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CITRUS BLIGHT DIAGNOSIS in SEVERAL SCION VARIETY/ROOTSTOCK COMBINATIONS of DIFFERENT AGES Roger H

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CITRUS BLIGHT DIAGNOSIS in SEVERAL SCION VARIETY/ROOTSTOCK COMBINATIONS of DIFFERENT AGES Roger H Proc. Fla. State Hort. Soc. 91:56-59. 1978. CITRUS BLIGHT DIAGNOSIS IN SEVERAL SCION VARIETY/ROOTSTOCK COMBINATIONS OF DIFFERENT AGES Roger H. Young, H. K. Wutscher took up less water by gravity injection than healthy trees Agricultural Research, or than trees afflicted with other diseases (6). This finding Science and Education Administration, indicated that the general water-stressed condition of United States Department of Agriculture, diseased trees, which often exhibited leaf wilt (1), was due 2120 Camden Road, Orlando, FL 32803 to impaired water translocation. Leaf zinc deficiency symp toms in trees with blight led Smith (1974) to evaluate zinc M. Cohen distribution in diseased trees (12). He found as much as 8 University of Florida, Institute of Food and Agricultural Sciences, times more zinc in outer layers of trunk and limb wood in Agricultural Research Center, trees with "blight" than in healthy ones, indicating impair P.O.Box 248, Ft. Pierce, FL 33450 ment of normal zinc translocation. Most recently, Wutscher et al. (1977) combined trunk water injection and analysis S. M. Garnsey of zinc and water-soluble phenolics in trunk wood to dis Agricultural Research, tinguish blight from other diseases, such as exocortis, tristeza, Science and Education Administration, xyloporosis, or foot rot, or from water damage or burrowing United States Department of Agriculture, nematodes (16). 2120 Camden Road, Orlando, FL 32803 This paper reports that trees of several scion variety/ rootstock combinations of different ages, grown on the ridge Additional index words, xylem dysfunction, wilt disease, and in the flatwoods that exhibit blight symptoms, all have young tree decline. similar patterns of water uptake, zinc accumulation, and in creased phenolics. It also reports further evidence that these Abstract. Eighty-four citrus trees exhibiting visual blight common symptoms occur in the absence of tristeza virus. symptoms were evaluated with corresponding healthy trees with the diagnostic tests of water-uptake and zinc and Materials and Methods phenolic levels in trunk wood. Trees with 'blight7 had less Plant material. Citrus trees of different ages, both healthy water-uptake and higher zinc and phenolic levels in the and exhibiting visual blight symptoms (5), located on the wood than healthy trees. These patterns were found in ridge and in the flatwoods sections of the state were evalu diseased trees 6 to 50 years old growing on the ridge and ated. Care was taken in selecting test trees to avoid other on the flatwoods. Diseased trees characterized as "blight/7 ailments such as foot rot, exocortis, tristeza, water damage, "young tree decline" (YTD), and "sandhill decline" by visual poor nutrition, lightning damage, etc. Scion varieties in symptoms, tree age, rootstock, or location had similar cluded Citrus sinensis (L.) Osbeck, cvs. 'Hamlin/ 'Valencia/ patterns of water uptake and zinc and water-soluble phenolic 'Parson Brown/ and 'Pineapple'; C. paradisi Macf. cvs. levels. Blight was diagnosed by the above techniques in the 'Marsh/ 'Duncan/ and 'Ruby'; C. reticulata Blanco cv. varieties 'Valencia', 'Hamlin', 'Pineapple', and 'Parson Brown' Dancy and hybrids 'Temple' and 'K-Early'; C. reticulata X oranges, 'Marsh', 'Duncan', and 'Ruby' grapefruit, 'Dancy' C. paradisi cv. 'Orlando'; and C. reticulata X (C. paradisi X tangerine, 'Nova', 'Robinson', 'Temple', and 'Orlando' C. reticulata) cvs. 'Robinson' and 'Nova.' Rootstock species hybrids; and rootstocks of sour and sweet oranges, rough included C. reticulata cv. Cleopatra; Poncirus trifoliata (L.) and Milam lemons, Cleopatra mandarin, trifoliate orange, Raf. trifoliate orange; C. sinensis X P. trifoliata cvs. Carrizo Carrizo and Troyer citranges, Rangpur hybrid and Columbia and Troyer; C. limon (L.) Burm. f. cvs. rough lemon and Sweet lime. Trees with blight were found with and without Milam; C. aurantium L., sour orange; C. reticulata hybrid tristeza virus. cv. Rangpur; C. aurantiifolia (Christm.) Swing, cv. Colum bia Sweet lime; and C. sinensis cv. Hamlin. One of the earliest reports of "blight" in Florida was in Blight diagnosis. Zinc and water-soluble phenolics in 1883 by Manville (10). He described a limb blight with trunk wood were determined by methods of Wutscher et al. characteristic leaf wilt on bearing trees over 16 years of age. (16), and water uptake was determined by methods of Subsequent reports by Swingle and Webber in 1896 (14), Cohen (6) and Young and Garnsey (17). Analyses were con Rhoads in 1936 (11), and Childs in 1953 (4) described a ducted from March 1976 to August 1978. Water uptake is blightlike disease with visual symptoms similar to those expressed as ml/hr or ml/24 hr, zinc as ppm, and water- reported by Manville. The terms "young tree decline" soluble phenolics as mg/g dry weight of tissue. (YTD) and "sandhill decline" (SHD) were coined in the Tristeza virus indexing. Buds or fruit with 1 cm of 1960's to describe a similar disease of relatively young bear stem attached were harvested from suspected trees with ing trees grown in the flatwoods and on the ridge, respec blight the day water uptake was measured and trunk wood tively. The characteristics distinguishing blight, YTD and samples were taken. Tristeza virus indexing was ac SHD are tree age, rootstock, and location. Blight was complished by either grafting buds into 'Mexican' lime in thought generally to affect trees older than 15 years on dicator seedlings (15) or by an enzyme-linked immuno- sour orange rootstock. Young tree decline was thought to sorbent assay (ELISA) of fruit stem bark (3). For ELISA affect young bearing trees on rough lemon in the flatwoods, tests, OD405 readings twice or more the value for healthy and SHD was thought to affect rough lemon-rooted trees control extracts were considered indicative of tristeza virus of all ages on the ridge (7). Visual symptoms, however, are infection. similar (5). Precise diagnostic techniques have recently been de veloped to complement visual observations of leaf wilt and Results and Discussion zinc deficiency in diseased trees. Cohen, 1974 (6), demon Blight diagnosis. Eighty-four trees exhibiting visual strated that trees afflicted with blight, YTD, and SHD symptoms of "blight" were evaluated with corresponding 56 Proc. Fla. State Hort. Soc. 91: 1978. healthy-appearing trees in the same groves for water-uptake exact cause of the disease is found or until new evidence is and trunk zinc and water-soluble phenolic levels. Diseased presented to establish more than 1 syndrome, the term trees took up significantly less water and had increased zinc "blight" has precedence for this decline syndrome as pro and water-soluble phenolic levels in outer trunk wood com posed by Smith (13). pared to healthy trees (Table 1). In all but 2 cases, zinc Presence of tristeza virus in trees with blight. Trees of content of trunk wood of diseased trees was greater than several scion/rootstock combinations which showed typical that of healthy trees; water-uptake was always lower, and visual blight symptoms and were diagnosed as being positive water-soluble phenolic levels greater in the trunk of diseased for blight by water uptake and zinc and phenolic assays were trees. indexed for tristeza virus (Table 2). As expected, most trees Reduced water uptake and increased zinc and water- tested were infected with tristeza virus, but some trees with soluble phenolics in trunk wood were found in diseased blight of 'Hamlin' and 'Marsh' on sour orange, 'Nova', trees ranging in age from 6 to 50 years, in trees planted on 'Dancy' and 'Valencia' on rough lemon, 'Valencia' on deep, sandy soils of the ridge and on shallow soils of the Carrizo, and 'K-Early' on Cleopatra mandarin were indexed flatwoods. Typical diagnostic patterns were found in 'Va free of tristeza virus. While indexing may have failed to lencia', 'Hamlin', 'Pineapple', and 'Parson Brown' oranges, detect CTV in one or more individual trees, it is unlikely 'Duncan', 'Marsh', and 'Ruby' grapefruit, 'Dancy' tangerine, that all 15 CTV-negative trees were positive. Blighted trees and 'Temple', 'Nova', 'Robinson', and 'K-Early' hybrids. that indexed free of CTV were reported earlier by Garnsey Typical trees with blight were found on rootstocks of rough and Young (9). Feldman and Hanks (8) indexed over 100 and Milam lemon, sweet and sour orange, Carrizo and trees, both healthy and exhibiting blight, and found a high Troyer citrange, trifoliate orange, Cleopatra mandarin, percentage of trees with tristeza virus. Two trees were found Rangpur hybrid, and Columbia Sweet lime. free of the virus, but it was not indicated whether these were Differences in zinc content of wood between diseased healthy or exhibiting blight. and healthy trees appeared to vary with rootstock suscepti Previous reports have indicated that the physiology of bility to blight. The more susceptible rough lemon and trees affected by blight and tristeza is- quite dissimilar. Carrizo citrange rootstocks showed much greater differences Blight is a xylem disorder with restricted water and zinc in zinc levels than did the more tolerant sour and sweet movement (6, 9, 12, 16, 17). Phloem function is apparently oranges and Cleopatra mandarin. Although insufficient normal and blighted trees have adequate root carbohydrate numbers of groves were available for similar observations reserves (9). Conversely, tristeza is primarily a phloem dis with other rootstocks diagnosed for the presence of blight, order which reduces root carbohydrate reserves (9), but does this observation may provide a clue to determining root- not restrict water and zinc movement in the xylem (6, 9, stock susceptibility. Similar differences were not found with 16). Though tristeza virus is now widespread in Florida and water-uptake or water-soluble phenolic levels in wood. most trees exhibiting blight carry this virus, blight was While these results do not indicate degree of susceptibility, common for many years when tristeza virus was not present they do confirm the presence of the disease in trees of these or was quite rare.
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