UNIVERSITY OF CALIFORNIA PUBLICATIONS COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA FURTHER EXPERIMENTS IN PLUM POLLINATION BY A. H. HENDRICKSON A tent of white mosquito bar was built over a pair of adjoining Formosa and Wiekson plum trees. A hive of bees was enclosed in the tent for six days during the blossoming season to test the value of the honey bee as a pollen distributor. BULLETIN No. 352 December, 1922 UNIVERSITY OF CALIFORNIA PRESS BERKELEY, CALIFORNIA 1922 David P. Barrows, President of the University. EXPERIMENT STATION STAFF HEADS OF DIVISIONS Thomas Forsyth Hunt, Dean. Edward J. Wickson, Horticulture (Emeritus). , Director of Resident Instruction. C. M. Haring, Veterinary Science, Director of Agricultural Experiment Station. B. H. Crocheron, Director of Agricultural Extension. C. B. Hutchison, Plant Breeding, Director of the Branch of the College of Agriculture at Davis. H. J. Webber, Sub-tropical Horticulture, Director of Citrus Experiment Station. William A. Setchell, Botany. Myer E. Jaffa, Nutrition. Ralph E. Smith, Plant Pathology. John W. Gilmore, Agronomy. Charles F. Shaw, Soil Technology. John W. Gregg, Landscape Gardening and Floriculture. Frederic T. Bioletti, Viticulture and Fruit Products. Warren T. Clarke, Agricultural Extension. Ernest B. Babcock, Genetics. Gordon H. True, Animal Husbandry. Walter Mulford, Forestry. James T. Barrett, Plant Pathology. Fritz W. Woll, Animal Nutrition. W. P. Kelley, Agricultural Chemistry. H. J. Quayle, Entomology Elwood Mead, Rural Institutions. H. S. Reed, Plant Physiology. L. D. Batchelor, Orchard Management. W. L. Howard, Pomology. *Frank Adams, Irrigation Investigations. C. L. Roadhouse, Dairy Industry. R. L. Adams, Farm Management. W. B. Herms, Entomology and Parasitology. John E. Dougherty, Poultry Husbandry. D. R. Hoagland, Plant Nutrition. G. H. Hart, Veterinary Science. L. J. Fletcher, Agricultural Engineering. Edwin C. Voorhies, Assistant to the Dean. DIVISION OF POMOLOGY W. L. Howard G. L. Philp W. P. Tufts L. C. Barnard E. L. Overholser W. P. Duruz A. H. Hendrickson C. L. Austin F. W. Allen M. J. Heppner J. P. Bennett L. H. Day * In cooperation with Division of Agricultural Engineering, Bureau of Public Roads, U. S. Department of Agriculture. FURTHER EXPERIMENTS IN PLUM POLLINATION By A. H. HENDRICKSON A previous report* of the experiments on the pollination of certain shipping varieties of plums contained data on a few of the older kinds of shipping plums and on the important varieties of prunes. The most successful combinations tested at that time were given, and the importance of the common honey bee as a distributor of pollen in prune orchards was demonstrated. The present report gives the results obtained from three years 7 work at Vacaville, Solano County, and near Newcastle, Placer County, on other varieties, many of which have recently become prominent. It also further emphasizes the value of honey bees in orchards of shipping plums. OUTLINE OF EXPERIMENTS Essential details of the technique of the methods used were described in the previous report.* The work was undertaken chiefly to obtain data on the effectiveness of certain combinations of varieties of plums in producing satisfactory crops. Hence, if the fruit remained on the tree to maturity, the cross was considered successful whether a viable seed was produced or not. Varieties able to set and mature fruit when pollinated with their own pollen are considered in this discussion as self -fertile. If unable to set fruit with their own pollen, they are considered as self -sterile. Two varieties which cannot be crossed one upon the other so as to produce fruit are considered as inter-sterile. The work in 1920 was carried on in the orchards of Mr. H. B. Naylor and Mr. Millard Sharpe of Vacaville, California. The former orchard is located on a hillside several hundred feet above the valley floor. The latter is in the valley proper, in the center of a large fruit- growing area. In both cases the varieties of plums used were, for the most part, topworked on old apricot trees. In 1921 and in 1922 the work was conducted on what was known as the Cook ranch, owned by Mr. J. F. Dudley, about two miles south of Newcastle in Placer County. In the Cook orchard, situated at an elevation of about 600 feet in the Sierra foothills, the soil is a decomposed granite, typical * Hendrickson, A. H., Plum Pollination. Calif. Agr. Exp. Sta. Bull. 310, July, 1919. 248 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION of a large area in that district. The trees were of various ages and were practically all on peach root. Many had previously been top- worked at least once when the first graft proved unsatisfactory. The Vacaville orchards were not irrigated. In Placer County the trees were irrigated in shallow furrows about every week or ten days during the growing season. In both districts, during the years the experi- ments were carried on, the trees for the most part blossomed heavily, but the resulting set of fruit was small. Weather conditions. — The weather conditions in 1920 were such as to bring about a long blossoming season. During the last half of February, a succession of cold cloudy days occurred which held back the later varieties. The first variety to bloom was Formosa, which began to open February 11. The last variety was President, upon which the last pollinations were made on March 19. At Newcastle, in 1921, the season was propitious for a heavy set of fruit. The polli- nation season commenced with the Formosa variety on February 26 and ended March 9 with the California Blue. During the entire season the weather was, on the average, fairly warm. There were a few foggy days, however, with a light shower on March 5. In 1922 the season was the latest that had been known for many years. The Wickson, which ordinarily blossoms the last week in February, did not open until March 10. The California Blue, which blossomed in 1921 on March 9, did not open until April 5. Throughout the season the weather was cold and rainy. Three inches of snow fell during the night of March 10. The month of April was marked by a long period of severe, drying northwest winds. In general, the weather was too cold and wet for the bees to work upon the flowers and these insects remained in the hive for the greater part of the time. It is also prob- able that the unseasonably low temperatures affected the pollen, which was scanty in many varieties and showed, on the average, poor germinating power. PRESENTATION OF DATA Germination of pollen. — The pollen used was gathered from well developed buds a day or two before it was evident that they would open. It was carefully dried and kept in loosely stoppered glass vials until used. In all cases the pollen was used as soon after drying as possible. Likewise the blossoms were pollinated within a day or two after emasculation. Blossoms which had been emasculated four or five days were discarded in favor of those more recently prepared for pollination. All pollen used was tested for viability. The percentage BULLETIN 352 ] FURTHER EXPERIMENTS IN PLUM POLLINATION 249 of germination seemed to be closely related to the general character of the weather at and slightly before the blossoming season. During 1920 and 1921, when the weather was generally warm and clear while the trees were in blossom, the pollen from the different varieties showed a high percentage of germination. The character of the pollen used these two seasons was reflected in the excellent results secured from many combinations. On the other hand, the 1922 season was very cold and rainy, and the pollen was scanty and showed poor germinating power. The resulting sets in many cases were not so high as similar crosses made the two previous seasons. Normal set of plums. — "Normal set" is the term used to designate the percentage of flowers which set and mature fruit when pollinated by chance, i.e., by insects or wind. It is that set produced naturally under open orchard conditions without artificial assistance. The normal set of a variety is used as the basis for judging the results of hand pollination. It also gives an idea as to the relative numbers of pollen-carrying insects present, as it has been repeatedly observed that the percentage of set is higher when these insects are plentiful than when they are scarce. In cold or rainy seasons, however, as was the case in 1922, there may be an abundance of bees present without a corresponding increase in the crop because the bees are not active under these conditions. The normal sets for the varieties tested during 1920, 1921, and 1922 are given in table 1. The percentage of fruit set as indicated under the columns "Per- centage matured" does not tell the whole story. A small percentage of set on varieties that have a profuse bloom often produces a heavier crop than a large percentage of set on varieties having a small number of blossoms. For example, in 1920 the Apex (often called a plumcot) with a somewhat scattering bloom and a 7.5 per cent set nevertheless matured a medium crop of fruit. Beauty, with 15.5 per cent set in 1920 at Vacaville, required heavy thinning. In 1921, with 1.6 per cent set, the crop was fair, but in 1922, with 2.4 per cent set, it was very light, due to the smaller number of blossoms produced on each tree. The Burbank crop in 1922, as a result of an average set of 6.3 per cent, required heavy thinning. This variety usually blossoms profusely and a satisfactory crop may be obtained from a comparatively light set. Duarte produced a fair crop with 4.0 per cent set in 1921, but only a light one in 1922 when the set dropped to 1.2 per cent. El Dorado produced a light crop in 1922 as a result of a 2.3 per cent set. 250 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION Formosa, as a rule, blossomed rather lightly and set erratically. In three years' trial it was noticed that a set of more than 5 per cent gave a satisfactory commercial crop, while if the set were below that figure the trees did not carry as many plums as they were capable of TABLE 1 Normal Set of Fruit on Plum Varieties at Vacaville in 1920 and at Newcastle in 1921 and 1922 1920 1921 1922 Variety No. of blos- soms used No. of fruits matured Per- centage matured No. of blos- soms used No. of fruits matured Per- centage matured No. of blos- soms used No. of fruits matured Per- centage matured Apex 2311 2085 175 323 7.5 15.5 Beauty 1652 27 1.6 1552 1882 2617 567 1982 2056 2030 1926 38 119 32 13 178 83 53 59 2.4 Burbank 6.3 Duarte ... 2227 89 4.0 1.2 El Dorado 2.3 Formosa 1997 1742 2210 8 128 191 0.4 7.3 8.6 1697 2486 1114 13 59 13 0.8 5.4 1.2 9.0 Formosa 4.0 Gaviota 2.6 Kelsey 3.1 Methley 2268 2063 915 2599 2101 2198 709 110 10 135 215 98 31.0 5.3 1.1 5.2 10.2 4.1 Prize Santa Rosa 1269 2162 69 52 5.4 2.4 2047 64 3.1 Santa Rosa Upright Wickson 2283 57 2.5 California Blue 878 2002 287 31 32.7 1.6 1311 1558 847 2310 343 10 186 136 26.0 Diamond 0.6 1468 986 1440 427 72 68 29.1 7.3 4.8 22.0 Grand Duke 1860 95 5.1 5.9 Pond (Gros) 1447 2258 173 48 11.9 President 1827 59 3.2 1884 1112 63 6 3.3 0.5 2.1 Standard Tragedy 1580 2077 127 18 8.1 0.9 1751 3 0.2 2110 t 7 0.3 maturing. The crop resulting from a 9.0 per cent set in 1922 was the largest which had been obtained on this variety for many years. Gaviota, with 8.6 per cent set, produced an excellent crop at Vacaville in 1920, but in the two following seasons at Newcastle, sets of 1.2 per cent and 2.1 per cent, respectively, produced only light crops. Kelsey bore a fairly heavy crop with 3.1 per cent. Methley was tested only BULLETIN 352 ] FURTHER EXPERIMENTS IN PLUM POLLINATION 251 one year and set 31.0 per cent. Observations have further shown this variety to be a heavy and regular producer. Prize produced a satis- factory crop with a set of 5.3 per cent. Santa Rosa in three years' trials was found to require a set of at least 5 per cent to produce a profitable crop. This variety is inclined to drop heavily before ripen- ing, so that an apparently heavy set early in the season does not give as high a yield as expected. Upright produced an excellent crop with Fig. 1. — a. The California Blue is self- fertile and sets fruit abundantly with its own pollen, b. A typical cluster of Formosa plums resulting from crossing with Wickson. a 10.2 per cent set. Wickson bore a satisfactory crop with a 4.1 per cent set in 1920, but a 2.5 per cent in 1922 did not produce as heavy a crop as the trees were capable of maturing. In the lower part of the table where the European kinds are listed, California Blue is shown to be an excellent bearer. Observation of this variety in other orchards further substantiated these data. In 1921 and in 1922 the bloom of Diamond was fairly profuse, but the percentages of set were low. The resulting crops hardly paid for har- vesting. Giant, in the two years ' trials, gave a high percentage of set each time. This variety, like the California Blue, has a reputation for heavy bearing. Imperial in one year's test gave a fair crop with a 252 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION set of 4.8 per cent, and Pond (Gros prune) produced a heavy crop with a set of 11.9 per cent. For three years the set obtained on President averaged a trifle less than 3 per cent. The crops from tnis percentage of set were fair, but not so heavy as the trees could have matured. Quackenboss set only 0.5 per cent, which gave a very light crop. Standard in one year's trial gave a fair yield with a set of 8.1 per cent. Although Tragedy blossomed heavily, the average set was less than one per cent in each of the three years the experiments were conducted. This variety has long been considered a shy bearer in many sections of California. Results of self-pollination. — Profitable production of plums is closely correlated with the self -sterility or self -fertility of the variety in question. Results during the past three years as well as those reported of previous years show decisively that most Japanese and many European plums are self -sterile and need cross-pollination. The need for cross-pollination is indicated in table 2. A study of table 2 shows that three Japanese plums, Beauty, Methley, and Santa Rosa, of the eleven varieties tested were capable of setting a limited amount of fruit in certain years with their own pollen. To this list may be added Climax, which was previously reported as being self -fertile. It must not be inferred, however, that it is eminently satisfactory to plant even these varieties in large blocks of one kind. More regular crops may be expected when they are interplanted with other varieties of the same species. Some of the European plums are distinctly self -sterile, some are self -fertile, and the remainder, although able to set and mature a limited amount of fruit when self -pollinated, are greatly benefited by cross-pollination. The Imperial, President, Quackenboss, Standard, Tragedy, and Washington gave distinct evidence of being self -sterile. Although usually considered self -sterile, Pond (Hungarian or Gros prune) was able to set a small crop with its own pollen in 1922. Grand Duke, in two years out of three, showed that it was able to set a small percentage of blossoms with its own pollen. On the other hand, California Blue (fig. 1, a), Giant, and Yellow Egg produced abundant crops with their own pollen. The experimental data obtained with these three varieties were substantiated by the obser- vation of many growers, who have noted that they are consistently heavy bearers. The status of the Diamond, which is one of the important blue plums, was not definitely settled. In 1920 the set from self-pollination was 15.8 per cent ; while in 1921 and in 1922 at Newcastle the evidence pointed to distinct self -sterility. In view of BULLETIN 352 ] FURTHER EXPERIMENTS IN PLUM POLLINATION 253 the fact that this variety has a reputation for shy bearing, the safest course, probably, is to plant it with other mid-season kinds, as dis- cussed later. TABLE 2 Self-Pollination of Plum Varieties at Vacaville in 1920 and at Newcastle in 1921 and 1922 1920 1921 1922 Variety No. of blos- soms used No. of fruits matured Per- centage matured No. of blos- soms used No. of fruits matured Per- centage matured No. of blos- soms used No. of fruits matured Per- centage matured Apex 326 234 Beauty 371 471 196 427 345 17 1 4.6 0.2 258 324 8 3 1 Duarte.. El Dorado Formosa 861 432 308 280 173 Gaviota Kelsey Methley 370 300 478 317 33 4 8.9 0.8 Prize Santa Rosa 458 30 6.5 261 3 1.2 Upright California Blue 328 612 53 1 16.2 0.2 274 310 139 427 37 15 13.5 Diamond 310 291 470 231 259 355 49 9 7 15.8 3.1 1.5 Giant 10.8 Grand Duke 315 4 1.3 Imperial .« Pond (Gros) 50 185 109 2 4.0 President 524 556 0.2 Quackenboss 330 359 411 219 1 25 0.3 11.4 Tragedy 202 216 Washington Yellow Egg Results of cross-pollination. — On the whole, the results of the crosses were decisive. This was particularly true in the 1921 season, while at Vacaville, the preceding season, there was doubt as to the effectiveness of a few combinations. From the number of crosses with each variety during the three seasons, it is a comparatively simple matter to choose combinations that will cross-pollinate effectively. The results follow in table 3. The general results obtained are so evident from an examination of the table that it seems unnecessary to discuss each cross in detail. It is important, however, to point out certain features regarding 254 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION several varieties about which there has been much complaint, and also to discuss some cases in which the results of the experiments were not satisfactory. Complaint regarding Japanese varieties has centered chiefly on the Formosa and the Gaviota. The three years' work showed that the Formosa may be effectively pollinated by Beauty, Burbank, Santa Rosa, and Wickson (fig. 1, 6). The Gaviota was most Fig. 2. — a. A cluster of Gaviota plums resulting from crossing with the Beauty. The Gaviota, however, could not be pollinated with the Formosa, b. Eight Tragedy plums set from thirty-two blossoms pollinated with Grand Duke pollen. The blossoming seasons of these two varieties usually overlap to a sufficient extent to make this combination successful. effectively crossed by Beauty (fig. 2, a), Burbank, Duarte, and Wick- son. Among other important early plums, both Beauty and Santa Rosa were successfully pollinated by several Japanese varieties and also by Tragedy. The success or failure of the crosses for the remain- ing varieties is easily determined from the column in table 3 entitled "Percentage matured.' ' BULLETIN 352 ] FURTHER EXPERIMENTS IN PLUM POLLINATION 255 TABLE 3 Cross-Pollination of Plum Varieties at Vacaville in 1920 and at Newcastle in 1921 and 1922 1920 1921 1922 Cross No. of blos- soms used No. of fruits matured Per- centage matured No. of blos- soms used No. of fruits matured Per- centage matured No. of blos- soms used No. of fruits matured Per- centage matured Apex X Beauty 347 346 269 351 389 392 180 284 2 3 1 20 64 0.6 0.8 0.3 11.0 22.5 X Burbank X Formosa X Gaviota X Santa Rosa X Tragedy Beauty X Apex X Burbank 335 67 20.0 X Duarte 203 266 258 221 373 290 300 6 7 9 6 6 11 3.0 X El Dorado 2.6 X Formosa 321 300 300 379 308 332 39 22 7 63 23 7 12.1 7.3 2.3 16.6 7.5 2.1 297 349 371 352 301 51 48 13 43 65 17.2 13.8 3.5 12.2 21.6 X Gaviota 4.1 X Santa Rosa 1.6 X Tragedy 2.1 X Wickson 3.6 Burbank X Beauty X Duarte 445 343 226 206 15 5 8 11 3.4 X Formosa 1.4 X Gaviota 3.5 X Kelsey 5.3 X Methley 247 1 .4 X Santa Rosa 430 162 37.7 X Tragedy 327 12 3.7 Duarte X Beauty 189 180 148 24 5 9 12.7 .8 6.1 323 313 281 228 297 289 298 332 232 17 1 39 20 5.3 X Burbank X El Dorado X Formosa 0.3 X Gaviota 19 33 16.6 X Kelsey X Santa Rosa 13.1 X Wickson 6.0 X Tragedy 256 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION TABLE 3— (Continued) 1920 1921 1922 Cross No. of blos- soms used No. of fruits matured Per- centage matured No. of blos- soms used No. of fruits matured Per- centage matured No. of blos- soms used No. of fruits matured Per- centage matured El Dorado X Burbank 113 117 127 106 4 12 10 3.5 X Formosa X Santa Rosa 9.4 X Wickson 9.5 Formosa X Apex 319 291 19 20 6.0 6.9 X Beauty 474 183 321 25 33 17 5.3' 18.0 5.3 97 336 349 ....284 11 4 2 17 X Burbank 3.3 X Duarte 1.1 X El Dorado X Gaviota 321 441 1 48 0.3 10.9 312 524 400 49 14 9.3 3.5 318 332 314 X Santa Rosa 0.6 X Tragedy 5.4 X Upright 186 325 315 320 6 10 3.2 3.1 X Wickson Gaviota X Apex X Beauty X Burbank 369 15 4.1 336 22 6.6 212 286 249 253 328 293 256 26 6 1 12 21 12.3 X Duarte 2.4 X El Dorado 0.4 X Formosa 218 187 4 4 1.8 2.1 436 X Santa Rosa 4.1 X Tragedy 349 9 2.6 8.2 X Upright 272 317 16 18 5.9 5.7 X Wickson 303 25 8.3 235 202 200 197 71 206 114 10 11 5 2 5 3 2 4.3 Kelsey X Beauty 5.5 X Duarte 2.5 X El Dorado . 1.0 X Formosa X Gaviota 2.4 X Santa Rosa 1.4 x Trasredv 1.7 BULLETIN 352 ] FURTHER EXPERIMENTS IN PLUM POLLINATION 257 TABLE 3— (Continued) 1920 1921 1922 Cross No. of blos- soms used No. of fruits matured Per- centage matured No. of blos- soms used No. of fruits matured Per- centage matured No. of blos- soms used No. of fruits matured Per- centage matured Methley X Apex 285 149 34S 395 379 311 352 43 19 84 93 128 15.0 12.7 24.0 23.6 33.8 X Beautv X Burbank X Tragedy X Wickson Santa Rosa X Apex X Beauty 192 16 8.4 192 333 332 159 302 264 232 2 4 X Burbank 6 X Duarte X El Dorado X Formosa 943 331 7 11 0.8 3.3 375 24 6.4 1.3 X Gaviota X Tragedy 254 6 2.4 X Upright 325 271 292 290 307 295 291 2 3 7 24 2 2 2 0.6 1.1 2.4 8.3 0.7 0.7 0.7 X Wickson 271 30 11.1 308 4 1.3 Upright X Formosa X Gaviota X Santa Rosa . . X Wickson Wickson X Beauty 103 215 336 265 353 4 1 4 15 25 3.9 X Duarte 0.5 X Formosa 317 308 307 350 293 14 17 6 42 3 4.4 5.5 1.9 12.0 1.0 1.2 X Gaviota 5.7 X Santa Rosa 7.1 X Tragedy X Upright California Blue X Diamond ... 258 288 64 13 24.8 4.5 X Gr. Duke X Pond (Gros) 142 4 2.8 X President 176 86 21 12 11.9 13.9 X Quackenboss 258 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION TABLE Z—(Co?itinued) 1920 1921 1922 Cross No. of blos- soms used No. of fruits matured Per- centage matured No. of blos- soms used No. of fruits matured Per- centage matured No. of blos- soms used No. of fruits matured Per- centage matured Diamond X California Blue 239 356 81 118 33.9 33.1 X Grand Duke 298 305 223 49 15 18 16.5 4.9 8.1 339 30 8.8 X Imperial X President 383 466 60 188 15.7 40.4 328 405 315 158 8 84 55 2.4 X Quackenboss X Tragedy 26.7 Giant X California Blue 34.8 X Gr. Duke 133 226 310 8 2 27 6.1 0.9 8.7 X Pond (Gros) 96 257 5 51 5.2 X President... 19.8 Grand Duke X California Blue . 352 325 108 87 30.7 26.8 X Diamond X Giant 504 263 131 3 26.0 1.1 274 14 5.1 X President 411 303 82 67 19.9 22.1 363 268 34 3 9.4 X Quackenboss 1.1 X Standard 608 145 23.9 X Tragedy 318 75 23.6 302 111 255 36 6 28 11.9 Pond (Gros) X California Blue 14.4 X Giant 224 124 1 2 0.5 1.6 11.0 X Gr. Duke President X California Blue 292 483 109 129 37.3 26.7 X Diamond 295 323 291 323 23 18 7.1 5.6 333 121 217 101 433 461 1 4 3 4 15 0.3 X Giant 3.3 X Gr. Duke 350 82 23.4 1.4 X Pond (Gros) 4.0 X Quackenboss 457 112 24.5 X Tragedy 3.2 Quackenboss X California Blue 233 555 532 369 89 106 108 47 38.2 19.1 20.3 12.7 86 94 173 1 3 27 1.2 X Gr. Duke 3.2 X President.... 15.6 Bulletin 352 ] further experiments in plum pollination 259 TABLE 3— (Concluded) 1920 1921 1922 Cross 'No. of blos- soms used 331 320 334 363 389 372 No. of fruits matured Per- centage matured No. of blos- soms used No. of fruits matured Per- centage matured No. of blos- soms used No. of fruits matured Per- centage matured Standard X Diamond 35 49 13 10.6 15.3 3.9 X Gr. Duke X Imperial Tragedy X Apex X Beauty 298 271 271 225 357 301 306 247 355 264 24 113 X Burbank X Diamond 6.7 X Duarte X El Dorado X Formosa 337 323 171 53.0 X Gr. Duke 517 12 2.3 31.8 X Kelsey X Methley 344 X Santa Rosa 291 298 207 258 X Wickson 366 1 0.3 It is also important to note some of the questionable combinations. Apex in 1920 proved to be an uncertain pollinator for Formosa and Santa Rosa and, in turn, was ineffectively pollinated by them. El Dorado, Formosa, Gaviota, and Kelsey, in general, were scanty pollen producers and were therefore not reliable pollinators for other Japanese varieties. On the other hand, Beauty, Burbank, Duarte, Methley, Santa Rosa, and Wickson produced a large amount of viable pollen and have proved to be excellent pollinators. One outstanding result was the apparent inter-sterility of Formosa and Gaviota which during three years' trials set very poorly when crossed. This was the first case of inter-sterility found among Japanese plums. Tragedy, for three years, was found to pollinate effectively the different Japanese varieties. When the reciprocal cross was tried, failure resulted in every case : the Japanese varieties were unable to cause the Tragedy to set fruit. From a practical standpoint this fact is unfortunate, as Tragedy is self -sterile and usually blossoms too soon to be pollinated by the early blossoming European plums. The inability of the Japanese varieties to pollinate Tragedy may be due 260 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION to the length of the pistil of the latter. According to Knight,* lack of fertilization is sometimes due to the fact that the pollen tube does not grow down the style and fertilize the ovule before the latter is beyond the receptive stage. Evidently Tragedy pollen grows down the comparatively short pistils of the Japanese sorts quickly enough to fertilize the ovules before they begin to disintegrate, but in the Fig. 3. — a. A typical cluster of Diamond plums resulting from crossing with the Tragedy. Satisfactory results were also obtained by crossing the Diamond with the Grand Duke. b. Typical branch from the Beauty tree inclosed with a hive of bees in a mosquito-bar tent with a Santa Rosa tree. The stems without fruits indicate where plums which set originally were removed in thinning. reciprocal cross the growth may be so slow that fertilization does not take place. Tragedy, however, was very effectively pollinated by both Grand Duke (fig. 2, b) and Diamond, although in some seasons it was difficult to get this combination, due to the differences in blossoming season. In the Placer County district, Diamond has long been known as a shy bearer, but on account of its excellent shipping qualities growers were loath to graft it over. From the data it can readily be seen that * Physiological Aspects of the Self-Sterility of the Apple. Hort. Sci., 1917, p. 101. Proc. Am. Soc. BULLETIN 352] FURTHER EXPERIMENTS IN PLUM POLLINATION 261 this variety can be made to produce when cross-pollinated by any one of several mid-season European plums. The best results were obtained with Grand Duke, President, California Blue, and Tragedy (fig. 3, a). As many combinations of European varieties were tried each year as possible. In no case was there any evidence of inter-sterility, although in one or two instances light yields resulted from certain crosses in one of the three trials. For example, Quackenboss in 1921 was shown to cross effectively with several other mid -season plums, but the follow- ing year, the same combinations being used, the resulting sets were low. Observations over a period of three years showed that the European varieties reported in this bulletin produced pollen abund- antly. Each variety could be used to pollinate the others provided the blossoming of both occurred at approximately the same time. The results obtained with this type of plums as shown in the preceding table were marked. In addition to the data obtained in the experiment proper, the beneficial results of mixed plantings were clearly evident to even a casual observer. In the Cook orchard, both in 1921 and 1922, the set of fruit on different varieties was invariably best on the rows where two varieties adjoined. Many reports received from growers sub- stantiate these observations. The use of bees as pollen distributors. — At Newcastle, in 1921 and 1922, mosquito-bar tents (frontispiece) were erected over adjoining trees of different varieties in a manner similar to that used in previous experiments with prunes in the Santa Clara Valley.* In 1921 one pair of trees consisted of a Formosa and a Wickson, the other a Formosa and a Gaviota. A colony of bees was kept in each tent for six or seven days during the blossoming season. At first the bees tried to escape, but later went to work on the blossoms. The trees under the tents, as far as could be observed, blossomed and produced leaves in a normal manner. The only climatic differences within the tent that could be noticed as compared with conditions in the open orchard were a slightly reduced light intensity and a slower air movement. It is doubtful that these differences were great enough to influence the setting of the fruit. The results are given in table 4. In the case of the Formosa and Wickson trees the resulting set was so heavy as to be very noticeable even after thinning. The Formosa enclosed with bees and a Wickson tree matured 7.3 per cent, as compared with 0.8 per cent which was the average under open orchard conditions. The Wickson matured 11.2 per cent, which is a * The Common Honey Bee as an Agent in Prime Pollination. Calif. Agr. Exp. Sta. Bull. 291. 262 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION very heavy yield for that variety. Through an oversight, no counts were made on adjoining normal Wickson trees, so that no comparison can be made with the yield obtained hy normal pollination. Observa- tion showed, however, that the Wickson tree with bees to carry pollen from a Formosa, set the heaviest crop of all trees of that variety. Throughout the orchard the crop of Formosa was heavier on the row of trees adjoining the Wickson than in the interior of the block. TABLE 4 Behavior of Trees Enclosed under a Mosquito-Bar Tent with a Hive of Bees in 1921 Variety Formosa: Formosa: Wickson: Formosa: Gaviota: Gaviota: Average set in open orchard Enclosed with bees and a Wickson tree Enclosed with bees and a Formosa tree Enclosed with bees and a Gaviota tree. Enclosed with bees and a Formosa tree Average set in open orchard No. of blossoms used No. of fruits matured 1697 13 1646 120 1513 169 1148 2 1119 14 1114 13 Per- centage matured 0.8 7.3 11.2 0.2 1.3 1.2 In the experiment with Formosa and Gaviota trees no increase in crop was produced. The Formosa tree actually set a smaller per- centage of fruit than the average for the trees of this variety in the open. Together with the data obtained from hand pollination, the results on these trees showed that the Formosa and Gaviota are apparently inter-sterile or are at least unsafe for interplanting for pollination purposes. In 1922 a Beauty and a Santa Kosa tree were enclosed in one tent, and a Grand Duke and a Diamond in another. A hive of bees was kept in each tent for six or seven days during the blossoming season, as was done the preceding season. The results are given in table 5. The Beaut}^ (fig. 3, b) tree in this trial set a very heavy crop which had to be thinned severely. The percentage of set was approximately four times as high as the average percentage of set in the orchard exposed to chance pollination. The Santa Rosa tree in the tent failed to respond as expected, but the difference in percentage of set as compared with other Santa Rosa trees in the open was in favor of the tree in the tent. In the second tent, the percentages of set for both the enclosed Diamond (fig. 4) and the Grand Duke trees as compared with the orchard average showed conclusively the value of bees in distributing BULLETIN 352 ] FURTHER EXPERIMENTS IN PLUM POLLINATION 263 pollen from one variety to another. The light crops produced under open orchard conditions were undoubtedly due to lack of pollen distribution, as but few honey bees were present in the neighborhood and only rarely were they seen on the trees adjoining those used in the tent experiment. TABLE 5 Behavior of Trees Enclosed under a Mosquito-Bar Tent with a Hive of Bees in 1922 Variety- Beauty: Beauty: Santa Rosa: Santa Rosa: Diamond: Diamond: Grand Duke: Grand Duke: Average set in open orchard Enclosed with bees and a Santa Rosa tree . Average set in open orchard Enclosed with bees and a Beauty tree Average set in open orchard Enclosed with bees and a Grand Duke tree. Average set in open orchard Enclosed with bees and a Diamond tree No. of blossoms used No. of fruits matured 2297 87 2532 432 2273 17 2130 38 1759 17 1386 249 2310 136 1533 213 Per- centage matured 3.8 17.1 0.7 1.8 0.9 18.0 5.9 13.9 Blossoming dates. — The time of blossoming of practically all varieties given in this report showed a great deal of variation during the three years of observation. In 1922 the season was practically four weeks later than it was in 1921. The varieties opened in approxi- mately the same order both years. The 1920 season was different from the other two in that the first varieties blossomed earlier than usual but the last ones opened later than was expected. In other words, the season was exceptionally long, and distinct gaps occurred between many varieties that ordinarily overlap each other. Three years' observation has indicated that certain varieties of plums included in this report could be placed in groups blossoming closely enough together for cross-pollination purposes. The first group would contain the early blossoming Japanese varieties, which open in approximately the following order: Formosa, Santa Rosa, Upright, Wickson, Gaviota, and Beauty. The second group would include the Duarte, El Dorado, Kelsey, Apex, Methley, Prize, and Burbank. In all seasons there was considerable overlapping in the blossoming season of these varieties. It is therefore also possible to use the early members of the second group with the late members of the first. Such early sorts as Formosa or Santa Rosa, however, could not be consistently pollinated by such late bloomers as Methley or Burbank. 264 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION Undesirable combinations previously mentioned should be avoided, even though the varieties blossom together. Before planting the grower should consult table 3 to find out if the desired combinations have proved satisfactory experimentally. The blossoming season of the Tragedy closely coincides with several of the late Japanese varieties. It also usually extends into the blossoming periods- of both the Grand Duke and Diamond. The remainder of the European varieties may be placed in two fairly distinct groups with relation to pollination season. The first group would include Grank Duke, Diamond, Quackenboss, Standard. Im- perial, and President, and the late group California Blue, Pond (Gros prune), Giant, Yellow Egg, and Washington. In certain seasons there might be sufficient change in the order of blossoming to permit Pond and Giant to be pollinated by President or Standard, but in normal seasons those varieties blossoming later than President are not effectively pollinated by members of the early group. SUMMARY 1. In addition to varieties previously reported the following Japanese plums were found to be self -sterile : Apex, Duarte, El Dorado, Formosa, Gaviota, Prize, and Upright. 2. In three years' experiments, Formosa and Gaviota were shown to be inter-sterile. 3. Beauty, Methley, and Santa Rosa seemed to be self -fertile, at least to a limited degree. 4. Apex, El Dorado, Formosa, Gaviota, and Kelsey were generally found to be scanty pollen producers and therefore of questionable value as pollinators of other Japanese varieties. 5. Beauty, Burbank, Duarte, Santa Rosa, and Wickson seemed to be the most effective pollinators for the Japanese varieties. 6. Among the European varieties, Imperial and Tragedy were again shown to be self-sterile. Likewise President, Quackenboss, Standard, and Washington proved unable to set fruit with their own pollen. 7. Diamond set well with its own pollen one year, but failed to do so the other two. For practical purposes it must be considered self -sterile. 8. Pond (Hungarian or Gros prune) when pollinated with its own pollen did not set any fruit in 1920, but set a light crop in 1922. It is probably partially self -fertile. BULLETIN 352 ] FURTHER EXPERIMENTS IN PLUM POLLINATION 265 Fig. 4. — Characteristic branches of the Grand Duke and of the Diamond from trees inclosed in a mosquito-bar tent in which a hive of bees was kept during the blossoming season. 266 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 9. California Blue, Giant, and Yellow Egg were shown to be self -fertile. Grand Duke, contrary to results obtained at Davis where it was found to be self -sterile under certain conditions, was shown to be able to set a small percentage of fruit with its own pollen. 10. All European plums blossoming in midseason or later appeared to be able to cross-pollinate effectively. 11. Tragedy was able to pollinate the Japanese varieties but was not pollinated by them. 12. The presence of honey bees materially aided in setting heavy crops on the following combinations of varieties ; Formosa and Wick- son; Beauty and Santa Rosa; Diamond and Grand Duke. Observa- tions, furthermore, showed that many other combinations were also benefited by these insects. 13. Inter-pollination of Formosa and Gaviota by means of bees proved unsuccessful. The interplanting of these two varieties for cross-pollination purposes cannot be recommended. 14. The results showed that whenever a particular cross had been made for two or three successive years the percentage of set was usually lowest in 1922, a fact probably due to the prolonged period of cold rainy weather during the blossoming season of that year. ACKNOWLEDGMENTS The writer is indebted to Messrs. Millard Sharpe and H. B. Naylor,. of Vacaville, to Messrs. J. F. Dudley, E. Van Riper, and William Lower, of Newcastle, for the use of their orchards in conducting the experiments. Special thanks are due to Mr. Dudley for cooperation in furnishing material used in building the mosquito-bar tents. To Messrs. E. W. Everett, L. H. Henderson, and E. Van Riper, advanced students in Pomology, the writer conveys his thanks and appreciation for assistance in counting and pollinating the blossoms. STATION PUBLICATIONS AVAILABLE FOE FKEE DISTRIBUTION BULLETINS No. No. 253. Irrigation and Soil Conditions in the 319. Sierra Nevada Foothills, California. 321. 261. Melaxuma of the Walnut, "Juglans 324. regia." 262. Citrus Diseases of Florida and Cuba 325. Compared with those of California. 263. Size Grades for Ripe Olives. 268. Growing and Grafting Olive Seedlings. 328. 270. A Comparison of Annual Cropping, Bi- 330. ennial Cropping, and Green Manures 331. on the Yield of Wheat. 332. 273. Preliminary Report on Kearney Vine- 334. yard Experimental Drain. 275. The Cultivation of Belladonna in Cali- 335. fornia. 276. The Pomegranate. 336. 278. Grain Sorghums. 279. Irrigation of Rice in California. 337. 280. Irrigation of Alfalfa in the Sacramento 339. Valley. 283. The Olive Insects of California. 340. 285. The Milk Goat in California. 286. Commercial Fertilizers. 341. 287. Vineerar from Waste Fruits. 342. 294. Bean Culture in California. 343. 297. The Almond in California. 344. 298. Seedless Raisin Grapes. 299. The Use of Lumber on California Farms. 345. 304. A study on the Effects of Freezes on Citrus in California. 346. 308. I. Fumigation with Liquid Hydrocyanic 347. Acid. II. Physical and Chemical Prop- erties of Liquid Hydrocyanic Acid. 348. 310. Plum Pollination. 352. 312. Mariout Barley. 313. Pruning Young Deciduous Fruit Trees. 353. 316. The Kaki or Oriental Persimmon. 317. Selections of Stocks in Citrus Propa- gation. Caprifigs and Caprification. Commercial Production of Grape Syrup. Storage of Perishable Fruit at Freezing Temperatures. Rice Irrigation Measurements and Ex- periments in Sacramento Valley, 1914-1919. Prune Growing in California. Dehydration of Fruits. Phylloxera-Resistant Stocks. Walnut Culture in California. Preliminary Volume Tables for Second- Growth Redwoods. Cocoanut Meal as a Feed for Dairy Cows and Other Livestock. The Preparation of Nicotine Dust as an Insecticide. Some Factors of Dehydrater Efficiency. The Relative Cost of Making Logs from Small and Large Timber. Control of the Pocket Gopher in Cali- fornia. Studies on Irrigation of Citrus Groves. Hog Feeding Experiments. Cheese Pests and Their Control. Cold Storage as an Aid to the Market* ing of Plums. Fertilizer Experiments with Citrus Trees. Almond Pollination. The Control of Red Spiders in Decidu- ous Orchards. Pruning Young Olive Trees. Further Experiments in Plum Pollina- tion. Bovine Infectious Abortion. CIRCULARS No. No. 70. Observations on the Status of Corn 172. Growing in California. 173. 82. The Common Ground Squirrel of Cali- fornia. 174. 87. Alfalfa. 175. 110. Green Manuring in California. 111. The Use of Lime and Gypsum on Cali- 178. fornia Soils. 179. 113. Correspondence Courses in Agriculture. 126. Spraying for the Grape Leaf Hopper. 181. 127. House Fumigation. 136. Melilotus indica as a Green-Manure 182. Crop for California. 144. Oidium or Powdery Mildew of the Vine. 183. 148. "Lungworms." 184. 151. Feeding and Management of Hogs. 188. 152. Some Observations on the Bulk Hand- 189. ling of Grain in California. 190. 155. Bovine Tuberculosis. 193. 157. Control of the Pear Scab. 198. 159. Agriculture in the Imperial Valley. 199. 160. Lettuce Growing in California. 201. 161. Potatoes in California. 202. 164. Small Fruit Culture in California. 165. Fundamentals of Sugar Beet Culture 203. under California Conditions. 205. 166. The County Farm Bureau. 206. 167. Feeding Stuffs of Minor Importance. 208. 169. The 1918 Grain Crop. 170. Fertilizing California Soils for the 1918 209. Crop. 210. Wheat Culture. The Construction of the Wood-Hoop Silo. Farm Drainage Methods. Progress Report on the Marketing and Distribution of Milk. The Packing of Apples in California. Factors of Importance in Producing Milk of Low Bacterial Count. Control of the California Ground Squirrel. Extending the Area of Irrigated Wheat in California for 1918. Infectious Abortion in Cows. A Flock of Sheep on the Farm. Lambing Sheds. Winter Forage Crops. Agriculture Clubs in California. A Study of Farm Labor in California. Syrup from Sweet Sorghum. Onion Growing in California. Helpful Hints to Hog Raisers. County Organizations for Rural Fire Control. Peat as a Manure Substitute. Blackleg. Jack Cheese. Summary of the Annual Reports of the Farm Advisors of California. The Function of the Farm Bureau. Suggestions to the Settler in California. CIRCULARS — Continued No. 212. 214. 215. 217. 218. 219. 224. 225. 228. 230. 232. 233. 234. 235. 236. 237. No. Salvaging Rain-Damaged Prunes. 238. Seed Treatment for the Prevention of 239. Cereal Smuts. Feeding Dairy Cows in California. 240. Methods for Marketing Vegetables in California. 241. Advanced Registry Testing of Dairy Cows. 242. The Present Status of Alkali. 244. Control of the Brown Apricot Scale 245. and the Italian Pear Scale on Decid- 246. uous Fruit Trees. Propagation of Vines. 247. Vineyard Irrigation in Arid Climates. 248. Testing Milk, Cream, and Skim Milk for Butterfat. 249. Harvesting and Handling California 250. Cherries for Eastern Shipment. Artificial Incubation. 251. Winter Injury to Young Walnut Trees during 1921-22. Soil Analysis and Soil and Plant Inter- 252. relations. 253. The Common Hawks and Owls of Cali- 254. fornia from the Standpoint of the Rancher. 255. Directions for the Tanning and Dress- ing of Furs. The Apricot in California. Harvesting and Handling Apricots and Plums for Eastern Shipment. Harvesting and Handling Pears for Eastern Shipment. Harvesting and Handling Peaches for Eastern Shipment. Poultry Feeding. Central Wire Bracing for Fruit Trees. Vine Pruning Systems. Desirable Qualities of California Bar- ley for Export. Colonization and Rural Development. Some Common Errors in Vine Pruning and Their Remedies. Replacing Missing Vines. Measurement of Irrigation Water on the Farm. Recommendations Concerning the Com- mon Diseases and Parasites of Poultry in California. Supports for Vines. Vineyard Plans. The Use of Artificial Light to Increase Winter Egg Production. Leguminous Plants as Organic Fertil- izer in California Agriculture.