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UNITED STATES DEPARTMENT OF AGRICULTURE

Contribution from the Bureau of Plant Industry
WM. A. TAYLOR, Chief

Washington, D. C. PROFESSIONAL PAPER September 14, 1917

EXPERIMENTS IN THE CONTROL OF POTATO LEAK. '

‘By Lon A. Hawkins, Plant Physiologist, Plant Physiological and Fermentation
Investigations, Bureau of Plant Industry.

CONTENTS.

Page. Page.
MHECAISAOFZANiSMS: 2 2 -\s<- Ss oa.055 eee a ele 1 | Results ofthe experiments.................-. 3
Methods ofinvestigation..............-.----- Zits COnclusionsis28 Sais Lewes acca aa 5

THE CAUSAL ORGANISMS.

In a paper ? on a disease of potatoes commonly known as leak, the
disease was described and the literature on the subject reviewed. It
was shown that the tuber rot typical of this disease could be caused
by Rhizopus mgricans Ehrenb., as Orton * found, but that the causal
organism was more frequently Pythiwm debaryanum Hesse. The
fungus obtains entrance through breaks in the skin of the potatoes,
and infection under field conditions usually takes place by some in-
fected soil getting into wounds made in harvesting. Preliminary
experiments in 1915 indicated that the disease could be controlled
by carefully sorting out all wounded tubers. As the disease is of
considerable economic importance in the delta region of San Joaquin
County, Cal., it was considered advisable to continue these experi-
ments. This bulletin deals with further experiments in the control
of the disease, together with some additional work on the causal
organism and its occurrence in the delta soils.

1 The work described in this bulletin was carried out as a part of the potato-disease project of the Office
of Cotton and Truck Disease Investigations. The writer wishes to acknowledge his indebtedness to Mr.
Carson C. Cook, of Stockton, Cal., for his hearty cooperation in this work.

2 Hawkins, L. A. The disease of potatoes known as “‘leak.’’ In Jour. Agr. Research, v. 6, no. 17, pp.
627-640, 1 fig., pl. 90. 1916.

8 Orton, W. A. Potato diseases in San Joaquin county, California. U.S. Dept. Agr., Bur. Plant Indus.
Cire. 23,14p. 1909.

Decay of potatoes due to Rhizopus nigricans. (Abstract.) In Science, n. s., v. 29, no. 753,
p. 916. 1909.

1903°—Bull. 577—17

2 BULLETIN 577, U. S. DEPARTMENT OF AGRICULTURE.

The fungus was isolated from the rotten tubers and inoculations
made, as in the previous year. The results were similar to those
obtained in 1915. Forty isolations of Pythium debaryanum were
made in the 45 attempts. The pathogenicity of the organism was
demonstrated in many instances by inoculating it into sound tubers
and reisolating it after the potato had rotted. Inoculations were
made by inserting small quantities of soil from various potato fields
into tubers. About 50 per cent of the inoculations produced the
disease from every sample of soil obtained from land which had been
reclaimed and farmed for some years and had not been burned over
recently.

METHODS OF INVESTIGATION.

It is considered among the growers that potatoes would not leak
if grown on new land (that is, land recently reclaimed) or on burned
land (that is, land on which the peat soil had caught fire and burned
more or less deeply). This theory was tested. A number of inocula-
tions were made from samples of soil from an area that had been
burned over. In no case was there any infection. Samples of soil
from two areas of new land, one of which had never been farmed,
while the other had been planted to potatoes the previous year, were
tested. Two potatoes out of eight inoculated with soil from the land
most recently reclaimed rotted, and Rhizopus mgricans was isolated
from the tubers. All other inoculations gave negative results.
Potatoes from such lands are not immune when inoculated with
Pythwum debaryanum. The apparent immunity of potatoes grown
on new land or burned land seems to be due to the absence of the
organism from the soil or to its presence only in limited areas.

The general plan of the experiments to demonstrate a method for
the control of the disease was to sort the potatoes in the field and
store the injured tubers under conditions as nearly approaching
those of a commercial warehouse as possible. The sound tubers
were to be shipped in the usual way. It was planned to examine
the cars of sound potatoes after they reached the market or after
they had been shipped and sufficient time had elapsed for the in-
cubation of the fungus.

The farm selected for the experiments was one on which a number
of crops of potatoes had been grown. Several cars of the 1916 crop
had been shipped and considerable damage from leak had been re-
ported. In the experiments here described the potatoes were
harvested and sacked in the field in the usual way. They were then
hauled to the levee, sorted, resacked, and the sound potatoes shipped.
The tubers that had been wounded by the removal of branches
(knobs) and by the digging fork were sorted out, and each lot was
sacked and stored separately. The wounded tubers remained in a
well-ventilated warehouse on the levee until sufficient time had

EXPERIMENTS IN THE CONTROL OF POTATO LEAK. 3

elapsed for the incubation of the fungus in any inoculated tubers.
They were then resorted and the number of rotten potatoes in each
sack noted. The wounded tubers which had not rotted were sacked

and sold.
RESULTS OF THE EXPERIMENTS.

The first car of potatoes included in these experiments was har-
vested and sorted on September 4 and 5.° For the car of sound
potatoes (270 sacks), 349 sacks of field-run potatoes were required.
Of the 79 sacks sorted out, 43 sacks were of tubers which had been
wounded by the removal of knobs, and 25 sacks had been injured
with the digging forks. The other 11 sacks represent shrinkage
due to the drying of the potatoes and to the stretching of the sacks
used in resacking.

This car of sound potatoes was shipped to Oklahoma. It was
impossible for the writer to examine the potatoes after the car left
Stockton, Cal. No leak had been reported by the consignee on
October 1, so it is reasonable to suppose that the potatoes arrived in
good condition.

The injured tubers sorted out were all stored in the warehouse
until September 13, when part of them were sorted and the unin-
fected potatoes sold. The remaining tubers were sorted on Septem-
ber 18, 14 days after the first of the lot were harvested.

The second and third cars of potatoes were harvested and sorted
from September 8 to 12, inclusive. For the second car (270 sacks),
326 sacks of field-run potatoes were required. Of the 56 sacks
sorted out, 20 sacks were of tubers that had been injured by the
digging forks, and 31 sacks were those from which knobs had been
removed. The other 5 sacks represent shrinkage. The car of sound
potatoes was shipped to Barstow, Cal., and was examined there by
the writer six days after the potatoes had been harvested. No leaky
tubers were found in any of the 20 sacks examined. Some leak was
reported from Phoenix, Ariz., where the car was finally sent.

The third car of potatoes in these experiments was sorted imme-
diately after the second. It required 325 sacks of field-run potatoes
for the car of sound potatoes. Of the 55 sacks sorted out, 20 sacks
were of tubers that had been injured by the removal of knobs, and
21 sacks were of those injured by the digging forks. The other 4
sacks represent shrinkage. The car of sound potatoes was shipped
to Bakersfield, Cal., where it was examined six days after most of
the potatoes in it had been harvested. No leaky tubers were found.
The car was diverted from Bakersfield to Los Angeles, where it was
reported to be leaking. It was then shipped to El Centro, Cal.,
where the potatoes were examined by the writer. The merchant to
whom the potatoes were sold said that they arrived in fair condition

4 BULLETIN 577, U. S. DEPARTMENT OF AGRICULTURE.

and that about 1 sack in 20 contained diseased tubers. A number
of sacks were examined; most of them were free from disease; in
only one sack was there more than one diseased tuber, and all the
diseased tubers found had been wounded with the digging fork.
The amount of leak in the car was considered too small to warrant
sorting.

The injured potatoes from the second and third cars were stored
in the warehouse until September 18. They were then sorted and
the uninfected potatoes sold. The data obtained from sorting the
1,000 sacks of potatoes as they came from the field are summarized
in Table I, which includes the data already given, together with that
obtained in sorting the wounded tubers.

TaBLE I.—Experiments with 1,000 sacks of potatoes harvested in the usual way in Cal-
ufornia, showing the results of sorting for the control of potato leak.

Rotten tubers (percentage based on estimate of 200

Field Tubers injured— | tubers to each sack) in three lots of those injured— | In-
Car of | run dure
sorted | pota- |Shrink Tl a | ea apes
tubers | toes | age in By removal of knobs. By fork wounds. ete
(270 | re- |sorting| By - In wee
sacks). | quir- OV-| fork | field ae
ed. oot | wounds.| 74" |Maxi-/Mini-| , Joo, |Maxi-[Mini-| , 100 {sorted
*{mum./mum.| * 8&- |mum.|mum. ge.

Per | Num-| Num-| Num-| Per |Num-|Num-| Num-| Per
Sacks.| Sacks. |Sacks. | Sacks. | cent. | ber. | ber. | ber. | cent.| ber. | ber. | ber. | cent. |Sacks.

INOW Eee} 349 11 43 25 /19.4+) 23 2} 12.5 | 6.25 61 14 40 | 20 56.5
No. 2....| 326 5 31 20 |15.6+) 40 8| 17 8.5 75 21 48 | 24 40

No. 3..-.| 325 4 30 21 st 6+} 35 10 | 19 9.5 68 28 49} 24.5] 40

Table I shows that something more than 19.4 per cent of the pota-
toes in the first lot sorted were wounded in harvesting, either by
the removal of branches (knobs) or by the digging fork. The
percentage of injured tubers in the other two lots, 15.6, is not quite
so great. In all, 170 sacks of potatoes out of 1,000 were injured.
That such a percentage of the potatoes should be injured in harvest-
ing indicates exceedingly poor methods. Nor does this 170 sacks
represent all the injured potatoes, as the tubers injured by fork
wounds were supposed to have been sorted out in the field. Much
of this damage might be eliminated with proper care, but as long as
the potatoes are harvested with forks it is probable that there will
be a high percentage of injured tubers. The digging of potatoes
with machinery is practiced very little in this region, though potato
harvesters adapted to this type of soil and these conditions are said
to be available.

Of the 170 sacks of injured potatoes, 104 sacks (10.4 per cent of
the total of 1,000 sacks) were injured by the removal of knobs. Break-
ing off the knobs is a common practice in this region and is recom-
mended by the potato buyers, as it results, of course, in a smoother
potato. The tubers injured in this way are considered to be no more

EXPERIMENTS IN THE CONTROL OF POTATO LEAK. 5

subject to leak than sound ones. That this view is erroneous is evi-
dent from the results obtained in these experiments. Of the first
lot, 6.25 per cent of the potatoes injured in this way became infected,
while in the second and third lots the percentage was somewhat
higher, being 8.5 and 9.5, respectively. The practices of removing
these branches and of rubbing the raw surface in the soil, as is
sometimes done, or sacking them, so that soil gets into the fresh
wound, are harmful. If the broken surface is exposed to the air
until the wound has had time to cork over, there is much less danger
of infection.

Not so many potatoes were wounded with the digging forks as by
the removal of knobs, but the percentage of rotten ones among
the fork-wounded tubers was much higher. Out of 1,000 sacks har-
vested, 66 sacks were wounded with the digging forks. Of these,
from 20 to 24.5 per cent rotted. The fact that the deep wounds
made with the digging fork do not dry so readily and are more lia-
ble to be filled with infected soil than wounds made by the removal
of knobs seems to account for this higher percentage of infection.

CONCLUSIONS.

It is evident from these experiments that the sorting out of all
wounded tubers would practically insure a shipment from damage
by this disease. It is probable that it would be impracticable in
commercial work to sort carefully enough to eliminate all the
wounded tubers. A few wounded potatoes slipped through in these
experiments in spite of the careful sorting. It is not difficult, how-
ever, to remove 96 to 98 per cent of them. Such sorting, according
to the percentages of rot occurring in the wounded tubers in these
experiments, would result in an average of about one rotten potato
in 10 to 25 sacks. This would be nearly complete control.

It has therefore been demonstrated that the leak may be con-
trolled by the elimination of all wounded tubers from the shipment.
The best and most economical means of accomplishing this end would
be to avoid wounding the tubers in harvesting them. Wounded
tubers should be sorted out and shipped separately or, better, stored
for a week or more to allow the fungus to incubate, and then sorted
and the uninfected potatoes sold.

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