STEWART, WHEATON, REESE: MURCOTT COLLAPSE 15

Summary sprayer, It follows that a good spray program should: A good spray program maximizes sprayer 1. Minimize time for intergrove travel and time efficiency and minimizes costs consistent other nonproductive operations. with satisfactory control of insects and diseases. 2. Minimize gallons applied per acre and From the standpoint of the sprayer, better con maximize ground speed and effective width trol is usually associated with increasing gallons consistent with satisfactory control. applied per acre and decreasing ground speed and effective width. In contrast, timely appli LITERATURE CITED cations and minimal per-application costs are 1. Savage, Z. 1968. Thirty-five years of citrus costs and associated with decreasing gallons applied per returns in Florida, 1931-1966. Univ. of Fla. Agr. Ext. Serv. Econ. Series 68-3. p. 10. acre, increasing ground speed and effective 2. Simanton, W. A. 1962. Losses and production costs width. Therefore, the optimum results are attributable to insects and related anthropoids attacking citrusin Florida in 1960-61. Coop.Econ. Insect Kept. 12:45. achieved at some moderate grove capacity of the p. 1182.

'MURCOTT1 COLLAPSE DUE TO NUTRITIONAL DEFICIENCIES

Ivan Stewart, T. A. Wheaton, and R. L. Reese that removal is necessary. Trees less severely affected usually recoverrapidly during the fol University of Florida lowing growing season but fail to set a crop. Citrus Experiment Station, I FAS This paper presents studies and observations Lake Alfred made on this problem.

Abstract Survey

'Murcott' citrus trees bearing heavy crops Leaf and fruit samples were taken in 15 often collapse during the time of fruit maturity. groves to establish whether 'Murcott' collapse In some cases, this disorder is sufficiently severe was related to nutrition of the trees. The sam to kill some of the trees. The trouble apparently ples were taken during the time of fruit matur is caused by starvation for nitrogen and potas ity from individual trees showing collapse and sium. This variety responds to much higher rates from adjacent healthy trees with lighter crops. of these 2 elements than do common varieties Many of the trees were 8 years old or younger. of oranges and grapefruit. Results of the leaf analyses indicated that both N and K levels were lower in leaves from trees Introduction showing collapse (Table 1). Leaves from af Collapse is a common problem of 'Murcott' fected trees averaged 1.83% N, as compared trees. It occurs on trees with heavy crops and is with 2.37% in those from healthy trees. Potas first noticeable in December and January when sium was also low in leaves from collapsed trees, the fruit begins to mature. The initial symptom averaging 0.25% K; and the most severe cases is mottling of the leaves similar to that asso ranged below 0.20%. These are extremely low ciated with potassium deficiency. This is fol levels of K in citrus leaves. Leaves from trees lowed by partial defoliation and fruit drop. Much that appeared healthy averaged 0.89% K. of the fruit remaining on the trees may be Fruit samples were also taken from a num yellow to green incolor and of small size. The ber of groves. Analyses of various part of the longer the fruit remains on the tree, the greater fruit indicated significantly higher amounts of the amount of collapse. In the most severe N,K, Mg, and Ca in the fruit peel from col cases, the trees may die or suffer such damage lapsed trees than from healthy trees. Quality determinations showed that juice from collapsed Florida Agricultural Experiment Stations Journal Series trees contained 12.6% soluble solids compared No. 3129. 16 FLORIDA STATE HORTICULTURAL SOCIETY, 1968

Figr. 1. 'Murcott' tree showing: symptoms of collapse.

with 14.2% for that from healthy trees. The plots in September (Table 2). The grove had a acid was about the same, being 0.85% and history of collapse, and many of the trees had 0.84%, respectively. been replaced due to this disorder. Many of the trees in the survey were receiv

ing approximately 150 pounds each of N and Results K2O per acre per year. The results of the leaf analyses indicated that the heavy-bearing trees The fruit was harvested during the second were extremely deficient in both N and K and week of February. The greatest difference to be suggested that higher fertilizer rates were seen among fertilizer treatments was the general needed. condition of th heavy-bearing trees. Trees with large crops and receiving the lowest rates of N

Field Experiment showed typical symptoms of collapse. The leaves turned yellow; there was defoliation with fruit Based on the above survey, a fertility experi drop and limb dieback. The symptoms were ment was established on 10-year-old 'Murcott' much less severe on trees receiving 200 pounds trees on rough lemon rootstock growing east of of N; and those receiving the highest rate of N, Haines City. Nitrogen and potash were applied and 200 pounds or more of K20, had dark green at the rates of 100, 200, and 400 pounds per acre foliage and very little leaf or fruit drop. Trees per year in all combinations to 22-tree plots, receiving 100-pound rates of K.,0 showed symp replicated 4 times. All trees received 50 pounds toms of collapse. per acre each of N and K20 in February and Yields for the first year were quite variable, in May. In July, differential rates were com ranging between no fruit to over 9 field boxes menced, followed by additional fertilizer to some per tree, with an average of 3.5 boxes. The STEWART, WHEATON, REESE: MURCOTT COLLAPSE 17

Table 1. Nitrogen and mineral content of 'Murcott1 leaves and fruit from collapsed trees and from adjacent healthy trees*

Leaves Whole fruit Seed Juice Peel

% dry wt.

Healthy K .89 1.46 .78 1.37 1.41

Collapsed K .25** 1.58 .90* 1.30 1.69**

Healthy Ca 3.53 .39 .30 .06 .62

Collapsed Ca 4.95** .47 .29 .08 .82**

Healthy Mg .70 .17 .23 .15 .18

Collapsed Mg 1.10** .22** .22 .18** .27**

Healthy P .15 .15 .24 .12 .15

Collapsed P .11** .16 .24 .12 .16

Healthy N 2.37 1.08 2.28 .73 1.02

Collapsed N 1.83** 1.21** 2.29 .87 1.19**

* Value significantly different from "healthy" at 5% level.

**Value significantly different from "healthy" at 1% level.

great variation in yield among individual trees condition due to fertility rates. Visual ratings was apparently due to alternate bearing. Since based on the amount of foliage showed that those samples were taken at random, the differential which had large crops the previous year and had yield due to alternate bearing had an overriding received the lowest rate of N remained in a very effect on the fertility treatments. As a result, poor condition, and it was necessary to severely there were no significant differences due to prune many of them (Table 3). This variation fertilizer application on yield, size, fruit quality, in tree condition, because of differential fertiliza or leaf analyses. tion, undoubtedly will be reflected in future The failure to measure differences in yield crops. and fruit quality the first year due to fertilizers Conclusions is not unusual. However, the following Septem ber, there was an obvious difference in tree The studies reported in this paper are pre- 18 FLORIDA STATE HORTICULTURAL SOCIETY, 1968

Table 2. Rates and timing of N and K20 for 'Murcott1 Table 3. 'Murcott1 tree condition 7 months after fruit field experiment. harvest as affected by differential fertilizer rates.

Feb. May July Sept. Total

lbs/A lbs/A Rating Rating

50-50 50-50 0-0 0-0 100-100 100 2.62 100 1.74

50-50 50-50 0-100 0-0 100-200 200 1.25** 200 1.82

50-50 50-50 0-150 0-150 100-400 400 1.25** 400 1.57

50-50 50-50 100-0 0-0 200-100 **Values significantly different from 100 lbs/A rate at 1% level. 50-50 50-50 100-100 0-0 200-200

I. Trees rated as follows: 1. Dense foliage. 50-50 50-50 100-150 0-150 200-400 2. Thin foliage. 3. Thin foliage and dieback 50-50 150-0 150-0 400-100 50-50 of small limbs. 4. Dieback of large 50-50 50-50 150-100 150-0 400-200 branches.

150-150 150-150 400-400 50-50 50-50 by Knorr (1) and Knorr and Price (2). A dis ease called fovea was described in which some of liminary but suggest that 'Murcott' trees have the symptoms were similar to those we found a much higher N and possibly higher K20 fer from nutritional stress. Later, a 'Murcott' de tilizer requirements than common varieties of cline was also shown to have similar symptoms oranges, grapefruit, and tangerines. Usually, to those described in this paper (3,4). In view 200 pounds per acre per year or less of N and of our finding that nutritional stress can greatly K20 are sufficient for mature citrus trees. In damage and even kill 'Murcott' trees, it is be one 'Murcott' grove which had received 300 lieved that the previous work on virus problems pounds per acre per year, each of N and K20 will need to be reviewed with this in mind. for several years, the leaves contained less than In California, the 'Wilking' and 'Kinnow' 2% N and only 0.24% K. In another grove inter- mandarins (6), and in Australia, the 'Emperor' planted with 'Hamlin' oranges, and receiving 150 mandarins (5), have been reported to have pounds each of N and K20, the 'Murcott' leaves similar symptoms to those we have described as contained 2% N and 0.28% K, while the orange due to nutritional starvation in 'Murcotts.' leaves from trees on the same cultural program contained 2.23% N and 1.23% K. Further results LITERATURE CITED will be needed to establish optimum rates of 1. Knorr, L. C. 1959. Povea—disease of concern to "Murcott' growers. Citrus Ind. 40:5-7, 16. fertilizer for 'Murcotts.' However, present in 2. Knorr, L. C, and R. J. Collins. 1966. 'Murcotts' in dications are that they may require up to twice distress. Citrus Ind. 47:17-19. 3. Knorr, L. C, and R. J. Collins. 1966. 'Murcott" de as much N and K2O as mature orange and grape cline. Fla. Agr. Exp. Stas. Annu. Rept. p. 259. fruit trees. It is also suggested that very heavy 4. Knorr, L. C, and W. C. Price. 1959. Fovea—disease of the 'Murcott.' Citrus Mag. 22:16-19, 26. crops should not remain on the trees longer than 5. Lloyd, R. H. 1962. Mandarins, results of fruit thin ning and sizing trials with 2,4,5-T on 'Early Imperials' and is necessary for the fruit to mature. 'Emperors.' Citrus News. Nov. :125, 130. Problems with 'Murcotts' have been discussed 6. Rosedale, D. O., M. P. Miller, and R. G. Platt. 1968. Mandarin decline. Calif. Citrog. 53:126, 128.