THE NATIONAL NURSERYMAN 
687 
APPLE CROWN-GALL IN THE NURSERY AND ORCHARD* 
GEORGE G. HEDg|cOCK 
Pathologist U. S. Dept. Agriculture 
The apple crown-gall, including hairy-root, is the most 
prevalent disease of apple trees in the nurseries of the 
United States. Owing to the continued agitation due to 
the enforcement of laws to prevent the dissemination of 
the disease it has become one of the best known diseases. 
It is found in nurseries in varying intensity in all sections 
of the country where nursery stock is grown. It is essen¬ 
tially a nursery disease, and its importance in the apple 
orchard has evidently been greatly overestimated. A 
strict enforcement of our State nursery inspection laws 
would result in the destruction of more than a million trees 
annually. 
The writer has studied the various forms of the disease 
in nursery and orchard experiments for a period of seven 
years; this study has included especially the effect of the 
disease and its control and prevention. In these exper¬ 
iments there were grown, under the personal supervision 
of the writer, 143,763 apple trees in nursery plots, and 2,077 
trees in orchard plots, in a number of localities in Nebraska, 
Kansas, Arkansas, Missouri, Iowa, Illinois, Virginia, and 
the District of Columbia. The description of these exper¬ 
iments with the full results will be published in a bulletin 
from the Bureau of Plant Industry, now in press. A few 
of the most important conclusions are given in this paper. 
In former addresses to this Association the writer has 
described two forms of crown-gall; the soft and the hard; 
and four forms of hairy-root: the simple, the woolly-knot, 
the broom-root, and the aerial. More recent investiga¬ 
tions by the writer and others—-Smith, Townsend and 
Brown, of this department—tend to show that ultimately 
all these forms of disease will be proven to be manifesta¬ 
tions of one disease, and relatedAo the crown-gall on grapes, 
raspberries, blackberries (B. P. I. Bulletin 131, Pt. Ill, 
1908), the stone fruits, and possibly the quince. In case 
of the soft and hard forms of crown-gall the relation is un¬ 
doubted. The soft galls either rot away and disappear or 
become hard galls as they grow older. Hard galls often 
develop into the woolly-knot form of hairy-root, which 
has been shown by the writer to be identical with the aerial 
knots on apple and quince trees (B. P. I. Circ. 3, 1908). It 
is easy to see that the same stimulus acting on young cells 
under the bark might result either in increased callus or 
gall production from ordinary cells, and in increased pro¬ 
duction of roots or root-buds where incipient root centers 
are affected. In the former case galls would result, in the 
latter, some form of hairy-root. 
Crown-gall is a bacterial disease and is contagious through 
the soil in wounds. No proof has been found that it can 
enter the tissues of apple trees without wounds of some sort. 
There is reason to believe that in case of the aerial form of 
*Paper printed in the Proceedings of the American Association of Nurserymen 
for the meeting held in Denver, June 8 to io, 1910. 
hairy-root, the woolly-aphis is concerned in the spread of 
the organism causing it. Aphis galls must not, however, 
be confused with crown-gall, since they arise directly from 
the stimulus accompanying the bite of the insect. In root¬ 
grafting the extensive cut surface in the union favors the 
entrance of disease germs. 
In our nursery experiments with root-grafted trees, 
crown-gall invariably developed on trees in the nursery 
rows in considerable amount the first year, but not to any 
extent the second and third years. On the other hand, 
hairy-root forms develop chiefly the second and third years. 
The total per cent of diseased trees was considerably less 
the second year, and decreased further the third year. 
Some of the galls developed hairy-root from their surface; 
others rotted away and the trees recovered. The decrease 
of disease the second and third year upon close analysis 
showed two results: i. A recovery of trees from milder 
forms of disease. 2. Although the disease was readily com¬ 
municated to root grafts the first year, it did not spread to 
an appreciable extent to healthy trees in the nursery rows 
the second and third years. 
The effect of the disease on trees in the nursery rows var¬ 
ied. In some cases of trees badly diseased with the simple 
form of hairy-root there was a pronounced stunting effect 
on the growth. In other cases with woolly-knot forms an 
increased growth and apparent vigor of the trees resulted. 
On the average, however, trees diseased with either crown- 
gall or hairy-root were very slightly smaller, but did not 
show any marked difference in the foliage. 
Experiments with orchard trees were carried out in two 
orchards till the trees were eight years old. These as a 
consequence furnish data only upon the effect upon the 
growth of young trees, and no data in our experiments were 
obtained upon the effect upon bearing trees as to whether 
diseased trees bear earlier than healthy ones, or whether 
they bear as well. 
In our orchard experiments a considerable per cent of 
trees diseased with crown-gall recovered entirely from the 
disease; many others developed hairy-root of the woolly- 
knot form from the surface of hard crown-galls. A few of 
the healthy trees became diseased. The results as a whole 
from the orchards showed plainly two facts: 1. More trees 
recovered from the disease than became diseased. 2. The 
disease spread very slowly, if at all. 
Careful measurements of healthy and diseased trees in 
the experimental orchards showed that the effects of the 
growth on the diameter was very slight, and as follows: 
in six years’ growth in the two oldest orchards the healthy 
trees averaged six one-hundredths of an inch greater in 
diameter than those diseased with hairy-root, and twenty- 
nine one-hundredths of an inch greater than those diseased 
with crown-gall. The outward appearance of the trees, 
