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Fruit tree pollination


Fruit tree pollinationVarious Reasons Why Flowers Do Not Set - Fruit-growers have seen trees which blossom full, but do not set a fair amount of fruit. It is a practical point to know the causes of this loss and the best way to prevent it. In the first place, but a small percentage of the blossoms set fruit, even in the most favorable seasons and with the most productive varieties.

In blossoming time a Japanese plum tree is a mass of white, carrying scores of flowers on a single branch; yet scarcely a dozen fruits may set on that twig, and some of those must be removed or the tree will over-bear. The normal failure in the setting of fruit blossoms may be due to a number of causes; as poorly nourished fruit-buds, lack of pollination, or winter injury to the pistils which cannot be seen with the eye alone. It is usually a distinct advantage to the fruit-grower, as it saves thinning.

The wholesale failure in the setting of fruit is often called self­sterility. It must have other varieties ear it in order to bear well. But it appears that self-sterility in orchard fruits is often confused with the unfruitfulness resulting from other causes. The influences which sometimes make trees unfruitful, which are often confused with the unfruitfulness resulting from self-sterility, are (1) heavy wood growth, (2) the attack of fungi on the blossoms, (3) frosts, (4) unfavorable weather during the blooming season. It should also be said that a tree is not self-sterile when it does not blossom. Why trees do not bloom is generally due to poor management.

Blossoms May Drop Because of Heavy Wood Growth - Young trees generally set little or no fruit the first few years, when they are growlng fast, although they may blossom full. With most varieties this early dropping of the blossoms occurs only two or three seasons, but Northern Spy and a few other varieties of apples are often unfruitful ten to thirteen years from this cause. Older trees may show the same results if stimulated too highly with nitrogenous fertilizers. The logical remedy is to check this excessive growth of wood by with­holding nitrogen or by putting, the orchard into sod for a few years.

The direct cause of this unfruitfulness is not known. The stamens and pistils are usually well developed and pollen may be produced in abundance. Since young; trees drop their blossoms as badly in a mixed orchard where other pollen is available, as when alone, the trouble probably lies more with the pistils than with the pollen. Up to this limit of excessive growth, there is a fairly constant relation between the vigor of a tree and its productiveness. Lack of vigor causes much more unfruitfulness than excessive vigor. If a tree is unhealthy or dying because of poor nourishment, few of its blossoms are strong enough to set fruit. The same results may follow if the tree is exhausted by over-bearing.

Blossoms May Be Killed by Fungi - If the weather is warm and wet in early spring, conditions are favorable for the growth of fungi and it sometimes happens that fruit blossoms are "blasted" by the early growth of these parasites. The killing of fruit blossoms by fungi need not occur, especially if one thorough application is made to the trees before the buds open.

Winter and Spring Frost May Injure the Blossoms - The unfruitfulness arising from winter or spring frost injury is sometimes with self-sterility. A common form of injury is that in which the pistil is blackened and stunted, having made no perceptible growth during the opening of the flower. These pistils always drop from the tree soon after the petals have fallen.

Another and not less common form of injury is that in which the pistil has made a partial growth but has no well developed ovary. Unless a careful examination is made, blossoms like this would not be considered as winter-injured. The killing of the pistils is the most common form of winter injury to fruit buds.

The injury to fruit blossoms from cold is of all degrees. During the opening of a normal flower, the pistil grows. It is often taken for granted that if this growth occurs the pistil is uninjured; but it may be that even though a pistil reaches its full size, it may, yet be so injured that it cannot develop into fruit. Some of the imperfect development of flowers which is attributed to winter injury may be caused by unfavorable conditions during the previous season, when the buds were being formed, yet it is likely that winterinjury to pistils is more common and more serious than appears at first sight.

Rain May Injure Fruit Blossoms - The unfruitfulness which often follows a rain during the blooming season is sometimes confused with self-sterility. A careful fruit-grower watches the weather anxiously when his trees are in blossom, for he knows this is the most critical period in the growth of the crop. Injury to fruit blossoms from rain is common wherever fruit is grown, but is particularly serious along the Pacific Coast and near the shores of the Great Lakes. It has been estimated that more fruit is lost in California from cold rains during blooming time than from all other causes combined. Like winter injury to fruit buds, there is no way of preventing this loss except to secure amore favorable location, since it is not in man's power to prevent rain.

If a rain comes while the trees are in full bloom the pollen is washed from those anthers which have already opened, and is thus prevented from reaching the stigma. Should the rain be a short one, no serious harm need result from this loss of pollen, for the unopened anthers will burst and pollination will begin again soon after the sun comes out. The washing away of pollen has very little influence in decreasing the setting of fruit, particularly when the rain is short. There will generally be enough pollen to supply the pistils before or after the rain.

The poor setting of fruit which often follows a long rain and sometimes a shower is due more to a loss of vitality in the pollen or to some mechanical injury to the pistils; also, in large measure, to the fact that bees and other insects which promote the beneficial cross­pollination between varieties are absent. If the rain lasts for several days, the pollen may lose its vitality. It is also natural to suppose that a hard rain may wash off, dilute, or otherwise injure the juices of the stigma so that the pollen cannot germinate after it falls upon the stigma. Perhaps a long "spell" of wet weather may even kill the pistils after they have been fertilized.

Thus a rain during the blooming season may decrease the setting of fruit in four ways: (1) By, preventing the pollen from reaching the stigma, both because it is too wet to fly and because pollen-carrying insects are absent. This is important only when the rain lasts several days and most of the pistils pass their receptive state before the rain ceases. (2) By destroying the vitality of the pollen. (3) By injuring the stigma. (4) By preventing fertilization or the germination of the pollen because of low temperature.

The Blossoms May Be Injured by Strong or Drying Winds - Near the sea and large lakes, fruit blossoms may be whipped off by very severe winds. In such cases a mixed windbreak of deciduous and evergreen trees may be used to advantage. Drying winds during the blossoming season are not common in the east but are often serious in some parts of the west.