Gummy bark and transmissible bud-union disorders
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DESCRIPTION AND BACKGROUND
A number of miscellaneous but graft transmissible disorders affecting the trunk or bud-union have been described. These include:
Gummy bark or phloem discoloration of sweet orange
scions on sour orange rootstock as reported from Egypt
Bud-union crease of sweet orange scions on rough lemon rootstock as reported from Egypt (Nour-Eldin,1968).
Bud-union crease of sweet orange scions on rough lemon rootstock as reported from Florida (Grimm, Grant and Childs,1955; Bridges and Youtsey,1968).
Abnormal bud-union of sweet orange scions on rough lemon rootstock as reported from South Africa (McClean,1974).
The above disorders share the following characteristics:
They are graft-transmissible.
They are rarely present in nucellar selections, and can be avoided by use of nucellar budlines.
Bud-union problems induce similar bud-union crease, and brown to reddish-brown lines at the interface of the sweet orange scion with the rough lemon or sour orange rootstock.
They are probably not related to exocortis, cachexia, psorosis, tristeza or vein-enation pathogens.
Seedling or short-term indexes are not available and have not been developed for these transmissible disorders.
In addition, a serious bud-union crease of sweet orange on rough lemon has been reported to cause a decline of this stionic combination in India (Bhutan), Bakhshi and Knorr,1972), and bud-union crease of a number of sweet orange varieties was observed in Brazil (Salibe and Cereda,1984) and was associated with a decline of these trees; the researchers report that comparable bud-unions on nucellar selections were normal, and trees showed no decline.
A graft-transmissible agent in kumquat induces a bud-union crease in Parson's Special mandarin on Volkamer lemon rootstock and appears to be a new graft-transmissible disease (Vogel and Bové,1988).
The gummy bark disease was first reported as phloem discoloration of sweet orange by Nour-Eldin (1956). It was found on numerous sweet orange varieties in Egypt. This disease has since been reported in many North African and Near Eastern countries including Saudi Arabia, the Sudan, Libya, Iran, Morocco, Greece and Turkey. Trees are stunted to varying degrees, and sometimes severely reduced in size. The characteristic symptom of sweet orange scions on sour orange rootstock is shown in Figures 100 and 101. If the outer bark is carefully scraped, reddish-brown, gum-stained tissue can be seen under the bark in various degrees of severity (Figure 101). Discoloration and gumming are severe just above the bud-union and can extend to 60 cm or more above the union. If the bud-union area is scraped, a gum line may occasionally be seen similar to that in Figure 103b.
The sour orange rootstock will not show the gummy bark symptom, but will occasionally show slight stem pitting (Nour-Eldin,1968). In addition to this very characteristic symptom, severe stem pitting can be seen when the bark is cut away to expose the wood. This is a common symptom, and the pegs on the under surface of the bark may show gum impregnation.
Transmission has been demonstrated by bud-graft inoculation to healthy sweet orange scions on sour orange rootstocks. After about five years a reddish-brown line can be noticed at the bud-union, and the stem-pitting symptoms appear a year or two later (Nour-Eldin,1968).
Nour-Eldin (1968) noted that when rough lemon was used as the rootstock, a bud-union crease always developed, suggesting that "gummy bark of sweet orange trees grafted on sour orange rootstock and bud-union constriction of sweet orange on rough lemon rootstock are caused by the same agent". The constriction or ring at the bud-union of sweet orange/sour orange shows up after three or four years. He also reported (Nour-Eldin,1980) that gummy bark specimens usually contain cachexia, exocortis and psorosis, and although evidence suggests that these diseases are not causal, no "pure" source of gummy bark has been isolated or developed.
There is an urgent need for the isolation of this disease free of other pathogens and the development of a rapid index test specific for the detection of gummy bark. Bové reported that gummy bark was widespread and a serious problem in many countries of the Near East, based on his many observations in that region. (Statement made at the 10th IOCV conference Valencia, Spain, November1986.)
Bud-union crease of sweet orange scions on rough lemon rootstock in Florida
Grimm et al. (1955) reported and described a bud-union crease of sweet orange scions on rough lemon rootstock in Florida citrus, and suggested the possible virus nature of this problem. Bridges and Youtsey (1968), in further studies of this bud-union abnormality, showed that it was not correlated with infection by exocortis, cachexia, tristeza, psorosis or vein-enation pathogens, and also suggested its possible viral nature. They found bud-union crease on 118/130 Valencia and Pineapple sweet orange scion on rough lemon rootstock, whereas 0/130 nucellar sweet orange scion selections on the same rootstock showed no bud-union crease. They concluded that "a factor responsible for the abnormality was transmitted from parent to progeny".
Abnormal bud-union of sweet orange scions on rough lemon rootstock in South Africa
McClean (1974) reported and described a bud-union crease of sweet orange scions on rough lemon rootstocks occurring on many sweet orange varieties grown in South Africa, and considered it to be the same as a disease reported in Israel in 1937. Propagations of 45 trees representing 12 varieties all showed the bud-union crease symptom (Figure 103). Transmissions by buds from infected trees to 24 trees of four varieties induced positive symptoms in 19 trees, whereas none of the ten non-inoculated trees showed symptoms. Symptoms were evident within ten years.
A gummy bark disease was observed on sweet orange scions on rough lemon rootstock in South Africa; it appears identical to that reported in Egypt (Figure 101). There have been no studies on the transmission of this disorder.
Bud-union crease of Palmer navel orange on trifoliate rootstock in South Africa
A bud-union problem was observed with many Palmer navel orange trees grafted on trifoliate orange (Figure 104). It creates a distinct brown line and crease very similar to that induced by the citrus tatterleaf virus (see Tatterleaf, Figure 57). The cause of this brown line disease is not known, but it is transmissible and possibly virus-related.
In all of these bud-union diseases, a virus or viroid may be causal. Current indexing procedures are long term, taking from three to ten years or longer. Selection of budwood for entry into a pathogen-free foundation budwood programme may be difficult without a rapid index for these diseases. Until more rapid indexes are developed in those areas of the world where bud-union problems are suspected, buds should be carefully selected from nucellar bud-lines or from mother trees on rough lemon or trifoliate rootstocks whose many progeny trees have a history of being negative for bud-union problems. All major scion sources used in a given area should be propagated on major rootstocks in a variety-rootstock block to verify that bud-union problems (either transmissible or non transmissible) will not occur.
Bud sources can be indexed by inoculation into a sweet orange such as a Valencia, propagated on to a rough lemon rootstock and put into the field. Trees should be well cared for and soil fertility should be of the best for rapid growth. Where tristeza is not present, sour orange should be added as an additional rootstock for the indexing of gummy bark disease. The need for a more rapid index for these bud-union problems is self-evident.
There are a number of bud-union incompatibility and over-growth problems not the result of transmissible agents. One such problem (see Figure 105) shows the severe bud-union overgrowth of a mandarin on Troyer citrange in the Central Valley of California. Numerous transmission trials (Roistacher, unpublished) have shown that this condition is not caused by a pathogen but is an incompatibility probably aggravated by warm temperatures. Mandarins are especially susceptible and begin to decline after about 15 years.
GUMMY BARK AND BUD-UNION ABNORMALITY DETECTION
Gummy bark: sweet orange/sour orange. Bud-union crease: sweet orange/rough lemon.
A minimum of 4, preferably 6 or 8.
Train as trees for field planting.
First symptoms: 3 to 10 years.
Gummy bark: reddish-brown gum in lightly scraped bark of sweet orange scion above the bud-union. Severe pits may be present in the sweet orange scion.Bud-union crease: indentation at the bud-union circling the trunk with pits and gum in the crease line.
Bhutani, V.P., Bakhshi, J.C. & Knorr, L.C.1972. Quantitative biochemical changes in healthy and decline sweet orange trees associated with bud-union decline. In Proc. 5th Conf. IOCV, p. 229-233. Gainesville, Univ. Fla. Press.
Bridges, G.D. & Youtsey, C.O.1968. Further studies on the bud-union abnormality of rough lemon rootstocks with sweet orange scions. In Proc. 4th Canf: IOCV, p. 236-237. Gainesville, Univ. Fla. Press.
Grimm, G.R., Grant, T.J. & Childs, J.F.L.1955. A bud-union abnormality of rough lemon rootstock with sweet orange scion. Plant Dis. Rep., 39: 810-811.
McClean, A.P.D.1974. Abnormal bud union between some sweet oranges and rough lemon rootstock: evidence of cause by a transmissible pathogen. In Prae. 6th Canf. IOCV, p. 203-210. Riverside, IOCV.
Nour-Eldin, F.1956. Phloem discoloration of sweet orange. Phytopathal., 46: 238-239.
Nour-Eldin, F.1968. Gummy bark. In Indexing procedures for 15 virus diseases of citrus trees. p.50-53. Agric. Handbook No.333. Washington, D.C., Agr. Res. Service, USDA.
Nour-Eldin, F.1980. Gummy bark. In Bové, J.M. & Vogel, R., eds. Description and illustration of virus and vilus-like diseases of citrus. A collection of colour slides. Paris, I.R.F.A. SETCO-FRUITS.
Salibe, A.A. & Cereda, E.1984. Limitation on the use of Volkamer lemon as rootstock for citrus. In Prac. 9th Conf. IOCV, p. 371-374. Riverside, IOCV.
Vogel, R. & Bové, J.M.1988. Graft transmission from kumquat of an agent inducing bud-union crease in Parson's Special mandarin grafted on Volkamer lemon rootstock. In Proc. 10th Conf: IOCV, p. 367369. Riverside, IOCV.
FIGURE 100 Gummy bark symptoms on Jaffa orange in Syria (Photo:R. Vogel)
FIGURE 101a Gummy bark symptoms in bark of navel orange near Tarsus, Turkey
FIGURE 101b A section of bark taken from a gummy bark-infected navel orange near Tarsus, Turkey, showing gum pockets in the bark, and the appearance of these same gum spots (c) when the bark is sliced tangentially
FIGURE 101c A section of bark taken from a gummy bark-infected navel orange near Tarsus, Turkey, showing gum pockets in the bark, and the appearance of these same gum spots (c) when the bark is sliced tangentially
FIGURE 102 Bud-union crease of Valencia on rough lemon rootstock in Florida (Photo: S.M. Garnsey)
FIGURE 103a Transmissible bud-union abnormality of Tomango sweet orange on rough lemon rootstock in South Africa. Cuts made into the bud-union bulge show indentation and severe staining surrounding the union
FIGURE 103b Close-up showing brown line totally around trunk
FIGURE 104 Bud-union crease of Palmer navel on trifoliate rootstock in South Africa. The condition is transmissible
FIGURE 105 Bud-union overgrowth of mandarin on Troyer citrange in the Central Valley of California. This condition is attributed to environmental conditions typical for most mandarins and tangelos in the Central Valley. Trees decline in about 15 years
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