Resent research has shown that amidopropyl amine surfactants are excellent adjuvants for fungicides. Traditionally, they are known as great adjuvants for aminoacids and phenoxies, however, to our knowledge they haven’t been assessed as adjuvants for fungicides. Fungicides are known to be hydrophobic and are often formulated as emulsifiable concentrates, suspension concentrates and dry formulations. For the latter two formulation types it’s known that an adjuvant can improve efficacy substantially. So how does the amidopropyl amine adjuvant work for fungicides? To investigate this, we employed a method called quartz crystal microbalance (QCM-D). We came up with a hypothesis on the adjuvant mechanism that explains the surprising field trial results.
We performed field trials using several adjuvants for comparison, among them were commercial benchmarks such as Silwet L-77 (organosilicone based) and Aspa-80 (NPE based). One novel amidopropyl amine adjuvant used was called Adsee C80W and its structure and typical properties can be seen in Figure 1. Our tests were performed at ICAR- National Center for grapes, Pune, India, and was done in a randomized block design using four replications. The first spray was given at the onset of disease, which in this case as powdery mildew. Subsequently 1-2 sprays were given at 5 to 7 day intervals depending upon disease severity using a knapsack sprayer. The water volume used was 1000 l/ha with an adjuvant concentration of 0.1% and with an active concentration of Azoxystrobin of 0.02 (v/v)%, Fluopyram + Tebuconazole 0.56 (v/v)%, Metrofenone 0.2 (v/v)% and Dimethomorph (1g/l). The concentration of the formulations is shown in Figure 2. The results were judged by the percent disease index (PDI) of powdery mildew on leaves and bunches as and when disease appeared in the experimental plots. Other observation parameters like chlorosis, necrosis, wilting, scorching, hyponasty and epinasty for phytotoxicity were taken after 1, 3, 5, 7, and 10 days after the first application only and no such signs were observed. From the field trials we can clearly see that when Adsee C80W was applied, the percent disease index (PCI) was at its lowest, indicating this to be the best performer in controlling powdery mildew on grapes. This was true for all trials, regardless the type of formulation or AI used, and clearly better than Aspa 80 and Silwet L-77, as shown in Figure 2.
Figure 1.The adjuvant Adsee C80W and typical surface chemistry properties.
Figure 2. Field Trials done in 4 repetitions using randomized block design for a) Azoxystrobin 250 g/L SC, b) Fluopyram + Tebuconazole (200+200) g/L SC, c) Metrafenone 50 g/L SC, d) Dimethomorph 50 g/L WP
To address the dearth of knowledge with regards to adjuvant mechanism, we utilized the use of a nanoscale acoustic sensing technique, QCM-D, to further probe the mechanistical actions of the adjuvants. The QCM-D method measures both frequency and dissipation of a sensor surface comprising a quartz crystal covered with a gold electrode and treated with a model wax applied by spin coating, which is the surface that interacts with the adjuvant (Figure 3A).
Figure 3. Showing the theory for QCM-D and the results. A) A QCM-crystal covered with gold; this surface is modified to create a model wax onto the surface. B) The QCM crystal from the side where the quartz material oscillates due to the current passing the crystal. C) How the results look like, the frequency change says if something is absorbing or desorbing, the decay when current is shut off says something about the softness of the absorbed layer: slow decay-soft material, fast decay-hard material. D) The upper graph showing the difference in thickness change due to different adjuvants. The lower picture shows the raw data of frequency and dissipation for Adsee C80W. (Figure 3A-C Richter et.al. 2018)
It is possible to make the quartz crystal oscillate by applying an AC voltage (Figure 3B). A change in mass of the quartz sensor will directly change the resonance frequency (a lowering in frequency means an increasing mass) and a change in dissipation will reflect the viscoelastic properties of the adhering layer (i.e. a higher dissipation means a softer adsorbed layer). The second technique used in this study is a Franz Cell technique which measures the penetration from a donor cell through a membrane consisting of the upper part of the leaf of a Clivia Minata, to an acceptor cell where cumulative concentration is measured over time.
So why is Adsee C80W such a great adjuvant for fungicides as proven in Figure 2? For fungicides, an optimal penetration is desired rather than a maximum penetration. Our knowledge of fungi tells us that they enter the plant through the leaf surface under wet conditions. Depending on the type of fungus, growth stage and stem of the fungus it will distribute itself at different depth. That is everything between subcuticular to a vascular level. With such knowledge, we propose a hypothesis that Adsee C80W distributes the active ingredient optimally both for contact mode of action and systemic mode of action. The basis for stating this is that the adjuvant has the properties to both form a hydrogel at the surface as shown by QCM-D (Figure 3) and to penetrate the wax as shown by Franz cell results in figure 4, we see that the adjuvant Adsee C80W is able to penetrate the leaf, to the acceptor cell.
Figure 4. Franz cell results showing the penetration rate (Y- cumulative amount in acceptor cell) of Tebuconazole through wax from the plant Clivia minata over time (x-axis)
Figure 5. Schematic picture on how the adjuvant works on a leaf surface, where A is the proposed mechanism for Adsee C80W and B the scenario for a traditional nonionic adjuvant where red symbolizes AI in its molecular form, thus active to interact with the fungus. Blue symbolizes active ingredient in a precipitated form, thus less active material.
Adsee C80W is novel adjuvant for fungicides. The current study shows that Adsee C80W is the best performer for controlling powdery mildew on grapes, the adjuvant shows increased efficacy for several active ingredients such as Azoxystrobin, Floupyram, Tebuconazole, Metrafenone and Dimethomorph. Based on the field trials and the complementary measurements using QCM-D and Franz Cell a hypothesis is presented on the adjuvant mechanism. The adjuvant promotes uptake, which is important for systemic mode of action but at the same time it promotes the ability to form a hydrogel on the surface, which is a property that promotes a contact mode of action. This dual action is what makes Adsee C80W the best performing adjuvant as shown in our field trial results.
Richter RP, Rodenhausen KB, Eisle NB, Schubert M,. (2018) Elipsometry of functional Organic surfaces and films, 52, pp 391-417
This article will be published in AgroPages '2021 Formulation & Adjuvant Technology' magazine to be published this May.
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