in the EU.

Following EU non-renewal of approval of an active substance, there is a separate process to review the relevant EU MRLs — maximum residue limits. In the case of mancozeb, the group of dithiocarbamates needs to be reviewed as a whole. This ongoing process is being closely monitored by Hortgro.

A proposed reduced EU MRL for mancozeb is unlikely to affect the 2022–23 season.

Source:

• Fresh Notes (193) March 2022.