1960] ROBINSON, et a / : CHEMICAL CONTROL OF DISEASES 203

C H E M I C A L C O N T R O L O F B L A C K L E G , DRY R O T A N D V E R T I C I L L I U M W I L T O F P O T A T O 1

D. B. ROBINSON, G. W . AYERS AND J. E. CAMPBELL 2

I NTRODUCTO N

Serious losses in seed potato production in Prince Edward Island occur annually from the diseases blackleg, dry rot and Verticillium wilt. Blackleg (Erwinia atroseptica) (van Hall) Jennison) is by far the most important disease of the three. It caused the rejection for seed purposes of more than 5500 acres in the period 1954-1958. Smaller, but consistent, losses are caused each year by dry rot (Fusarium sambucinum f.6 Wr. and Fusarium coeruleum ( 'Lib.)Sacc.) and by wilt (Verticillium albo-atrum R. &B. ) .

Individual control measures for each of these diseases have been reported previously (1, 8, 9). This paper presents further results obtained over the past four years with various chemicals. In these trials the incitants of the three diseases were present both singly and in combination with each other

~ATERIALS AND METHODS

The pathogens used in the trials were isolated from diseased material. The fungi were maintained on potato dextrose agar and the bacteria on nutrient agar. Inocula consisted of a condial or bacterial suspension made from freshly growing cultures, except that in a few cases dry rot inoculum consisted of a suspension of macerated diseased tuber tissue.

All field trials were planted in randomized block designs on land where adjacent plantings had been relatively free of the dry rot. wilt and blackleg pathogens. In most of the trials each treatment comprised 200 plants in a four-replicated block. Seed tubers were obtained from crops known to be free of the diseases under study. The varieties Sebago, Keswick and Irish Cobbler were used. These varieties were chosen because of their differing susceptibility to the pathogens in the tests, as follows:

Verticillium Erwinia F. sambucinum F. coeruleum Irish Cobbler ............ -~- + + Sebago ........................ + -4- -4- Keswick ...................... -1- -~-

Unless otherwise noted, the seed pieces were sprayed with a suspen- sion of inoculum or were immersed in it; then they were air dried and treated with the desired chemical. Where inocula or chemica!s were used in combination, they were kept at the same concentration as when used singly.

Data on the incidence of wilt and blackleg were obtained by counts of infected plants during the growing season. Data on dry rot, unless

1Accepted for publication November 9, 1959. Contribution No. 57, Research Laboratory, Research Branch, Canada Department of Agriculture, Charlottetown, Prince Edward Island.

2plant Pathologist, Plant Pathologist, and .Associate Plant Pathologist respectively.

204 AMERICAN POTATO JOURNAL [Vol. 37

otherwise noted, was obtained by examination of tubers after two to six months in storage. Such tubers were harvested with an elevator digger, bruised on a specially designed platform, and stored in jute sacks at 40-44 ~ F. Dry rot severity was recorded by rating tubers in each treat- ment lot for number and extent of lesions, weighting and summing the figures and converting to a percentage daum. Percentage data referring to the incidence of any of the diseases under study are presented as an index obtained by the conversion of percentage figures to degrees of an angle as given by Snedecor (10).

A number of the chemicals used were tested for effectiveness in vitro. Filter paper discs were saturated with a solution of the chemical and placed on agar media that had been seeded with one of the pathogens. When the test organism was bacteria, streptomycin assay agar was used, otherwise the assay medium was clarified potato dextrose agar. Tim comparative effectiveness of cheMcals tested in this way was measured by comparing the inhibition zones produced after 12 to 48 hours incubation at 24 ~ C.

The chelnical preparations used in these trials, and their active ingredients, were as follows:

Agristrep: streptomycin sulfate 26.7%. Acid mercuric chloride: 1 qt. hydrochloric acid: 6 oz. mercuric chloride:

25 gals. water. Agristrep dust: streptomycin sulfate 0.24%. Agri-mycin 500, 10%: streptomycin sulfate 10~ , copper 1 0 9 . oxytetra-

cycline 1%. Agri-mycin 500, 42.4~ : streptomycin sulfate 1.755%, copper 42.4%, oxy-

tetracycline 0.176%. Arasan dust: tetramethyl thiruamdisulphide 50%. Captan: n-trichloromethylmercapto-4-cyclohexene-12 dicarboximide 50%

and 75% wettable . Dithane Z78: zinc ethylene bisdithioearbamate 65%. Karathane : dinitro-phenylcrotonate 22.5%, other nitro phenols and deriva-

tives, chiefly dinitrophenol, 2.5%. Kolo 100: sulfur 75.4%, 2-3 dichloro-1, 4-naphthoquinone 3.5%. Phygon X L : 2,3-dichloro-1, 4 naphthoquinone 50%. Spergon : tetrachloro-para-benzoquinone 97%. Semesan Bel: hydroxymercurinitrophenol 12.5~ and hydroxymercuri-

chlorophenol 3.8%. Sulfuron: sulfur 97%. Terramine: phenol, coefficient 20. Lime: equivalent CaO, 74.1~e.

RESULTS Blackleg

Several fidd trials were carried out with this disease using the variety Sebago. The treatments used and tile blackleg control obtained in two such trials are presented in Tables 1 and 2.

1960] R O B I N S O N , et al: C t t E M I C A L CONTROL OF DISEASES 2 0 5

TABLE 1.--Effect of some seed treatment chemicals in controlling blackleg in the variety Sebago.

Inoculated Seed Non-inoculated Seed Seed Percentage Blackleg Percentage Blackleg treatment 1 non-emergence index non-emergence index

Check--no treatment ...............

Spergon .................... Semesan Bel ............. Captan 50VV ............ Mercuric

chloride ................ Acid mercuric ..........

chloride ................. Agristrep dust ........ Agristrep dust

Jr Captan 75W ._.

8,0 2,5

24,0 1.5

48.0

56.0 4.5

9.5

18.1 18.1 17.5 17.9

2.5

3.1 0

3.5 1.5

36.5 3.0

68.5

72.5 6.0

9.5

20.5 4.1 3.7

22.1

7.9

5.8 0

L.S.D. P.-=0.05 ... 6.5 10.8

1Application rates: Spergon 1 lb. to 16 gal.; Semesan Bel 1 lb. to 6.25 gal.; Captan 50V~r 1 lb, to 25 gal. ; Mercuric chloride 1 qt. to 25 gal. ; acid mercuric chloride 6 oz. HC1, 1 qt, HgCI~_ to 25 gal. ; Agristrep dust 0.25 per cent streptomycin.

TABLE 2.--The effect of several fungicides in controlling blackleg in the variety Sebayo.

Percentage Blackleg Seed treatment non-emergence index

Check--no treatment .................................... Phygon--1 1b./25 gal .................................... Dithane Z78--1 lb./40 gal ........................ Spergon--I lb./16 gals ................................ Semesan Bel--1 lb./6M gals .....................

28.4 7.2

10.6 23.2 6.9

33.5 23.4 33.2 37.6 3.5

L.S.D. P=0.05 ........................................ 6.3

In the first trial (Table 1), the t reatments were applied to cut seed. Because of unfavorable weather the seed lots had to be held for 36 to 40 hours before plant ing and under these conditions the mercuric chloride and Semesan Bel treatments markedly reduced the stand. Wi th inoculated seed, t reatments containing mercuric chloride or streptomycin significantly reduced disease. W i t h non-inoculated seed, all t reatments except Captan 50W significantly reduced blackleg incidence.

In the second trial (Table 2) tile seed pieces were contaminated with suspensions made from tubers infected with blackleg and dry rot and the chemical t reatments were applied to the whole tubers. U nde r these con- ditions Phygon increased the emergence and slightly reduced blackleg incidence whereas Semesan Bel markedly reduced both seed-piece decay and blackleg. Other materials used were ineffective.

In another trial some fungicides were combined with Agris t rep (100 ppm streptomycin) in the t reatment of inoculated seed. The results, pre- sented in Table 3, show that acidulated mercuric chloride, Semesan Bel

206 AMERICAN POTATO JOURNAL [Vol. 37

and a combination of Agristrep and Semesan Bel were the only treatments that significantly reduced the diseases. Once again, the mercuric chloride treatment was quite phytotoxic to the seed pieces, reducing emergence. Combinations that included Agristrep gave the best stands.

In two other field trials Terramine, Karathane, Sulfuron dust and liquid, Kolo 100, and hydrated lime have all been found ineffective in controlling blackleg. In these trials, however, the preparations that con- tained streptomycin at 100 ppm gave moderate control of the disease, especially in the early part of the growing season.

TABLE &---Effect of several seed treatments on emergence rout blackleg development in the variety Sebago.

Percentage 2 Blackleg Seed treatment 1 non-emergence index

Check--no treatment ........................ Acid mercuric chloride ..................... Agristrep + Semesan Bel .............. Agristrep + Captan 50W ............ Agristrep + Spergon ...................... Agristrep ............................................. Captan 50 W ...................................... Semesan Bel ....................................... Spergon ...............................................

10.0 14.6 2.6 1.0 1.0 5.3 3.6

10.6 19.6

39.5 10.4 7.4

31.0 46.4 37.8 35.8 22.3 38.6

L.S.D. P=0.05 ............................ : 7.6

1Application Rates : as given in Table 1 except that Agristrep was applied as a solution containing 100 ppm streptomycin.

2Based on 300 seed pieces per treatment.

Dry Rot and Verticillium Wilt In previous work (1) Semesan Bel, used as a seed treatment a t

planting time, was found to give a high degree of dry rot control in potatoes stored from the resulting crop. This finding has been substantiated by subsequent trials over a three year period in which several fungicides have been tested comparatively for dry rot control in parallel trials using the variety Sebago, susceptible to F. sambucinum f.6 and the variety Keswick, susceptible to F. coeruleum. Semesan Bel has also been shown to be effective in controlling Verticillium wilt (8). In the present work the seed pieces were inoculated with both the dry rot and wilt organisms. Some of the fungicides tested, and their effectiveness in controlling these diseases, are given in Table 4. Seed treatment with mercury compounds invariably gave the best control of both wilt and dry rot but their effect in reducing vigor or stand resulted in a lowered yield in the variety Keswick even though wilt and dry rot were controlled. With this variety the treatments Spergon and Phygon actually increased wilt incidence, and in addition Phygon markedly reduced emergence. In the variety Sehago a high degree of wilt control was given by acid mercuric chloride and this apparently compensated for its phytotoxicity so that there was no reduction in yield.

The direct effect of several fungicides in preventing storage rot in harvested tubers was tested. Control of this disease is particularly impor- tant in seed eye-indexing work and in the grading of seed lots because

1960] R O B I N S O N , et al: C H E M I C A L C O N T R O L OF DISEASES 2 0 7

TABLE 4. - -The effect of some seed treatments on the control of Verticillium wilt and dry rot.

Percentage Wil t Dry rot Plot yield Seed treatment 1 non-emergence index index in lbs.

A - - Variety Keswick

Check--no treatment ....... 15.0 Semesan Bel ....................... / 9.7 Captan 50W ........................ 7.4 Spergon ............................... I 4.1 Phygon ............................... [ 29.7 Dithane Z78 ....................... 10.6

26.5 2.3

24.6 33.1 37.6 40.2

13.0 4.6

10.2 14.5 12.4 11.9

44.1 51.3 61.2 52.5 28.0 41.4

11.1 4.5 11.2

B - - Variety Sebago

Check--no treatment ....... 0.3 Semesan Bel ....................... 0.9 Acid mercuric chloride ._.1 11.9 Agri-mycin 500 I

(42.4% Cu) .................... 0.3

29.2 3.5 0.0

32.3

14.0 4.0 2.1

11.0

8.2 7.1

1Application Rates: Semesan Bel 1 lb./6.25 gal.; Captan 50W 1 lb./25 gal. ; Spergon 1 lb./16 gal. ; Phygon 1 lb./25 gal. ; Dithane Z78 1 lb./40 gal. ; Acid mercuric chloride 6 oz, HC1, 1 qt. HgCI=/25 gal. ; Agri-mycin 500--2400 ppm copper, 100 ppm strepto- mycin.

in t h e s e o p e r a t i o n s t h e r e is a b u n d a n t o p p o r t u n i t y f o r i n f ec t i on of cu t s a n d b r u i s e s by Fusarium w i t h r e s u l t a n t s e v e r e losses d u r i n g t he s u b s e q u e n t s t o r a g e pe r iod . T w o t r i a l s w e r e c a r r i e d out .

Trial 1. S e b a g o t u b e r s , e ach w i t h a n eye exc i s ed for i n d e x i n g w e r e su r f ace i n o c u l a t e d w i t h a s p o r e s u s p e n s i o n of F. sambucimtm f.6, a l l owed to d r a i n , t h e n t r e a t e d w i t h a f u n g i c i d e e i t h e r by d i p p i n g in a s o l u t i o n o r d u s t i n g t he cu t su r face . T h e d r y ro t i n c i d e n c e a f t e r t he s t o r a g e p e r i o d was as f o l l o w s :

Dry rot Tuber treatment index

Check--no fungicide ........................................................................................... Agri-mycin 500 (10% Cu) - - 60 ppm Cu : 60 ppm streptomycin ........ Captan (50% W ) - - 1 lb./25 gal ................................................................. Arasan (50% dust) - - to cover cut surface ................................................ Captan (7% dust) - - to cover cut surface .................................................. Semesan Bel - - 1 lb./6.25 gal ........................................................................ Agri-mycin 500 (42.4% Cu) - - 2400 ppm Cu : 100 ppm streptomycin ..

L.S.D. P=0.05 ........................................................................................

69.8 75.1 64.4 63.6 61.3 36.9 19.8

4.9

S e m e s a n Bel , a n d A g r i - m y c i n 500 c o n t a i n i n g 42 .4 p e r c en t coppe r , g a v e a h i g h d e g r e e of d r y r o t c o n t r o l w i t h n o i n h i b i t i o n of s u b s e q u e n t s p r o u t i n g a l t h o u g h A g r i - m y c i n wes s l igh t ly p h y t o t o x i c as i n d i c a t e d b y s o m e p i t t i n g of the t u b e r su r faces . T h e o t h e r m a t e r i a l s w e r e r e l a t i ve ly inef fec t ive in c o n t r o l l i n g rot .

208 AMERICAN POTATO JOURNAL [\701. 37

Trial 2. \u tubers and cut seed respectively were inoculated with a spore suspension of F. sambucinum f.6, then treated with Semesan Bel and held in storage at 40 ~ F. At the end of five months an examination for dry rot showed that considerabIe fungaI and bacterial rot had occurred in the cut seed but that the whole tubers were unaffected. These whole tubers, when cut into seed pieces, began to develop dry rot whereas seed pieces cut from inoculated, Semesan Bel-treated tubers remained sound and plots planted with them developed uniform stands with complete plant emergence.

Combined Fungicidal-Bactericidal Treatments Verticillium wilt, blackleg and dry rot often occur together in potato

fields and a seed treatment designed to control any one may not be effective against the others. Attempts were therefore made to control these three diseases sinmltaneously by combinations of a bactericide and fungicide. In field trials, seed lots of the variety Sebago were inoculated singly and in all combinations with the respective pathogens. A bactericide and fungicide were then used singly and in combination on half of each inocul- ated seed lot with suitable untreated and uninoculated checks included. Two separate trials were carried out, one in 1957 with Agristrep and Captan 50W, the other in 1958 with Agristrep and Semesan Bel.

Trial 1. Agristrep, (100 ppm streptomycin) and Captan 50 per cent wettable, at 3 pounds per 100 gals. water, were used. Dry rot inoculum was applied as a suspension of macerated, dry rotted tubers taken from storage. This inoculum incited a high incidence of bacterial decay in the seed pieces and this was augmented by the addition of blackleg bacterial suspensions. Dry rot and wilt development was slight in both treated and untreated lots so that the effects of seed treatments were inconclusive. The chemicals, alone or in combination, did not significantly control black- leg except in the case where blackleg and dry rot inoculum were applied together on the seed pieces. This combination resulted in 44.2 per cent blackleg, but when such seed was treated with a mixed suspension of Agristrep and Captan the incidence of blackleg was reduced to 22.6 per cent.

Trial 2. Agristrep, (100 ppm streptomycin) and Semesan Bel were used and all inoculum consisted of pure culture suspensions of the patho- gens. In this trial there was no appreciable seed piece decay. Emergence was uniformly high in all treatments, with a high incidence cf wilt and moderate dry rot and blackleg developlnent. Some of the inoculum and treatment combinations and their effects are given in Table 5. In no case was there any statistical evidence that combinations of inoculum increased the incidence of any disease. Significant control of wilt and dry rot was given by Semesan Bel and of blackleg by Agristrep. Diseases caused by combinations of bacterial and fungal inoculum were controlled by combina- tions of the bactericide and fungicide.

Tests in vitro, using the assay plate technique (Fig. 1), generally supported these findings. Growth of E. alroseptica was suppressed by materials containing streptomycin or mercury but not by Spergon, Phygon or Captan 50W. F. sambucinum was inhibited by mercury compounds and by Agri-mycin 500 (42.4 per cent copper) but was not affected by Spergon,

1960] ROBINSON, et al: CItEMICAL CONTROL OF DISEASES 209

TABLE 5. - -The effect of Agristrep and Semesan Bel on the incidence of dry rot, blackleg and UerticiIlium wilt.

A - - Dry rot

hloculum 1 Seed treatment ~ Dry rot index

F . . . . . . . . . . . . . . . . . . . . . . .

EF ................... FV .................... EFV ................ F . . . . . . . . . . . . . . . . . . . . . . . .

EFV ..................

l l o l l e

n o n e

none none

Semesan Bel Semesan Bel + Agristrep

9.4 13.8 11.5 7.8 3.1 3.1

L.S.D. P=0.05 4.9

B - - Wilt

Inoculum I Seed treatment Wilt index

V . . . . . . . . . . . . . . . . . . . . . .

EV .................... FV .................... EFV .................. V . . . . . . . . . . . . . . . . . . . . . . . .

EFV ..................

L.S.D. P=0.05

nolle n o n e

nolle llone

Semesan Bel Semesan Bel q- Agristrep

49.2 45.6 41.7 46.6 7.7

13.8

6.7

C - - Blackleg

Inocutum l

g . . . . . . . . . . . . . . . . . . . . . . . .

EV .................... EF .................... g . . . . . . . . . . . . . . . . . . . . . . . .

EFV ..................

L.S.D. P=0.05

Seed treatment

none none Done

Agristrep Agristrep + Semesan Bel

Blackleg index

13.0 13.1 10.4 3.3 0.0

8.4

1Suspensions of: E--Erwinia atroseptica, F--Fusarium sambucinum, V--Uerticillium albo-atrum.

2Application Rates: Semesan Bel, 1 1b/6.25 gal. ; Agristrep, 100 ppm streptomycin.

Phygon, Captan 50 W or s treptomycin preparations. V. albo-atrum reacted similarly to F. sambucinum except that slight inhibition was exer ted by Captan 50W and Phygon. In some of this work, the filter paper discs were impregnated with combined suspensions of materials and plates seeded with combinations of the organisms. No addit ive inhibitory effects were observed to result f rom this, the materials apparently acting inde- pendently of each other.

Agr imyc in 500, in a solution containing 100 ppm streptomycin and 2400 ppm copper, was tested in a greenhouse planting with Sebagoes in which seed pieces were inoculated with a combination of wilt and blackleg inocnlum. Some individual control of each disease was achieved but the Agr imycin was again solnewhat phytotoxic, reducing the vigor and growth of the plants. It was of interest that, under these conditions, blackleg incidence was 24.8 per cent when a combined inoculum was used, but only 9.0 per cent when blackleg inoculum alone was used, suggesting that reduced vigor may have predisposed the plants to blackleg infection.

210 AMERICAN POTATO JOURNAL [Vol. 37

FIGURE 1.--Comparative inhibition in vitro given by A--Semesan Bel; B--mercuric chloride (1:1000) ; C--Agristrep (100 ppm streptomycin), Pairs of paper discs satur- ated with the respective chemical were placed on agar seeded with: upper left--F. sambucinum f.6, upper right--E, atroseptica, lower--V, albo-atrum. Center disc on each

plate saturated with water.

D i scuss ioN

Seed treatment is by no means general practice in many potato grow- ing areas. In a recent survey (6) it was found that only a few locations in Canada and the United States had more than 50 per cent of the acreage treated, and in many of the major potato producing areas little seed treatment is being carried out. Where seed-borne bacterial or fungal diseases are not a serious problem there seems to be little evidence that seed treatment is beneficial. In trials in western Washington with many of the common fungicidal seed treatment chenficals, under conditions where seed piece decay was not encountered, none of the treatments gave con- sistently improved stands (5) . Oil the other hand, seed or tuber treatment has proven very beneficial in certain localities and against certain diseases, notably for bacterial seed-piece decay and blackleg in Maine (2) , for

1960] ROBINSON, et al: CHEMICAL CONTROL OF DISEASES 211

Fusarium seed-piece decay in Long Island (3) and for Fusarium storage rots in Long Island and Great Britain (3, 4).

In the present work significant control of blackleg, dry rot and Verticillium wilt has been achieved with seed treatments. In most trials, moderate to good control of blackleg was given by mercuric chloride or Semesan Bel. In some cases this control was more than offset by phyto- toxic effects that reduced plant emergence (Table 1). Such injury was accentuated when unfavorable weather forced a delay in planting. Agri- strep, applied to give 100 ppm streptomycin, gave some control of blackleg except in one instance (Table 3). In this case it is notable that a combination of Agristrep and Seniesan Bel controlled blackleg very effec- tively and that this combination was also very effective in later tests (Table 5) where both fungal and bacterial inoculum were present. It is noteworthy that Agristrep usually promoted emergence and gave good control of blackleg early in the growing season but that its protection apparently lessened with time so that considerable blackleg often developed in August and September.

Of the materials tested, only Semesan Bel and mercuric chloride gave effective control of seed-borne Fusarium or Verticillium. Semesan Bel and Agri-mycin 500 (42.4 per cent copper) gave the best control of Fusarimn rot in stored tubers. Phygon seriously reduced emergence and yield in the variety Keswick (Table 4) whereas Captan increased stand and yield. Neither material gave effective disease control. In these trials the ineffectiveness of Arasan, Spergon and Dithane Z78, as contrasted with the effectiveness of an organo-mercury material, is in agreement with other reports (4, 7).

There is some evidence that more blackleg develops when the seed pieces are contaminated with a combined inoculum than when the blackleg organism is present alone. This trend was clearly present in only one year in field trials but nevertheless points to the possibility that fungal or other organisms present on the seed piece may predispose the plant to attack by E. atroseptica. This would explain the apparently contradictory results occasionally obtained. For example, Spergon was ineffective in controlling Fusarium rot and blackleg (Tables 1, 2 and 3) where the blackleg organism was present alone or in combination with Fusarium, but Spergon effectively controlled a natural epidemic of blackleg in one test (Table 1) where non-inoculated seed was used. In this case it may be that seed-borne organisms were suppressed that, if unchecked, would have predisposed the plants to infection by blackleg bacteria. Similarly. the effectiveness of Semesan Bel in controlling blackleg may depend to some extent on its fungicidal activity in addition to its considerable bacteri- cidal action.

Captan solutions proved relatively ineffective against all three diseases. Combinations of Captan and Agristrep also gave no control of seed piece decay and blackleg except in one instance where dry rot and blackleg inoculum were present together on the seed pieces. Here again the fungicidal action of Captan may have indirectly controlled blackleg by suppressing fungal invasion, thus reducing infection courts for the blackleg pathogen.

In general, Semesan Bel has proven in these trials to be the best all-round seed treatment chemical. It was consistently effective against

212 AMERICAN POTATO JOURNAL [Vol . 37

dry rot and wilt and partially so against blackleg, but under sub-optimum planting conditions its phytotoxicity oftinies reduced its value. Greater effectiveness, and a more widespread use, of seed treatment in potatoes will probably follow the development of a material of low phytotoxicity having both fungicidal and bactericidal properties. Progress niay also conie through the use of a fungicide on whole tubers during storage followed by a bactericide on seed pieces applied at time of planting.

SUMMARY

A number of chemical proprietary compounds were tested for the control of blackleg, dry rot and Verticillium wilt in potato. In most, but not all instances, blackleg was controlled by treatment of seed pieces with mercuric chloride, Semesan Bel or Agristrep. Phygon, Spergon, Dithane Z78, Captan, Terramine, Karathane, Sutfuron, Kolo 100, and Lime gave little or no control.

Dry rot and Verticillimn wilt originating in seed stocks were con- trolled by seed treatment with Semesan Bel and acid mercuric chloride. Captan, Spergon, Phygon, Dithane Z78 and Agr i -mycin 500 proved ineffective. Dry rot in stored potatoes was most effectively controlled by tuber t reatment with Semesan Bel or Agr i -mycin 500 (42.4 per cent copper) .

Combinations of two or more of these diseases were controlled by Semesan Bel alone and by combinations of Semesan Bel and Agristrep, but not by combinations of Captan and Agristrep. Pronounced phytotox- ieity always followed treatment with mercuric chloride, and occasionally fol- lowed treatment with Semesan Bel, but under good cultural conditions Semesan Bel gave marked disease control and did not adversely affect stands.

LITERATURE CITED

1. Ayers, G. W. and D. B. Robinson. 1956. Control of Fusarium dry rot of pota- toes by seed treatment. Am. Potato J. 33: 1-5.

2. Bonde, Reiner and Paulo de Souza. 1954. Studies on the control of potato bacterial seed-piece decay and blackleg with antibiotics. Am. Potato J. 31 : 311-316.

3. Cunningham, H. S. and Otto A. Reinking. 1946. Fusarium seed piece decay of potato on Long Island and its control. N. Y. Agr. Exp. Sta. Bull. 721.

4. Foister, C. E., A. R. Wilson and A. E. W. Boyd. 1945. Control of dry rot of seed potatoes by dusting. Nature 156 : 394-395.

5. Gould, C. J, and T. E. Randall. 1950. Potato tuber disinfection tests in western Washington. Am. Potato J. 27:249-256.

6. Hoyman, W . G . 1957. Potato seed treatment. In Potato Handbook. Disease Control Issue. Vol 2:13-17. The Potato Association of America.

7. Lutz, J.M. 1953. Fusarium tuber rots of late potatoes as related to injuries and certain chemical treatments. Am. Potato J. 30: 131-134.

8. Robinson, D. B. and G. W. Ayers. 1953. The control of Verticillium wilt of potatoes by seed treatment. Can. J. Agr. Sci. 33: 147-152.

9. Robinson, D. B. and R. R. Hurst. 1956. Control of potato blackleg with anti- biotics. Am. Potato J. 33: 56-59.

10. Snedecor, G.W. 1946. Statistical Methods. Iowa State Coll. Press. 4th ed.