Effects of Seaweed Sprays on Citrus Fruit Production


Robert C.J.Koo
University of Florida. IFAS
Citrus Research and Education Center

Steven Mayo
Mayo Grove Management

Additional index words. Citrus, fruit yield, fruit drop

Abstract. The effect of seaweed sprays were evaluated on orange and grapefruit trees. Trees sprayed with seaweed extracts increased fruit production 10-25% over that of the control. A combination of prebloom, postbloom, and summersprays appeared to give greater response in fruit production than other timing treatments. Trees sprayed with seaweed ex tracts had fewer premature fruit drop than the control. Fruitsize from seaweed treated trees were equal or larger than fruit from the control trees, despite the fact that higher fruit production is usually accompanied by smaller sized fruit. In general, rind color break occurred earlier in fruit from the seaweed treated trees than fruit from the control trees.

Seaweed-based products have been used on agricultural crops in the United States as an organic fertilizer supplement. Increased yield of poato and corn with seaweed sprays have been reported (Chapman and Chapman, 1980). Increased shelf life of peaches have been reported from trees sprayed with seaweed extracts (Skelton and Senn, 1966). In Floridatrials, seaweed sprays increased total yield as well as early maturity of tomatoes (Csizinszky, 1984).

In citrus, under controlled greenhouse conditions, seaweed extract sprayed at a dilution of 1:25 parts of extract and water corrected Mg, Mn, Zn, and B deficiency symptoms (Ait-ken and Senn, 1964). A 3-yr field study of seaweed extract on young Sunburst tangerines showed a 30% increase in fruit production (Koo, 1988). Studies were initiated in 1991 to study the effects ofseaweed sprays on oranges and grapefruit. 

This paper summarizes the data from 3 experiments over a 2-to 3-yr period.

Materials and Methods
Three experiments were initiated in central (Polk County), east coast (Martin County) and southwest (Hendry County) regions of Florida to study the effects ofseaweed sprays on oranges and grapefruit growing under different production conditions. Experiment 1 was conducted on Washington Na vel orange [Citrus sinensis (L) Osbeck] and Ruby Red grape fruit {CitrusparadisiMacf) on sour orange {Citrus aurantium) rootstock near Frostproof, Florida. The trees were planted in 1980 on single beds at a 17/2 x 30 foot spacing. Experiment 2 was located near LaBelle, Florida using Ruby Red grapefruit {Citrus paradisi Macf) on Carrizo citrange [Poncirus trifoliata (L) Raf x C. sinensis] rootstock planted on double beds. The trees were planted in 1987 at 15 x 20 foot spacing. Experi ment 3 was located near Indiantown, Florida using Hamlin and Pineapple oranges. Hamlin orange [Citrus sinensis (L) Osbeck] on Cleopatra mandarin (C. reticulata Blanco) root- stock were planted in 1980 on double beds with a tree spacing of 15 x 22lA feet. Pineapple orange [Citrus sinensis (L) Os beck] on rough lemon {Citrusjambhiri) rootstock were plant ed on single beds in 1962 using a 20 x 25 foot spacing. Treatments ofall 3 experiments are listed in Table 1 together with the mineral compositions ofthe seaweed extracts. Exper imental design in all 3 experiments was complete randomized block. Each treatment was replicated 4 times using 6-tree plots. Experiments 1 and 2 were designed to obtain informa tion on the best timing to apply seaweed extracts. Experiment 3 was a comparison of seaweed sprays and gibberellic acid as foliar sprays.

Fruit production data were collected at the time of fruit harvest. Fruit samples were collected 1-2 weeks before fruit was harvested to measure quality. Premature fruit drops were counted 3-4 times a year at 5-6 week intervals. Only the freshly dropped fruit were counted. Fruit size and fruit color were measured both in the laboratory and in the field. Leaf sam ples were collected to evaluate the ability of seaweed extracts to supply plant nutrients. Four- to five-month-old spring flush leaves from nonfruiting terminals were collected in 1991 and 1992 for this purpose. Data from all 3 experiments were ana lyzed by analysis ofvariance on an annualbasis.The multiyear
average data were not analyzed. They are listed only to show trends if any.

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Results and Discussion

Fruit production. Increased fruit production from seaweed treatments were found in all 3 experiments (Table 2). In Expts. 1 and 2, trees treated with seaweed extract sprays pro duced 10-25% more fruit than the control trees over a 3-yr period. This is in agreement with previous studies on Sunburst tangerine and Valencia orange (Koo, 1988). The only exception to increased fruit yield, however, was found in the grapefruit yield of 1992 in Expt. 1. The block was hedged every other middle in the spring of 1992. It was not known to what extent the hedging operation may have negated the treatment effects of that year.


Among the seaweed treatments, Tr. 2 had produced morefruit than Tr. 3. Both treatments received 2 applications of BM-86 and one application of MZ-63 a year. In Tr. 2, BM-86 was applied as a prebloom spray and again as a postbloomspray. It was followed by an MZ-63 spray in the summer. In Tr.3, BM-86 was sprayed during postbloom and again in the fallwith MZ-63 sprayed in the summer. These findings would suggest the best timing is to apply 2 BM-86 sprays in the spring followed by MZ-63 in the summer. Treatment 4 in Expt. 1 was intended to determine if seaweed will supply plant nutrients,especially microelements, to the trees. It will be discussed un der leaf analysis. Fruit production was not affected by the omission of nutritional sprays. Apparently sufficient plant nutrients were supplied by seaweed extract sprays.

In Expt. 3, seaweed treated trees (Tr. 2) produced morefruit than the gibberellic acid (Tr. 3) or the control trees. The

gibberellic acid treated trees produced slightly more fruitthan the control trees but the differences were not signifi cant.

Part of the differences in fruit production could be related to premature fruit drops (Table 3). Data in Table 3 includ ed only the freshly dropped fruit found on the days when fruitdrops were counted. The control trees had more fruit drops than the seaweed treated trees. This was true in all 3 experiments, although much of the differences were not significant because of tree-to-tree variations. gibberellin acid treated trees in Ex[t. 3 had the lowest number of premature fit drop which was substantiated by a previous study on grapefruit (Fergueson et al., 1982).

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Fruit quality. Both internal and external fruit qualities were measured. No consistent trends were found in juice quality measurements and the data are not reported in this paper. Fruit size was measured both in the packinghouse and in the field. Fruit size and weight measurements showed fruit from the seaweed treated trees were equal or larger than fruit from the control trees (Table 4). These differences were small. Higher fruit production is usually accompanied by smaller fruit; however, seaweed treated trees had higher fruit production and somewhat larger sized fruit than the control trees. Peel color was measured in the laboratory and the data were inconclusive.

Rind color break was a field measurement of percent of fruit on the trees that showed traces of yellow or orange color. Seaweed treated trees had a higher percent of fruit showing color break earlier than fruit from the control trees. Similar observations were reported on Sunburst tangerine (Kool, 1988) Fruit from the gibberellin acid treated trees in Expt. 3 had the lowest color break which was consistent with previous findings (Fergueson et al., 1982).

Leaf analysis. Leaf samples were collected to evaluate the ability of seaweed extracts to supply certain nutrient elements as claimed on the product label. Evaluations were made in the greenhouse on sour orange seedlings, one-year-old Osceola trees, and mature trees in Ext. 1.Nutrient element absorbtion was very apparent in the greenhouse and young green studies where the tree root systems are restricted (Table 5). Seaweed extracts were sprayed on the mature trees at a much lower rate, which was reflected in the small difference in mineral composition between leaves from treated and control trees.

Nitrogen was absorbed by leaves to varying degrees in all 3 studies. Magnesium showed no uptake in any of the studies. Manganese absorption was apparent in the greenhouse and the young tree studies but not in the mature tree study. Zinc absorption was consistent in all 3 studies. Similar trends were found in a previous study (Kool, 1988). Copper absorption was confounded by high Cu concentration in the leave of mature trees. Iron was not listed on the product label but the trees seemed to have absorbed some Fe in all 3 studies. Boron absorption was found in all 3 studies. It would be safe to conclude that B and nutritional sprays can be omitted from the fertilizer program when seaweed extract sprays are used.

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