1985) 273

POTATO VINE KILL: STEM-END D I S C O L O R A T I O N EFFECTS ON RUSSET BURBANK 1

J .L. Halderson, D.L. Corsini and L.C. Haderlie ~

Abstract

The relationship of pota to (cv. Russet Burbank) tuber stem-end discol- orat ion to speed of vine kill and maturi ty of vines when killed was studied for three years at several sites in Idaho. Discoloration due to leafroll was separately determined. Vine killing with sulfuric acid, dinoseb, endothall and hand pulling produced a range in killing speeds f rom fast to slow. Vine kill speed alone did not affect amounts of discoloration. Vines that died naturally f rom frost or other causes had as much discoloration (14~ over all treatments) as f rom any chemical or mechanical vine killing method. The tendency for tuber discoloration increased when vines were less mature when killed.

Introduction

Stem-end discoloration (SED) is an important factor in determining the quality of potatoes (Solanum tuberosum L. cv. Russet Burbank) for pro- cessing or fresh usage.

Much of the research work concerning tuber SED was done in the 1940's and 50's. H o y m a n (7) was one of the earlier researchers to study dis- coloration as it related to speed of vine kill. He concluded that discoloration was related to rapidity of vine kill with more mature potatoes being less af- fected. He also found that discoloration developed within two days of kill. Callbeck (1) studied potatoes grown on Prince Edward Island and deter- mined that dinitro compounds produced the most pronounced discoloration. He also found that less mature potatoes discolored more. Rich (11) also found that more rapid killing with certain chemicals caused increased dis- coloration and reported 47~ discoloration in Russet Burbank when dinoseb was used. Cunningham, et al. (3) conducted a more detailed study and concluded that more rapid killing tended to produce the greatest vascular discoloration with the exception of rotobeating and hand pulling. They differentiated between stem-end browning, leafroll net-necrosis and vascular discoloration. They also concluded that no increase in discolora-

tApproved for publication as paper No. 8434 of the Idaho Agricultural Experiment Station. ZRespectively, Assoc. Research Prof., Agric. Engr.; Research Plant Pathologist, USDA-ARS; Assoc. Research Prof., Weed Science, University of Idaho, Research and Extension Center, Aberdeen, ID 83210. Accepted for publication February 21, 1985. KEY WORDS: Stem-end discoloration, vine kill speed, potato, vine age.

274 AMERICAN POTATO JOURNAL (Vol. 62

tion occurred in storage. Some of their tests resulted in 85 to 90% vascular discoloration from dinoseb vine killers.

Several more recent studies have addressed discoloration but mostly f rom the perspective of mechanical vine pulling. Callihan (2) concluded that high temperatures had to be present to produce discoloration when vines were killed rapidly (within 24 hours). He also noted that dry soil conditions seemed to enhance discoloration levels. Grassert and Vogel (4) in a culinary study in Germany determined that weekly mechanical vine killing, four or five weeks before normal maturity, did not produce discoloration. They found no significant change in discoloration in storage for as long as six months. Misener and Everett (8) attributed SED in one instance to diquat but found relatively low levels from mechanical pullers and other chemicals. However, in spite of instances of SED, vine killing has generally been shown to improve tuber quality (9).

Because of reports of rapid vine kill causing SED, this defect was carefully monitored in a comprehensive study of the effectiveness of various vine killing procedures in Idaho. The major objective of this research was to determine whether improved mechanical vine removers, or very fast acting chemicals such as sulfuric acid, 1 could be developed and used without asso- ciated quality problems. Experiments were conducted on vines of different degrees o f maturity. The characteristics measured were: rate of desiccation, yield and grade, specific gravity, seedpiece vigor, and stem-end discolora- tion. Discoloration due to leafroll net necrosis was distinguished from physiological SED. This report discusses the effects of vine kill methods on SED. Other aspects of this series of experiments are reported separately (5, 6).

Materials and Methods

Detailed procedures for the entire experiment are described by Halder- son, et al. (6). In brief, potatoes were grown under field conditions at several sites in Idaho (Tetonia, Shelley, Aberdeen and Kimberly). Cultural practices were as recommended by the University of Idaho and others (10) to produce optimum yield and quality. All test plots were routinely irrigated by sprinklers except for the Aberdeen location in 1983 which was gravity ir- rigated. The effects of five vine kill treatments were determined in a ran- domized complete block design when rolling was excluded. A split-plot design was used for two years when rolling was included. All vine killing treatments were applied the same day at any given site. Sulfuric acid, dinoseb and diquat (or endothall) were selected to provide a relative range of fast, medium and slow desiccation, respectively.

Tubers were stored at approximately 6 C until early December when fifty U.S. No. 1 tubers were cut on the stem end to determine discoloration.

1Sulfuric acid is not presently registered in Idaho for use as a vine kill chemical.

1985) HALDERSON, et aL" STEM-END DISCOLORATION 275

A rating index of 0, 1, 2, 3, and 4 was used to evaluate discoloration (Figure 1). A " 0 " rating showed no visible discoloration, 1 = trace; 2 = distinct but less than 1.25 cm penetration (still U.S. No. 1); 3 = serious, penetration over 1.25 cm but less than �89 length of tuber (a U.S. No. 2); 4 = serious with pen- etration over �89 length of tuber (a cull). In those plots with severe SED, ten tubers were chosen f rom the number 4 rating (number 3 rating used also if number 4 contained less than 10 tubers) and were grown in the greenhouse to determine incidence of leafroll. The amount of leafroll was determined by visual examination for the 1981 tubers and by the ELISA test for 1982 and 1983 tubers.

FIG. 1. Rating index for stem-end discoloration (SED) of Russet Burbank potato.

Results and Discussion

Sulfuric acid gave the most rapid rate of vine kill of the chemicals tested (Table 1). Hand pulling or cutting is not included in the table because 100~ of the vines were immediately removed at, or just below, the soil sur- face with these treatments to simulate an ideal mechanical vine removal. Chemicals were applied during a wide range of temperatures during the three years but soil moisture at application time was never deficient. All of the chemicals were effective as determined two to three weeks later.

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TABLE l . - - Potato vine desiccation rate fol lowing four chemical treatments at Kimberly, Idaho in 1983.1

% Desiccation Vine Kill Days after treatment Method n 2 4 11 20

No treatment 24 11 21 62 Endothall 16 50 79 92 Diquat 8 55 79 92 Dinoseb 24 64 85 92 Sulfuric acid 24 79 89 95

1Data are means combined over rolling treatments and two vine maturities. 2n = number of observations included in data.

T o t a l s t em-end d i s c o l o r a t i o n ( tubers in r a t ing ca tegor ies 2, 3 and 4) was no t adverse ly a f fec ted by the speed o f vine kill (Tab le 2). The a m o u n t o f SED f rom mechan ica l vine r e m o v a l was s imi lar to tha t for sul fur ic ac id which t ended to be lower than d inoseb . N a t u r a l dy ing f r o m disease or f ros t was the s lowest ki l l ing p r o c e d u r e and p r o d u c e d as much SED as any o ther t r ea tmen t .

TABLE 2. - - The effect o f vine kill method on percent total stem-end discoloration (SED) over 12 location years in Idaho.

% Total Vine kill method SED

Sulfuric Acid 12 Hand Pulled 12 Slow ChemicaP 13 Natural Death 14 Dinoseb 15

LSD 5% n.s.

~Diquat or Endothall

Some p lan t ings at ce r ta in loca t ions in cer ta in years had s igni f icant dif- ferences in to ta l SED a n d in ser ious SED ra t ing (ca tegor ies 3 and 4) a m o n g vine kil l t r ea tmen t s (Tables 3 and 4). Tuber s wi th ser ious SED wou ld be g r a d e d as U.S . No . 2 's o r culls, whereas m o d e r a t e SED w o u l d no t be con- s idered a ser ious g rad ing defect . The levels o f ser ious SED shown in Tab le 4 w o u l d be enough to t ake the ent i re lot o f p o t a t o e s out o f g rade . As po in t e d out in Tab les 3 and 4, there were s igni f icant t r e a t m e n t d i f fe rences in b o t h t o t a l a n d ser ious SED at s o m e loca t ions , bu t these were no t cons is ten t as to t r ea tmen t .

1985) HALDERSON, et al: STEM-END DISCOLORATION 277

TABLE 3. - - Locations where there were significant effects o f vine kill me thod on the amount o f total s tem-end discoloration (P= 0.05).

TETONIA '81

SHELLEY'82

SHELLEY'82

ABERDEEN'83

Immature vines (100 days old): Natural Death and Hand Pulled, 3 to 4 x higher SED than other treatments.

Mature vines (112 days): Natural Death and Sulfuric Acid, 2x higher SED than hand pulled.

Immature vines (81 days): Dinoseb, 1.5 x higher SED than hand pulled.

Immature vines (76 days): Dinoseb, 2 to 3 x higher SED than all other treatments.

TABLE 4. - - Locations where there were significant effects o f vine kill me thod on the amount o f serious s tem-end discoloration (P= 0.05).

SHELLEY '81

SHELLEY '82

TETONIA '81

Mature vines (115 days old): Natural Death and Dinoseb treatments had 5~/0 serious SED

Immature vines (81 days old): Dinoseb, Diquat and Sulfuric Acid treatments had 5% serious SED

Immature vines (100 days old): Natural Death had 4% serious SED

Although there appeared to be little direct effect of chemicals or mechanical pulling on SED, there were up to three-fold differences among locations (Table 5). There was no relationship between the amount of SED and leafroll infection at the 12 location-years reported in Table 5. Tubers from the Kimberly location in 1981 and 1983 had high levels of leafroll in- fection. Therefore, these locations were not included in the summary of Table 5. The Kimberly location in 1981 and '83 had, by far, the most serious internal defect problems across all treatments (27% and 34%, respectively, for mature vines), which points out the relative seriousness of leafroll related internal necrosis compared with physiological SED in the Russet Burbank cultivar.

After eliminating leafroll as a factor in the large differences in stem- end discoloration among the locations, other factors were considered. One major variable tested in the experiment was vine maturity. When data from two vine maturities were combined (Table 2), speed of vine desiccation did not correlate with SED results. However, when total SED was considered as a function of vine age (days from planting to vine kill) (Table 5), there was a significant negative correlation (r = - 0.42, P = 0.01). This relationship was even stronger for serious SED (r = - 0.60, P = 0.01). This means that a por-

278 AMERICAN POTATO JOURNAL

TABLE 5. - - Percent total stem-end discoloration (SED) for twelve location-years in Idaho.

(Vol. 62

Location-Year Days from planting aT0 Total to vine kill SED

Shelley '82 81 23 Aberdeen '81 80 19 Shelley '82 112 18 Shelley '81 115 17 Aberdeen '81 118 14 Aberdeen '83 119 13 Aberdeen '82 99 12 Kimberly '82 111 10 Kimberly '82 141 9 Aberdeen '83 76 9 Aberdeen '82 129 7 Tetonia '81 100 7

t ion of the var iabi l i ty in serious SED (r2= 0.36) can be accounted for by physiological age of the vine at vine kill, with less ma tu re vines having more SED problems. The r ema in ing var iabi l i ty a m o n g locat ions and growing seasons is p robab ly due to c o m b i n e d cul tural an d e nv i r onme n t a l factors no t tested in these exper iments . Since soil mois ture , at the t ime of vine kill, was no t al lowed to become deficient , mois ture stress should no t have been a fac-

tor in SED. In conc lus ion , it did no t appear tha t vine pul l ing or vine kil l ing by the

most rapid chemical means (sulfuric acid with roll ing) increased the prob- abi l i ty of developing physiological s tem-end discolora t ion.

A c k n o w l e d g m e n t s

The au thors express apprec ia t ion to the Idaho Po ta to C o m m i s s i o n for fund ing of the research, to James L. Norris a nd Dr. Phil J. Petersen for statistical assistance and to Phi l ip W. Leino for general assistance t h roughou t the project .

Literature Cited

1. Callbeck, L.C. 1948. Current results with potato vine killers in Prince Edward Island. Am Potato J 25:225-233.

2. Callihan, R.H. 1980. Vine killers and weed control in potatoes. Potato School Proceedings, Univ of Idaho, College of Agric, Moscow, ID 83843.

3. Cunningham, C.E., P.J. Eastman and M. Goven. 1952. Potato vine killing methods as related to rate of kill, vascular discoloration and virus disease spread. Am Potato J 29: 8-16.

4. Grassert, V. and J. Vogel. 1981. The effects of defoliation on the culinary quality charac- teristics of potatoes. Potato Res 25:321-327.

1985) HALDERSON, et al: STEM-END DISCOLORATION 279

5. Haderlie, L.C., P.W. Leino, J.L. Halderson and D.L. Corsini. 1984. Chemical desiccation of potato vines. Proc West Soc Weed Sci 37:159.

6. Halderson, J.L., L.C. Haderlie and D.L. Corsini. 1985. Potato vine kill: pulling, chemical killing and rolling effects on yield and quality of Russet Burbank. Am Potato J 62:281- 288.

7. Hoyman, W.G. 1947. Observations on the use of potato vine killers in the Red River Valley of North Dakota. Am Potato J 24:110-116.

8. Misener, G.C. and C.F. Everett. 1981. Vine pulling as a means of top killing potatoes. Am Potato J 58:103-109.

9. Murphy, H.J. 1968. Potato vine killing. Am Potato J 45:472-478.

10. Potato Grower Handbook, Washington State Univ., Pullman, WA 99163.

11. Rich, A.E. 1950. The effects of various defoliants on potato vines and tubers in Washing- ton. Am Potato J 27:87-92.