tt-134 BY DON MEEHAN

FRUIT TREE POLLINATION

This is the time of year when we begin to see lots of partially developed fruit fall off trees. This is especially noticeable on cherry trees. There are a number of reasons for this, but the one that is most common is poor pollination. When I use the term 'pollination' I am referring to a complete process which includes fertilization. Because this is such a common problem here, I think it worthwhile to focus on what is happening. In doing so I have borrowed an article written by one of my co-workers in Chelan County, Paul Tvergyak. Pollination is the transfer of pollen from the anther of a flower to the stigma of the same or another flower. Fruit tree pollen is sticky and heavy. It does not move well with wind, but insects, particularly honeybees, carry pollen very well. Pollen itself is a good source of food for bees and, along with nectar, is a the basis for bees being attracted to flowers. A number of factors influence pollen quality and if less than 70 percent is viable, then your pollen source is probably weak. If viability drops below 20 percent germination, then your pollen source is of little use. Poor tree nutrition may reduce pollen quality, but by far the biggest factor is low temperatures between bud break and half-inch green in bud development on apples. At this time, pollen is particularly sensitive to chilling or cold injury. The surface of the receptive stigma is also sticky, and when pollen adheres to it the pollen can germinate. When stigmas are receptive, usually for 3 to 7 days, they are shiny and moist-looking. After that time they dry out, turn brown, and are no longer useful. When the pollen germinates it first forms a pollen tube which grows down the style to the ovule. Under good temperature conditions, about 60 to 80 degrees F., this might take 48 hours. With cooler, rainy weather like we have had this spring, pollen tube growth could take a week, and the risk of poor or no fertilization becomes greater. This type of weather also negatively influences bee activity. Strong colonies may reach full flight at 60 degrees F. but weaker ones may not fly until 72 degrees. Since it takes about 25 to 30 bee visits per tree per minute to set a crop, it doesn't take too much cool weather to have a negative impact on pollination and germination. The next step in the process occurs when the sperm cell from the pollen moves down the pollen tube, joins with the egg cell, and fertilization occurs, resulting in a seed. The sperm cell actually follows the pollen tube rather than waiting for it to complete its growth. Therefore, the same weather factors that influence pollen tube growth affect the sperm cell's trip to the ovule. The process of pollination and fertilization comes down to some bottom lines that you can influence. First is the quantity and quality of available pollen. You provide that by having planted or grafted additional pollinizers. Remember that cross pollination can not take place if your pollinizers bloom at a different time of the spring than your fruit trees. And don't forget that most fruit trees require a cross pollinator of a different variety. Apple varieties like Gravenstein, Jonagold, and Mutsu have sterile pollen. Some cherry varieties are cross incompatible. These are Bing, Royal Ann and Lambert. European type plums can not rely on Japanese plums as pollinizers. Those trees which you are particularly interested in acting as pollinizers need to be pruned as if you wanted good fruit production from them, and then when fruit is set it needs to be thinned or removed to assure vigorous return bloom the next year. Obviously, pollinating insects are critical. Bumble bees and wild bees will operate in cooler temperatures, but their numbers are often far too low to make a significant difference. This is why commercial orchards, beet seed growers, and berry growers rent bees during bloom time. Poor fertilization or pollination results in lack of seed development, which influences the growth of associated fruit parts. In berries this is often seen as irregular-shaped berries. Fruit quality and yield will almost always be less when pollination is poor.




Return to Main WSU Page

Return to Articles Index

Providing Educational Support to the Island County Community