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Why Whey Works Against Powdery Mildew

Spraying with milk and whey works against powdery mildew. For the cultivation of cucumbers, courgettes, pumpkins, melons and grapes, milk and whey are therefore regularly used as a biological alternative to common fungicides. But what is their mode of action? And how effective are they? And if it works, why are they not used widely in viticulture?

Whey is a slightly acidic liquid that is produced during the preparation of cheese when the milk is curdled. Whey still contains many of the components that are also found in milk, but no longer the fats or the milk protein casein, these are processed in the cheese. Nevertheless, whey still contains an exceptionally high amount of amino acids and proteins, including lactoferrin. It is therefore certainly not a waste product and is widely used in the food and pharmaceutical industry for, among other things, the production of Rivella, ricotta, or as a source of amino acids and proteins for nutritional supplements and baby food. But what in these harmless food products causes the death of whole fungal populations?

Whey is produced during the curdling of the milk

Whey is produced during the curdling of milk

The mode of action

It has long been known that milk works against fungi. What one does not know is which component of milk exactly ensures this effect. It appears that various components of milk, including whey (protein), lactose and lactoferrin, compete with fungi1-3. According to the scientific literature, the effect of milk against powdery mildew seems to be based on two (conflicting) effects:

  • The nutrients in milk stimulates the growth of microbial life3,4
  • Oxygen radicals that are released under the influence of sunlight from, among others, the proteins lactoferrin and lactoperoxidase reduce microbial life5.

Milk and milk products contain many amino acids and proteins and are a breeding ground for fungi, bacteria and yeasts. The theory is that the growth of these (benign) microbes on the plant provides a natural counterbalance to the harmful powdery mildew fungi. They have a so-called entomopathogenic effect on the growth of fungi that cause powdery mildew.

In contrast, the free oxygen radicals released from proteins such as lactoferrin and lactoperoxidase are harmful to microbial life and inhibit the growth of fungi. Oxygen radicals are molecules with an oxygen atom that has an unpaired electron. These radicals arise under the influence of, for example, sunlight and are very unstable. The oxygen atom wants only paired electrons and will therefore react with random other molecules to achieve this. As a result, the oxygen radicals damage the cells of an organism, and in this case also the spores and hypha of powdery mildew5.

Researchers have demonstrated this also by incorporating the lactoferrin gene in grain and in the tobacco plant. As a result, these crops started to produce lactoferrin and became less sensitive to fungi6. Due to this modification, these crops are now genetic modified organisms, which is generally not desirable for new grape varieties (especially since hybrid varieties are already often (unjustly) viewed with suspicion).

READ ALSO: Breeding, genetic modification and CRISPR in the vineyard

The effectiveness of milk and whey against powdery mildew

So instead, we stick to spraying milk and whey dilutions in the vineyard. Multiple studies of different crops have assessed the effectiveness of milk and whey in controlling powdery mildew. Table 1 provides an overview of these studies.

Table 1. Overview of the effect of milk and whey on powdery mildew in different crops. Dilutions are always in water. The effect of the treatments is usually expressed in “area under the disease progress curve” (AUDPC) where the progression of the disease is compared to the untreated plants.

Product (Application and concentration)Crop (greenhouse or outside)Fungus that causes powdery mildewEffect on powdery mildewRef.
Milk (1-2x per week, different dilutions of 5-50%)Zucchini (greenhouse)Sphaerotheca fuligineaProgression is reduced by 90% when treated twice a week with a dilution of 3.8 – 11.4%7
Milk (1x preventative before infection, 30% dilution)Zucchini (greenhouse)Podosphaera xanthiiReduction of progression by 88%3
Milk (every 7-12 days and after 25mm of rain, 50% dilution)Pumpkin (Outside)Podosphaera xanthiiReduction of progression by 50-70%8
Whey (2x per week, 10-30% dilution)Cucumber, zucchini (greenhouse)Podosphaera xanthii71-94% less diseased plants depending on the whey concentration used9
Milk (20 and 50 days after infection, 5.10 or 20% dilution)Soya bean (greenhouse)Erysiphe diffusaReduction of progression by 40-72%10
Milk (weekly, 10% dilution)Kale (greenhouse)Erysiphe polygoni DC.Reduction of progression by 30%4
Milk powder (once every two weeks, 30g/L ~ 20% dilution)Grape (Viognier) (greenhouse)Erysiphe necator15g/L reduces progression by 65-96%, 30g/L milk powder prevents powdery mildew contamination1
Whey powder (1x every two weeks, 15, 30 or 45g/L)Grape (Viognier) (greenhouse)Erysiphe necator15 and 30g/L reduce progression by 58-79%, 45g/L whey prevents powdery mildew contamination1
Milk (1-2x every two weeks, 10% dilution, 300-600L/ha)Grape (Verdelho) (Outside)Erysiphe necator79% of the harvest is acceptable compared to 87% when treated with sulfur (3g/L)2
Whey powder 1-2x per two weeks, 45g / L, 300-600L/ha)Grape (Verdelho) (Outside)Erysiphe necator76% of the harvest is acceptable compared to 87% when treated with sulfur (3g/L)2
Milk (1-2x every two weeks, 10% dilution, 300-900L/ha)Grape (Chardonnay) (Outside)Erysiphe necatorReduces progression by 40% at harvest11
Whey protein powder (1-2x per two weeks, 25g/L, 300-900L/ha)Grape (Chardonnay) (Outside)Erysiphe necatorReduces progression by 46% at harvest11

A number of comments must be made about these research results. They have been carried out on different crops, with different concentrations and number of applications, against different fungi (all of which cause powdery mildew), in different climates, in the controlled conditions of a greenhouse, or outside in the open field. In addition, the composition of milk and whey solutions is always different because the milk can be either pasteurized, whole or skimmed, fresh or made from milk and whey powder. So there are quite a few variables that can influence the effect. Either way, what emerges clearly from these studies is that:

  1. Milk and whey reduce powdery mildew infestation
  2. The effect depends on the concentration and the number of sprays
  3. Complete coverage of the leaves with milk / whey is necessary for proper functioning
  4. Milk and whey are especially effective when there is a low disease pressure. Under high disease pressure and / or when sensitive grape varieties are used, sulfur and conventional fungicides are (much) more effective
  5. It is not possible to limit the amount of powdery mildew to commercially acceptable values ​​(<5%) with milk and whey under high disease pressure and when using sensitive grape varieties.

In particular in the greenhouse milk and whey have a major effect on disease intensity. The progression of infestation, that is, the rate at which powdery mildew spreads, is largely inhibited by spraying with milk and whey. However, due to rain, wind, sun, organisms and other factors in the vineyard, there can be a large difference in effect between the controlled environment of a greenhouse and outdoor treatments. An important factor in the effect of milk and whey is the extent to which they cover the parts of the plant. If the coverage is not good, for example due to rain or a too dense canopy, it still leads to unacceptable amounts of powdery mildew in the vineyard2. Furthermore, milk and whey must be sprayed preventive and do not work curative. Therefore, the progression of the disease and the spread of powdery mildew through the vineyard continues despite the milk and whey treatments. Although, one study in the greenhouse shows that under a low disease pressure treatment with high concentrations of milk and whey (30g/L and 45g/L) can reduce the contamination with powdery mildew1.

Powdery mildew on grapes

Powdery mildew on grapes. Maccheek via CC BY-SA 3.0

The usability of milk and whey in viticulture

Milk and milk products are an interesting alternative to common fungicides in viticulture. Their application is inexpensive, not harmful to the environment, and spraying can be done with existing equipment. In addition, due to its nutritional value, whey even appears to stimulate the growth of soil fungi such as mycorrhiza12. Whey (but not milk) has been registered within the EU as a basic substance since 2016 – a food product that may also be used as crop protection. However, it is not yet registered for use in the open field, for grapes or for the Erysiphe necator fungus that causes powdery mildew in grapes13,14. The use of milk and whey in the vineyard is simply not yet allowed.

However, if whey would be allowed for use in the vineyard, what concentration is needed? And does this correspond to the current EU regulation for use of whey in crop protection?

Required whey concentration

According to the EU regulation, 6 to 30 liters of whey dissolved in 1000 to 1500 liters of water may be used for spraying one hectare. Whey consists of 6% of solids, so 1 liter of whey equals about 60 grams of dried whey powder. This corresponds to the 60 to 80 grams of active substance that may be present in the (liquid) whey. With this data, the highest possible concentration that can be used for spraying is 2,4 g/L (i.e. 30 liters of whey x 80 gr in 1000 liters of water). This concentration is in stark contrast to the 45 g/L whey that is recommended for use in the vineyard1,2. In addition to the fact that this concentration is probably too low for an effective effect, whey may also be used only 3 to 5 times, with a minimum interval of 7 days. An advantage is that there is no safety period, so the grapes can be sprayed with whey until harvest. Spraying could therefore be used in combination with other conventional pesticides, especially towards the end of the season.

Read also: How Plasmopara viticola breaks through the grapevines’ defense mechanisms

Effect on the wine?

When the grapes are sprayed with milk or whey close to the harvest, residues can end up in the must. At higher concentrations, milk and whey even leave a visible layer on the grapes1,2. Although this is not harmful to consume, and does not seem to have any effect on the pH, acidity and sugar content of the must2, it could still have an effect on the wine. To my knowledge, however, there are no studies on the effect of milk and whey residues on the taste and quality of wine, or on the effects of these milk proteins on the winemaking process.

Cool climates

Milk and whey work against powdery mildew. Studies in Australian vineyards show promising results2,11. But the researchers themselves also comment that the effectiveness of milk and whey should also be tested in vineyards with higher humidity, less sun intensity and more rainfall2. Research into the effect of these milk products in vineyards in cooler climates is therefore highly desirable. Full protection in the open field by milk and whey against powdery mildew seems unlikely (especially with the current permitted concentrations in the EU), but their application could reduce the use of conventional plant protection products. However, in order to enable the use of whey and milk in the vineyard, they should first be authorized as plant protection products for viticulture.

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References
1.   Crisp P, Wicks TJ, Lorimer M, Scott ES. An evaluation of biological and abiotic controls for grapevine powdery mildew. 1. Greenhouse studies. Australian Journal of Grape and Wine Research. 2006; 12:192-202
2.   Crisp P, Wicks TJ, Bruer D, Scott ES. Australian Journal of Grape and Wine Research. 2006; 12:203-211
3.   Medeiros FHV, et al. Microorganisms, application timing and fractions as players of the milk-mediated powdery mildew management. Crop Protection. 2012; 40:8-15
4.   Martins SJ, et al. Dual role of milk on aphid and powdery mildew control in kale. Scientia Horticulturae. 2016; 203:126-130
5.   Crisp P, Wicks TJ, Troup G, Scott ES. Mode of action of milk and whey in the control of grapevine powdery mildew. Australian Plant Pathology. 2006; 35:487-493
6.   Lakshman, DK, Natarajan S, Mandal S, Mitra A. Lactoferrin-derived resistance against plant pathogens in transgenic plants. Journal of Agricultural and Food Chemistry. 2013; 61, 48, 11730-11735
7.   Bettiol, W. Effectiveness of cow’s milk against zucchini squash powdery mildew (Sphaerotheca fuliginea) in greenhouse conditions. Crop Protection. 1999; 18:489-492
8.   Ferrandino FJ, Smith VL. The effect of milk-based foliar sprays on yield components of field pumpkins with powdery mildew. Crop Protection. 2007; 26;657-663
9.   Bettiol W, Silva HSA, Reis RC. Effectiveness of whey against zucchini squash and cucumber powdery mildew. Scientia Horticulturae. 2008; 117:82-84
10. Perina FJ, et al. Essential oils and whole milk in the control of soybean powdery mildew. Ciência Rural. 2013; 43(11):1938-1944
11.  Savocchia S, Mandel R, Crisp P, Scott ES. Evaluation of ‘alternative’ materials to sulfur and synthetic fungicides for control of grapevine powdery mildew in a warm climate region of Australia. Australasian Plant Pathology. 2010; 40(1):20-27
12.  Demir S, Ocak E. Effects of whey on the colonization and sporulation of arbuscular mycorhizal fungus, Glomus intraradices, in lentil (Lens orientalis). African Journal of Biotechnology. 2010; 8(10):2151-2156
13. Het College voor de toelating van gewasbeschermingsmiddelen en biociden (Ctgb) (https://www.ctgb.nl/onderwerpen/basisstoffen/documenten/instructies-gewasbeschermingsmiddelen/2018/10/08/basisstof-wei) accessed on 13-02-2020
14.   EU pesticides database (https://ec.europa.eu/food/plant/pesticides/eu-pesticides-database/public/?event=activesubstance.detail&language=EN&selectedID=2374) accessed on 13-02-2020

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