Whitewashing your fruit

The Agricultural Research magazine in November 2000 reported the findings of some interesting research in the USA, where a special type of kaolin clay (marketed as the product “Surround”), sprayed over apples and pears protected them from sunburn and increased yield dramatically, as well as keeping insect pests away.  The research was carried out by D. Michael Glenn, Gary J. Puterka and Michael E. Wisniewski .

Kaolin safe to ingest (in fact, it is often taken as an anti-nausea medication). The idea of spraying trees with kaolin was originally conceived as an organic method that would repel insects and prevent disease organisms from entering fruit, by putting a physical barrier over the fruit. However the research revealed another advantage.  Apples harvested from treated trees were an average of 17 percent larger than fruit from trees left untreated. Some types of pear trees yielded twice as many pears with no decrease in fruit size. Other trials revealed that using the kaolin improved the colour and raised the sugar content of Arizona-grown lemons, while grapes reached the desirable sugar content sooner than those left bare.

It seems that the specially processed kaolin coating reflects the heat-producing infrared wavelengths of the sun, as well as the burning ultraviolet rays. Keeping the tree cooler increases fruit yield and helps prevent fungus. Some of the research has also shown that kaolin-coated trees photosynthesized up to 30 percent faster than uncoated trees.

Kaolin also seems to be proving very successful against insects, as it seems they don't like biting or crawling on a plant or tree covered with the kaolin dust. The researchers reported in their article that the kaolin seems to sticks to insects’ wings, legs, and mouth parts, causing them to leave to find a more favourable place to feed and lay eggs. They state that “In studies around the country, codling moths, apple maggots, plum curculio, leafhoppers, Japanese beetles, rose chafer, thrips, and rust mites—not to mention pear psylla—have fled whitewashed crops in search of greener pastures.” The white leaves also seem to make it harder for insects to recognise their favorite hosts. After several years of research trials, the product is proving effective against many types of insects.

The researchers reported that it is important to get good coverage of the crop with the kaolin product. They have investigated ways to improve kaolin's ability to mix with water, so that it sticks better. Areas of low rainfall are particularly adaptable to this new technique, because in rainy areas the kaolin will eventually get washed off by rain.

Finally, another feature of the new product is its ability to reduce frost damage, because of the water repelling qualities of the kaolin coating. Leaves that are have less water droplets on them will not suffer as must tissue damage from freezing.

In summary, although the kaolin spray is still in the early days of testing, it appears that it can:

• control fungal diseases
• control insect pests
• prevent sunburn on fruit and nuts
• prevent heat stress
• increase yields
• increase photosynthesis
• prevent frost damage

 

http://www.ars.usda.gov/is/AR/archive