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The ever increasing world population, along with continued deterioration of arable land, scarcity of fresh water, and increased abiotic and biotic stresses pose serious threats to global agricultural production and food security (Jagendorf, 2002).

The cost of pesticides has skyrocketed; and many chemical companies have decided not to continue development of certain products because of the high costs involved. The risk of contaminating groundwater and the resistance of consumers to purchasing commodities that have been genetically engineered or treated with pesticides and/or inorganic fertilizers, add to the dilemma. Fruit processing plants are also becoming reluctant to process fruit that has been treated with pesticides.

Despite focused efforts to improve the production of agricultural crops amidst biotic and abiotic stresses by traditional breeding, success has been limited. This can be attributed to the fact that tolerance to these stresses involves complex traits influenced by coordinated and differential expression of a network of genes (Jagendorf, 2002). The above dilemma has spurred researchers to investigate means and ways of enhancing existing resistance mechanisms in plants.

These investigations have led to the discovery of products that not only enhance resistance to several pathogens including fungi, bacteria and viruses, but also enhance yield and internal and external quality of agricultural crops. This phenomenon is termed ‘systemic induced resistance’ and has been extensively investigated in plants (Agostini et al., 2003; Van Loon, 2000).

At the application of the product (biostimulant or plant activator) a mobile signal is generated and transported to other parts of the plant where it enhances the mechanisms normally functioning to limit infection caused by fungi, bacteria and viruses (Van Loon, 2000).

The physiology of the effects on yield and quality are complex and not fully understood but may be ascribed to increased photosynthesis, inhibition of stress hormone (abscisic acid) production, increased nutrient availability through enhanced residue mineralization and effects on plant hormones regulating fruit set.

Biostimulants are a group of naturally occurring compounds that are used to enhance crop yield and quality. These compounds stimulate inherent physiological processes such as cell elongation and division, and optimize other desirable traits specific to the crop.


Effect of Crop-Set^TM on yield and quality of selected fruit crops

CITRUS

Sweet oranges (Valencia, Navel)

Trials conducted in Florida, California, Jamaica and South Africa demonstrated that two applications of Crop-Set™ applied at recommended rates (8 oz per acre) at petal fall and when fruit is golf-ball size not only increased yield but also increased fruit size, internal quality (sugar content) and juice content of the fruit. Yield increases (lbs per acre) varied between 10 and 55%. Fruit size increases varied from 6 to 125% depending on the age and variety.

Fruit size of 10 year-old Valencia trees in Florida increased by 125% in the 64-48 size distribution class; in South Africa the increase in size was 45% with yield increases of 12 and 55%, respectively (Table 1). These increases led to an overall increase in profit to the citrus growers.

For example, a trial with navel oranges conducted in Florida during 1999 demonstrated that the cost of two applications of Crop-Set™ was $45.00 per acre (including spray costs), but the net profit to the grower was $450.00 per acre. The lowest net profit realized by citrus growers in the above trials was $200 per acre.


Tangelo (Minneola tangelo)

California field trials demonstrated that two applications of Crop-Set™ at the rate of one pint per acre had a dramatic effect on the fruit size and color of Minneola tangelo trees. The yield of fruit in the size class 80-40 increased by 18%, with 18% less fruit being produced in the smaller 100-150 size classes. Crop-Set™ also increased the number of export cartons by 31% over the controls (Table 2).

This indicated that internal and external quality was improved by the treatment. Leaf analyses indicated that the levels of nitrogen, phosphorous, potassium, magnesium, copper and boron were significantly higher in the Crop-Set™ treated trees than the untreated controls. These findings support research demonstrating that the plant extracts used in Crop- Set™ enhance mineralization of nutrients in the soil.


Grapefruit

Four trials were conducted in 10-12 year-old orchards in South Africa and one in Texas.

Grapefruit trees are inherently extremely susceptible to citrus tristeza virus (CTV), which causes stem pitting in trunk and branches resulting in reduced fruit size, yield and economic lifespan. In the presence of this disease the economic life span of grapefruit trees is between 10-15 years. The pigmented varieties e.g. Star Ruby, Rio Red and Rosé are more sensitive to CTV and can succumb to the disease within four years following planting.

CTV is thus a major limiting factor in grapefruit production in countries such as Australia and South Africa where the disease is endemic. The only practical means of reducing the effects of this disease at present is by means of mild strain cross protection commonly termed pre-immunization. Plants are inoculated with a mild strain of the virus in the nursery, which then protects the plant from the effects of severe strains that are transmitted to the trees by aphids (Marais and Kotzé, 1985).

In humans and animals, defenses against pathogens are often activated by natural or artificial immunization by using sub-minimal doses of the pathogen or by artificial injection of pathogen proteins and other antigenic substances.

The result of immunization is the production of antibodies against the pathogen. Plants do not have immune systems like humans and animals that produce antibodies but they do have biochemical defense systems that are activated by signals transmitted to plant defense systems when infection by pathogens takes place or when elicitors such as ISR 2000™ are applied to plants (Tosun, 2002). The systemic induced resistance evoked is active against bacteria, fungi and viruses (Van Loon, 2000).


Table 1. Effect of Crop-Set™ on fruit size distribution (% of total) of sweet orange varieties in different localities.





Table 2. Effect of Crop-Set™ on the number of export cartons of Minneola tangelos in various size categories.





The results obtained in the grapefruit trials in South Africa illustrate the combined effects of biostimulation and systemic induced resistance invoked by the application of Crop-Set™ against the effects of CTV.

There is no documented evidence of any measure other than mild strain cross protection, which will reduce the effects of this virus on yield and fruit size in grapefruit. Crop-Set™ applications increased the yield of Marsh grapefruit by more than 80% and the fruit size of size distribution 40-27 by 400%.

The yield of Rosé grapefruit trees was increased by >40% and fruit size (40-27) by >12%. Star Ruby grapefruit, which is the most sensitive to CTV (Marais and Breytenbach, 1996), reflected a yield increase of >80% and fruit size (40-27) increase of >90% (Table 3).

The effect of Crop-Set™ was not as dramatic on the Rio Red grapefruit trees in Texas owing to the absence of CTV. The increase in yield was 8.5% with a 22% increase in fruit size distribution 48-27.


GRAPES

Eleven trials in total were conducted in grape vineyards, nine in California and two in Brazil.

Varietal differences were observed in the effect of Crop-Set™ on grapes, but in spite of this growers netted good profits from the treatment. The increase in yield in a Flame vineyard was 4%, but the increase in Grade 1 fruit was 25%, which realized a net profit of $420.00 per acre.

In a Red Globe vineyard the yield increase was only 3% with an increase of 1% in Grade 1 and 6% in Grade 2 fruit, but this difference still realized a net profit of $480.00 per acre. The yield increase in a Crimson grape vineyard in California was 25% with an increase of over 90% in Grade 1 fruit. This netted the grower a profit of $960.00 per acre (Table 4). The two trials in Brazil produced increases in yield of 13 and 15% with a berry size (5/8-3/4 inch) increase of 7%.


Table 3. Effect of Crop-Set™ on number of export cartons (15 kg) of different grapefruit varieties in South Africa.


*TSR = Texas Star Ruby



Similar increases in berry size were recorded in the other trials in California. In several cases the sugar content of the grapes was also found to increase following the application of Crop-Set™.

In the variety Perlette, the sugar content was 11% higher than in untreated grapes. The sugar content of Red Globe grapes was found to increase by 6% following the application of Crop-Set™.

In the other trials there was either no increase or only slight increases in sugar content. Increases in yield and berry size and bunch quality were more consistent than those in sugar content.


Table 4. Effect of Crop-Set™ on yield, quality and net profit of different grape varieties in California.





PRUNES

The data from nine trials conducted in California during 2000 and 2001 demonstrated that the application of Crop-Set™ at the rate of 1 pint per acre consistently increased the fruit size, green and dry weight.

Increases in green weight of 14 to 30% were recorded for the Moyer variety and dry weight increases between 15 and 46% were noted for the French variety. This increased the value of prunes by approximately $240.00 per ton.

Fruit size increases of 3 to 11% were recorded for both French and Moyer varieties resulting in significantly less (19%) culling of undersized fruit (<1 1/16 in.).

The increase in sugar content varied from zero to 6%. In three of the trials fruit was earlier in maturing while in three of the other trials it was later in maturing than the untreated controls.


AVOCADO

Trials in South Africa, California and Florida demonstrated that Crop-Set™ application at a rate of 8 oz per acre increased yield and net profit to the growers. The increases in yields from mature trees (15-20 years old) were 12.6%, 31% and 11% for South Africa, Florida and California, respectively.

Net profits realized were $1500, $4750 and $1700 per acre, respectively. The increase in yield from a 7 year-old orchard in Florida was 6%, realizing a net profit of $450 per acre.


POME FRUIT

Apples

The application of Crop-Set™ had a dramatic effect on the production of pome fruit in all trials conducted. In the three trials conducted in the state of Washington, Crop-Set™ increased the production of apples in size classes 80, 88 and 113 by 11%, 32% and 140%, respectively.

In the two trials in California, the yield increases in the same size classes were 62% and 250%, respectively. This was accompanied by a 6% increase in sugar content of the fruit. In the state of New York, yield increased by 16%, while in South Lebanon, where three applications of Crop-Set™ were applied, the yield of size class 80, 88 and 100 increased by 138% with an increase in sugar content of 15%.


Pears

In a single trial conducted in California, Crop-Set™ increased yield of Shinseiki pears by 14.9% and fruit size by 3.8%. This was accompanied by an increase in weight of the fruit by 13.6%. The increase on return was 87 boxes per acre, which realized $1392.00 per acre.


STONE FRUIT

Peaches

Crop-Set™ applications increased the overall yield of peaches in six trials conducted by between 6% and 12%, coupled with a significant increase in the fruit size class ≥2.5 inches by 2.5% for the Starn variety and by 67% for Red Globe. Enhanced fruit size was accompanied by improved color and an increase in sugar content (3.5%). The Red Globe trial realized a net profit of more than $700 per acre.


Nectarines

Two trials involving two different varieties, Arctic Snow and Grand Pearl, were conducted in California. Application of Crop-Set™ to the Arctic Snow variety did not result in larger fruit but increased the weight of fruit by 4% and the sugar content by 5.5%. The Grand Pearl variety responded with a 4% increase in weight, an 8% increase in sugar content and a 55% increase in the number of fruit in size class 60/64. Fruit color was improved in both varieties.


Apricots

Crop-Set™ applications improved yield, increased fruit size and net profit to the grower in all four trials conducted in California. The mean values for these criteria were 12.2% for yield, 9.2% for increase in size and $261.00 for net profit per acre.


Almonds

In a single trial in California Crop-Set™ increased the fruit set of almond by 10% over the untreated control.


Cherries

Results from a single trial in California indicated that Crop-Set™ increased the yield of Bing cherries by 32% and the fruit size by 130%. A net profit of $540.00 per acre was realized by the grower.


Olives

In a trial conducted in South Lebanon, the application of Crop-Set™ improved the oil content of olives by 24%.


Cantaloupe

At two trial sites in California the application of Crop-Set™ increased yields by 8 to 14% when cantaloupe was planted early in the season, between 35 and 38% when planted mid-season and between 1 and 3.5% when planted late in the season (Table 5).

Results recorded in an Arizona trial showed an increase in yield of 27% (>110 boxes per acre), with fruit being larger and more uniform in size. In the three trials conducted in Texas, yield increases of 8-14% were recorded.


Table 5. Effect of Crop-Set ™ on the yield of cantaloupe (cartons/acre) planted during the mid-season in California.

Acknowledgments

The author wishes to acknowledge valuable assistance from Monterey Chemical Company, CA., ACDS Research Inc. NY.,University of California West Side Research and Extension Center, CA., University-Saint Joseph, Kaslik, Lebanon, S&S Farm Supply, SC., Stan Bauer & Son, CA., John Bower and Piet van Rensburg of Capespan International, South Africa, Salty’s Agricultural Services, South Africa, Lykes Agri Sales, Florida, and Geoff Frank, Improcrop USA Inc.


References

Agostini, J.P., P.M. Bushong and L.W. Timmer. 2003. Greenhouse evaluation of products that induce host resistance for control of scab, melanose and Alternaria brown spot of citrus. Plant Dis. 87:69-74.

Jagendorf, A.T. 2002. Trehalose accumulation in rice plants confers high tolerance levels to different abiotic stresses. Proc. National Academy of Science Vol.99(25).

Marais, L. J., and J.M Kotze. 1985. Stem pitting of grapefruit. An evaluation of cross protection trials in South Africa using the Nartia mild strain of citrus tristeza virus. Citrus and Subtropical Fruit Journal 619:6-11.

Marais, L.J. and J.H.J. Breytenbach. 1996. The effect of tristeza stem pitting on the Star Ruby grapefruit industry in southern Africa. Citrus Journal 6:19-25.

Tosun, N. 2002. Disease control with a yeast elicitor in conjunction with fungicides. Proceedings of the BCPC International Symposium, Brighton, UK.

Van Loon, L.C. 2000. Systemic induced resistance. In: Mechanisms of resistance to plant diseases. (A.J. Slusarenko, R.S.S. Fraser and C.C. Van Loon, eds). Kluwer Academic Publishers, the Netherlands.

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