CITRUS ROOTSTOCKS FROM THE PAPEDA GROUP by W. P. Bitters, C, D. McCarty and D, A. Cole* Many citrus and near-citrus relatives have been tried as rootstocks. Of interest are some of the species and hybrids of the subgenus Papeda, the fruits of which differ from citrus mainly in that they are unedible because the pulp vesicles contain an acrid oil that gives the fruit a disagreeable flavor. Among the most widely used as rootstocks from the Papeda group are the naturally occurring hybrids Yuzu and Citrus macrophylla or Alemow. =, C. macrophylla has gained wide acceptance in California and Arizona as a rootstock for lemons. Seedlings of macrophylla are very vigorous and require a minimum of nursery care. Macrophylla buds well with all scion varieties of citrus, but its susceptibility to tristeza and cachexia rule it out for use in most areas except with lemons and limes. Macrophylla is highly resistant to Phytophthora and tolerant of exocortis. It does well on all soil types from sandy to heavy clay loams and has a high degree of resistance to calcareous and saline soils. *W. P. Bitters and D. A. Cole are members of the Department of Plant Science, CRC, UCR. C.D. McCarty is a member of the Agricultural Extension Service, UCR. -2- It shows perhaps the highest boron tolerance of all citrus rootstocks, and is also capable of absorbing micronutrients in slightly greater quantities than other citrus. Hence, trees on macrophylla are deep green in color and seldom show leaf patterns that denote micronutrient deficiencies. Young lemon trees on macrophylla are vigorous and precocious in bearing. One problem is that fruiting is so heavy that severe limb breakage occurs. Bud unions have been good with all scion varieties tested, and yields have been heavy with navels, Valencias, Dancy tanger¬ ine, grapefruit. Eureka and Lisbon lemon, and lime. Fruit from trees grown on macrophylla are larger than average. However, fruit quality is below average and with some varieties could even be classified as poor. Dancy, for example, granulates badly when macrophylla is used as a root- stock. Soluble solids and citric acid from fruit of trees grown on macrophylla are low, being comparable to fruit when rough lemon is used as a stock. Current recommendations are for use only with lemons and limes. Tree vigor is good while the trees are young, but older trees show no more, and perhaps even less, vigor than trees budded on rough lemon. Lemon trees on macrophylla are relatively short-lived. Eureka on this stock has a productive life of 20 to 25 yeairs, and the life of Lisbons may be only slightly longer. Macrophylla is very sensitive to cold and trees on this rootstock do not fare as well as trees on rough lemon when subjected to freezing temperatures. -3- Macrophylla is an interesting rootstock, and hybrids with trifoli¬ ate orange may combine the good qualities of both. Testing of such hybrids is in progress. ^ The Yuzu orange (£. junos or C, ichangensis x C, reticulata var. Austera?) sometimes called Kansu, is used in Japan as a secondary root- stock for the Satsuma mandarin. Satsumas on trifoliate orange bear early and abundantly but begin to decline after 10 to 20 years. It is then common practice to inarch with Yuzu, after which the trees again become productive and long-lived. Yuzu was tried briefly as a rootstock for Satsuma mandarins in the early 1900*s along the gulf area of the southeastern United States and was not considered as good as the trifoliate orange. Its use was dis¬ continued, and only recently has there been a revival of interest in California in as a Yuzu rootstock. It has been tried to a limited extent in New Zealand and Algeria. Yuzu is low in polyembryony, and seedling populations must be care¬ fully rogued in order to insure rootstock uniformity. Seedlings of Yuzu grow slowly and usually remain in the nursery two years before they are large enough to bud. They do not bud easily early in the spring but there seems to be little difficulty when budding later in the season. Young budded trees grow slowly and this characteristic holds over into early orchard years. This probably accounts for the growing of Satsumas on trifoliate orange in Japan and subsequent inarching with Yuzu. Orchard trees on Yuzu are slow to come into bearing. In California trials, fruit quality is better with most citrus -4- varieties on Yuzu than when the rootstock is sweet or Sour orange. Soluble solids and acid are almost as high as when trifoliate is the rootstock. Yuzu does best on rich, deep soils, Gummosis resistance of Yuzu is good, being comparable to sour orange or better. It is not affected by citrus canker and shows some resistance to oak root fungus. Since it is used as an inarching stock in Japan, it could be assumed that it would do fairly well as a replant in old citrus soil. In California its replant growth has been comparable to sour orange. It is not resistant to citrus nematode. Trees on Yuzu are nearly as resistant to cold as those on trifoli¬ ate. In tests in California tristeza tolerance has been uncertain. This might be due to seedling variability; certainly it has done well in Japan in the presence of tristeza. In California, trees on Yuzu were noticeably greener in foliage color than nearby trees on other stocks. Analysis indicated the iron 1 content of leaves of trees grown on Yuzu was twice as high as the iron content of trees grown on sweet orange and grapefruit. Leaves of trees on Yuzu were also higher in manganese. It picked up less calcium and might be a good stock to try on calcareous soils. However, it picked up more boron and sodium and should not be used under saline conditions. The bud union with scions on Yuzu is good. There is a slight bulge at the union and the stock may be slightly larger than the scion and slightly fluted. Because of its good attributes, interest in Yuzu as a rootstock -5- should be maintained. Further expeirimentation may indicate areas of use in other countries besides Japan. The Ichang lemon, also known as the Ichang pummelo and the Shangyuan, is probably a hybrid of C» ichangensis x C. grandis var. ? and is the C. wilsonii described by Tanaka. Apparently it has not been tried commer¬ cially anywhere in the world and only to a limited extent in experimental plots. It has been under observation for 13 years in California and its response is similar to Yuzu. It is low in polyembiyony, and seedling plantings must be severely rogued in order to insure rootstock uniform¬ ity. Seedling plants are slightly more vigorous than Yuzu and handle better in the nursery. Citrus trees on Ichang are slightly larger than comparable trees on Yuzu but they are still small as far as citrus trees go. The rootstock is slightly larger (more than Yuzu) than the scion;- there is less bulge at the union than with Yuzu and less tendency for ridging. Occasionally orange trees on Ichang show tristeza symptoms. Whether this is t3^ical or only the reaction of unrogued genetic variance is not known. It shows some resistance to oak root fungus, and interest has been expressed in it as a rootstock for lemons. Tests for this purpose would be worthwhile. Certainly it would not be wise to use it with susceptible scions in tristeza areas. Citrus ichangensis has never been tried as a rootstock, probably because of its general unavailability and its extreme variability. of its extreme However, because hardiness, the greatest within the genus -6- Citrus, and because of its resistance to Phytophthora, it should be used in hybridization studies for the development of new rootstocks. Citrus latipes is raonoembryonic and quite variable but not so much so as C, ichangensis. Its seedlings are dark green in color, quite vigorous, and bud well with citrus. It is a hardy stock and has attributes similar to ichangensis. However, when inoculated with tristeza, orange scions budded upon it declined and its use certainly cannot be recommended for oranges. Citrus sudachi and tosu have been tried as rootstocks for oranges. Inoculated with tristeza these trees declined, The sudachi gave a better performance than the tosu. However, there seems little to recom¬ mend these for further trial. Citrus celebica var. southwickii remotely resembles a shaddock. When tested with orange scions and inoculated with tristeza, the trees declined severely. Citrus hystrix has also been budded with citrus and tested for tristeza tolerance. These trees were veiy susceptible and stem-pitted severely. They were also susceptible to gummosis. The trees are not hardy and produce fruit of poor quality. The trees are uniform and the bud union is good. Citrus macroptera. These seedlings are very uniform and bud well with all citrus. The stock is more vigorous than Citrus hystrix but it is susceptible to Phytophthora, Orange trees budded upon it were very susceptible to tristeza. Lemons, however, did quite well and it may be worthy of further testing, C. macroptera is quite susceptible to cold damage. -7- Citmis webberii var, Montana gave interesting and encouraging results as a rootstook. It was included in a rootstock trial v/ith Washington navel orange and at the end of 30 years was one of the better performers. Because of its willowy growth habit, it is difficult to grew and bud in the nursery. However, the bud union is good and quite smooth. Yields have been good and fruit quality is average. Its Phytophthora resistance is fairly good, but it is slightly susceptible to tristeza and shows mild stem pitting when inoculated with this virus.