Applying Roundup to the Base of Lemon Tree Canopies: Preliminary Effects on Leaves, Flowers, Fruitlets, and Yield

Abstract

The effect of Roundup on lemon trees was evaluated by repeatedly spraying 0.5, 0.75, 1, 1.25, and 1.5 lb. a.i./acre on the bottom 20 to 24 inches of the tree canopies over a three year period. The Roundup applications caused significant leaf injury in the sprayed area of the canopies and there was also significant defoliation of branches at the higher Roundup rates in all three years of the study. In 1996, flower and fruitlet counts were not affected by the Roundup applications and the 1998 data were inconclusive. However, flower and fruitlet counts in 1997 in the sprayed zone of the canopy were significantly reduced by Roundup and the effect increased with increasing Roundup rate. The 1996 and 1997 yield data indicated that Roundup applied to the bottom 20 to 24 inches of the tree canopies did not significantly affect lemon yield. The preliminary data suggest that accidental drift or misapplication of Roundup on to lemon trees when spraying weeds on the orchard floor has no short-term effect on grove productivity.

Introduction

Weeds in Yuma Mesa lemon groves have been managed by frequently disking and by applying preemergence (i.e., soil applied) and postemergence (i.e., foliar applied) herbicides. Disking orchards occasionally damages tree branches and may damage shallow tree roots. Disking also incorporates plant debris into the soil surface creating a breeding habitat for eye gnats on the Yuma Mesa. Thus, the use of herbicides has increased in recent years. Many preemergence herbicides including Goal, Prowl, Solicam, Surflan, Treflan, and Visor can be used on sandy soils without injuring citrus trees. Postemergence herbicides are used to control weeds that escape control by preemergence herbicides or mechanical cultivation, or to control all weeds in groves that are not treated with preemergence herbicides. One of the most popular postemergence herbicides in recent years has been Roundup and occasionally some foliar injury symptoms caused by Roundup can be found on the skirts of trees.

The use of foliar or postemergence applications of Roundup for weed control in Arizona citrus orchards is a matter of some controversy. Proponents of Roundup use praise its lack of soil residual effects on trees and its effectiveness as a weed killer, particularly on difficult to control perennials such as bermudagrass and purple nutsedge. Proponents of Roundup either consider the incidental treatment of citrus foliage with Roundup as inconsequential or use application methods that minimize Roundup spray contact with citrus foliage. Opponents of Roundup use in citrus orchards feel that even small amounts of Roundup spray drift to citrus foliage can have adverse affects on fruit drop and yield, particularly when multiple applications are made in a year. Thus, the objectives of this study were to evaluate the effect of simulated drift of Roundup spray on to citrus foliage (or inadvertent application of Roundup to citrus foliage) when multiple postemergence applications of Roundup per year are made for weed control.

Materials and Methods

This experiment was initiated in the spring of 1995 in a flood irrigated lemon grove on the Yuma Mesa (soil series: Superstition Sand) managed by Marlin Farming. The grove was in full production and contained trees with scions of 'Limoneira 8A' Lisbon lemon on Citrus volkameriana rootstock. Weed control in the grove prior to the beginning of the experiment was accomplished using postemergence applications of Roundup. The treatments included an untreated control and five rates of Roundup; 0.5, 0.75, 1.0, 1.25, and 1.5 lb. of active ingredient (a.i.) per acre or 0.5, 0.75, 1.0, 1.25, and 1.5 quarts of Roundup per acre. The six treatments were arranged in the orchard using a randomized complete block experimental design with nine blocks. Each plot consisted of a single tree. Trees were not skirted so the canopies extended down to the ground. The experiment began with the first set of Roundup applications on 6/6/95. Subsequent applications were made on 9/1/95, 3/6/96, 7/31/96, 11/22/96, 5/14/97, 8/15/97, 11/19/97 and 3/20/98. The Roundup was applied with a CO₂ pressurized backpack sprayer at about 20-psi using a single TeeJet 8002EVS nozzle with the nozzle orifice pointing at the tree canopy (i.e., oriented 90 degrees from the horizontal). The nozzle was held 10 inches above the ground at about the edge of the canopy and the applicator paced around the perimeter of the tree at a speed of 3 miles per hour. This method of application resulted in about a 20 to 24 inch spray band extending upward from the ground around the perimeter of the canopy at the base of the tree.

The data collected included visual estimates of leaf and canopy injury symptoms, counts of blooms and fruitlets (fruit with a diameter less then 1-cm) per unit branch length, and lemon yield per tree. Visual estimates of canopy injury symptoms were made on 4/26/96, 5/14/97 and 4/18/98 in all treatments and blocks. On each evaluation date, two separate ratings were made; one for the sprayed canopy zone (0 to 2 feet above the ground) and one for the portion of the canopy that was not directly sprayed (greater than 3 feet above the ground). Similarly, bloom and fruitlet counts were made in both the sprayed and non-sprayed canopy zones in February to May of 1996, 1997 and 1998. In each tree (i.e., plot) in both the sprayed and non-sprayed zones, one branch between 15 and 50 inches in length was selected. Beginning at early bloom (i.e., flowers evident but not yet open), the length of the shoot and number of flowers and fruitlets were counted every two weeks until no more flowers were produced. Flower and fruitlet counts were made on (month/day) 2/29, 3/12, 3/26, 4/9, 4/23, 5/7, and 5/21 in 1996; on 2/28, 3/14, 3/28, 4/11, 4/25, and 5/9 in 1997; and on 3/25, 4/3, 4/22, and 5/8 in 1998. Flower and fruitlet counts were made for all treatments in blocks one to six. Trees were harvested individually in all treatments and blocks. In 1996, there was a ring harvest (11/4/96) followed by a "strip" or complete harvest (12/2/96). In 1997, there were two ring harvests (10/20/97 and 11/21/97) followed by a complete harvest. Unfortunately, the complete harvest was done without our knowledge, and thus we did not obtain yield data from the final harvest. We estimate that about 20% of the total yield was not measured in 1997. Fruit harvested from each tree was placed in one or two field boxes. Five full field boxes were weighed individually to obtain the average weight of fruit in a full box. For the rest of the field boxes, the portion of a full box was visually estimated and the weight calculated based on the average weight of a full box.

Results and Discussion

Roundup applications to the bottom 20 to 24 inches of the tree canopies caused visible injury in the sprayed zone 1, 2, and 3 years after Roundup applications were initiated but had little effect on leaves in the upper portion of the tree canopies (Table 1). The injury symptoms were primarily narrow "strapped leaves" and branch defoliation but there was also some discoloration or bleaching of sprayed leaves immediately after some applications. The degree of injury at the base of the tree canopies increased with increasing rates of Roundup (Table 1). The degree of Roundup induced defoliation observed at the higher application rates was severe at certain times of the year and probably limited the uptake and subsequent translocation of Roundup in the trees. The lack of injury symptoms in the upper portion of the canopy suggests that there was not enough translocation or movement of Roundup from the sprayed zone to unsprayed portions of the trees to cause foliar injury symptoms. The injury observed in the control trees on 4/26/96 was due to Roundup applied prior to the start of the experiment. This injury declined in 1997 but an inadvertent application of Roundup in the experiment area in 1997 caused the increased injury observed on 4/18/98.

The effect of Roundup on flower production was variable. In 1996 after three Roundup applications, Roundup had no effect on flower production in either the sprayed or unsprayed portions of the tree canopies as judged by the counts collected from single branches in each canopy zone (data not shown). In 1997 after five Roundup applications, Roundup significantly affected flower production in the sprayed or bottom portion of the trees but did not affect flower production in the upper portion of the canopies (Table 2). There was substantial flower production only on 2/28/97 and 3/14/97, and on both evaluation dates, the higher rates of Roundup significantly reduced the number of blooms produced compared to the untreated control (Table 2). In 1998 after eight Roundup applications, flower production appeared to be largely unaffected by increasing Roundup rate in the sprayed portion of the tree canopies except for the data collected on 4/3/98 (Table 3). There was a substantial amount of variation in the flower data as indicated by coefficients of variation of 50% or more making it difficult to interpret the 1998 data. In addition, there was substantial flowering prior to 3/25/98 but no data was collected before this date. However, as in 1997, it did not appear that the Roundup applications affected flower production in 1998 in the unsprayed portion of the tree canopies.

Fruitlet production in 1996 in the sprayed and unsprayed zones, like flower production, was not affected by the Roundup applications (data not shown). The number of fruitlets (fruit with a diameter less than 1 cm) in the sprayed canopy zone was significantly affected by Roundup in 1997 with fewer fruitlets present on branches sprayed with the higher rates of Roundup (Table 4). Although there was also a decrease in fruitlets on 4/22/98, overall, the data were not conclusive on other sampling dates in 1998 (Table 5) due to the large amount of variation in the data as discussed above in relation to flowering. As with the injury symptoms and flower counts, Roundup did not affect fruitlet production in the unsprayed or upper portion of the tree canopies.

Spraying Roundup on the bottom 20 to 24 inches of the tree canopies did not reduce total lemon yield per tree in 1996 at any of the application rates (Table 6). Although the final strip harvest in 1997 was not measured, spraying Roundup on the base of the tree canopies also did not appear to reduce lemon tree yields in 1997 (Table 7). Yield data was not collected in 1995 because Roundup had been applied only twice before harvest and it seemed unlikely that there would be a yield effect. In both 1996 and 1997, increasing Roundup rate had no effect on the yield of the first or second ring picks or the percentage of the total crop (partial total in 1997) picked on the first harvest date (Table 6. Table 7). The lack of yield effects contrasts with the obvious foliar symptoms in the sprayed portion of the tree canopies caused by Roundup in all years of the study (Table 1) and with flower and fruitlet effects noted in 1997, and to a lesser degree, in 1998. These yield data suggest that a significant amount of Roundup does not translocate out of the sprayed portion of the canopy to affect other parts of the tree. These data also suggest that the bottom portion of the tree canopy may not contribute significantly to yield or that loss of fruit in the sprayed portion of the canopy is compensated for by increased fruiting in the unsprayed portion of the canopy.

It is noteworthy in regard to the yield data that the defoliation and narrow strapped leaves on the lower branches caused by the higher Roundup rates may limit that amount of Roundup absorbed. Defoliation might also be expected to reduce flower and fruitlet production because less leaf area is available to feed developing flowers and fruits. We consider the data contained in this paper to be preliminary and plan to continue the study to more accurately determine the effect of foliar Roundup applications on lemon trees. However, this study indicates that current weed control practices involving the use of Roundup in lemon groves on the Yuma Mesa probably do not have a significant effect on tree yields.

Acknowledgement

The authors wish to thank the Arizona Citrus Research Council and the Yuma County Pest Abatement District for financial support of this project. This is a progress report for project 97-03 'Effects of Foliar Applied Roundup on Lemon Physiology and Yield - 1997'.