Stone fruit (summerfruit)

Stone fruit (summerfruit) crops grown in New Zealand

Stone fruits have been valued for their fruit for many years. Apricot originated from China more than 4000 years ago and spread through old trade routes to the Far East and the Mediterranean. Peaches also originated from China more than 3000 years ago and spread by trade throughout South-western Asia. Nectarines are more recent but even they have been around for at least 2000 years.

Cherries (both sweet and sour) and European plums originated in Europe, but the time of this event is uncertain. The Japanese plum originated in China and was introduced to Japan about 1500. Some European plums are also known as prunes because they are suitable for drying.

Site selection
In New Zealand, stone fruits do well in areas with cold winters and hot dry summers (so long as the fruit can escape spring frosts and be irrigated). Peaches, nectarines and some Japanese plums grow well at sea level, but apricots, European plums and sweet cherries tend to produce light crops and excessive vegetative growth in climates where there is inadequate winter chilling.

Most stone fruit require naturally fertile, free-draining soils. Peaches and nectarines fail to thrive in wet soil conditions. Plums (and stone fruits grown on plum rootstocks) are the most tolerant of the wet soils. Apricots need to be planted on elevated land, not only for soil drainage but also air drainage. If planted on elevated ground, they not only escape the harshest cold weather but the foliage and fruit dry out more quickly, thus avoiding some of the more debilitating diseases. In dry climates, some irrigation will be necessary, especially during the establishment phase. Cherries tolerate higher rainfall than most of the stone fruits and are less prone to fungal diseases.

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Fruit
The fruit can be eaten at the soft ripe stage, straight from the tree, but if they are to be transported or stored inside they need to be picked at the firm but coloured (no green) stage. Fruit picked at this stage often improve in flavour after spending a few days at room temperature while their acid content declines. Most stone fruit have a short shelf life (up to 5 days) at room temperature. Fruit can be held in the refrigerator or coolstore for up to 3 weeks but their flavour is lost if the fruit are stored any longer.

When grown in the home garden, the fruit can be eaten fresh or processed. Apricot is probably the most versatile of the stone fruit crops for processing and can be used for jam, nectar, preserving, drying or freezing.

Varieties
Recommended varieties for the home garden are noted below. Those marked with an asterisk (*) may not be available through garden shops because of restrictions relating to plant royalties

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Diseases
The success of many stone fruit in New Zealand depends on their resistance to several diseases, including brown rot Monilinia fructicola, bacterial spot Xanthomonas arboricola pv. pruni, and leaf curl Taphrina deformans. Some varieties have some resistance to these diseases but others require sprays to keep the trees alive and productive. It is important to select varieties with good resistance to diseases. It is also important to search out any local knowledge on disease resistance. Most diseases of stone fruit occur on all five crops within the stone fruit group. Susceptibilities then depend on the choice of variety.

Management recommendations

• Apply copper programme
• Prune out dead wood
• If possible, protect blossom and small green fruit from frost

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Management recommendations

• Avoid overhead sprinkling during the day in summer
• Apply copper programme in autumn

Brown rot
causes major fruit losses at harvest both
on the tree and postharvest
Brown rot is probably the most important and limiting of the stone fruit diseases. It is particularly severe in seasons and regions where there is high rainfall either over flowering or close to harvest. A combination of warm and wet conditions at these times can cause high fruit losses with the fruit collapsing within a few days. Tip die-back may also occur with fruit rots at harvest time and the brown leaves are left hanging on the tree. Spray programmes include fungicides to be used during flowering and within 21 days of harvest. Modern fungicides are remarkably effective against this disease. Sulfur can be used on most stone fruit (except apricots) but will not be adequate in a high risk year.

Management recommendations

• Remove all remaining fruit from the tree (healthy or otherwise)
• Remove or mulch prunings

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Peach leaf curl
causes reddening and distortion of leaves
of peaches and nectarines
Can severely debilitate trees if uncontrolled over several years. Rarely a problem on flowering peaches. Leaves become thickened and curled. Later infections turn the leaf white then black before they fall off. Growth can become stunted.

The application of one key spray can prevent this problem if is timed correctly. The best timing is when the first signs of leaf development begins (`leaf bud movement’). This takes place before flowering. The most common treatment for leaf curl is copper, but ziram, thiram or dithianon can also be used. Brown rot fungicides are generally not effective against peach leaf curl. Some resistance to copper has developed in the pathogen that causes peach leaf curl in Hawke’s Bay.

Management recommendations

• Apply copper at the critical time
• Remove primary red-leaf infections before they reach the `white bloom stage’.

Silver leaf
causes silvering of leaves and tree decline

All stone fruit are susceptible to the fungus, Chondrostereum purpureum, which causes silver leaf and tree decline. It has other common hosts, including apples, pears, raspberries, gooseberries, silver birch, poplar, willow and gorse.

Not all stone fruit varieties are equally susceptible and silver leaf is less severe in drier climates.

Leaves become silvered and the epidermal layer can easily be separated from the leaf. Infected branches die as the disease spreads through the tree. The disease starts when fresh pruning cuts are infected during wet weather. It therefore occurs where pruning is carried out in wet conditions, particularly in late winter or early spring.

Management recommendations

• Prune in fine weather in autumn using clean cuts
• Treat all large cuts with fungicidal wound dressing
• Inject infected trees with dowels treated with biochemical or chemical treatments developed for the treatment of silver leaf

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Pests of stone fruit (summerfruit) in New Zealand

Aphids
Each of the stone fruit (summerfruit) crops has its own aphid species with the exception of apricots. Aphids were introduced into New Zealand on their host plants when those plants were introduced from Europe. Despite the number of different aphid species, there is some similarity in the life cycle. The most common aphids, which are found on peaches and nectarines (green peach aphid), cherries (black cherry aphid) and plums (leafcurl plum aphid), overwinter as eggs on fruit spurs on their primary host plants. The eggs hatch in spring producing several generations of wingless aphids that feed on the young shoots. Aphid feeding causes leaves to curl, protecting the aphid populations inside. The leaves become sticky with honeydew that is excreted by the aphids. In November, the first winged forms start to develop. These start their migration to the summer hosts – usually on annual or perennial ground cover weeds. A number of generations are completed on the secondary host but in the autumn, winged females return to the primary host where they produce male and females. This second generation of females lays eggs near the buds on the trees.

Dr Jill McLaren has investigated the timing of these events on stone fruit trees in Central Otago and found it was very similar for each species. For instance, the eggs of three species (green peach aphid, black cherry aphid and leafcurl plum aphid) all hatch during August. However, the colonisation of peaches and nectarines by green peach aphid ends earlier (late November) than for black cherry aphid on cherries or leafcurl plum aphid on plums (January-February). A longer period of infestation on their fruit-tree hosts gives these aphids a greater chance to produce damage. It also means that black cherry aphids can be present in high numbers during cherry harvest, making this a most unpleasant operation on susceptible varieties. Predators such as ladybirds do reduce aphid numbers, but complete biological control cannot be relied on in certain varieties. Fortunately some varieties, especially flowering cherries, peaches and almonds as well as sour cherries, seem to be untroubled by aphids.

Research has shown that it is very difficult to control aphids after leafcurling symptoms have started to appear because the aphids are so well protected. However, good control can be obtained by applying oil at the end of egg hatch - in the last week of August in Central Otago. This timing works well for black cherry aphid, green peach aphid and leafcurl plum aphid in Central Otago, but may need to be applied earlier in northern districts to avoid damaging flower buds or new tips with oil.

Research is continuing on alternative timing of control measures including autumn applications and spring controls.

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Leafrollers
Leafrollers are common on many garden and fruit crops where their habit of rolling leaves and fruit together brings them to the attention of consumers and gardeners. They attack all the stone fruit (summer fruit) crops and their white webbing and feeding cavities can be seen on both the fruit and foliage. A number of leafroller species attack stone fruit in New Zealand. Most are native to New Zealand, but one, lightbrown apple moth, is Australian. Research has shown that lightbrown apple moth occurs throughout the country but there is variation between districts in the species of native leafrollers found. Generally, at least two native leafroller species are found in each fruit growing region.

Leafrollers are a quarantine pest on export crops but rarely cause serious damage on home garden crops. If they are a problem, it is useful to apply control measures at the correct time. There are usually two to three generations per year and the early caterpillar (larval) stages are more susceptible to sprays than the pupal or adult stages. Therefore, it is useful to know when these are happening. Researcher Dr Max Suckling at HortResearch, Lincoln, has developed pheromone traps that release copies of the female pheromone. These traps catch males and give information on the time of flight of the males of each species. Records of the time of the first male activity in each generation are important in that the first larvae are likely to appear within about 14 days. Pheromone traps are often available at garden shops or outlets such as FruitFed Supplies. Note, the traps catch only males and will do little for population control.

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New Zealand flower thrips
New Zealand flower thrips, Thrips obscuratus, can damage nectarines between flowering and shuck fall (a period of six to seven weeks) by feeding on the developing fruitlet. Both the immature larval stages and adults can cause damage. Thrips cause cosmetic damage to the skin, leaving irregular scars and russet patterns that carry over until harvest. In summer, adult thrips can damage the skin of peach, nectarine and apricot fruit, causing small white spots that can give the fruit a pale and silvered appearance. Research has shown that the adults find flowers by both sight and smell. They are attracted to pale-coloured flowers, such as those in the native bush. In the orchard they are attracted to the white or pale pink flowers of stone fruit and white, yellow or pale pink traps. Colour may not be the only factor to consider; odour can play a dominant role. Trials have shown that apple blossoms are not as attractive to thrips as cherry or nectarine flowers, yet they all have white or pink flowers. Furthermore, New Zealand flower thrips are often found on white roses when few are found on red roses. However, it is not known if the attraction of the white rose is based solely on colour. Do red roses have the same smell as white ones to a thrips?

It is worth noting that apart from nectarines, thrips do not damage fruit because their feeding occurs on the disposable parts of the flower, the petals, pollen and anthers.

New Zealand flower thrips need to feed on pollen in order to reproduce. Therefore they are unable to overwinter in areas where there are no sources of pollen. In practice, the area needs to be bare of gorse because this plant is a significant source of pollen for thrips during winter. Gorse flowers give the thrips population an early start in areas such as Canterbury.

Dr Jill McLaren, based in Central Otago, carried out a series of trapping experiments and found that New Zealand flower thrips set off in flight between 10 am and 3 pm, especially on fine warm days (maximum temperature above 17°C). Once they were airborne these very small insects (<0.1 mm long) could travel passively for long distances in air currents and establish where they landed.

Thrips also undertake short-distance flights, not only to find new flowers (food sources) but also to be `comfortable’ as they seek to avoid direct sunlight, low humidity (e.g. north-west wind) or rain. To do this they can move within the tree or hide within floral or vegetative parts of the plant. On any day, New Zealand flower thrips move around within a nectarine tree, from the top and south side of the tree in early morning, to the north side later in the morning, to the centre and west in the afternoon, and return to the top and east side at the end of the day.

Thrips feed on fruit (sweet juices) in summer. Most of the adults found on fruit and flowers are female. They reproduce rapidly and most complete a series of rapid generations over summer. Dr McLaren’s research showed that they can complete seven generations in Central Otago where their growing season is restricted to eight to nine months. The first generation in spring is completed in less than 30 days on nectarines in Central Otago.

It is relatively easy to reduce the numbers of New Zealand flower thrips with the simplest insecticides such as soap or oil, but it is difficult to keep them away for long from attractive flowers or ripe peaches. This is a problem for export fruit (New Zealand flower thrips is a quarantine pest) but should not be a problem for fruit from the home garden.

Nectarines are the only crop at risk from thrips infestation in spring. The critical times are during flowering and for two to three weeks after petal fall. Note that the choice of insecticide for use during flowering is problematic because of the risk of killing bees. Read the label before use.

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Mites
Mites are unlikely to be a problem on stone fruit in a home garden because predatory mites should be present in good numbers. In the 1970s and 1980s, however, a team of researchers tracked the cause of sudden mite “flares” in commercial orchards back to the use of certain pesticides (insecticides, fungicides and even miticides). These had been responsible for killing the predatory mites that had always controlled the plant-feeding mites. Today, all pesticides are screened for their impact on mites before they are released commercially, but some of the older products are still available for use in the home garden. Look for information on the label that says they are 'IFP compatible’.

The most important mite on pipfruit is European red mite, Panonychus ulmi, and its key predator is Typhlodromus pyri. On stone fruit the most important mite tends to be two-spotted mite, Tetranychus urticae, and its key predator is Galendromus occidentalis. There are a number of other general predators of mites that may be very important in mite control such as the whirligig mite, Anystis baccarum.

One of the key differences between Panonychus ulmi and Tetranychus urticae is that the latter overwinters as a diapausing adult female on the tree or on the ground cover. In this diapausing state two-spotted mite is resistant to any miticide and remains that way until October when it starts to feed. In contrast, the European red mite overwinters as an egg on the tree and hatches during October in all regions. European red mite eggs can be killed by treatments such as oil in the late dormant period but two-spotted mite is unaffected and cannot be treated until it starts to feed again. The diapausing female of two-spotted mite is orange, but in its summer form it is green with two spots (gut contents).

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Cherry slug
Contrary to its name and maggot-like appearance, cherry slug is in fact the larva of a sawfly and a member of the Hymenoptera (wasps and bees). As an orchard pest it is unusual in that it overwinters in a pupal cocoon in the soil. Cherry slug can skeletonise the leaves of cherry or plum trees in February or March, presumably reducing yields in the following year. It has two other important hosts that could be present in a garden: pear and rowan. Research by Drs Howard Wearing and Jill McLaren on cherry slug in Central Otago found that only one generation per year could be detected in that region but in Hawke’s Bay, Dr Jim Walker found two generations per year with the earliest generation causing some damage. Cherry slug is also known as pear slug and can be a problem because many of the newer insecticides do not target cherry slug.

Earwig
Earwigs can cause some damage to the fruits of apricots, peaches and nectarines. The damage produced is typically a circular cavity. Earwigs can shelter inside the stones of stone fruit, especially those with split stones, and generally cause some alarm among people handling the fresh fruit. However, earwigs are also a useful predator, feeding on pests such as leafrollers. They are nocturnal, feeding during the night and sheltering by day. They are easily trapped in corrugated cardboard traps tied around the trunk.

For further information see the website www.Hortnet.co.nz/key/

Book
McLaren G.F.; Tate, G.F.; Wood, G.; Grandison, G.; Horner, I. 1999: Summerfruit in New Zealand. Management of pests and diseases. HortResearch, AGMARDT, Summerfruit New Zealand Inc. University of Otago Press. 136 p.

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