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Citrus Rootstock Problems recommendations change as developments within citrus industry reflect influence of rootstock on tree and fruit W. P. Bitters and 1. D. Batchelor The citrus rootstock problem in Cali- trees budded on sweet stock. Yields of Rough lemon generally grows well as fornia has changed since the recent ad- Valencias and grapefruit on sour orange a replant and growers frequently over- vent of quick decline and must be have been comparable to that of trees look its bad points because of this one re-evaluated. budded on sweet stock. Yields of Eureka favorable characteristic. Rootstocks are known to have a bear- lemons have been 10% to 20% smaller ing on the resistance of lemon trees to on sour orange than trees budded on Grapefruit decline and collapse and the problem of sweet orange. The quality of fruit is com- replanting old citrus soils with young parable to that of fruit budded on sweet Grapefruit rootstock has not been used trees may be solved to some degree by orange or grapefruit stocks. Fruit size extensively in California. Oranges budded the use of proper rootstocks. of Washington Navel and Valencia or- upon it are susceptible to quick decline. Among other factors to be considered anges are larger on sour orange stock Lemon collapse occurs more commonly in evaluating rootstocks are their effects than fruit from trees on most other root- on grapefruit root than on other roots. on yields, on fruit size, on fruit quality, on stocks. As a stock it is not as resistant to gum- cold resistance of the trees and fruit, Trees budded on sour orange stock are mosis as sour orange root. In all the root- on longevity of the trees, and on the salt more hardy than trees budded on Rough stock trials conducted by the University tolerance of the trees. lemon and grapefruit stocks. Sour orange of California Citrus Experiment Station grows well in heavy soils and grows bet- the yields of all species and varieties ter as a replant than sweet orange. It is budded on grapefruit root have been from Sweet Orange more tolerant to salts than sweet orange. 15% to 30% less than those varieties Sweet orange rootstock has given ex- Many trees in California budded on budded on sweet orange or sour orange cellent results in California with all sour orange root are 50 to 55 years old. roots. However, occasional good orchards species and varieties of citrus budded on In California this rootstock should only on grapefruit root do occur in California. it. Fruit quality of such trees is good and be used for lemons, particularly Lisbons, There is a great deal of variability in equal to that of fruits grown on trees or possibly nucellar Eureka strains. grapefruit varieties and perhaps one with budded on sour orange and grapefruit more favorable rootstock characters than others may be found. stocks, and far superior to the quality of Rough lemon fruit on Rough lemon roots. Recent stu- In California grapefruit should be used dies indicate that fruit size of oranges Rough lemon is a rootstock which has only for lemons and then with some re- on sweet orange stock tend to be slightly been overrated in California. Trees on luctance in view of the prevalence of smaller than those from trees budded on this stock-with few exceptions-mature lemon collapse upon some strains, par- most other rootstocks. early in their orchard history and decline ticularly in coastal areas. Trees on sweet orange stock are long in appearance, vigor and production at lived and are equally as hardy as trees an age varying from 10 to 25 years. There Trifoliate Orange on grapefruit or sour orange, and are are some good orchards on Rough lemon somewhat more hardy than trees on in California, but they appear to be an In commercial orchards trees budded Rough lemon. exception rather than the rule. upon trifoliate orange are extremely va- Sweet orange stock does best on sandy Oranges budded on Rough lemon stock riable in size and appearance. Trifoliate loam soils. It does not grow very well are tolerant to quick decline but with orange stock itself is subject to exocortis as a replant in old citrus soils. Recent grapefruit tops the trees are apparently disease which seriously affects the size, studies indicate that sweet orange is more susceptible. Lemons budded upon it are health and productive capacity of trees sensitive to soil salinity than is sour or- predisposed to early and heavy outbreaks budded upon it. ange. of shellbark. As a stock Rough lemon is Oranges budded on trifoliate orange Sweet orange is still a good rootstock no more resistant to gummosis than sweet appear to be fairly tolerant to quick de- and its use should be continued in Cali- orange. cline. fornia, particularly in the lighter, virgin Yields of various species and varieties In trials at the Citrus Experiment Sta- citrus soils. budded on Rough lemon have usually tion the yields of Valencia oranges on been good for the first 10 years, but com- trifoliate stock have been equal to that monly no higher than trees budded on of Valencia trees budded on sweet stock Sour Orange sweet orange. Fruit sizes of varieties even though the trees budded on trifoliate The resistance of this rootstock to gum- budded on Rough lemon are good but no are smaller. Navel trees on trifoliate or- mosis is one of its major assets however, larger than fruit from trees on sour or- ange have yielded about the same as those oranges or grapefruit on sour orange are ange. budded on sweet orange. Fruit size of susceptible to quick decline. Fruit quality of orange varieties and oranges budded on trifoliate stock is Eureka lemons budded on sour orange grapefruit grown on Rough lemon stock larger than average. The quality of fruit root tend to develop shellbark and de- is extremely poor. of trees on trifoliate orange is high. Trees cline at an early age. Yields of navel or- Most trees on Rough lemon stock are on trifoliate orange are hardy and will anges in some field trials on sour orange relatively short lived, particularly in the stand more cold than trees budded on stock have been 15% to 20% greater than coastal area. Continued on page 12 8 CALIFORNIA AGRICULTURE, FEBRUARY, 1952 ROOTSTOCK apparently has inherited some of the good turity. They cease to grow suddenly but qualities of both. It is highly resistant remain green, turgid, and firmly attached Continued from page 8 to gummosis. for several days. Finally many of the sweet orange or sour orange roots. Tri- Oranges budded on it appear to be fruits turn yellow, shrivel, and drop from foliate orange grows well as a replant and tolerant to quick decline. The trees come the vine. some strains are fairly resistant to nema- into bearing early and bear good crops In fruits which drop, abscission always todes. It grows best in acid sandy loam of large fruit of excellent quality. The occurs several days after the ovary ceases soils, but is fairly susceptible to injury trees are more resistant to cold than trees to grow, and thus appears to have a sec- by high salt content in the soil. budded on sweet orange or sour orange. ondary role in preventing fruit set. Present use of trifoliate orange as a Its ability to grow as a replant in old Embryo sac development, pollen-tube rootstock should be restricted to oranges citrus soils has been outstanding. growth, and the early stages of seed devel- and for limited replanting purposes only. Use of this stock should be restricted opment were studied in growing fruits to oranges and grapefruit. Lisbon lemons and in drops. For the insect-pollinated are growing well on it, but Eurekas have flowers on unthinned vines, there was no Cleopatra Mandarin not as yet proved adapted to it. evidence that fruit drop was caused by the misfunction of any of these processes. The Cleopatra mandarin is a stock W.P. Bitters is Assistant Horticulturist, Uni- which has done well with all species and ersity of California College of Agriculture, Riv- The changes which bring about fruit varieties in experimental trials of the erside. drop apparently first affect the growth Citrus Experiment Station. L. D. Batchelor is Professor of Horticulture, of the fruit as a whole and then the de- Oranges and grapefruit budded on University of California College of Agriculture, velopment of structures within the ovule. Cleopatra stock are tolerant to quick de- Riverside. The sequence is just the reverse of what cline. Cleopatra root is equally as resist- could be expected if processes associated ant as sour orange to gummosis. No other with fertilization or embryo or endo- diseases are known to be a factor. Lenioii BLACKBERRIES sperm development were the cause of fruit shellbark seems to be less severe on trees Continued from page 4 drop. budded on Cleopatra than on Rough Fruit set in this test did not appear lemon, grapefruit or sour orange stock. vigorous five to seven canes per plant to be limited by the number of ovules fer- Lemon decline is less pronounced in trees need be trellised. tilized. Counts of fertilized and nonfer- budded on Cleopatra than on other stocks Trellising should be done soon after tilized ovules were made from sections observed. harvest, and with as little breakage of of 13 growing fruits, and from sections Yields of all varieties budded on Cleo- canes as possible. If tip-pruned to eight to from 13 comparable drops. patra have been equally as good as those 10 feet at the time of trellising, the sup- Of 78 ovules in the fruits growing nor- varieties budded on sweet orange.

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