MBFi: Advanced green chemistry fungicide Agri-Cure SP for the control of Citrus Black spot for South African Citrus producers

By Jonathan Etherington & SB Coetzee

Agri-Cure SP is a highly advanced green chemistry technology fungicide with both fungicidal and fungistatic properties. This makes it a multi functioning product which can be used as an effective contact fungicide with preventative, and in certain cases, curative properties on established infections. The active ingredients in Agri-Cure SP are recognised as GRAS ( generally regarded as safe) compounds by the EPA (Environmental Protection Agency in USA) and are exempt from residue analysis due to the nature of the ingredients. This gives us a product which has a zero day withholding period but at the same time has a long stable residue of close to 25 days on the crop, without wash off via heavy rain. MBFi's green chemistry range is a perfect f'it to the global movement towards the limiting of residues without reducing the eff'icacy of products used to control diseases and insects on citrus crops.

In the development of Agri-Cure SP, MBFi has conducted more than 36 trials on citrus black spot throughout South Africa. MBFi has put Agri-Cure SP through vigorous trialing from various interval sprays, stand alone applications, in combination with various Strobilins and in programs with copper hydroxide to formulize the best viable alternative to replace older and traditional hard chemistries available in the market.

Agri-Cure SP is a multi-combination of Metallic bicarbonate, humectants and wetting agents. The active ingredient, Potassium Bicarbonate, was approved by the Department of Agriculture, Land Reform and Rural Development in the late part of 2021 for use as a fungicide for the control of CBS on Citrus and is registered under Act 36 of 1946.

How does Agri-Cure SP work?

Agri-Cure SP is an effective dehydrating and pH correcting active ingredient that acts as a fungicide for the control of fungal diseases.

Agri-Cure SP has both fungicidal and fungistatic properties due to its multiple modes of action. The fungicidal action is due to the hypertonic dehydration and the fungistatic ability is due to osmotic gradient pH disruption within the pathogen.

Hypertonic dehydration

Hypertonic dehydration (Figure 2) kills the pathogen and spores. All cells prefer to be in a state of equilibrium with their outside environment, where the osmotic pressure in the cell is equal to that of their outside environment (isotonic state). In this state there is no movement of water into or out of the cell. Potassium bicarbonate creates a high osmotic pressure on the leaf surface resulting in the movement of water out of fungal spores or hyphae. In the process the cells become dehydrated and shriveled, resulting in death.

Osmotic gradient disruption

Under favorable environmental conditions, fungal spores germinate and form hyphae. During this process, the spore absorbs water through its wall. The wall initially grows as a spherical structure and from the wall of the spore a germ tube bulges out, enveloped by a wall of its own that is formed as the germ tube grows. At the growth tip the cell wall extends continuously to produce a long hyphal tube. The cytoplasm within the apical zone is filled with numerous vesicles. For these wall vesicles to reach the cell wall to be incorporated cytoplasmic movement is critical. Cytoplasmic movement is dependent on an osmotic potential gradient within the hyphae (Figure 3). After Agri-Cure's application the osmotic pressure on the outside of the hyphae is greater than the osmotic gradient the hyphae can produce within itself. This leads to reduction to no movement of the cytoplasm that in effect stops all hyphal growth and spore germination.

pH at work (Fungistatic)

The environmental and internal pH levels affect several critical structures and functions in fungi. Tight control of cytosolic and organelle pH is critical for viability in all cells. Fungal pathogens prefer mostly acidic environments, at high pH levels proteins and enzymes are denatured, lipids (fats, oils and hormones) are hydrolyzed and bonds that hold the genetic material (DNA) together tend to dissociate. Agri-Cure SP shifts the pH Balance to 8.5 inducing the metabolic changes in a fungal pathogen stopping growth and limiting the spread and development of the disease and symptoms.

The use of Agri-Cure SP as a standalone product for the control of CBS

In head to head comparisons of the eff'icacy of Agri-Cure SP compared to Mancozeb in commercial orchards in the 2017 and 2021 citrus seasons, Agri-Cure SP outperformed mancozeb in 2021 on Nardorcotts and Lemons with 71% vs 68% and 88% vs 80% (Graph 1) respectively after four applications spaced at 25 day intervals. In the 2017 season the same spray program yielded good comparison results of Agri-Cure SP against Mancozeb with 80% vs 83% and 91% vs 93% on Midnight Valencias in Hectorspruit and Marble Hall respectively. The overall average between the four trials over 4 locations showed that Agri-Cure's eff'icacy as a stand-alone replacement for Mancozeb is justif'ied as the overall results showed that Agri-Cure SP gave control of 81% vs 82% of mancozeb in a stand-alone fungicide program. These results were in high inoculum pressure environments with an average of 49.5% disease incidence across the untreated control blocks.

Incorporation of Agri-Cure SP in an industry standard spray program for CBS control

In the 2021 season Agri-Cure SP was slotted into an industry standard spray program to replace Mancozeb. The industry standard practice is the stand alone application of mancozeb in mid-October followed by two mancozeb applications in conjunction with a Strobilurin fungicide and mineral oil at the beginning of November and mid-December, and a final standalone mancozeb application at the end of January. Agri-Cure SP was slotted into the standard industry spray program at 200 g/100 L in place of the mancozeb and the results showed the high eff'icacy of Agri-Cure SP in the control of CBS (Graphs 2-4). Agri-Cure SP was also tested for eff'icacy in combination with the three most highly used Strobilurin fungicides registered for control of black spot on citrus, this included Azoxystrobin, Pyraclostrobin and Trif'loxystrobin. These trials were done on Eureka lemons, Nardorcott's and Valencia's in Nelspruit, Mooinooi and Groblersdal in the 2021 season.

Agri-Cure SP in combination with Azoxystrobin showed high eff'icacy across lemons, Nardorcott's and Valencia's with high inoculum pressure in the untreated controls sitting at 65% disease incidence. Over all three varieties Agri-Cure SP in combination with azoxystrobin gave control of 97% vs 97% for Mancozeb in combination with azoxystrobin and mineral oil (Graph 2).

Agri-Cure SP in combination with Trif'loxystrobin showed high eff'icacy across lemons, Nardorcott's and Valencia's with high inoculum pressure in the untreated controls sitting at 64% disease incidence. Over all three varieties Agri-Cure SP in combination with Trif'loxystrobin gave control of 97% vs 97% for Mancozeb in combination with Trif'loxystrobin and mineral oil (Graph 3). On the Lemons the Agri-Cure SP in combination with Trif'loxystrobin and mineral oil gave complete control of CBS with just a four spray application approach that did not include a 5th late spray of Agri-Cure SP as stand alone.

Agri-Cure SP in combination with Pyraclostrobin showed high eff'icacy across lemons, Nardorcott's and Valencia's with high inoculum pressure in the untreated controls sitting at 64% disease incidence. Over all three varieties Agri-Cure SP in combination with Pyraclostrobin gave control of 99% vs 97% for Mancozeb in combination with Pyraclostrobin and mineral oil (Graph 4). On all three varieties Agri-Cure SP in combination with Pyraclostrobin and mineral oil outperformed mancozeb in combination with Pyraclostrobin and mineral oil. On the Lemons the Agri-Cure SP in combination with Pyraclostrobin and mineral oil gave complete control of CBS with just a four spray application approach that did not include a 5th late spray of Agri-Cure SP as stand alone.

MBFi is committed to the industry to provide green chemistry which is effective, reduces residues and are more environmentally safe. From these results it is evident that with all the challenges forced onto our citrus producers with the non-renewal of mancozeb in the EU, the producers can rest assure that local knowledge and development of novel fungicides will be enabling them to still combat CBS as effectively as they have been in the past without the risk of MRL's playing a roll. Agri-Cure SP has proven itself as an effective fungicide for the control of CBS on citrus. For more technical information and recommendation on Agri-Cure SP, please contact an MBFi Agent for further information.