The Australian Improvement Programme

P. Broadbent and L. R. Fraser citrus bud selection in New South Wales eases. Budwood from the nominated trees (N.S.W.) began in 1928 when grower and was screened for psorosis virus using nurserymen's organizations formed the sweet seedlings as indicators and N.S.W. Bud Selection Society. Selected for xyloporosis using seedling Orlando candidate trees were assessed by officers trees. Indexing for psorosis was of the N:S.W. Department of Agriculture done in the glasshouse but indexing for over a 3-year period for cropping, fruit xyloporosis was done in open ground quality, tree size, vigor and genetic screening plots at the N.S.W. Department variability. of Agriculture Horticultural Research Sta- Two major developments in the mid tion, Somersby. 1940's considerably affected bud selec- The supply of budwood from ap- tion: a) root rot caused by Phytophthora proved trees required the owner's con- citrophthora (Sm. & Sm.) Leon. caused sent, so that, as properties changed extensive losses of trees on Rough ownership, the sources of budwood often and sweet orange rootstocks during a became unavailable. In 1968 a mother- succession of wet seasons and led to a tree block was established at the N.S.W. sudden increase in demand for trees Department of Agriculture Horticultural budded on (Fraser, Research Station, Dareton. Trees were 1949); b) scalybutt was recognized as a planted 1.66 m apart in rows spaced at bud-transmitted virus disease affecting 6.6 m. was used as the trees on trifoliate orange rootstock rootstock for Eureka lemon and trifoliate (Benton et al., 1950). orange for all other scions. Scion material Use of trifoliate orange stocks necessi- included in the mother-tree block is listed tated selection of trees free of exocortis in table 1. Due to salting problems, virus. Candidate trees had to be more Troyer is now replacing trifoliate than 10 years old and growing vigorously orange as a rootstock. on trifoliate orange rootstock with no The increase in incidence o/f Australian signs of scaling. Each tree had to conform dieback (Broadbent and Frper, 1976) to other bud selection standards. Diffi- during the past 3 years has added to the culties in obtaining budwood from the problems of maintaining mother-tree few trees meeting these requirements led blocks. Fortunately trees with dieback to the introduction of a Parent Tree are self-indexing and transmission by Registration Scheme in 1953. This budding from infected trees is very low. scheme provided two classes of registra- Mother trees are regularly sprayed with tion: a) trees free from exocortis virus; 30 per cent w/v dimethoate in an attempt b) trees on stocks other than trifoliate to keep down populations of possible orange, which could not be guaranteed vectors, and are examined closely in free from exocortis virus. spring and autumn for the first symptoms Trees were nominated for registration of dieback. To date five trees of Seminole by growers or nurserymen. Nominations tangelo with early symptoms of were referred by the Bud Selection Soci- Australian dieback have been removed ety to the N.S.W. Department of Agricul- from the N.S.W. mother-tree block. ture for inspection over 3 years by a plant As a precautionary measure, an addi- pathologist and a horticulturist, Nomi- tional mother-tree block is to be estab- nated trees and trees in a 13 m radius were lished on a wider spacing at the Viticul- inspected for mutations and virus dis- tural Research Station, Griffith, N.S.W. Surveys, Citrus Improvement, and Other Subjects

AUSTRALIAN CITRUS REPOSITORY TABLE 1 SCION BUDWOOD AVAILABLE FROM A repository of virus-tested citrus is to THE N.S.W. CITRUS be established soon at the Horticultural MOTHER-TREE BLOCK Research Station, Dareton. In addition, Accession trees will be maintained in an insect-free Scion number glasshouse at the Biological and Chemical Research Institute, N.S.W. Department of Agriculture, Rydalmere. Scion material Davis for these trees was collected at the time Marsh of virus screening. Marsh This repository will form the nucleus Thompson of virus-tested mother trees for Australia. Wheeny It will be the responsibility of each citrus VALENCIA growing state in Australia to establish and Newton maintain multiplication units of this Newton material. New South Wales has the re- St. lves sponsibility for indexing material for NAVEL inclusion and for establishing and main- Bellamy taining the repository. Herps Candidate trees of the major cornmer- Houghton (Hutchinson) cia1 varieties of citrus are being screened Lane's Late for inclusion in the repository. As all Leng citrus trees in Australia are infected with tristeza virus, candidate trees will prefer- LEMON ably carry a mild strain of this virus. Doncaster Lisbon Grapefruit (Thompson and Marsh) must Lambert nucellar Eureka carry a mild strain of stem-pitting virus as Prior Lisbon evidenced by a very mild West Indian Taylor nucellar Eureka reaction and freedom of the scion Villa Franca parent from trunk pitting, stunting and MANDARIN malformed or small fruit. Emperor Freedom from exocortis virus will be Hansen assumed if the scion parent has grown Hickson vigorously on trifoliate orange for 10 Imperial years and no reaction is produced on Kara 60-13 or Arizona 861. Both Late Emperor requirements are necessary as severe Silverhill satsuma strains of tristeza virus often interfere TANGELO with the exocortis reaction on Etrog Minneola citron; and some scions which produce Seminole dwarf trees with no scaling of the trifol- iate orange rootstock give a positive reaction on Etrog citron (Broadbent et Leng Ellendale al., 1971). Indexing for psorosis, crinkly-leaf, and tatter-leaf viruses is being done in the glasshouse using seedlings of sweet or- orange rootstock in nursery containers. ange, Eureka lemon, Excelsa, and Rusk Field trees will be observed for 8 years citrange. but it is hoped that results for xyloporo- Xyloporosis indexing is being done on sis will be achieved in 2 years using Orlando tangelo seedlings in the field and container-grown Ellendale tangorltrifol- on Ellendale tangor trees on trifoliate iate orange plants. Seventh IOCVConference

LITERATURE CITED

BENTON, R.J., F.T. BOWMAN, L. FRASER, and R.G. KEBBY 1950. Stunting and scaly butt of citrus associated with Poncirus trifoliata rootstock. N.S.W. Dept. Agr. Science Bul. 70, 20 p. BROADBENT, P., and L. R. FRASER 1976. Australian citrus dieback. (this volume) BROADBENT, P., L.R. FRASER, and J.K. LONG 1971. Exocortis virus in dwarfed citrus trees. Plant Dis. Rep. 55: 998-99. FRASER, L. R. 1949. Gummosis disease of citrus in relation to its environment. Proc. Linn. Soc. N.S.W. 74: 5-18.