304 AMERICAN POTATO JOURNAL [Vol. 32

P O T A T O V A R I E T Y Y I E L D S , T O T A L SOLIDS, A N D C O O K I N G Q U A L I T Y AS A F F E C T E D BY D A T E OF V I N E K I L L I N G 1

R. V. AKELEY, 2 F. J. STEVENSON, a AND C. E. CUNNINGHAM 4

In Maine it is common practice to kill potato vines while they are still green. The grower of certified seed kills the vines the last week in August or the first week in September to avoid the rapid spread of viruses that occurs toward the end of the growing season. The grower of table stock kills the vines early to prevent development of oversize tubers, to permit the skins to toughen, and thus obviate excessive skinning at harvest time, and in blight years to prevent dry rot. These are desirable objectives but they are gained usually at the expense of yield, dry-matter content, and cooking quality. For 3 years, 1950 to 1952, tests were conducted on the Aroostook Farm, Presque Isle, Maine, to determine the effects of date of vine killing on yield, total solids, and quality of chips and french fries.

MATERIAL AND METI-IODS

The varieties used in these tests were Irish Cobbler, Sebago, Chip- pewa, Katahdin, Teton, Mohawk, Kennebec, and Green Mountain. They were planted on May 5 in 1950 and 1951 and on May 6 in 1952. The vines were killed on 4 different dates. A randomized split block, with 4 replications was grown for each of the 4 dates of vine killing. Each single plot consisted of 25 plants spaced t0 inches apart, in rows 34 inches apart. The size of seedpieces ranged from 1.5 to 2.0 ounces. A fertilizer, analyzing 8-16-16 and containing 1 per cent magnesium, was applied at the rate of 1,400 pounds per acre. Regular cultural methods for the area were practiced in growing the crop. The vines on the first block were pulled about August 15, those on the second August 25, and those on the third on or about September 4. The vines on the fourth block were allowed to grow until they were killed by frost or the plants were considered mature enough to harvest. The average date of vine killing on the last block was September 18.

Each year all tubers were harvested on the same date. They were graded to U. S. No. 1 size and weighed. Twenty tubers each weighing about 8 ounces were taken from each 25-hill row to be tested for specific gravity and quality of chips and french fries. The 20-tuber samples were weighed in air and water and the specific gravity was calculated. The specific-gravity data were converted into percentages of solids by the use of a table giving the average percentage of dry matter expectec( for each specific-gravity reading.

After being weighed in air and water the 20-tuber samples were dried and placed in storage at a controlled temperature of 40~ After 6 to 8 weeks in this storage the potatoes to be made into chips and french fries were reconditioned at a temperature of 60~ for 35 days or more.

For chips 2 center slices slightly less than M inch thick were cut from each of 5 tubers. The 10 slices were rinsed with water to remove excess starch, blotted dry with paper towels, and cooked in pure vegetable oil

1AccePted for publication May 3, 1955. 2.a,4Senior Horticulturist, Principal Geneticist, Horticultural Crops Branch, Agricul-

turaI Research Service, U.S.D.A., Beltsville, Md. and Assistant Agronomist, Maine Agricultural Experiment Station,, Orono, Me., respectively.

1955] AKELEY, et at: EFFECT OF VINE KILLING ON POTATO YIELD 305

(Prinaex) at a temperature ranging from 335 ~ to 345 ~ F. They were removed from the cooked when bubbling stopped.

For french fries two ~ - i nch square cores were taken from the center of each of 5 tubers. The 10 pieces were fried in Pr imex for 4 ~ minutes at a temperature ranging from 375 ~ to 385~ A 2-tray deep-fat cooker with thermostatic controls was used for cooking both chips and french fries.

The same 2 men judged the chips and french fries in all tests. They used the classifications employed by processors in Caribou, Maine. The chips and french fries were grouped for color in 7 classes, ranging from class 1 for those with a very deep brown burnt appearance to class 7 for those with a very light tan ahnost white color. The texture of the french fries was judged by applying pressure with the index finger. The soggiest pieces were grouped in class 1, intermediate in class 2, and the firmest, in class 3.

YIELDS

The 8 varieties in the test differed in season of maturity. I t might be assumed that the yield of an early var iety, such as Irish Cobbler, would be higher on the first date of vine killing than those of any of the late varieties. It is interesting to note that the late-maturing variety Kennebec produced 404 bushels per acre compared with 339 for Irish Cobbler, a difference of 65 bushels on the first date of vine killing, August 15. On that date, Chippewa, Teton, and Mohawk were in the same class as Irish Cobbler so far as yield was concerned, but Sebago, Katahdin, and Green Mountain were significantly lower in yield than the early-maturing Irish Cobbler. On September 18, all 7 late varieties significantly outyielded Irish Cobbler (Table 1).

The mean yield of Irish Cobbler for the 3-year period increased 45.1 per cent from August 15 to September 18. The corresponding increase for Kennebec was 66.6 per cent and for Green Mountain 123.5 per cent. The increases for the other 5 varieties ranged from 73.6 per cent for Chippewa to 92.5 per cent for Mohawk. The increases varied somewhat with the season. In 1950 the mean yield for all varieties increased 90.0 per cent; in 1951, 70.2 per cent; and in 1952, 80.8 per cent. The mean for all varieties for the 3-year period increased from 326 bushels per acre on August 15 to 584 on September 18, or 79.1 per cent.

These data indicate the loss in yield that can be expected by the grower of certified seed who kills the vines about the middle of August to avoid heavy infections with viruses that usually occur toward the end of the growing season. However, growers of table stock do not ordinarily kill the vines until the last week in August or later. The Irish Cobbler increased less than 10 per cent from August 25 to September 18, but the other varieties increased from 32.2 per cent for Kennebec to 47.5 per cent for Mohawk. In 1950 the mean yields for all varieties were 397 bushels per acre on August 25 and 553 on September 18, an increase of 39.3 per cent. In 1951 the corresponding means were 527 and 691, an increase of 31.1 per cent, and in 1952 the means were 381 and 508, an increase of 33.3 per cent. The mean for the 8 varieties for all tests combined were 435 bushels per acre for August 25 and 584 for September 18, an increase of 149 bushels, or 34.3 per cent. This represents the loss that can be expected if the vines are killed about August 25.

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Killing the vines 14 days before they matured or were killed by frost resulted in significant losses in yield in all varieties except Irish Cobbler. Chippewa, Katahdin, and Teton revealed increases in yield that were significant at the 5 per cent level, and Sebago, Mohawk, Kennebec, and Green Mountain showed highly significant increases from September 4 to 18. The means for all varieties for these 2 dates for 3 years were 523 and 584, a difference of 61 bushels per acre, or about 11.7 per cent. This is a highly significant difference.

TOTAL SOLIDS CONTENT

It is generally believed that total solids content of potatoes increases as the plants approach maturity. This is probably true in the early stages of tuber development but may or may not be true during the last 4 weeks of the life of the plant. Much depends on the season, as can be seei~ from the data in table 2.

In 1950 in Maine a warm dry period occurred in May, soon after the potatoes for these tests were planted. This warm weather caused the potato plants to emerge quickly and grow rapidly during the early part of the season. The total solids content reached a maximum on August 15, about 4 weeks before they were harvested. On that date the mean for the 8 varieties was 19.9 per cent and on September 18, 19.6 per cent. The difference is not significant.

The 1951 season was characterized by a late cool spring, followed by cool rainy weather during much of the summer. Although the test plots were planted about the same date as in 1950 they did not get such an early start. Consequently, the dry-matter content did not reach a maximum on August 15 but was comparatively low at that date. I t increased gradually from 17.6 per cent on August 15 to 18.7 per cent on September 18, a highly significant increase.

The 1952 season was cool and wet at the time of planting and the plants in the test plots did not get such a good start as they did in 1950. During the latter part of July and the greater part of August the weather was hot and dry. The mean dry-matter content for all varieties on August 15, 1952, was the lowest during any of the 3 years. On August 15 it was 16.7 per cent but it increased to 18,2 per cent by August 25, and 19.7 per cent by September 4; and by September 18 it was 18.9 per cent. It is difficult to account for the difference between the dry matter for September 4 and that for September 18. Evidently an increase in yield between these two dates occurred without a corresponding increase in the percentage of total solids.

The data for each of the 3 years indicate that in 1950 there was no loss in dry-matter content resulting from early vine killing, but in 1951 and 1952 significant losses in percentage of total solids were sustained when the vines were killed as early as August 15. The differences between the percentages for August 25 and September 18 were not significant.

The mean of all varieties for the 3-year period was 18.1 per cent total solids when the vines were killed on August 15; 18.6 per cent on August 25 ; 19.3 per cent on September 4; and 19.1 per cent on September 18. The differences between the percentages for the first 2 dates and those for the last 2 were highly significant. Although seasons differ and the

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percentages of total solids vary with the seasons, it is evident that over a period of years significant losses in dry-matter content are sustained if the vines are killed early.

COLOR OF CHIPS

The chips were rated in 7 classes according to the degree of browning. A rating of 7 signified a light tan, almost white color, and the lower the number the greater the amount and intensity of the brown color. The chips in class 1 were very dark brown with a burnt appearance. Such chips would have no commercial value. Chips in classes 4 to 7 are usually preferred by the trade.

The chips of all varieties except Irish Cobbler improved in color from August 15 to September 18. The early maturity of the Irish Cobbler is reflected in these results. The other varieties matured later than the Irish Cobbler, and the color of their chips improved as they approached maturity. The improvement in the color of the chips made from the late- maturing varieties was highly significant when the data for the first and second dates of vine killing were compared with those of the fourth as revealed in table 3.

The chips of Irish Cobbler and Kennebec, which graded higher than class 4 when the vines were killed on August 15, would be acceptable to the trade at any of the dates of vine killing. The chips made from Katahdin tubers for the first 3 dates of vine killing were too dark in color to be preferred by the trade, but when the plants of this variety were allowed to mature or be killed by frost its chips graded higher than 4. The color of the chips made from Green Mountain for all dates of vine killing graded too low to be acceptable to the trade.

The seasonal differences are indicated by the means of all the varieties for each of the 3 years. In 1950 there were no significant differences in the data for any o( the 4 dates of vine killing. In 1951, and again in 1952, highly significant differences were found. When the means for all the varieties for the 3-year period are considered there is no significant difference between the first and second dates of vine killing, but there are highly significant differences between the first 2 and the last 2 dates.

The color of chips varies with the variety and with the maturity of the potatoes. Chips with satisfactory color can be made from some varieties when they are relatively immature, but the color of chips made from most varieties improves as the tubers reach maturity.

COLOR OF FRENCH FRIES

The same criteria were used in judging the color of french fries as were used in classifying the color of the chips. French fries in color classes 5 to 7 are preferred by the trade.

The color of french fries made from Kennebec averaged 5 for a mean of 4 dates of vine killing for three years. The mean 4.8 and 4.9 for August 15 and August 25, respectively, were not significantly different from 5.0 and 5.2, the means for September 4 and September 18. It is evident that french fries with an acceptable color were made from the tubers of Kennebec in any stage of maturity reached from August 15 to September 18. Irish Cobbler was a close second, and no significant differences were

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312 AMERICAN POTATO JOL'RNAL [Vol. 32

found between the means for any of the dates of vine kill ing. The color of the chips made f rom Green M o u n t a i n at any of the dates of vine ki l l ing would not be acceptable to the t r ade as shown in table 4.

Seasonal differences are apparen t in the color of the french fries. In 1950, j udg ing f rom the means of all variet ies , there was no improvement in color t h roughou t the tests. Howeve r , in 1951 and 1952 the improvement in color f rom Augus t 15 to Sep tember 4 was highly significant.

TEXTURE OF FRENCH FRIES

Data on t ex tu re of french fries were taken for 2 years as you will observe in table 5. The french fries made f rom Ir ish Cobbler, Katahdin , Mohawk, Kennebec, and Green Mounta in were on the average in No. 3 or highest class in firnmess of tex ture . The other 3 var ie t ies , Chippewa, Sebago, and Teton, ra ted on the average 2.7, 2.8, 2.9, respectively. The tex ture of the french fries as not closely associated with date of vine kil l ing or season. T h e r e was little or no difference between any 2 of the 4 dates for any of the variet ies , and the resul ts for each of the 2 years and the means for both years were very similar.

SUMMARY

In Maine g rowers of certified seed potatoes often kil l the vines dur ing the last week in Augus t or the first week in September to avoid the rap id spread of v i rus diseases that may occur toward the end of the g rowing

TABLE 5.--Texture of french fries made from 8 varieties of potatoes at 4 dates of vine killing, Presque Isle, Maine, 1950 and 1951.

Variety and Means

rish Cobbler

;ebago

~hippewa

:atahdin

"eton

~ohawk

:ennebec

;reen Mountain

/lean All Varieties

1950

1951

2 Years

Mean Dates of Vine Killing 2-Year Means

Aug. 15 Texture

Class No. 3.0

2.8

2.7

3.0

2.9

3.0

3.0

3.0

2.9

2.9

2.9

Aug. 25 Texture

Class No. 2.9

2.9

2.8

2.9

2.8

3.0

3.0

2.9

2.9

2.9

2.9

Sept. 4 Texture

Class No. 2.9

2.7

2.6

3.0

2.8

2.9

3.0

3.0

2.9

2.8

2.9

Sept. 18 Texture

Class

No. 3.0

2.6

2.8

3.0

2.9

3.0

3.0

2.9

2.8

2.9

2.9

4 Dates Mean

Texture Class

No. 3.0

2.8

2.7 3.0

2.9

3.0

3.0

3.0

2.9

2.9

2.9

1955] A K E L E Y , et al: EFFECT OF V I N E K I L L I N G ON POTATO YIELD 313

season. The grower of table stock kills the vines early to prevent develop- ment of over-size tubers, to permit the skins to toughen, and thus obviates excessive skinning at harvest time, and in blight years to prevent dry rot.

Data for 3 years of tests with 8 varieties of potato and 4 dates of vine killing, August 15, August 25, September 4, and September 18, indicate that early vine killing resulted, on the average, in lower yields, lower percentages of total solids, and inferior color of chips and french fries. The texture of the french fries was not affected to a measurable degree. Varietal and seasonal differences were apparent. The early- maturing Irish Cobbler was less affected by early vine killing than most of the late varieties. Kennebec, a late-maturing variety, yielded higher than the Irish Cobbler on August 15 and increased rapidly from that date to the end of the growing season. Green Mountain, another late variety, produced relatively low yields on August 15 but its yields increased rapidly thereafter.

In 1950 the percentage of total solids was about as lfigh on August 15 as it was at any of the other vine-killing dates, but in 1951 and 1952 the dry-matter content of all varieties except Irish Cobbler increased significantly after August 15.

On the first date of vine killing the color of chips from only 2 varieties, Irish Cobbler and Kennebec. was acceptable. The chips made from Katahdin improved in color as the variety approached maturity. The color of the Green Mountain chips was not acceptable at any vine-killing date.

The color of the french fries followed the same pattern in all tests as the color of the chips.