LEAF DROP FOLLOWING SPRAY APPLICATIONS

THOMPSON: LEAF DROP 29 Control in Citrus Groves. Fla. Agri. Expt. Sta. Agronomy. Mimeo Rept. 58-3. 3. Burt, E. O. and J. T. McCown, 1959. Controlling Perennial Grasses in Citrus Groves. So. Weed Conf. Proc. 12: 93-99. 4. King, J. R., W. A. Simanton and D. W. Kretchman, 1958. Chemical Control of Weeds in Florida Citrus Grove Ditches. Proc. Fla. State Hort. Soc. 71: 157-166. 5. Simanton, W. A., 1958. Weed Control in Citrus Groves. Ann. Rept. Fla. Agr. Expt. Sta. pp. 256-257. 6. Simanton, W. A. and J. R. King. 1956. Weed Con trol Problems in Florida Citrus Groves. Proc. The Soil and Crop Sci. Soc. of Fla. 16: 297-304. LEAF DROP FOLLOWING SPRAY APPLICATIONS ON CITRUS W. L. Thompson Florida Citrus Experiment Station Lake Alfred Leaf drop following the application of sprays to citrus trees in Florida is often of concern to people in production. A light leaf drop is of no consequence, but an excessive drop may cause dead wood and decrease leaf area to the extent that production is affected. A so-called "natural leaf drop" usually occurs just before or during the time a new flush of growth is coming out. Oil sprays are the most common cause of leaf drop. As far back as 1895 Hubbard (2) stated that kerosene was a shock to the tree and sometimes caused leaf drop and dead wood, especially on trees in a weakened condition from scale insects. Thompson (3) reported heavy leaf drop fol lowing oil sprays applied prior to cold weather where the trees were heavily infested with purple scale, Lepidosaphes beckii (Newm.), but a very light leaf drop where there was only a light scale infestation. Ebeling (1) and others have reported leaf drop following oil sprays in California. Leaf drop has also been reported from materials other than oil. Thomp son (4) found that 2 gallons of lime-sulfur per 100 gallons usually hastened the drop of old chlorotic leaves. However, excessive leaf drop following application of insecticides is the exception rather than the rule. Methods-. Data presented in this paper were collected from scale control experiments. Usually leaf drop records were taken only when it was observed that there was more leaf drop following some treatments. Dropped leaves were collected from the inside of a triangular frame that covered about 1/32 of the area under the tree. This frame reached from the trunk of the tree to the outside edge Florida Agricultural Experiment Station Journal Series, No. 971. of the limbs. One sample was taken from each quadrant of a tree, or a total of about 1/8 of the area. Where periodic samplings were made, all leaves were raked from under the trees after each count. Unless otherwise desig nated, each block of experimental trees was divided into 3 or 4 sections and all treatments were randomized in each section. The total number of count trees ranged from 6 to 12 per treatment. In this paper the term "oil" or "percent oil" refers to the oil in the dilute oil emulsion. Results. Oil or parathion: Tree condition appeared to be a factor associated with leaf drop in duplicate plots in a grapefruit grove sprayed with oil emulsion just before and during a period of natural leaf drop. In 1953, excessive leaf drop followed 1.3 percent oil sprays applied January 15 and February 15. Leaf drop was not severe for nearly four weeks after the January spray but was severe during the next three weeks. In an adjacent plot, sprayed February 15, leaf drop was excessive immediately after the spray, but a leaf drop was in progress at the time the oil was applied. In a third plot the oil spray was delayed until March 16 after leaf drop had already occurred. Comparatively few leaves dropped after the application and the total number of dropped leaves was much less than in the other two plots. A fourth plot had not been sprayed with oil since the previous April. Here there was the least leaf drop, but the pattern of increased drop was the same as in the other three plots (Figure 1). In this 1953 experiment, 0.25 pound actual parathion per 100 gallons was also applied January 15, February 15 and March 16 to other plots. There was a higher than average leaf drop after the January spray. The Feb ruary and March parathion sprays caused con siderably less drop than oil, but the drop on

30 FLORIDA STATE HORTICULTURAL SOCIETY, 1959 l300r 1200 - ACCUMULATIVE LEAF OROP FOLLOWING OIL SPRAYS ON GRAPEFRUIT TREES gress at the time the sprays were applied. The weather was dry and the humidity was rel atively low, but the grove had been irrigated about a week before application and the trees showed no sign of wilting. Although the top 6 inches of soil was dry, it was moist from 6 inches to 4 feet and from 4 feet to 5 feet it was moist enough to ball. Leaf drop was observed in all plots, but the heaviest drop occurred where oil and parathion were in cluded in the spray. The next heaviest drop followed the spray supplemented with 0.8 percent oil. The amount of leaf drop was significantly less where oil was not included, but the progression of drop was the same. Figure 2. Results. Oil plus parathion or Malathion: In the summer of 1958, records were made of dropped leaves following oil, oil plus para thion, and oil plus malathion mixtures sprayed on mature Hamlin orange trees. Applications were made July 18, August 8 and August 21. J.-I5 J.-27 F.-ll F.-24 M.-2 M.-I6 SAMPLING DATES g% these two plots was still higher than in the unsprayed check. In the three oil-sprayed plots an average of 50 percent of the dropped leaves were af fected with greasy spot, but only 29 percent were affected in plots that were not sprayed. It is believed that the severe leaf drop in plots sprayed in January and February was due to the high percentage of leaves affected with greasy spot plus the effect of the oil. The spring growth in these plots was sparse and the leaves were small, which was the type of growth that follows severe defoliation. Results: Oil and oil plus parathiom-the combination of a low dosage of oil and parathion is an excellent scalicide that is used rather extensively, but sometimes this com bination causes excessive leaf drop. On March 28, 1956, post-bloom sprays containing 1.5 pound of basic copper sulfate plus 0.75 pound of lead arsenate per 100 gallons were applied on grapefruit trees. This mixture was supple mented with 0.8 percent oil in one case and with 0.43 percent oil plus 0.15 pound actual /parathion in another. A leaf drop was in pro The leaf drop was not heavy after any of the treatments and the leaves that did drop appeared to be old. Nevertheless, significantly more leaves dropped in plots sprayed with oil on July 18 than where parathion was used or 1000 900 800 2 700 i 600 * 400 300 200 ACCUMULATIVE LEAF DROP FOLLOWING OIL 8 OIL-PARATHION SPRAYS ON GRAPEFRUIT TREES-MARCH, 1956 MO.43% OIL + 0.15 Ib PARATHION/100 GAL. 2«0.8% OIL 3* NO SCALICIDE LSOJIJI!I* 204!li 283 100 8 16 21 DAYS AFTER SPRAY 28

THOMPSON: LEAF DROP 31 in untreated checks. The greatest leaf drop followed an application of 0.7 percent oil plus 0.15 pound actual parathion per 100 gallons and there was a trend to less drop where 0.5 percent oil was added to the para thion. There was no significant difference, however, in leaf drop between either 0.5 percent or 0.7 percent oil plus parathion and 1.3 percent oil alone. The August 8 applications produced less leaf drop than the July 18 applications, but the drop was significantly higher than the check in all treatments except 1.3 percent oil. Leaf drop was significantly higher following 0.7 percent oil plus parathion than 1.3 per cent oil alone or an oil-malathion mixture. Following the August 21 sprays, there were no significant differences among treatments but the trend of drop was the same as in the other two tests. Table 1. Results. Oil and phosphatic compounds: On April 22, 1959, mature Valencia orange trees were sprayed with several organic phos phate compounds. These materials were used alone and mixed with 0.5 percent oil emulsion. Oil emulsion at 0.7 and 1.3 percent oil was also applied for comparison. Two pounds of Cop-film (34 percent metallic copper) and 1 pound of 65 percent zineb per 100 gallons were included in all sprays to control melanose and rust mite. A record was made of dropped leaves two weeks after application. In this experiment both dosages of oil produced a significant amount of leaf drop. Delnav, malathion and Korlan did not ac centuate a leaf drop, but Ethion did. There was a significant leaf drop where all of these materials were used with oil, but this drop was due to the oil rather than the mixtures. Where Ethion was used with oil, however, the leaf drop was due partly to the oil and partly to the Ethion. Table 2. Results: Lime-sulfur and copper com pounds: Sometimes leaf drop is caused by spray combinations that are believed to be comparatively safe. Before 1955 a mixture of lime-sulfur and copper sulfate or other copper compounds was thought to be safe on citrus and was in commercial use. In the spring of 1955, however, reports were received of severe Table 1. Leaf Drop Following Oil, Oil-Parathion and Oil- Malathion Sprays on Hamlin Orange. Materials in Percent or of Active Ingredients per in Pounds 100 Gallons 1 Number of Dropped Leaves per Unit Area Spray Dates July 18 Aug. 8 Aug. 21 Oil emulsion 1.3% oil 225a 75b 200c Oil emulsion 0.7% oil + 0.,15 lb. parathion 306 157 295 Oil emulsion 0.5% oil + 0..15 lb. parathion 177 114 236 Oil emulsion 0.7% oil + 0,.5 lb. malathion... 80 110 Parathion 0.15 lb. 93. No scalicide 22 18 - L.S.D. 19:1 135 58 ns 99:1 173 80 ns Days after spray: a b - 14-12 c - 42

32 FLORIDA STATE HORTICULTURAL SOCIETY, 1959 leaf drop and fruit bum following sprays of and was heaviest with lime-sulfur plus copper lime-sulfur plus copper compounds. On April oxide. Table 3. 15, 1955, grapefruit trees were sprayed with Results-.-Miscellaneous mixtures :-Leai drop lime-sulfur used alone or mixed with copper was not noticeable in a block of grapefruit sulfate, basic copper sulfate and copper oxide, sprayed May 16, 1958, with 1.3 percent oil Basic copper sulfate plus wettable sulfur was alone or with oil mixed with the following comalso applied. Leaf drop was heavy where pounds in amounts per 100 gallons: zineb 0.5 lime-sulfur was mixed with copper compounds pound; basic copper sulfate, 1.5 pound; zineb Table 2. Leaf Drop Following Oil and Mixtures of Oil and Phosphatic Compounds Applied on Valencia Orange Trees. Materials in Percent or Pounds Active Ingredients per 100 Gallons No. Dropped Leaves Sprayed April 22, 1959 Per Unit Area Oil emulsion 1.3% oil Oil emulsion 0.7% oil Delnav1 0.5 lb. Delnav 0.5 lb* + oil emulsion 0.57. oil Ethion2 1.0 lb. Ethion 0.5 lb. + oil emulsion 0.5% oil Malatbion3 0.5 lb. Malathion 0.5 lb. + oil emulsion 0.5% oil Malathion 0.5 lb. + oil emulsion 0.7% oil Korlan4 0.25 lb. Korlan 0.25 lb. + oil emulsion 0.5% oil 417 287 143 473 304 449 78 135 369 97 236 Mo scalicide 33 L.S.D. 19:1 99:1 174 231 Heutral copper 2 lb. + zineb 1 lb. per 100 gallons included in all sprays. XDelnav «33% 2,3,p-dioxanedithiol-bis-(0,0-diethylphosphorodithioate). 2Ethion -47.1% 0,0,0,O'-tetraethyl S,S1 -methylene bisphosphorodithioate. ^lathion - 87% 0,0-dimethyl dithiophosphate of diethyl mercaptosuccinate. 4Korlan 25W - 25% 0,0-dimethyl 0-2,4,5-trichlorophenyl phosphorothioate.

THOMPSON: LEAF DROP 33 Table 3* Leaf Drop Following Mixture* of Lime-Sulfur and Copper Compounds Applied on Grapefruit* Materials in Gallons or Pounds Per 100 Gallons Sprayed April 19, 1955 No* Dropped Leaves Per Unit Area Lime-sulfur 2 gal* Lime-sulfur 2 gal. + 3 lb. copper sulfate Lime-sulfur 2 gal* + 1*5 lb* basic copper sulfate 3 Lime-sulfur 2 gal* 1 lb. copper oxide Wettable sulfur 5 lb. + 1.5 lb. basic copper sulfate 48 152 284 417 21 Mo treatment 25 L*S*D* 19:1 99:1 118 168 Percent metallic copper: 1 251 2-53X 3-75X and basic copper sulfate; and a combination of zineb, basic copper sulfate and 0.75 pound of lead arsenate. In two other experiments there was no significant difference in leaf drop between treated and untreated plots where the following sprays were applied October 9, 1958, to grapefruit and Pineapple oranges: 1.3 percent oil alone and mixed with 0.5 pound of zineb per 100 gallons; 0.7 percent oil plus 0.15 pound actual parathion and 0.5 pound of zineb; and 0.7 percent oil plus zineb and 0.5 pound actual Ethion. Although no data are available, it has been observed that heavy leaf drop sometimes occurs following an application of a mixture of parathion, DN Dry Mix and wettable sulfur. Discussion: Excessive leaf drop following the application of insecticides and other ma terials to citrus trees is the exception rather than the rule. It appears to be associated with normal shedding of old or diseased leaves and their drop is hastened by any spray that is a mild shock to the tree. A severe shock causes apparently healthy leaves to drop. Oils are the most common cause of leaf drop, but tree condition at the time application is made appears to be a factor where excessive drop occurs. With few exceptions, phosphatic compounds do not cause severe leaf drop and, except for parathion and Delnav, have not accentuated leaf drop when used with oil. In addition to mixtures of most phosphatic compounds with oil, mixtures of zineb, neutral copper compounds and lead arsenate with oil have been no more hazardous than oil alone. However, the emulsion may occasionally break in such mixtures and result in free oil that will cause a severe leaf drop. Lime-sulfur has promoted leaf drop and mixtures of lime-sulfur and copper compounds may be a severe shock to the trees and should not be used. Observations have also been made of leaf drop following mixtures of DN Dry Mix, parathion and wettable sulfur. Since there is now a wide variety of pesti cides to choose from, it is advisable to use the ones that are less likely to injure the

34 FLORIDA STATE HORTICULTURAL SOCIETY, 1959 trees, especially under conditions when leaf drop is likely to occur. Summary Leaf drop was caused or increased by various pesticides applied alone or in com bination sprays. The greatest drop occurred when pesticides were applied just before or during the time when normal shedding of old or weak leaves was in progress. Regardless of when sprays were applied, scalicides pro duced leaf drop in the following descending order: parathion plus 0.7 percent oil; Delnav plus 0.5 percent oil; Ethion plus 0.5 percent oil; parathion phis 0.5 percent oil; 1.3 percent oil; malathion plus 0.5 percent oil; Ethion; par athion; Delnav and malathion. Leaf drop was not increased where the fol lowing compounds were added singly or in combination to 1.3 percent oil; zineb, basic copper sulfate and lead arsenate. Significantly more drop occurred where either basic copper sulfate or copper oxide were added to 2 gallons of lime-sulfur per 100 gallons than with lime-sulfur alone. The mix ture of copper sulfate and lime-sulfur caused more drop than lime-sulfur alone, but the difference was not significant. LITERATURE CITED 1. Ebeling, Walter, 1950. Subtropical entomology. Lithotype Process Co., San Francisco, Calif. 2. Hubbard, H. G. 1885. Insects affecting the orange. U.S.D.A. 3. Thompson, W. L. 1958. Control of purple scale and whiteflies with lime-sulfur. Fla. Agr. Exp. Sta. Annual Report. 4. Thompson, W. L. 1941. The present status of scale insects infesting citrus. Proc. Fla. State. Hort. Soc. 54. EFFECT OF LEAD ARSENATE SPRAYS ON DEADWOOD, YIELD, FRUIT SIZE, AND DROP OF MARSH GRAPEFRUIT' E. J. Deszyck and S. V. Ting Florida Citrus Experiment Station Lake Alfred Arsenical sprays are generally used in Flori da for early legal maturity and high internal quality of grapefruit. The chief effect of such sprays is lowered acidity of the juice, and consequently raised ratio of soluble solids to acid (6, 7). Rates and time of application of arsenic sprays have been definitely estab lished and recommended (5) to the grower for use in the production of legally early mature grapefruit (4) or high quality fruit lln cooperation with the Florida Citrus Commission. Florida Agricultural Experiment Stations Journal Series, No. 970. Table 1. Effect of lead arsenate on yield of Marsh grapefruit during nine seasons*. Treatment Lead Arsenate lb./loo gal. April 10 1951 March 27 1952 March 17 1953 March 1954 Harves 31 April 1955 t Date 12 March 28 1956 January 1957 8 January 1958 30 April 15 1959 Eight Year None 13.6 11,.1 10.9 13.3 12.1 8.4 12.9 10.7 10.1 11. 8 0.4 12.4 9,.6 10.0 13.1 9.9 6.6 11.9 8.4 9.9 10. 7 1.25 12.8 10.,0 10.2 11.6 9.2 8.0 11.3 8.1 10.3 10. 4 3.0 Fb 12.2 9.2 * 10.5 n.a. 10.5 9.1 n.s. 10.3 6.6 9.4 n.s. 9. 7 a Each result is respectively. the sean <»f 40, 30, and 15 tr< tea for the check,, 0.4 and 1.25, and 3 pounds lead arsenate, n.s. differences between neani > not significant. *P significant at 51 F significant at 1%