h-grower is mainly in preventives.

_The Yellows_ is usually regarded as a disease. Imagination has invented
many causes of this evil. Some suppose it to be produced by small
insects; others that it is in the seed. Again, it is ascribed to the
atmosphere. It has been supposed to be propagated in many ways--by
trimming a healthy tree with a knife that had been used on a diseased
one; by contagion in the atmostphere, as the measles or small-pox; by
impregnation from the pollen, through the agency of winds or bees; by
the migration of small insects; or by planting diseased seeds, or
budding from diseased trees. This great diversity of opinion leaves room
to doubt whether the yellows in peach-trees be a disease at all, or only
a symptom of general decay. The symptoms, as given in all the
fruit-books, are only such as would be natural from decay and death of
the tree, from any cause whatever. This may result from neglect to
supply the soil with suitable manures, and to trim trees properly, and
especially from over-bearing. This view of the case is more probable,
from the fact that none pretend to have found a remedy. All advise to
remove the tree thus affected at once, root and branch. We have seen the
following treatment of such trees tried with marked success. Cut off a
large share of the top, as when you would renew an old, neglected tree;
lay the large roots bare, making a sort of basin around the body of the
tree, and pour in three pailfuls of _boiling_ water: the tree will
start anew and do well. This is an excellent application to an old,
failing peach-tree. The sure preventive of the yellows is, planting
seeds of healthy trees, budding from the most vigorous, heading-in well,
supplying appropriate manures, and general good cultivation.

_Curled Leaves_ is another evil among peach-trees, occurring before the
leaves are fully grown, and causing them to fall off after two or three
weeks. Other leaves will put out, but the fruit is destroyed, and the
general health of the tree injured. Elliott says the curl of the leaf is
produced by the punctures of small insects. One kind of curled leaf is,
but not this. But we have no doubt that Barry's theory is the correct
one, viz., that it is the effect of sudden changes of the weather. We
have noticed the curled leaf in orchards where the trees were so close
together as to guard each other. On the side where the cold wind struck
them, we noticed they were badly affected; while on