What might be on a particular batch of nursery plants varies with the specific circumstances. Sometimes they appear to get a white deposit from being grown or displayed in plastic houses. Other times, of course they will have been deliberately sprayed with chemicals.

Here is a link that might be useful: The Myth of Foliar Feeding

It could just be deposits forming from irrigation water evaporating off of the leaves if over head irrigated. That tends to happen when you've got some calcium and magnesium in the water and bicarbonate a level greater than 250 ppm or so.

My guess is also irrigation water.

WhatÂs a coincident!!! I recently read most of the myth articles of Dr. Chalker-Scott (she) and found several interesting topics. Some of them are foliar spraying and Epson salt. Before, I did not believe that plants can absorb fertilizer through leave. After reading her article "The Myth of Foliar Feeding" and search on the internet, I think about it again: Wait a minute, a lot of people spray fertilizer on foliar to improve crop, "foliar feeding is 8, 10, or even 20 times more effective", "Foliar feeding is yet another agricultural practice best suited to intensive crop production", "Species differ widely in their ability to take up nutrients through their leaves", "It would obviously have benefit for those landowners with landscape fruit trees that perpetually have flower or fruit disorders associated with micronutrient deficiencies". So plants can absorb fertilizer from leave. In the article "Fertilizers sprayed on the leaves of trees and shrubs are more effective than soil applications" , thereÂs no statement said that plant doesnÂt absorb fertilizer. It only discuss about "LEAVESÂMORE EFFECTIVE Â SOIL" as in the title. After reading most of her articles, I love and learn a lot of information about how to grow plant ; however, the way she explains may give readers wrong impression. Somebody think "foliar spraying is bad" (not what she discusses). In another forum, some people use her article to state that "Epson salt should not be used in agriculture" (not what she said, she only said not abuse it).

Foliar spraying is good. Plants can absorb nutrient/fertilizer from leaf. How much, how often and when to apply fertilizer through the leaf is what we want to learn. Too much fertilizer can burn leaf. Wrong foliar spraying technique will not help growing plant. ItÂs the reason why I ask people in this forum who have been trying it to learn what fertilizer they use, how often and when. I know there are many masters in this forum who have tremendous live experience and try it. They produce good and healthy plants and sell them to us. I see powder (fertilizer?) on their plant leaf. Hope they can share their experience.
Cheer.
Kelvin

From above link (The Myth of Foliar Feeding):

 Tree and shrub species differ dramatically in their ability to absorb foliar fertilizers.
 Proper plant selection relative to soil type is crucial to appropriate mineral nutrition.
 Foliar spraying is best accomplished on overcast, cool days to reduce leaf burn.
 In landscape plants, foliar spraying can test for nutrient deficiencies, but not solve them.
 Micronutrients are the only minerals that are effectively applied through foliar application.
 Foliar application will not alleviate mineral deficiencies in roots or subsequent crown growth.
 Foliar spraying is only a temporary solution to the larger problem of soil nutrient availability.
 Minerals (especially micronutrients) applied in amounts that exceed a plantÂs needs can injure or
kill the plant and contribute to environmental pollution.
 Any benefit from foliar spraying of landscape trees and shrubs is minor considering the cost and
labor required

As I said the previous threat: "the way she explains may give readers wrong impression". Read it carefully and think more about it. Some critical points:
- Remember the key word in the title "more effective". The author discuss about itÂs better to fertilizer through root or foliar. She confesses that foliar spraying is good but not as good as root fertilizer.
- ItÂs an opinion of a professor. There are hundreds different opinions of other professors (Dr. Tukey is one of them). Remember there are thousand of experts in agriculture who have different opinion about foliar feeding. Google "foliar feeding" you will see it. Reading the "Physiology of tropical orchids in relation to industry" about the foliar feeding you will see how fertilizer is measure between uptakes from root and absorb from leaf of orchid and how fast it is transferred between root and leaf and you will see in this case her opinion is totally wrong.
- Finally, I am talking about combination of fertilizer both leaf and root. Before, I only fertilizer at root only but now I will also spray fertilizer, especially with orchids with bare root.

Cheers,
Kelvin

I sometimes have used foliar feeding, but it's been in a controlled and contained environment. I only use it in special circumstances, when availability of nutrients through the roots is compromised and as a temporary solution. Example would have been when I was bench growing mums one summer and we had week upon week of rainy and cool weather. The leaves were showing signs of nitrogen deficiency. No way was I going to insult them more by soaking them with pumped up irrigation. I agree with the above post entirely.

I see nursery and greenhouse used so interchangeably on these forums. In our local vernacular a nursery is a place where hardy stock is grown. It's not used for greenhouse situations. So, when you say you see white powder on nursery plants, in my mind that equates to perennial container stock. In the facility where I used to work at the growing farm, we used overheads for irrigation of nursery stock. All of the stock had white residue on it from the hard water drying on the foliage.

My experience says it could be either fertilizer residue or pesticides. The growing operation I used to work at irrigated entirely by overhead booms - both outdoor fields and greenhouses - and fertilizers were routinely included in the irrigation water (fertigation). And the crops were also on a regular pesticide program, which was accomplished by spraying. Both have the ability to leave a powdery residue on the foliage of various plants. As will hard water :-)

Once again, I'd have to say that Ms. Chalker-Scott tends to hold an unnaturally narrow view on the issue that has little basis on reality. Foliar 'feeding' obviously offers some benefits to plants, at least in their infancy, as that is often the only way these plants, produced in mass quantities by large growing operations, receive any fertilization. In a home garden with more mature and even established plants, she may have a more valid argument but it is not one that is shared unilaterally