CITRUS ROOT GROWTH IN SOILS SUBJECT

FORD: FLATWOODS CITRUS STUDIES 41 9. Reitz, H. J., C. D. Leonard, Ivan Steward, R. C. J. Koo, D. V. Calvert, C. A. Anderson, P. F. Smith, and G. K. Rasmussen. 1964. Recommended fertilizers and nu tritional spr.ays for citrus. Fla. Agr. Expt. Sta. Bulletin 536B. 10. Reitz, Herman J. and Wallace T. Long. 1952. Min eral composition of citrus leaves from the Indian River area of Florida. Proc. Fla. State Hort. Soc. 65: 32-38. 11. Reitz, Herman J. and Wallace T. Long. 1955. Water table fluctuation and depth of rooting of citrus trees in the Indian River area. Proc. Fla. State Hort. Soc. 68: 24-29. 12. Reuther, Walter and Paul F. Smith. 1951. Relation of fertilizer treatment to fruit quality of Valencia oranges. Proc. Fla. State Hort. Soc. 64: 29-35. 13. Reuther, Walter and Paul F. Smith. 1954. Effect of method of timing nitrogen fertilization on yield and quality of oranges. Proc. Fla. State Hort. Soc. 67: 20-26. 14. Sites, John W., I. W. Wander, and E. J. Deszyck. 1953. The effect of fertilization timing and rate of applica tion on fruit quality and production of Hamlin oranges. Proc. Fla. State Hort. Soc. 66: 54-62. 15. Sites, John W., I. W. Wander, and E. J. Deszyck. 1961. The rate and timing of nitrogen for grapefruit on Lakeland fine sand. Proc. Fla. State Hort. Soc. 74: 53-57. 16. Smith, P. F. and G. K. Rasmussen. 1961 Effect of nitrogen source, rate, and ph on the production and quality of Marsh grapefruit. Proc. Fla. State Hort. Soc. 74: 32-38. THE EFFECT OF ROOTSTOCK, SOIL TYPE, AND SOIL ph ON CITRUS ROOT GROWTH IN SOILS SUBJECT TO FLOODING Harry W. Ford Florida Citrus Experiment Station Lake Alfred Most of the poorly drained soils currently be ing developed for citrus are located in low, flat areas popularly called flatwoods. The soils of the flatwoods in the native state are covered with pines and palmettos and are generally fine sands, usually containing an organic pan. Flooding usually occurs during the summer months. Most flatwoods soils desirable for citrus were selected many years ago. The remaining areas contain soil types ranging from those suitable for citrus with proper drainage, to types that are submarginal. The line of demarcation between suitable and submarginal soils for citrus is not clearly estab lished. The following acid soils have been listed in order of decreasing desirability for citrus: Blanton, Scranton, Ona, Leon, Immokalee, Pomello, and St. Lucie fine sands (4). Changes in soil characteristics have been shown to influence tree development in flatwoods groves. Ford and Eno (6), after studying micro organisms, citrus, nitrate production, and nutrient content, reported that the charac teristic most closely related to tree size was quantity of citrus per unit volume of soil. Root distribution was extremely low in the leached zones of Leon and Immokalee fine sands. Under adequate drainage, tree size was increased when a thicker subsoil organic layer was present (4). Bryan (1) found that breaking the organic pan and adding lime resulted in greater growth than various fertilizer treatments. Florida Agricultural Experiment Stations Journal Series No. 1965. Limited field observations on the tolerance of citrus roots to flooding have been recorded (12, 13) but these workers did not consider the sub soil structure or ph. A study of the tolerance of roots to flooding was conducted in the greenhouse (1) but the results were difficult to interpret since the roots lived considerably longer than the time required to kill citrus roots in the field. This paper presents data from studies in the greenhouse indicating that root survival under flooded conditions is influenced by rootstock, soil type, and soil ph. Methods Experiment 1. Twelve sheet-metal columns, 6 inches in diameter and 2 feet in height were constructed. Polyethylene inserts were placed in the columns and wire screens secured to the bottoms. s from different horizons were used to construct 3 soil profile types. The first profiles were formed from soil secured in the Owens- Fisher grove at Largo and resembled a Leon fine sand containing a leached A2 horizon. The con structed profiles contained the following hori zons: inches gray topsoil; 6-16 inches, white leached horizon; 16-20 inches, organic pan; 20-24 inches, white sand below the organic pan. The second series of profiles consisted of 6 inches of gray Leon fine sand topsoil from the Owens- Fisher grove and a 6-24 inch zone of white washed sand. The third group of profiles were from a Lakeland fine sand. The inch zone was topsoil from a citrus grove and the 6-24 inch zone was secured from a virgin Lakeland subsoil. Rough seedlings were planted in the cylinders and grown for 11 months to determine

42 FLORIDA STATE HORTICULTURAL SOCIETY, 1964 if adequate soil moisture would eliminate the poor root growth that occurs in the leached horizon of Leon and Immokalee fine sands under field conditions. One 7-gram application of 6-4-8-3 fertilizer was mixed with the top soil of each con tainer. The profiles were kept moist with deionized water. Experiment 2, Ninety-six fiber pipe cylinders 3 feet long and 6 inches in diameter were cut longitudinally so that roots could be observed by removing one side of the columns. Each cylinder held 55 pounds of soil. Leon fine sand was ob tained from a grove near Bowling Green. Var ious horizons of the profile were collected sepa rately. Each horizon was divided into two groups. The first group received no treatment and the second contained sufficient calcium carbonate to raise the soil ph to to 7.0. The soil horizons were placed in the cylinders so as to resemble the Leon fine sand profile that existed in the field. The depth zones in the columns were: inches, topsoil; 12-18 inches, leached A2 horizon; 18-24 inches, organic pan layer; and 24-36 inches, leached white sand (C horizon). Seedlings of 'Estes' rough, 'Milam', sour orange, sweet orange, 'Cleopatra' mandarin, and P. trifoliata were planted in the containers and grown for 6 months. The plants received a soluble fertilizer solution at 2-week intervals and were watered with deionized water as needed. Experiment 3. Sixty fiber pipe cylinders were divided into two groups and filled with soil as outlined in Experiment 2. Seedlings of "Estes' rough, 'Milam', sour orange, sweet orange, and 'Cleopatra' mandarin were permitted to grow for a period of 6 months before they were subjected to flooding. The columns were placed in 55-gallon drums. Ground water was added slowly from the bottom until the drums and cylinders were full. The tops of the cylinders were sealed with plastic to minimize oxygen transfer. After 2 weeks, the water was drained from the drums and the plants permitted to grow for 4 weeks, after which the cylinders were dis mantled. Thus, it was possible to ascertain the exact location of live tissue and note the rate of regrowth of new roots. Experiment 4- Ninety-six fiber pipe columns were divided into two groups and filled with Leon fine sand as outlined in Experiment 2. Valencia oranges budded on 'Carrizo' citrange, 'Rangpur lime,' 'Milam', rough, P. trifoliata, sweet orange, sour orange, and 'Cleopatra' man darin were planted in the cylinders. After 9 months, the cylinders were flooded by the method outlined in Experiment 3. After 2 weeks of flooding, the root systems were permitted to grow for a period of 2 weeks before the cylinders were dismantled. Experiment 5. Thirty-two fiber cylinders were filled with Lakeland fine sand after being divided into two ph groups. The cylinders were planted with 'Queen' budded on rough and sour orange and grown for 6 months. The cylin ders were flooded for 2 weeks to compare root damage with that which occurred in Leon fine sand profiles. Results Experiment 1. Feeder roots of rough did not grow satisfactorily in the leached A2 zone of Leon fine sand even with adequate mois ture (Table 1). The seedlings all grew vigorously during the 11 months of the greenhouse experi ment. No evidence of deficiency symptoms oc curred. In general, concentrations of in the Leon profiles were identical to those ob served under field conditions (6). The extensive root development in the organic pan indicated that this layer was satisfactory for citrus root development under adequate drainage. Root growth in the washed sand was almost identical to that which occurred in the A2 horizon of Leon fine sand. In contrast, there were almost 3 times as many roots in the Lakeland subsoil as were found in the washed sand or Leon A2 horizon. Experiment 2. Root distributions of the six kinds of seedlings grown in Leon fine sand pro files are compared in Table 2. Root concentra tions were lower in the leached white layers than in the topsoil or organic pan even though both moisture and nutrients were adequate. The root development of rough was the most concen trated, while 'Cleopatra' mandarin and P. tri foliata were low in numbers of. Experiment S. The damage to citrus feeder roots following 14 days of flooding in Leon fine sand profiles is recorded in Table 3. In general, all of the stocks were damaged more severely at the low ph of 4.8 to 5.3 than at higher ph of to 7.0. 'Estes' and 'Milam' were the most water tolerant. 'Estes' and 'Milam' are new rootstocks released for use in burrowing nematode-infested areas (7). The of sour orange were se verely damaged at low ph. All roots below 3 inches were destroyed. All of the plants showed

FORD: FLATWOODS CITRUS STUDIES 43 Table 1. Root distribution SJ of 'Estes1 rough seedlings after 11 months in constructed soil profiles. Profile Depth Zone Feeder Roots (sons.) Lateral Roots (sniff ) Leon Fine Sand Leached horizon Organic pan Leached subsoil 6-16 16-20 20-24 3.7 1.2 4.0 1.4 4.7 1.3.6.2 Constructed Washed sand 6-24 3.6 1.1 4.4.8 Lakeland Fine Sand Subsoil horizon 6-24 2.8 3.3 3.3.7 Grams of dried roots per liter of soil. slight wilting and vein-clearing symptoms typi cal of water damage. Sweet orange was also severely damaged when the subsoil ph was below 5.3. The plants in acid soil showed slight wilt and vein-clearing symp toms. However, all plants were alive 4 weeks after flooding for 14 days. There were some visible wilt and leaf pattern symptoms at ph 7 with more damage to than was found on sour orange. The roots of 'Cleopatra' seedlings were se verely damaged at ph 5 and ph 7. All of the plants showed visible symptoms of water damage and recovery was poor. Fifty percent of the plants eventually died. Experiment 4. Data involving flooding the roots of budded citrus trees are shown in Table 4. The odor of hydrogen sulfide was detected in the soil of the ph 5 profiles after flooding. 'Carrizo' and 'Rangpur lime' were damaged less by flooding that the other rootstocks tested. There were no leaf patterns or other visible symptoms of flooding damage in the scions on these two Table 3. Seedling response after 14 days of flooding In Leon fine sand profile columns. Table 2. Feeder root* i in a Leon fine i land profile recon- strueted In fiber cylinders. Seedlings dh (Inches) Relative recovery 4 weeks after flooding Seedlings 1Estes1 rough Mil am' Sweet orange 'Cleopatra' mandarin P. trifollata Surface soil " 1.6 0.9 0.6 1.4 0.6 0.2 Gran is of Feeder Roots Leached Organic horizon pan 6-18" 18-24" 1.0 0.4 0.4 0.3 0.1 2.1 1.8 1.9 2.0 1.7 0.3. SJ Subsoil horizon 24-36" 1.2 0.8 0.7 0.1 1Estes1 rough Ml lam1 Sweet orange - 7.0-7.0-7.0-7.0 10 29 20 3 29 4 25 Beat Best Fair Expressed as graai s of dried 6 inches in diameter and 6 Inches < leep. in a column 'Cleopatra1 mandarin - 7.0 L 5 Poor Poor

44 FLORIDA STATE HORTICULTURAL SOCIETY, 1964 Table 4. Feeder root survival on rootstocks with sweet orange scions after 14 days of flooding in Leon fine sand profile PH columns. PH 'Carrizo' citrange P. trifoliata 15 'Rangpur lime' Sweet orange 12 19 'Milam1 26 28 Sour orange 9 18 Rough 24 'Cleopatra' 4 15 rootstocks. 'Milam' and rough showed some root injury from flooding and several plants were mildly wilted when flooding was terminated. Poncirus trifoliata was damaged at ph 5 but at ph showed excellent tolerance to flooding. Both sweet and sour orange rootstocks were dam aged. Again, root loss was more severe at a sub soil ph of 5. 'Cleopatra' was the least tolerant to flooding regardless of subsoil ph. Experiment 5. 'Queen* on rough and sour orange in Lakeland fine sand showed only slight root damage from water whether at ph 5 or ph (Table 5). Feeder roots were alive in the bottoms of the containers. stock was damaged and most severely at ph 5. Damage was not enough to produce foliar symptoms in the scions. All of the plants showed excellent re covery from the flooding treatments. In general, rough and sour orange showed less feeder root damage from flooding in Lakeland fine sand than occurred in Leon fine sand. Discussion The poor root growth found in the leached horizon of certain flatwoods soils was not entirely eliminated by providing adequate water and fer tilizer even though drainage was adequate. These data, together with observations in the field (3, 6) indicate that soils in flatwood areas that are planned for citrus should be surveyed to deter mine the nature of the profile. The extent of the white leached A2 horizon is an important factor in determining whether an area is suitable or un suitable for citrus. Poor feeder root growth may occur in white sand even though moisture and nutrients are adequate. Root damage resulted from flooding for 14 days. This was in contrast to Prevatt (11) who was unable to obtain extensive root damage over longer periods. The marked influence of subsoil ph on the flooding tolerance of citrus roots indi cates that subsoil ph should be considered in pre paring new land for citrus. Improved growth of citrus trees with subsoil liming was reported by Bryan (1). Part of the improved response at tained by Bryan may have been associated with improved water tolerance since the trees were subjected to high water during the first 18 months in the field. It has been commonly assumed that citrus roots are killed by flooding because of a deficiency of oxygen per se. Studies by Ford (5) suggest that sulfides formed by anaerobic bacteria which Table 5. Response of rough and sour orange after 14 days of flooding in Lakeland fine sand columns. ph Relative recovery 4 weeks Rough 4.8 7.0 4.8 7.0 21 24 after

COHEN, GRIMM, BISTLINE: CITRUS FOOT ROT 45 reduce sulfates can, under suitable conditions, rapidly kill citrus. Many of the raw materials for the reaction are supplied in the nutritional program and citrus could furnish the energy sources necessary for their own destruction. Root damage by sulfides is more rapid at low soil ph and in the leached horizon of the subsoil. The fact that rough showed higher flooding tolerance than sour orange was sur prising since it had been assumed from earlier field observations that sour orange was more tolerant to flooding than rough (12). Early observations were made in alkaline soil types and in somewhat finer textured soils. Rough is more susceptible to foot rot (10) than sour orange and foot rot is usually more prevalent under wet, humid conditions. Foot rot can be a complicating factor when attempting to evaluate the tolerant to flooding characteristics of a rootstock in the field. In general, the data are in agreement with Gardner (8, 9) who reported that rough and 'Carrizo' citrange were more tolerant to flooding than sour orange. The relatively poor feeder root growth of Poncirus trifoliata in Leon fine sand, together with the root damage that occurred at ph 5 when flooded, suggests that this stock should be carefully evaluated before planting on a large scale in the acid fine sands of the flatwoods. The satisfactory tolerance to flooding of 'Rangpur lime,' as indicated in these studies, combined with the encouraging results of rootstock trials by Cohen and Reitz (2), suggests that 'Rangpur lime/ should receive further study. Summary Poor growth of citrus occurred in the leached zone of certain acid soils of the flatwoods. In columns of Leon fine sands in the greenhouse, poor root growth was not entirely corrected by applications of adequate water and nutrients. Certain popular citrus rootstocks were grown in 3-foot fiber cylinders containing reconstructed Leon fine sand profiles at two ph levels. The soils were flooded for 14 days. Rough pro duced the best feeder and lateral roots; however, damage was more severe at low soil ph. 'Cleo patra' mandarin was severely damaged at both low and high soil ph. LITERATURE CITED 1. Bryan, O. C. 1962. The response of young citrus trees to lime mixed with the soil profile in Leon-Immokalee fine sands. Citrus Ind. Mag. 43(9) : 9-10. 2. Cohen, M. and H. J. Reitz. 1963. s for 'Valencia' orange and 'Ruby Red' grapefruit; Results of a trial initiated at Fort Pierce in 1950 on two soil types. Proc. Fla. State Hort. Soc. 76:29-34. 3. Ford, H. W. 1959. Root distribution of citrus State Project 663. Ann. Rept. Fla. Agr. Exp. Sta. 1959:219. 4. Ford, H. W. 1963. Thickness of a subsoil organic layer in relation to tree size and root distribution of citrus. Proc. Amer. Soc. Hort. Sci. 82: 177-179. 5. Ford, H. W. 1965. Products from bacterial metabo lism that affect citrus root survival in poorly drained soils. Proc. Amer. Soc. Hort. Sci. (In press). 6. Ford, H. W. and C. F. Eno. 1962. Distribution of mircoorganisms, citrus, nitrate production, and nutrients in the profile of Leon, Scranton, Immokalee, and Blanton fine sands. Proc. Fla. State Hort. Soc. 75: 49-52. 7. Ford, H. W. and W. A. Feder. 1964. Three citrus rootstocks recommended for trial in spreading decline areas. Fla. Agr. Exp. Sta. Cir. S-151: 1-8. 8. Gardner, F. E. 1961. Evaluation of citrus rootstocks for Florida citrus. Citrus and Veg. Mag. 24(10) : 12. 9. Gardner, F. E. 1961. Evaluation of citrus rootstocks for Florida citrus, II. Citrus and Veg. Mag. 24(11): 16. 10. Knorr, L. C, R. F. Suit, and E. P. DuCharme. 1957. Citrus Diseases in Florida. Fla. Agr. Exp. Sta. Bui. 587: 57-58. 11. Prevatt, R. W. 1959. Tolerances in certain citrus seedlings to free water in soil. Ph.D. Thesis. Univ. of Fla. 12. Sites, J. W., L. C. Hammond, R. G. Leighty, W. O. Johnson, and K. D. Butson. 1964. Information to consider in the use of soils of flatwoods and marshes for citrus. Fla. Agr. Exp. Sta. Cir. S-135-A. 13. Young, T. W. 1948. moisture and the citrus root system. Proc. Fla. Hort. Soc. 61: 74-79. FOOT ROT IN YOUNG GROVES Mortimer Cohen,1 Gordon R. Grimm2 and Fred W. Bistline3 Introduction circumference of the tree near the ground line is killed. Usually some gum exudation accompanies the killing of the bark. In Florida, foot rot is incited by Phytophthora parasitica. Foot rot disease is that condition in citrus trees in which part or all of the bark around the lassociate Plant Pathologist, University of Florida Citrus Experiment Station, Indian River Field Laboratory, Fort Pierce. 2P\ant Pathologist, U. S. Department of Agriculture, Horticultural, Field Station, Orlando. 3Horticulturist, Minute Maid Groves Corporation, Orlando. Florida Agricultural Experiment Stations Journal Series No. 1964. Foot rot disease has been very prevalent in young groves in the Indian River area in recent years and in certain groves has been extremely serious. One 40-acre area of young Valencia trees on Rough rootstock had 66% of the trees killed by foot rot, necessitating the planting of 2,600 replacement trees. Most affected plantings have had fewer diseased trees but foot rot is