Exocortis Disease of Citrus top-root relationships of trifoliate orange and its hybrids studied in search for cause of root disease

W. P. Bitters

the citrus root disease called exocortis years of age, but a few combinations were but remains partially attached to the liv- is present in most areas where trifoliate under 10 years. ing bark beneath. The bark lifts up in orange-Poncirus trifoliata-is used as The most striking symptom of exo- scales %(;” to f/g” thick, ’A‘‘ to 1” wide a rootstock. cortis is the shelling or scaling of the and 1” to 5” in length. A small amount Trifoliate orange stock is desirable for trifoliate orange bark. In trees which of gum is frequently present beneath the citrus because of its effect on yield, fruit develop scaling, the diameter of the stock scales. Scaling of the bark generally be- quality, fruit size, hardiness, resistance to is generally reduced to the point where comes extensive and severe. The trees nematodes and other characters. How- it is nearly equal to the diameter of the are seldom killed, but ultimately become ever, there is inadequate information on top and fluting of the stock is not con- worthless. all the attributes of some of its hybrids- spicuous. Top growth is usually, but not particularly the citranges-as rootstocks. always, retarded at the time scaling is In commercial orchards of California first evident. The degree of such stunting Observations general use of trifoliate stock has been varies with the scion variety and the time Of 128 trees on trifoliate orange restricted because of the extreme varia- a1 which scaling first appears. Scaling has roots-32 combinations-only three of bility in size and appearance of citrus not been observed on nursery trees but the combinations examined showed evi- trees budded upon it, and the frequent has occasionally appeared on seedling dence of exocortis. presence of exocortis. In part, the varia- trees. Five trees of Chase Eureka lemon at 22 bility in tree size and the presence of years of age were severely affected. The exocortis may be directly related. size and general appearance of the trees The merits of trifoliate orange as a Symptoms indicated the disease was of long dura- stock and the prospective use of it and its The first symptom of scaling is evident tion. They were markedly stunted and hybrids in California led to extensive in- below the bud union on the outer bark of thinly foliated. The trifoliate stock was vestigation into the nature of exocortis the stock. This may occur either at or of small diameter, badly scaling, and not to learn whether it might be prevented. below the ground line or at the bud union. fluted. Approximately 600 trees representing Bark scaling also extends below ground Two trees of Temple orange at 17 years 100 different combinations of trifoliate onto the roots. Frequently large roots and of age also were severely affected. orange, or its hybrids-as roots or as portions of the root crown may die. Twenty-five trees of the Valencia com- tops-were examined for symptoms of Small oblong or elongated areas of bination which were part of some Valen- exocortis in the rootstock and variety dead bark appear as the first visible cia orange rootstock trials were available plots at Riverside, during the winter of symptom of scaling. Such bark later sepa- for study. In 1947, at an age of 18 years, 1950-51. Most of the trees were over 20 rates from the margins of the live bark, Continued on next page

Trifoliate orange stock budded to Valencia Trayer citrange stock budded to CES Eureka lemon, Troyer citrange stock budded to Monroe orange, 22 years old, showing the develop- eight years old, showing an atypical citrange union, Lisbon lemon eight years old, showing a ment of the first stages of exocortis. the stock being smaller than the scion. healthy normal union and larger growth of the citrange stock. EXOCORTIS when examined in 1948. Symptoms were were conducted, so the nature of the con- undoubtedly evident sooner. The tops dition is debatable. However, in the Continued from preceding page were markedly stunted and the trees were other rootstock experiments several two of the 25 trees-during trunk meas- in very poor condition. Twenty-five stocks, a hybrid-shaddock crossed with urements-were observed for the first Washington Navel trees 22 years of age St. Michael sweet orange-and the Cle- time to display evidence of exocortis. The on Morton show no symptoms of the dis- mentine mandarin have occasionally dis- general health and vigor of these trees ease. Two navel trees out of 20 in a played bark irregularities which could had not been affected in 1951. The scal- younger planting-and with a different be confused with exocortis. Clementine ing of the bark was confined to a small top source-showed questionable symp- produces only gametic seedlings and the area of the trifoliate stock. The bud toms of exocortis at 34years of age, but shaddock hybrid produces a high per- source of these Valencias came from a this condition is not presently apparent. centage of gametic seedlings. Such irreg single tree which also provided budwood All other combinations with Morton ularities could be hereditary but that has for 15 trees of Savage citrange and for citrange are healthy. never been established as a fact. 10 trees of Cunningham citrange. The Of the four combinations on Troyer Savage and the Cunningham trees show citrange only those budded to the same no symptoms of the disease. strain of CES Eureka lemon as that used Genetic Factors In these trials no external evidence of on Morton citrange appear to be in dis- Genetic factors must be considered in the disease was found in 20 grapefruit tress. Twenty such trees planted in 1944 the examination of the exocortis problem. trees and 25 Washington Navel orange and 20 planted in 1945 were markedly Many F, seedlings are characterized by trees budded on trifoliate orange stock. stunted, off-color, and sparsely foliated the expiession of a wide range of char- This does not imply that different sources in 1951. No above ground shelling of acters. Exocortis itself may not be inher- of these same varieties would not impart the bark was evident. The poor condi- ent but there is a chance that some exocortis to the stock when budded upon tion of such trees was evident within a weakness may exist in certain progeny trifoliate orange. few years after planting. Similar trees of which predisposes it to such diseases. Of 69 trees on Cunningham citrange the same age but budded on sour orange None of the eight trifoliate orange hy- stock-16 combinations-only trees bud- stock have grown normally. In all other brid combinations budded on sour or- ded to the same strain of Chase Eureka combinations, growth of the Troyer cit- ange, sweet orange, or Sampson tangelo lemon as budded on trifoliate orange range stock is larger than the scion but showed any indication of exocortis. Many showed symptoms of the disease. The in this instance the Troyer stock is much of these trees are 33 years of age. Some bark scaling on the citrange stock of 10 smaller than the scion. grapefruit trees on sweet orange stock trees examined extended up to the ground Fifteen of the Eureka trees on Troyer planted in 1917 and topworked to Mor- line but not up to the bud union as it stock were pulled in February, 1952. ton citrange in 1944 have remained did with trifoliate orange. There was no Nine trees had evidence of exocortis at healthy. external evidence of the disease in the or below the crown roots. The bark was It appears probable from these inves- 6-8 inches of the stock exposed above stripped from the bud union area at this tigations that exocortis is not due to in- ground. These trees were much larger time. Not only was there a sharp line herent factors, but may be of a virus and in better health than the trees budded of demarcation at the bud union of the nature. on trifoliate orange. These trees were lemon and citrange, but the wood of the An experimental planting made in pulled in 1950 and it was observed that citrange was stained and discolored. 1951 at Riverside-using roots of trifoli- the bark shelling extended to the very Whether these symptoms are related to ate orange, Morton citrange, and Troyer extremities of the root system. exocortis is not yet established. citrange-will permit further investiga- Of 97 trees on Morton citrange- stock That some trees will grow" satisfac- tion of the problem of exocortis. examined-5 combinations-only one torily on Troyer is demonstrated by 40 1.p. Bitters is Associate Horticuburist, Uni- combination displayed definite symptoms Monroe Lisbon lemon trees. They are the versity o/ ~~li/~~~kcollege o~ ~~~i~~/~~~~~, of the disease. Of 13 trees of the CES same age as the Eureka lemons on Trover Riverside. Eureka lemon strain budded on Morton stock aid are growing normally. In 1951 The above progress report is based on R~ and planted in 1943, all showed severe they were better trees than similar tops search Projects 193~and 194. exocortis extending to the bud union the same age budded on sour orange. Twenty of the Monroe Lisbon lemon trees trifoliate orange stock budded to Eureko Morton citrange stock budded to Eureko lemon were pulled in February, 19527 and there lemon, 22 years old, showing the reduction in eight years old, showing the scaling extending was no evidence of exocortis on the sire of the stock. prodigious scaling and lack of to the bud union. Troyer roots. tluting. None of three combinations budded on Rusk citrange and Savage citrange had any indications of exocortis in 1951. A few trees of Washington Navel orange budded on Coleman citrange and ex- amined a year previously also showed no exocortis symptoms.

Seedlings and Cuttings Of 38 seedlings or cuttings examined there was no evidence of any bark disor- ders identified as exocortis. Several seed- lings out of the 17 Sanford citrange F,- second generation-seedlings examined in 1950 did show a peculiar type of rough scaling bark extending up the trunk and into the branches. No transmission tests