-61.

Some Disease Pro~ems of Citrus Rootstocks

E. C. Calavan

Introductio n

Except for certain disease problems, many citrus trees would be grown on meir own roots or on seeding stocks of tile samevariety, d1us eliminating many bud-union problems wh ich are currently important as causes of decline and tree loss. Also, sweetorange and mandarin trees on their own roots are very tolerant of most forms of tristeza. There are several otller diseases thatare rootstock-related which is to say mat rootstocks tolerant to d1ese diseases usually exist. Two of me very worst diseases, stub-born and greening can effect all kinds of commercial scions so severely that they cannot be contrmled by using disease-resistantrootstocks.

Let us take a look at some of the rootstock-related citrus disease problems in various parts of the world. We will concentrateon virus and mycoplasmalike diseases and their symptoms. We will ~so discuss some very severe disease pro~ems unrelated torootstock varieties. Symptoms of the diseases and varieties affected are shown in the Table 1 which f~lows this text.

Virus and Virus-Like Diseases

Tristeza. Tristeza probably has killed more trees than any other disease in the past 35 years, probably more than 30 million.Most of the losses occurred in Argentina, Brazil, Spain, and the United States. Some countries have not yet experienced exten-sive losses from tristeza. Australia, Japan, India, Indonesia, Philippines, Rhodesia, South Africa, and Taiwan are heavily infes-ted with tristeza virus and have long used tristeza-resistant rootstocks such as rou!l1lemon (India, Rhodesia, and South AfrICa),mandarin (Taiwan and Philippines) and trifoliate orange (Japan). Other countries, including those surrounding the Mediterranean(except Spain), Mexico and Central America, Chile, and part of the United States (Arizona, Texas, and parts of California) stillhave little or no tristeza present and still have millions of trees of the highly sensitive sweet orange/sour orange. Tristeza, if estab-lished in these areas, will eventually damage or destroy the trees of sweet orange/sour orange. Tristeza is known to be spreadingnow in Peru, Spain, Israel and the United States. It probably is spreading in Turkey. The disease has not yet killed half of thewort d's highly tristeza-sensitive trees. .

Symptmns of tristeza are varied, depending on the strain of virus, the variety of host and the environment. Usually the sourorange phloem is damaged near the bud union producing a girdling effect which results in starvation of the roots and gradual de.cline of the top. Pinholing often occurs in the sour orange bark just below the union. Sometimes the quick decline effect withsudden witing, occurs when a sudden and severe water stress develops. Starch disappears in the rootstock before it does cn thetop. All knlMn forms of tristeza cause some vein clearing and stem pitting of Mexican lime at cool to moderate temperatures.There are some mild isolates of tristeza v;rus that cause little damage to trees on sour orange rootstock. Some other rather severeforms of tristeza are called seedling yellows, stem pitting, and 'Hassaku' dwarf.

Seedling yellows causes yellowing and stunting of lemon, sour orange and grapefruit seedlings and usually some vein corkingof Mexican lime. It can damage mature trees of these varieties. Seedling yellows appears to be abundant in Brazil, Argentina,South Africa, Japan, India, Southeast Asia, Philippines, Australia, Hawaii, and Taiwan.

Stem pitting tristeza occurs on grapefruit and its close relatives, such as 'Hassaku', causing severe stem pitting, Marting ofthe plant, and snail fruit size. Tristeza also causes extensive stem pitting of sweet lime and 'Alemow' rootstocks, badly dc:mag-ing trees planted on these rootstocks. Stem pitting is most common in areas where Toxoptera citricidus (black citrus aphId) isthe principal tristeza vector. In some localities stem pitting causes considerable damage to sweet orange as well as to grapefruit.Tristeza stem pitting of gr~efruit and sweet orange is extremely rare in California and Fforida but tristeza is now killing grape-fruit trees on sour orange rootstock in both states. Where severe stem pitting of grapefruit or of limes is common it appearsadvisable to use a mid strain of tristeza for protection against stem pitting, as has been reported from Brazil. It is posS1b!e toobtain tristeza.free young trees by using tristeza-free buds propagated on clean stocks. However, in heavily infested areas theyoung trees soon become infected with whatever isolates are being moved by vectors. In sum situations inoculation with one ormore isolates of tristeza virus may be desirable.

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Psorosis. The name psorosis is applied to a group of diseases that are b"ieved to be r"ated to each other; in addition tocommon psorosis there are ~ind pocket, crinkly leaf and infectious variegation, Satsuma dwarf, concave gum, eruptive gum-mosis and ringspot. Of these, crinkly leaf, infectious variegation and Satsuma dwarf are known to be caused by viruses.

Common psorosis causes pimpling and scaling of the bar1< of mature trees of sweet orange, grapefruit, mandarin, and tan.gelo. Lemon, trifoliate orange, citranges, and limes may become infected but do not scale. In advanced stages the wood be-n.th lesions dies and discolors to develop 1h e "woodform" of psorosis. Psorosis was once the most important citrus virusdis.se in Florida and California. Thanks to budwood programs it is becoming a minor disease except in old orchards.

Inf~tious variegation causes crinkled and puckered leaves and some leaf variegation but is less common than the very sim-iar disease crinkly leaf. Lemons as well as oranges may be aff~ted. Infected lemon fruits tend to be rougher than normal.Occasionally these viruses are seed transmitted.

Blind pocket causes abrupt depre~ons in trunks of affected trees and aff~ts some trees of various varieties in Californiaand Florida, but is easily avoided bv using registered budwood.

Concave gum causes some stunting and results in formation of broad concavities on the trunk and large branches. An oak-leaf pattern is normally associated with this disease. .

Relatively little is known of eruptive gummosis and ringspot. Eruptive gummosis and pelf1.s ringspot may be spread by anunknown vector. Their geographical distribution is incompletely determined but they appear to be present in South and NorthAmerica.

Impietratura causes gumming in 1he peel of grapefruit and some other varieties in countries around the Mediterranean basin.A leaf symptom similar to that of psorosis is associated with it. There is proof of graft transmissibility but no proof of naturalspread.

Satsuma dwarf has a leaf symptom similar to psorosis and causes downward bending of leaf margins on Satsuma trees inJapan and Turkey.

Fortunately most psorosis diseases can be completely avoided by using only registered psorosis-free seed and budwoodmaterials.

Cachexia (Xyloporosis). Cachexia is important mostly on certain mandarin, mandarin lime, tangelo, 'Alemow', and Meetlime varieties when these varieties are used either as scions or as rootstocks. The virus can infect sweet orange, sour orange andgrapefruit but causes litde or no damage nor symptoms on these species. Cachexia infection is widespread in most of the citrusworld in old budlines but, since it has apparently no means of natural spread, it usually is found in seedling lines oliy as a re-sult of graft transmission. It is easily prevented by using cachexia.free stock but the most rapid indexing method requires about6 months to a year.

Symptoms of c.:hexia are lens-shaped pits in the wood, especially on the trunk and rootstock, usually with gum impregnatedirregularly in the bark, and pegs on the innerf.:e of the bark. Severely affected trees usually have chlorotic foliage and are stun-ted. In stocks or trees such as 'Alemow' and 'Palestine Sweet lime' the symptoms closely resem ~e those of tristeza but cachexiapits are broader and the bark contains more gum than with tristeza infection.

Cachexia is relatively unimportant in California because most popular budlines are of seedling origin. However, 'Alemow',the principal rootstock for lemons, reacts severely when budded with cachexia-infected lemon buds. If there were an efficientvector present many lemon orchards would be ruined by cachexia. Budwood programs in Florida and California have done muchto control cachexia.

Cristacortis symptoms are similar to those of cachexia but occur on sweet and sour orange scions and rootstocks. Pits areusually larger than for cachexia and gum deposits are p resent in the wood. Cristacortis has not been reported in the United Statesnor in most other citrus producing countries but is common in the Mediterranean area. No natural spread is known for cristacorti!

-63-

Woody gall or ~in enation. This virus disease causes rou~ woody galls on Citrus ~/kameriana rough lemon and on someother lemon and lime trees and rootstocks. Galls form especially at wounds on infected plants. Woody galls at the bud unionare very clMnmon on rough lemon rootstocks in Peru but damage to the tree appears to be minor in most cases. Vein enationson the leaves are found mostly on young, vigorous trees and are rarely seen in the orchard. The woody g~1 virus is transmittedby aphids, including Toxoptera citricidus, Aphis gossypii, and Myzus persicae. The use of vein enation-free budwood in cleanareas is advisable. In infested areas the use of clean budwood h~ps to improve the ~pearance of young trees on rough lemonrootstock but the trees soon become infected with the virus.

Stubborn. Stubborn is widespread in most arid and semiarid citrus areas of the world and has ruined sever. million treesduring the past 40 years; it rar~y kills trees but it lowers the quality and quantity of fruits. Stubborn is e~ecially destructive

in California and Arizona, in the warmer portions of Mediterranean countries, and in Peru.

The symptoms of stubborn are somewhat variable but include low yield, stunting, eccentric or acorn-shaped fruits, stylar-endgreening, blue albedo, seed abortion, stiff, upri~t growth of leaves and stems, small and sometimes mottled 15ves, premature de.foliation and drop of fruit in hot areas, and dieback in severely aff~ted trees.

There are about 2 million stubborn-inf~ted trees in C.ifornia and proba~y at least that many in the Mediterranean. Thedisease spreads readily by vectors, as yet unidentified, in some'California Ilx:ations. The vectors are believed to be leafhoppers.

All commercial varieties of citrus appear to be susceptible to stubborn regardless of the rootstock used, but sweet orange,grapefruit, and tangelo are the species most frequently attacked. Lemon and lime trees are rarely attacked by 1f1e stubborn

patllogen.

The apparent pathogen of stubborn, Spiroplasma citri, is unique in that it is mycoplasmalike, has many fo nns, and can becultured from diseased plants and from ins~ts suspected of being vectors. The suspected pathogen is sensitive to tetracycline81d to some other antibiotics. Thus far, attempts to control stubborn by antibiotic treatments have succeeded only with small

plants grown in hydroponic solutions containing tetracydine.

Most stubborn infections of inportance occur in young trees less than 4 years of age or from infected budwood. In areaswhere natural spread is relatively slow stubborn can be controlled by planting clean trees and destroying visi~y stubborn-diseasedones. It is not known whether plants other than citrus are hosts of stubborn.

Spiroplasma citri is smaller than most bacteria, motile at certain stages in citrus and ins~t hosts, and has a characteristic spiralform of very ~ender filaments at one stage of its development. In citrus this organism se~s to be confined to the sieve tubes.

Greening. Greening is by far d1e most dangerous and destructive citrus disease in the woltd. Greening is called by variousnames including blotchy motde in South Africa, leaf mottling or leaf motde yellows in d1e Philippines, citrus decline in India,likubin in Taiwan, and yellow shoot in China. The symptoms resemble stubborn but greening is far more detrimental d1an stub-born to citrus trees. Greening is vectored by the p syll ids Trioza erytreae in South Africa and nearby countries and by Diaphonnacitri in soud1ern, eastern and southeastern Asia, the Phiippines and Indonesia. Greening is present in most of d1e tropM:al andMrm subtropical citrus areas in 1he eastern hemisphere but has not been reported from d1e western hemisphere. In the westernhemisphere, only Brazil has reported having eid1er of the psyllid vectors. Climatewise greening should do well if established inthe western hemisphere, especially in Florida, Mexico and the Caribbean area.

Greening has completely ruined many milions of trees in the Orient and in South Africa during the past 20 years and contin-ues to ruin millions more. The apparent pathogen of greening is mycoplasmalike but it has a thicker covering around each cell,making it resemble bacteria in this respect. The greening organism apparently is more difficult to culture than is the stubborn

organism.

Greening attacks all important varieties of citrus regarcftess of the rootstock variety on whN:h they are propagated.

No control has been developed for greening but injections of tetracycline-HC1 appear to have improved tree condition andproduction of some greening-affected trees in South Africa. Reports from the Philippines indicate inactivation of the greeningpathogen in mandarin orange budwood dipped in tetracycline solutions. In India promising results have been obtained by spray-ing tetracycline antibiotics on greening-infa:ted citrus trees.

-64-

Miscellaneous virus or virus-like dis_es are listed in Ta~e 1

Bud-union Disorders

Disorders at the bud union may severely damage or kill citrus trees. Some such disorders are due to transmissible pathogens;others for want of a better term are called incompatibiities and are apparently not transmissiNe.

Among the transmissible diseases causing bud-union disorders are tristeza and citrange stunt. The latter is rare, except thatit is usually present in old budlines of '~yer' lemon and can cause a brown stain and crease at the bud unions of Meet orangeor mandarin scions on citrange or trifoliate orange rootstocks.

~ny g:ion-rootstock combinations are sometines or ~ways ill:ompatible. Among the most common of these is Meet orange(especially blood orange) on rough lemon. This combination is rather variable in its performance, certain budlines on certainrough lemon seedlings are apparently fully compatible while others are slightly to severely affected.

Calamondins and kumquats are incompatible with most other citrus varieties and form a bud-union crease with highly ab-normal xylem and phloem at the union. This disorder does not appear to be transmissible.

Some citranges and trifoliate oranges are highly ir-=ompatible with certain budlines of lemon, esp~ially 'Eureka' and 'Villa-franca' sel~tions. There also seems to be considerable incClmpatibiity between sour orange and some citranges. No scion root-stock combination can be considered fully compatible in any situation until it has been tested for 10 years or more in a givensituation.

Maintena~e of a strong, viable citrus industry in any country requires the prevention and/or contr~ of severe diseases. Fun.pi and bacterial diseases may be controlled by cultural and pesticidal measures but graft.transmissi~e and bud-perpetuated dis.eases can best be controlled by the use of clean propa~ting materials, resistant or tolerant varieties and com binations, and by thereplacement of worthless diseased trees with healthy ones. In some areas severe diseases such as stubborn and greening cannot becontrolled by any pract~al measures available at this time. Under such h~eless circumstances it is advisa~e to change to anothercrop.

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