Chemical Thinning

Chemical thinning is the regulation of crop load through the addition of PGRs that reduce flowering and crop load in established orchards. Used correctly, chemical thinners can have two dramatic effects: (1) increased fruit size and quality and (2) maintenance of annual bearing.

In pome fruit, chemical thinners are commonly applied between full bloom and the cessation of cell division, which occurs about a month after full bloom. The exact timing of thinner application depends on material chosen, cultivar, and local conditions. Table P3.1 lists the most widely used chemical thinners for pome fruit, as well as their benefits and weaknesses.

Chemical thinning of stone fruit has been far less successful than in pome fruit. In pome fruit, chemical thinning takes advantage of the inherent differences in seed count, fruit development, and sink strength to remove smaller-size fruit. Since stone fruit are single seeded and little difference exists among fruitlets, chemical thinning is more difficult. Fewer materials are available for commercial orchards. To circumvent this problem, thinning strategies that rely on prebloom or bloom thinning can be used. The major prebloom thinner available is gibberellic acid. Since stone fruit do not have mixed buds, flower differentiation during the previous summer determines whether a bud will be vegetative or reproductive. For years, tart

TABLE P3.1. A listing of the commonly used plant growth regulators for chemical thinning of pome fruit and some benefits and weaknesses of each

Material Type of PGR Benefits Weaknesses

TABLE P3.1. A listing of the commonly used plant growth regulators for chemical thinning of pome fruit and some benefits and weaknesses of each

Material Type of PGR Benefits Weaknesses

Carbaryl

Insecticide

Unlikely to

Can affect

overthin, useful on

beneficial insects

easy-to-thin

cultivars

Dinitro compounds Phytotoxicto

Useful for bloom

Environmental

(DNOC)

flowers

thinning, inexpen-

concerns, dry

sive

weather required

Ethephon

Ethylene-releasing

Long period of

Temperature

compound

activity, relatively

sensitivity, pre

inexpensive

mature ripening

Accelâ„¢

Cytokinin-plus-

Promotion of early

Relatively untested,

gibberellin mixture

season fruit growth

expensive

Naphthalene

Synthetic auxin

Inexpensive, long

Pygmy fruit,

acetic acid (NAA)

history of usage

temporary growth

suppression

Naphthalene

Synthetic auxin

Inexpensive,

Slower acting than

acetamide (NAD

useful on late-

NAA

or NAAm)

season cultivars

cherry growers have used gibberellic acid to maintain vegetative growing points, especially as trees age. Recently, this strategy has been employed to thin peaches, where excessive flowering increases thinning costs and depresses fruit size. Table P3.2 lists the three thinning strategies for stone fruit, the growth regulators used, and their strengths and weaknesses. Some of the PGR registrations listed are restricted to particular geographic areas. Before using any of the materials listed in Table P3.2, a grower should consult the pesticide label for specific recommendations for the crop and farm location (American Crop Protection Association, 2001).

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