Spring 00

Over the last 3 years flowering conditions in Holland were quite poor, resulting in a low fruit set of triploid apple varieties. This resulted in large differences between orchards, varying from a 20 percent crop to a full crop. From this information we can learn a lot about improving fruit set of triploid varieties under poor weather conditions.

The first step in ensuring a good fruit set and preventing an excessive June drop is to control the amount of extension growth. The aim is to create a well-balanced tree, which has no branches thicker than half the diameter of the stem. When pruning, the thick branches should be removed first.

Where a tree has too many thick branches, these should not be pruned at the same time. In these trees, pruning should be affected by removing all the thick branches in the upper half of the tree first and then working down the tree over 2 - 4 years. When in the bottom tier of the tree, branches, which are too thick and vigorous can all be removed, provided the rest of the branches are pruned lightly.

Where the extension growth is too long (many shoots of 50 cm and longer), a good way to retard growth is to root prune the trees. Where trees cannot be irrigated, it is better to root prune one side of the tree now and delay root pruning of the other side until the following year. As the common distance of tree spacing is 1.25 to 1.50 metres, root prune 40 - 50 cm from the stem at a depth of 30 - 40 cm. For trees which can be irrigated, root prune both sides in the same year at 40 - 50 cm from the stem. If satisfactory results are not seen, root prune a little closer the next year. When root pruning trees for the first time, caution is required. Root pruning will have a strong effect on fruit size and can put a heavy stress on trees resulting in a poor fruit set.

Root pruning is effective in the early spring period (end of February/early March). An alternative time to root prune is 4 - 5 weeks after full bloom. The effect of early root pruning is stronger. However,an advantage of late root pruning is that one can assess the crop first and adapt the root pruning to the volume of fruit on the tree.

Another possibility is to root prune one side early and if the extension growth after flowering is still too high, root prune the other side 5 weeks after full bloom. Table I presents the results of a root pruning experiment on Belle de Bokshop after bloom at the Experimental Farm in Numansdorp (Holland). The trees were planted in spring 1987, 1.5 metres apart as light feathered 1 year-old on M9 rootstock. Root pruning began from 1990 onwards on both sides of the trees at 40 cm from the trees and 30 cm deep. Root pruned trees had a much lower vigour and stayed smaller. The effect on fruit set per tree volume was much better for the root-pruned trees. In years with a poor fruit set this is a large advantage.

Root pruning usually is done with a coulter. Under good conditions it is possible to root prune two sides of a row at the same time, making it easier to keep the tractor straight. Over the last few years root pruning machines have been made which can lift the coulters hydraulically. This allows the grower to leave parts of rows, which are showing poorer growth. In Holland a large number of orchards are root pruned each year to keep the growth down. As no chemical retardants are allowed, this is the only way for retarding growth of apple trees in Holland.

The nutrition of the orchard has big impact on fruit set. While the balance of nutrients is vital, nitrogen and boron levels are considered to be most important.

Boron is important to extend the effective pollination period.

Pollen from trees with a good boron level stays viable over a longer period. Growth of the pollen tube through the style is strongly temperature related. For example with an average daily temperature of 7°C it takes 10 days before the pollen tube reaches the ovule, with an average temperature of 12°C it takes 5 days and with 15°C it takes only 2 days. From this point of view it is very important for the more northern fruit growing areas to keep the boron levels high. Common practice in Holland is to spray once post-harvest and 2 - 3 times pre-bloom with I litre Bortrac or I kg Solubor/hectare. Furthermore Boron is sprayed 2 - 3 times post-bloom to prevent russetting. The aim is to have a boron level of leaf samples on mid shoot leaves in July at 30 - 60 ppm.<

Nitrogen is particularly important for the number of days the ovule stays viable. On triploid varieties this is often a limiting factor. Nitrogen, which is applied in spring, cannot improve the viability of the ovule. Urea sprays in the pre-bloom period (3 - 4 sprays at 3 - 5 kg urea/hectare) can help a little, but creating a good nitrogen reserve in the flower buds is the main thing. Urea sprays applied post-harvest can do this (2 - 3 sprays at 15 kg urea/hectare). However when nitrogen levels in leaf samples are low, it's better to apply 30 - 50 kg N on the soil at the end of August or early September. This application does not effect fruit quality, but has a beneficial effect on the nitrogen reserve of the tree. By creating a good nitrogen reserve, a longer effective pollination period is created, ie., the longevity of the ovule minus the time required for pollen tubes to reach the embryo sac.

For good pollination, a minimum of 2 strong beehives per hectare is required. The strength of the beehive is very important. Under poor weather conditions, bees always keep their brood warm first, before foraging outside. Strong beehives can commence foraging from 12 - 13°C. Beehives should be placed on a wind-sheltered spot receiving the morning sun with their flight-opening Southeast.

Under Dutch conditions the minimum pollination is 10 percent of a variety which is not susceptible to biannual bearing, e.g. Discovery, Alkmene or Idared. Over the last 5 years, Malus varieties have become more popular because they have a high number of flowers. Their disadvantage is the short flowering period, which makes it necessary to plant 3 - 4 different Malus varieties in the same orchard. The main Malus varieties in Holland are Professor Sprenger, Evereste, Golden Hornet and Red Sentinel. To improve the length of their flowering period, prune the Malus pollinators at petal fall. This will result in more flowers on the one-year-old wood. During pollination in the following year, the "spurs" flower first, followed by the fruit bud on the one-year-old wood.

To improve pollination in an orchard, which has a small number of pollinating varieties, a possible short-term solution is to work with controlled pollination. This method was originally introduced in the USA and for the last 6 years has been used in Holland. With this method, (pollen which is imported from the USA), is brought into an orchard by applying it to a bee booster, which is placed before a beehive. When the bees are foraging, they transport the pollen to the apple flowers. In many tests, this pollen proved to be as viable as fresh pollen. However this method only works when bees are actually foraging. Nevertheless, for orchards with a shortage of pollination, this might be a way to improve this situation. Holland Fruit Advies is distributor of "Firmyield" pollen for northern Europe.

As climate is important for the effective pollination period and for the flying activity of the bees, windbreaks are considered essential to create a good fruit set. Over the blossom period a proper windbreak can make a difference of 2 - 3°C. This difference can be enough to make the difference between good pollination and poor pollination.

One can conclude that improving fruit set on triploid varieties requires attention to number of crucial points. When one of these factors is limiting, fruit set is often poor. So consider which factor eg., pollination, climate, nutrition, and vigour level are limiting fruit set in your orchard.

Modern intensive orchards with a tree density exceeding 600 trees per hectare which are growing vigorously should receive a Cultar foliar programme commencing in year 2 from green cluster stage.

To reduce extension growth the herbicide strip can be reduced to 0.5 metres (1½ feet) thereby increasing competition from the grass alley.

To encourage lateral branches to break from the bare wood on the central leader, run a fine hacksaw blade once just over the top of the buds from bud burst up to blossom time. Best results are obtained by doing this as close to bud burst as possible.

For modern orchards, tying down should be carried out in year 2. Only very strong laterals should be tied down in year 1. Vigorous feathers should be tied down to the horizontal. If the laterals are tied down below the horizontal all the "power" goes into the central leader preventing these branches filling their space. This would be ideal if your trees were planted in a bed system 1 - 2 metres apart.

The aim is for Bramley Seedling trees to fill their allocated space as quickly as possible and then to hold them in that space.

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