I was reading a rose book in a section regarding spraying neem oil to combat Japanese beetles, and was surprised when the book suggested spraying bottom to top and not top to bottom. It did not get into the rationale, however, and I thought conventional wisdom was to spray top to bottom, not just for neem but in general. My guess is that spraying bottom to top presumes you’d get the undersides of the leaves as well. Wondering if anyone can shed any scientifically-based light on this. Thanks!
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I don’t think it really matters if you start at the top or start at the bottom as long as you get the undersides of the leaves. Start at the top and go down the bush and then back up to get the undersides, or start at the bottom and go up getting the undersides of the leaves, then back down.
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Most often I start at the bottom and go up. My thinking is that any extra droplets of spray will go down and not on my arms etc. but I don’t think there is a proven difference.
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I always spray from the bottom (to get the soil, base and under the leaves), and work my way up the plant. The top leaves will drip down on the lower leaves.
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IF spraying Neem Oil to combat Japanese Beetles, spraying the bottoms of the bushes would be a complete waste of time and effort (not to mention product). You won’t find any Japanese Beetles in the lower half or a rose bush - unless your bush is 8 foot tall or taller 
However, for spraying fungicides for blackspot, I agree with the other posters to start at the bottom and work up. I try to get right near the bud union and point the spray wand UP in order to get the dense inside parts of the bush which is where problems can start. For powdery mildew I believe you can get by with only spraying the top-half of the bush.
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It depends on what I am spraying and what issue I am trying to address. I was taught to spray bottom up - in doing so the spray is landing on the top of some leaves so it helps the process along. Of course - as has been mentioned, if you are spraying for JP etc that are only on the top part of a plant - no reason to spray the bottom of leaves or the bottom of the plant.
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The great, late, past ARS president, Howard Walters, taught me to spray from bottom, spraying the bottom of the leaves, to the top, with finishing at the top. This applies to spraying fungicides, you get better coverage this way. But for Japanese Beetles, who usually prefer to be near the top of rose bushes, that would not apply. Did the neem oil work on killing the beetles?
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Thank you for this! For the beetles, if I can see them I just pick them off and drop them into soapy water rather than wasting spray on them.
For Japanese Beetles, I use a 1 qt. spray bottle (trigger spray bottle) with the insecticide ‘Eight’. I walk the gardens a few times each day (several times when very hot and sunny) and if I see a Beetle I spray and kill a Beetle. It’s targeted spraying so very little, if any, harm is done to beneficials. I know which varieties are their favorites, so I start with those. Each year there seem to be fewer Beetles. Grub control or milky spore really won’t help much in my area - lots are 1.5 acres minimum and all lawns. Unless everybody in the neighborhood followed suit I’m still going to see a fair number of Beetles. But I do kill every single one I see.