1954] AKELEY AND B U C K : BREEDING FOR LATE BLIGHT RESISTANCE 165

B R E E D I N G F O R R E S I S T A N C E T O L A T E B L I G H T 1

R . V . A K E L E Y AND t . W . B U C K 2

It is now more than 100 years since late blight, incited by Phytoph- thora infestans, destroyed many potato crops in Europe and North America and caused the Irish Famine. From that time to the present, research has been conducted intermittently to breed blight-immune varieties. Worthwhile progress has been made, but only partial success can be reported.

In the United States the work of breeding blight-resistant varieties began about the middle of the 19th century. Although many excellent varieties, some of which are grown commercially today, were produced during the last half of the century, none was resistant to late blight. Protective measures that permitted the growing of susceptible varieties were devised and the work of breeding for late blight resistance was allowed to lapse for a time.

In 1910 potato breeding was actively undertaken by the United States Department of Agriculture under the leadership of William Stuart. At that time the only disease resistance sought was to the late blight fungus. However, the work had not progressed far when it became evident that virus diseases had to be given chief consideration, and it was not until 1932 after the National Potato-Breeding Program was under way that emphasis could be placed once more on breeding for resistance to late blight. I t seemed advisable at first to study the reaction of the available cultivated varieties and parent material phenotypically and genotypically. many named varieties of Solanum tuberosum and thousands of seedling varieties were tested. Various degrees of resistance were found from high resistance to complete susceptibility to the races of the late blight fungus prevalent in Maine at the time.

MATERIALS AND METHODS

Since 1932 the materials used in breeding potatoes resistant to late blight by the United States Department of Agriculture in cooperation with the Maine Agricultural Experiment Station, Presque Isle, Maine, have gone through a number of phases: The testing of American varieties and crosses between some of them to determine their reaction to late blight; the use as parents of foreign varieties showing an intermediate degree of resistance; the introduction of seed of the German /47 varieties possessing a gene for immunity from the common physiologic races, and the use of these varieties as parents in the production of superior com- mercial varieties highly resistant to, if not immune from, the common races of the late blight fungus; and the use as parents of seedling varieties related to 5". demissum carrying two or more of its genes for immunity

aAccepted for publication December 15, 1953. An invitation paper presented at the Symposium on Potato Late Blight at the 40th Annual Meeting of the Potato Association of America in Madison, Wis., September 7, 1953.

2Senior Horticulturist and Geneticist, respectively, Bureau of Plant Industry, Soils, and Agricultural Engineering, United States Department of Agriculture, Plant Industry Station, Beltsville, Md.

166 AMERICAN POTATO JOURNAL [Vol. 31

in an attempt to breed commercial varieties resistant to all the physiologic races known at present in the United States.

The methods also have undergone a number of changes. Most of the early tests were conducted in the field at Presque Isle, Maine, and a few were made in the greenhouse at Beltsville, Maryland. The seedling varieties were tested in comparison with the President variety, which is intermediate in resistance, and with Green Mountain, which is extremely susceptible to the disease. In Maine the seedlings usually were grown in 5-hill lots. Each row of 5-hill lots was grown adjacent to a row of Green Mountain. As additional checks Green Mountain, President and the parents of the respective progenies under test were distributed through the field. The plants were sprayed at intervals during July and August with a water suspension of zoospores of the late blight fungus and if the weather was favorable a satisfactory epidemic resulted. In 1936, for example, the leaves of susceptible varieties were killed at least 30 days before the plants would have matured normally.

In the greenhouse at Beltsville, Maryland the humidity of the air was raised almost to the saturation point by the addition of steam just prior to the spraying of the plants with water suspension of zoospores.

Recently most of the tests for blight resistance have been conducted in greenhouses, some at Presque Isle and others at Beltsville. These are made in moist chambers, where a fine spray of water is applied to the plants at regular intervals. The water is automatically controlled by a time clock. In Maine selections from both blight-resistant progenies and seedlings in about the four-leaf stage are sprayed with a water suspension of zoospores of the common races of the late blight fungus.

The most promising survivors from this test are tested again at Beltsville, this time with each of the physiological races that have been isolated from Kennebec and other resistant varieties in Maine.

In other tests at Beltsville, the seedlings related to the HI varieties and to the species 5". demissum are sprayed in the seedling flats in about the four-leaf stage with a mixture of all the races mentioned above.

In these tests a fine water spray is applied by hand, the spore suspension is applied with an atomizer, and the flats are covered with glass.

RESULTS Resistance in Cultivated Varieties. The first indication in the

present program that resistance to blight could be found in the cultivated varieties was observed in a cross of two susceptible varieties, Chippewa x Katahdin. A number of selections of this cross were more "resistant than either parent. One selection was found to be moderately resistant to late blight in both vines and tubers and as its horticultural characters were satisfactory it was named Sebago and released to growers. I t is now an important American variety.

Sebago was back crossed to Katahdin. The mean for the population of 280 seedlings of this back cross indicated a resistance significantly higher than that shown in the Chippewa x Katahdin cross.

When President, which on the average showed a slightly higher degree of resistance than Sebago, was crossed with Katahdin a higher percentage of seedlings showed an intermediate degree of resistance than was found in the Sebago x Katahdin cross. One of the selections of the

1954] A K E L E Y A N D B U C K : B R E E D I N G FOR L A T E B L I G H T R E S I S T A N C E 167

President x Katahdin cross was named Saranac and released. Ackersegen, a German variety, was more resistant in the early tests than Sebago or President. A cross between Ackersegen and Katahdin produced a number of seedlings, some of which were more resistant than Sebago. One of these was named Calrose and released to growers in California.

The greatest resistance in this group of crosses was found in a cross between two resistant parents, a seedling of Katahdin selfed and a Japanese variety, Ekishirazu. More than 90 per cent of the seedlings of this cross were resistant but, the tuber shapes were so poor and the yields so low that none of the seedlings could be considered as a commercial possibility.

Resistance in 14/ Varieties. True seed of the //V varieties, sent to us by K. O. Muller formerly of Berlin Dahlem, Germany, has contributed two characters of great importance to our breeding program: immunity from the physiological races of late blight prevalent in most parts of the United States combined with genes for early maturity. The exact origin of the W varieties is not known, but it is supposed that they originated from crosses in which SoIanum demissum played a part as recent results indicate that they carry a factor for immunity identical with one obtained from S. demissum. Seedlings were grown in Maine from the seed received from Germany and subjected to a heavy epidemic of late blight induced by spraying the plants with a suspension of zoospores. Many seedlings came through the tests free from blight, whereas other seedlings and the susceptibile checks were dead as long as 30 days before they would have matured normally. Although many of these seedlings were highly resistant to blight, they were all low in yield and market quality. A few of the best of them were crossed with American varieties. Selections from a cross with Earlaine were among the best. A number of the latter had the resistance of the original FV varieties, were early, and were self-fertile. They yielded much more than the original seedlings but could not compete in this respect with the best commercial sorts. They were in turn selfed, sib-mated, and outcrossed to various commercial varieties and promising seedlings.

Regardless of the other parent, a number of seedlings that escaped infection with the common races of late blight fungus were found in three resulting progenies.

In this second series of crosses, a number of seedlings were found that combined immunity from the common races of the late blight fungus with high yield, goad cooking quality, and satisfactory market quality when grown in Maine. Three of these, Kennebec, Cherokee and Pungo, were named and released. With the release of these and a number of Dr. Reddick's varieties, the blight problem in the United States might have been solved if it had not been for the appearance of new physiologic races of the causal organi'sm.

About 25 years ago it was thought that there was only one biotype of the late blight fungus in North America and it was not until varieties and seedlings related to S. demissum were produced and showed various reactions to the disease that different races could be identified.

Varieties such as Kennebec seem to be immune from the common races of the late blight fungus, but they are susceptible to some of the more virulent races.

To get immunity from all races identified in the United States it

168 AMERICAN POTATO JOURNAL [Vol. 31

seems necessary to work with seedling varieties closely related to S. demissum. Multiple genes for immunity found in that species segregate or separate in crosses or back crosses with cultivated sorts. Some of the resulting seedling varieties carry only one of the genes, but others may possess two or three. These seedling varieties are far superior to the wild species in horticultural characters. By the use of these improved seedling varieties as parents, we are attempting to recombine the several immunity genes and to produce commercial varieties with the multiple genes for immunity found in 5". demissum. Some of the seedlings related to this species have been obtained from Cornell University, at Ithaca, New York, from the Pennsylvania Agricultural Experiment Station at State College, Pennsylvania, from Scotland, and from Germany.

One of the first crosses made in the latest phase of our work was between T15 and B 355-24. T15, one of Dr. Reddick's older seedlings, is five generations from S. demissum. I t is imnmne from the common races of late blight and from several of the more virulent races. B 355-24 is imnmne from the common races.

From the cross B 922 (T15 x B 355-24) 337 seedlings were grown. Of these 23 were selected as having commercial possibilities and tested for their reaction to late blight in the field. Eleven of these showed an imnmne reaction. Five of the latter escaped infection when inoculated with 6 virulent cultures at Beltsville, Maryland.

The five selections that showed inmmnity in the tests at Beltsville were sent to Cornell University, where L. C. Peterson tested them with his races B, C, D, BC, and BD. Three selections showed an immune reaction to B, C, D, and BD but not to BC. The other two were free from infection with all 5 races. With the parent material we have on hand we have produced thousands of seedlings apparently imnmne from the common races of late bilght.

In 1951. 15 crosses and 2 selfed lines were inoculated in the seedling stage with the zoospores of the common races in the greenhouse at Presque Isle, Maine. The data from these tests are given in table 1.

Of the seedlings of B 355-24 selfed 96 per cent showed an immune reaction, but only 38 per cent of those of B 922-3 selfed escaped infection in spite of the fact that B 922-3 is a selection from the cross T15 x B 355-24 and is immune from several of the virulent races. Eighty-four per cent of the seedlings from the backcross B 355-24 x B 922-3 did not become infected and were considered immune.

Of the seedlings of the cross Aquila x Kennebec 81 per cent were imnmne. Aquila is a German variety related to Muller's blight-resistant H7 varieties and Kennebec inherited its resistance from the same sources. A genetic hypothesis to explain these results has not been attempted, but the data do show that immunity from the common races is not difficult to obtain. Immunity seems to behave as a dominant.

The recent work of breeding for resistance to the more virulent races of the late blight organism is carried on in the greenhouses of the Plant Industry Station, Beltsville, Maryland. During September and October, 1952, the progeny of 60 crosses and 18 selfed lines were screened for resistance to late blight by the junior author and L. C. Cash. The parents of these crosses and selfed lines were obtained from different sources. Some were produced at Beltsville, and others were obtained from W. R. Mills.

1954] A K E L E Y AND B U C K " BREEDING FOR L A T E B L I G H T R E S I S T A N C E 169

TA/3LE 1 . - -Bl igh t reactions of crosses and selfed lines of potatoes inoculated in the seedling stage with the common races of the late blight fungus, x

Seedlings Parentage Inoculated Seedlings Resistant

Essex x B 505-44 ............................ Essex x B 606-3 ................................ Essex x B 754-16 .............................. B 355-24 x B 721-1 ......................... B 355-24 x B 1919-15 .................... B355-24 x B 922-3 ........................

B 341 x B 919-15 ............................ B 778-43 x B 922-3 ......................... B 922-4 x B 606-3 ........................... B 922-12 x B 919-15 ...................... B 931-2 x B 922-4 B 2135-28 x B 922-5iiiiii~iiiiiiiiiiiiiiiii Aquila x Kennebec ............................ B 920-7 x B 922-5 .............................. B 922-5 x B 725-37 ........................... B 355-24 selfed .................................. B 922-3 selfed ...................................

No.

763 458 147 333 866 766 256 417 200 105 220

1,435 I51 30 53

38O 228

No. Per cent 267 35 294 64 77 52

169 51 570 66 646 84 201 79 140 34 158 79 11 10

146 66 1,352 94

122 81 29 97 33 62

366 96 86 38

1The inoculations were made in a moist chamber in the greenhouse at Presque Isle, Maine, in 1951.

State College, Pennsylvania , L. C. Peterson, Cornell Univers i ty , Ithaca, New York, Wil l iam Black, P lant Breeding Station, Corstorphine, Scotland, and Wm. Rudorf, Max-P lanck Insti tute, Voldagsen uber Elze, Germany.

Seedlings were inoculated with a mixture of zoospores from five isolates of the late blight fungus made by E. S. Schultz from resistant varieties growing in the fields in Maine. The seedlings were inoculated in the seed flats at an early stage of development. The flats were covered with glass and the humidi ty was kept at a high level by frequent spraying with water from a fine nozzle. Seedlings upon which late blight developed were counted and discarded. Seedlings free from blight were considered resistant, potted, and grown to matur i ty in 3-inch pots.

A total of 23,446 seedlings from 60 crosses and 3,951 from 18 selfed lines was inoculated. F r o m the crosses 4,414 seedlings, or 18.9 per cent, and from the selfed lines 1,151 seedlings, or 29.9 per cent, were resistant.

As an i l lustration of the results obtained the data for a few of the crosses and selfed lines are given in table 2.

Kennebec is resistant to the common races and B 922-3 is resistant to several of the virulent races. W h e n these two were crossed only 13.9 per cent of the seedlings were resistant (tab!e 2) . However, when B 922-3 was crossed with others that are also resistant to some of the virulent races, the percentage of resistant seedlings was considerably higher, ranging from 34.2 per cent to 51.1 per cent.

B 922-6 is immune from the common races and several of the virulent races but when it was crossed with the very susceptible, Teton, only 0.4 per cent of the seedlings escaped infection. However, when it was crossed with B 2968-31, which is resistant to the common races, 41.9 per cent of the seedlings were resistant to the mix ture of common and virulent races.

170 AMERICAN POTATO JOURNAL [Vol . 31

TABLE 2.--Results of inoculatin 9 seedlings of potato crosses and selfed lines with a mixture of common and virulent races of the late

blight fungus, in flats, BeItsville, Maryland, 1952.

Parentage Seedlings Inoculated

I

I No. Kennebec (R1) x B 922-3 (R2) ........ I 3 NC-9 (R~) x B 922-3 (R=) ............ Ac 25949 (R~) x B 922-3 (R=) .......... Ac 25956 (R.2) x B 922-3 (R=) .......... B 922-6 (R2) x Teton (S) .................. B 922-6 (R2) x B 2968-31 ( R 1 ) . . . . . . . . . .

3 VW-9 (R~) x B 2968-31 (R,) ........ S. demissum (R~) x B 983-9 (R,) _.. B 922-3 (R2) selfed ............................... B 922-6 (R2) selfed ........................... Ac 25949 (R2) selfed ............................ ' Ac 25956 (R2) setfed ............................ / 3 VW-9 (R=) selfed ..............................

301 231 231 152 576 460 710 85

189 29

240 95

231

Seedlings Resistant

No. Per cent 42 13.9

118 51.1 79 34.2 73 48,0 23 0.4

193 41.9 280 39.4 80 94.1 55 29.1 28 96.5

148 61.7 82 86.3

145 62.8

R 1 = Resistant to the common races. R 2 = Resistant to some of the virulent races. S --Susceptible to a!l races.

A first generat ion hybrid between S. demissum and B 983-9 gave the percentage of resistant seedlings found in the crosses, 94.1.

The five selfed lines, the parents of which are resistant to some of the virulent races, produced resistant seedlings ranging from 29.1 per cent for B 922-3 to 96.5 per cent for B 922-6.

The results in these tests are promising, but they are by no means final. The tubers of most of the resistant seedlings of the crosses and selfed

lines were sent to John S. Niederhauser, the Rockefeller Foundat ion , Mexico City, Mexico. who gave a report on these seedlings later in the symposium, but according to a pre l iminary report received from him they did not show much promise in his tests.

F r o m reports received it is evident that some of the races found in Mexico are more virulent than those found thus far in the Uni ted States.

The new races have given breeding for resistance to late blight a set-back, but progress has been made despite many complications. This is evident in our greater knowledge of the heri tance of blight immuni ty and in the information that we have on physiologic specialization in the causal fungus.

W e have released to growers a number of varieties with an inter- mediate degree of resistance and others apparent ly immune from the common races, such as Cherokee, Pungo and Kennebec. These varieties may be grown in Maine for years before the newer races build up to the point where regular fungicidal t reatments are needed. Cherokee, Pungo and Kennebec have been tested in Maine for 13-years and no changes in their reactions to the late blight races prevalent in Maine have been noted.

Many tests and observations to this effect have been made. As an i l lustration one of these tests will be given in detail.

The year 1951 was very favorable for late blight in Maine and fields

1954] A K E L E Y AND B U C K : BREEDING FOR LATE B L I G H T R E S I S T A N C E 171

of susceptible varieties were severely injured despite attempts to control the disease with fungicides. That year six varieties, Katahdin, Sebago, Green Mountain, Cherokee, Pungo, and Kennebec, were tested to determine their reactions to late blight, with or without sprays.

A split block 4 x 4 latin-square design was used with four treatments: tri-basic copper, tri-basic copper -I- DDT, DDT, and no spray. The Teton variety unsprayed was interspaced in paired rows throughout the entire plot after every third row of the varieties being tested. This helped to prevent spray drift and wheel damage to the vines and provided a susceptible host throughout the plot on which a late blight epidemic could be built up. Inoculation was made on July 16 by placing potted Green Mountain that had been heavily infected with late blight between the paired rows of Teton. The weather was favorable and heavy infection was soon present on the susceptible Teton and other susceptible varieties in the unsprayed replications. The spray treatments were applied 11 times at weekly intervals during the season. The yields of U. S. No. 1 tubers are given in table 3.

On Katahdin, Sebago, and Green Mountain tri-basic copper -1- D D T produced significant increases in yields per acre above those of plots sprayed with D D T alone. Tri-basic copper alone increased yields of Green Mountain over the unswayed plots, but with Katahdin and Sebago the increases were not significant. On Cherokee, Pungo and Kennebec, how- ever, the yields of the plots sprayed with tri-basic copper + D D T were not significantly different from those of the plots receiving D D T alone; also the plots of those varieties sprayed with tri-basic copper did not yield significantly more than the unsprayed plots. This indicates that when a severe epidemic is caused by the races of the blight prevalent in Maine, Cherokee, Pungo and Kennebec produce as high yields without the application of a fungicide as they do when one is applied. However, an insecticide is beneficial on all varieties.

TABLE &--Yields of six varieties of potatoes given different treatments in a field artificially infected with late blight, Maine, 1951.

Yield of U. S. No. 1 Tubers per Acre

Spray Treatment Katahdin

Bus.

Tri-basic copper .. 516 Tri-basic copper + DDT.. 556

DDT ................... 453 None .................. 418

Mean 486

Sebag____2_~176 2' Bus. Bus.

483 608

600 I 645 423 I 537 411 j 495

571

G r e e n [ Cher~ Pun ~ _ 2nnebec Bus. Bus. Bus. Bus.

623 618 618

Mean

BIlS.

558

644 585 532

587

L.S.D. at 5 per cent level for two varieties with same treatment = 88.1 bus. LS.D. at 5 per cent level for two treatments on same variety= 99.5 bus.

172 AMERICAN POTATO JOURNAL [Vol. 31

DISCUSSION AND CONCLUSION

Frequent epidemics of late blight occur in Maine in spite of the improved equipment and fungicides devised to control the disease. As recently as 1951, heavy losses from late blight occurred in that state. Rains interfered with spray operations at times when they should have been performed. The results were that fields of susceptible varieties, such as Irish Cobbler and Green Mountain, were killed by blight before they produced maximum yields and there was an additional loss from blight rot in the tubers in storage. These losses contributed to the shortage of potatoes in the spring of 1952.

Under the conditions that prevailed in 1951 the value of blight- resistant varieties was clearly demonstrated since Kennebec was grown on hundreds of acres and there were few authentic reports of injury to this variety.

The work of breeding for resistance to late blight as summarized in this paper shows continued progress from the susceptible varieties with which the work started; through the intermediate classes of resistance represented by Sebago, President, and Ackersegen; to the immunity from the common races characteristic of the /IV varieties; and to the immunity from some of the more virulent races found in seedling varieties closely related to S. demissum.

With our improved breeding material, our increased knowledge of the breeding behavior of the several characters involved, our greater knowledge of the causal fungus, and our improved technics, future progress should be much more rapid, and in the near future we should have varieties possessing at least the multiple genes for immunity found in S'. demissum combined with the most desirable characters of the best commercial varieties.

S E L E C T I V E A N D N O N - S E L E C T I V E B U Y I N G I N A C C E P T A N C E

T E S T S O F P O T A T O E S 1

F. A. KRANTZ 2

Acceptance studies on potatoes have been made to secure information on consumer preferences and aversions to certain qualities and character- istics. This information is useful to growers, processors, distributors, and those engaged in the improvement of production practices and varieties, for one of their major objectives is to supply potatoes satisfactory to the

Accepted for publication November 11, 1953. 1Paper No. 3066 of the Scientific Journal Series, Minnesota Agricultural Experiment

Station. 2Department of Horticulture, University Farm, St. Paul 1, Minn.