Poison ivy has white berries in the fall, but the harmless Virginia creeper has red berries. The rhyme “Berries white, poisonous sight! Berries red, have no dread!” has been cited in print since at least 1902.
 
Similar poison ivy rhymes include “Leaves of three, let it be,” “Leaves of five, let it thrive,” “Hairy vine, no friend of mine” and “Raggy rope, don’t be a dope.”
 
   
Wikipedia: Toxicodendron radicans
Toxicodendron radicans, commonly known as poison ivy (older synonyms are Rhus toxicodendron and Rhus radicans), is a poisonous North American plant that is well known for its production of urushiol, a clear liquid compound found within the sap of the plant that causes an itching, irritation and sometimes painful rash in most people who touch it. The plant is not a true ivy (Hedera).
(...)
Various mnemonic rhymes describe the characteristic appearance of poison ivy:
 
1.“Leaflets three; let it be” is the best known and most useful cautionary rhyme. It applies to poison oak, as well as to poison ivy.
2.“Hairy vine, no friend of mine.”
3.“Longer middle stem; stay away from them.” This refers to the middle leaflet having a visibly longer stem than the two side leaflets and is a key to differentiating it from the similar-looking Rhus aromatica (fragrant sumac).
4.“Raggy rope, don’t be a dope!” Poison ivy vines on trees have a furry “raggy” appearance. This rhyme warns tree climbers to be wary. Old, mature vines on tree trunks can be quite large and long, with the recognizable leaves obscured among the higher foliage of the tree.
5.“One, two, three? Don’t touch me.”
6.“Berries white, run in fright” and “Berries white, danger in sight.”
 
Google Books
Stories of Woods and Fields
By Elizabeth Virginia Brown
Yonkers-on-Hudson, NY: World Book Company
1902
Pg. 80:
“All you have to do is look at the leaves in the summer and the berries in the fall,” said Fred. “If the leaves have five fingers, you may safely put your hand on them, for they belong to the friendly creeper; but if they have but three fingers, you must not touch them, for they are poison ivy. In the fall, the berries of the Virginia creeper are red, while those of the poison ivy are white.
 
“You can always tell the ivy from the creeper if you remember,—
 
“‘Fingers three,
Turn and flee!
Fingers five,
Let them thrive!
 
Berries white,
Poisonous sight!
Berries red,
Have no dread!’”
 
Google News Archive
27 July 1904, Pittsburg (PA) Press, “Poison Ivy,” pg. 4, col. 5:
Poison ivy has three leaflets, and Virginia creeper has five. The former has white berries, the latter purple.
(...)
There is a little jingle which I remember having at some time read, which, concisely, if not very poetically, explains these differences, and which, if committed to memory, will always serve to distinguish the poisonous and non-poisonous sumacs:
 
“Berries red,
Have no dread;
Berries white
Poisonous sight;
Leaves three
Quickly flee;
Leaves three, with berries red,
Fragrant sumac, have no dread.”
 
Google Books
The Poison Oak and Poison Ivy Survival Guide
By Sandra J. Baker
Medford, OR: Coleman Creek Press
2011
Pg. 13:
HISTORICAL RHYMES
(leaves changed to leaflets for botanical accuracy)
Leaflets three, let it be; leaflets five, let it thrive.
Berries red, have no dread; berries white, a poisonous sight.
Hairy vine, no friend of mine. (poison ivy vines)
Raggy rope, don’t be a dope. (poison ivy vines)
Red leaves in the spring, it’s a dangerous thing.
 
Trenton (MO) Republican-Times
June 21st, 2013
Leaves Of Three Are Not for Me
By Jeff Berti
Poison ivy. The name strikes fear in the hearts of many. But don’t let this plant keep you from enjoying the outdoors.
 
Poison ivy grows just about anywhere, including wet or dry woods, fields, fencerows, and gravel bars. It comes in many shapes and sizes, but usually is a vine or low shrub. The leaves typically have a glossy surface, and occur at the end of long leaf stems. In the fall, the leaves turn brilliant shades of red, orange and yellow. Remembering this jingle may help you identify the plant: “Leaves of three, let it be. Berries white, a poisonous sight.”