1. Are the White Powders on Grapes and Cucumbers the Same Thing?
The faint white powder on grape skins is called "fruit powder" or bloom. According to the Ministry of Agriculture, Forestry and Fisheries, it is a natural component made of fatty acids. It repels rain and morning dew to prevent disease and keeps the grape fresh by stopping moisture from escaping.[3] The grape produces this powder itself to protect its surface.
The white powder on cucumbers also has the same name: bloom. Although they look similar, it is explained that the contents of the cucumber powder are different from those on grapes. What exactly is the cucumber powder made of?
2. Silica Is the Main Component of the White Powder on Cucumbers
A research group at Okayama University reports that the main component of the white powder on the surface of cucumber fruits is silica (SiO2).[1] Silica is also a major component of sand and stone. A report from a testing station in Tochigi Prefecture states that in past analyses, silicic acid was the primary inorganic component of the powder.[2] Since it is not written that it consists only of silicic acid, we call it the "main component."
Where does this silicon come from? A 2025 paper reports that silicic acid absorbed by the plant's roots from the soil accumulates in fine hairs called glandular trichomes on the fruit's surface, where it emerges like powder.[7] This powder is simply a substance naturally present in cucumbers that has appeared on the surface, and it is described as having no danger.[4]
With this knowledge, a strange question remains. If what is absorbed from the soil becomes powder, then preventing absorption should result in powder-free cucumbers.
3. How Powder-Free Cucumbers Are Made by Grafting onto a Different Pumpkin Rootstock
Bloom-free cucumbers are popular with consumers because they are shiny. This type is made using a specific pumpkin as a rootstock. Rootstock is the plant that provides the roots when grafting. This type of pumpkin has the characteristic of absorbing less silicic acid from the soil.[1]
According to Okayama University research, in this rootstock pumpkin, the 242nd amino acid of a protein (transporter) responsible for taking in silicic acid had changed. Because of this, the protein was not transported to the cell membrane, so it could hardly absorb silicic acid from the soil. The cucumber grafted on top does not receive silicic acid and becomes a shiny fruit.[1]
The change was found in the rootstock pumpkin, not the cucumber. The mechanism that took in silicic acid in the roots affected the appearance of the fruit. This is how powder-free cucumbers became possible. But why did people start making powder-free cucumbers to such an extent?
4. Why Powder-Free Cucumbers Became the Mainstream Type
An article citing a seed company explains that bloom-free cucumbers became mainstream about 30 years before 2019. The reasons include that bloom looks like residual pesticides, it looks better in stores, and it has a longer shelf life. A 2019 article states that about 90% of cucumbers on the market were bloom-free.[4] Materials from the Agricultural and Livestock Industry Promotion Organization also state that bloom-free cucumbers became mainstream because bloom-free cucumbers keep well and look good.[5]
A 1991 report from the Tochigi Prefectural Agricultural Experiment Station noted that pumpkins for rootstock that produced almost no bloom had been bred, making it easier to grow cucumbers with less bloom. It was reported that market evaluation was high, and producers were growing bloom-free cucumbers year-round.[2] However, reading this as "bloom-free was widely distributed in 1991" is an overstatement. We can only say it became mainstream "about 30 years ago (as of 2019)."
The same report also notes that changing the rootstock led to issues with taste, plant vigor, disease, and yield. Have we traded something away for the sake of appearance?
5. How Powder-Free Cucumbers Relate to Disease Resistance and Silicon
In a test in Kumamoto Prefecture, powdery mildew appeared more easily on bloom-free rootstock and less on bloom-producing rootstock.[6] This is a result of comparing rootstocks and is not stated to hold true for all varieties or conditions.
A 2025 paper reported that if the BEC1 gene in cucumbers (a transporter that moves silicon out) mutates, bloom disappears, and the lack of silicon accumulation weakens resistance to brown spot fungus and low temperatures.[7] It seems that the silicon that becomes powder material is also related to the plant's ability to withstand disease and cold.
Interestingly, when bloom-free scions (the part grafted on top) were grafted onto normal rootstock, silicon absorption and resistance returned, and the fruit remained bloom-free.[7] This report suggests that by adjusting the roots, it may be possible to aim for both appearance and strength.
6. Comparing Cucumbers With and Without Powder at the Supermarket
While a 2019 article states about 90% of supermarket cucumbers are bloom-free, some stores still stock those with powder. If you find one, pick it up and compare the surface. If you cannot find one, and you have grapes at home, you can trace the white powder on the grape skin with your finger and compare it to the shine of a cucumber. Bloom-free cucumbers should look shiny, while those with bloom should look faintly cloudy white. If you have both, place them side by side and observe how the light hits them.
Ask three questions when looking: Where is the powder most concentrated? Is it more common around thorns or bumps? How do the appearance and texture differ between grape skin powder and cucumber powder? You might also think about how the story that less powder means more vulnerability to disease helps in the field.
The press release from Okayama University and the public reports from Tochigi and Kumamoto Prefectures are all free to read. They show specifically how farmers have tried to balance appearance and strength.