Abstract

While the public’s expectation of organic production is for unsprayed fruit, the rapid decline in the health and production of existing unsprayed stonefruit cultivars in neglected orchards is testimony to the need to take action against key problems. The most critical of these problems is infection from brown rot Monilinia fructicola (Wint.). Brown rot damages the tree and causes major crop losses. It is responsible for limiting outlets for organic stonefruit to local and processing markets. The most important insect pests are aphids, leafrollers and thrips. Trials have been carried out over the past three years at Clyde to find organically acceptable solutions for some of these problems.

Sulphur, when applied to nectarines at the same frequency and timing as conventional fungicides, achieved control of brown rot (after storage) similar to that with a conventional fungicide programme. Sulphur is reputed to be phytotoxic to apricots, but trials suggest that this effect is not serious, apart from delaying harvest. It is hoped that further trials with modern sulphur formulations will succeed in finding rates of application and growth stages of the crop when sulphur can achieve control of brown rot without causing damage.

The impact of most of the other diseases of stonefruit can be reduced to acceptable levels by the careful use of copper.

Trials on nectarines showed that pyrethrum and intermediate oil, applied separately in the spring, controlled green peach aphid Myzus persicae (Sulzer), but not New Zealand flower thrips Thrips obscuratus (Crawford). Ryania controlled thrips on nectarines at flowering time, but the New Zealand registration of this product is currently not proceeding. An alternative, organically acceptable product is being sought.

Control of leafrollers is difficult on organic orchards. Mating disruption, Bacillus thuringiensis and pyrethrum are under investigation. Despite the activity of natural enemies in most districts, they are inadequate for commercial leafroller control.

Control of oriental fruit moth, Grapholita molesta, with mating disruption has not been tested in New Zealand, but the success of the technique in Australia and North America suggests that it could be used successfully here. South Island orchards are at risk from the transport of infested fruit and nursery trees from the North Island. No formal inter-island quarantine procedures are in place for this pest.

The three protectants, copper, sulphur and oil, are valuable in the organic stonefruit programme and can, with the possible assistance of mating disruption, control most pests and diseases if applied correctly. Remaining problem areas include sulphur-sensitive crops (brown rot), leafrollers and thrips. Breeding and selection for disease tolerance in stonefruit crops remains a priority for the future, particularly for brown rot.

Introduction

Organic production of stonefruit, if it is to be economically viable, requires not only an understanding of organic principles but a knowledge of the timing of susceptible stages of the different pests and diseases. There is the same need for this understanding among organic growers as for growers of conventional stonefruit.

Some pest and disease problems are common to all stonefruit crops, e.g. brown rot Monilinia fructicola, but others are specific. For example, New Zealand flower thrips Thrips obscuratus (Crawford) damages only nectarines, and leaf curl Taphrina deformans (Berk.) attacks only peaches and nectarines.

Although the public’s expectation of ‘organic’ fruit is that it should be unsprayed, there are few stonefruit crops that will survive with just a supply of water and nutrients. There are even fewer that will produce a full crop every year. In the absence of sprays, we have observed in Central Otago that more than 50% of current cultivars of peaches, nectarines and Japanese plums die within the first 5 years, although this varies with cultivar. In the same time period, approximately 25% of sweet and sour cherries and apricots die, but less than 10% of European plums, e.g. cv. Greengage. The main causes of tree death are a mix of stonefruit blast, Pseudomonas syringae pv. syringae, brown rot, and silver leaf, Chondrostereum purpureum. Yields of those trees that survive is severely reduced by brown rot and leaf curl while the appearance of the fruit is downgraded by stonefruit blast, bacterial spot, aphids, leafroller and thrips (nectarines only). Reliance on natural pest and disease control is therefore unlikely to be economic for commercial-scale stonefruit production with current cultivars. European plums and sour cherries are the possible exception. Where these two crops are grown in dry climates, and silver leaf and brown rot are less prevalent, there is a real possibility for the production of low-input organic crops with existing cultivars.

Diseases

For those crops which require some protection to maintain satisfactory tree health, yields and fruit quality, the main diseases are brown rot, stonefruit blast, leafcurl, silver leaf and bacterial spot Xanthomonas campestris pv pruni (Smith). Before discussing the control of these diseases, it is important to emphasize that for organic orchardists it is helpful to select cultivars which have low susceptibility to disease, particularly to brown rot and stonefruit blast. While HortResearch is still developing the techniques to screen cultivars for susceptibility, local orchardists and home gardeners may be able to suggest crops and cultivars which are well adapted to the local climate and less susceptible to disease.

Several diseases can be controlled with copper, including stonefruit blast, bacterial spot (Young 1987) and leafcurl (Tate and Wood 1994). Information on their timing is available from conventional orchard spray programmes and will not be discussed further.

Silver leaf

Trials are continuing to develop methods of protecting trees against silver leaf, using both trunk injection and wound dressings which incorporate biological control agents (Spiers 1994).

Brown Rot

Application of control measures for brown rot is critical during the flowering and preharvest stages. In 1992/93, at Clyde, we compared a conventional fungicide programme of nine applications, carried out between flowering and harvest, with three modern formulations of sulphur on the same programme (Table 1).

The trial, which was carried out on nectarines, consisted of 3-tree plots which were randomized and replicated 5 times. Fifty fruit per tree were harvested from the central tree in each plot, and were stored for 2 weeks at 0^oC before being held in the laboratory at 20^oC. The number of fruits with rots was recorded daily and then removed. The cumulative number of rotten fruit per treatment was analysed after 7 days at 20^oC.

Table 1: Cumulative post-storage losses of ‘Fantasia’ nectarine and ‘CluthaGold’ apricot fruit due to brown rot. Phytotoxicity score on apricot (5 = no phytotoxicity, 4 = slight leaf cupping, 3 = severe leaf cupping, 2 = some defoliation, 1 = dead) on 16 January 1993. Numbers in the same column followed by the same letter were not significantly different at the 5% level.

A concurrent trial was carried out on apricot using single tree plots, with guard trees, replicated 4 times. Some leaf cupping was observed on the sulphur-treated trees and fruit maturity was delayed by at least a week. Maturity was delayed on all the sulphur-treated trees and complicated the assessment of brown rot susceptibility, when compared with the untreated and conventional programmes, because susceptibility to brown rot increases with maturity. Thirty apricot fruit per tree were placed in coolstorage for 2 weeks and assessed for rots after seven days at 20^oC. On nectarines, both Kumulus and Thiovit were as effective as the standard programme (P=0.01).

On apricots, there was no significant difference between treatments in the level of brown rot (P>0.05) but Kumulus and Sulflo showed less phytotoxicity to apricots than Thiovit (P<0.05). Overall, Kumulus showed promise for use on both crops for brown rot control.

More work is required on the phytotoxic effects of sulphur on apricot leaves, flowers and fruit before it can be recommended on this crop. Similarly, the effect of sulphur on cherries and plums requires investigation. On sulphur-sensitive crops such as these, we are seeking organically acceptable alternatives to sulphur for brown rot control.

Sulphur is reported to be toxic to predatory mites overseas (Field 1978; Hoy and Conley 1987) and some delayed toxicity was demonstrated for the most common predatory mite on stonefruit in Central Otago, Galendromus occidentalis (Nesbitt) (McLaren and Fraser 1993).

We tested the effect of 4 applications of sulphur per season over two years on G. occidentalis and its plant-feeding prey, European red mite Panonychus ulmi (Koch) and two-spotted mite Tetranychus urticae Koch on nectarines (Figure 1). Numbers of G. occidentalis and two-spotted mite were not affected over two years, but European red mite increased significantly on the sulphur-treated trees in the second year (P<0.05).

The long term effect of sulphur would probably depend on the frequency of sulphur use over a number of years and the ability of the predator to survive this level of exposure.

On the positive side, there is evidence that long-term exposure of G. occidentalis to sulphur can lead to the development of resistance to this material (Hoy et al. 1984; Hoy and Standow 1981).

Orchard hygiene is an important factor in the control of brown rot, requiring the removal of all mummies in late summer and pruning of cankers in the winter.

Figure 1: Mites per leaf over two years on nectarine cv. 'Fantasia' sprayed 4 times with sulphur (200g/100 litres) between October and December in 1990 and 1991. The mites included the predator Galendromus occidentalis, and its two prey species, two-spotted mite Tetranychus urticae, and European red mite, Panonychus ulmi

Pests

The main pests of stonefruit are aphids, leafrollers, thrips and armoured scales. Their relative importance depends on where and how the fruit is to be marketed. Local and processing outlets can tolerate the presence of some insects and their damage, but export markets have a nil tolerance for many pests, particularly leafrollers and thrips.

Aphids

Each crop, with the exception of apricots, is colonized by different species of aphids. The most common aphid on peaches and nectarines is green peach aphid, Myzus persicae (Sulzer), and on cherries, its close relative, Myzus cerasi (F.), the cherry aphid. The black peach aphid, Brachycaudus persicae (Passerini), also occurs on peach while the leafcurl plum aphid, Brachycaudus helichrysi (Kaltenbach), is the most common on plum.

On most crops, aphids overwinter as eggs on the host plant, at the base of the flower or leaf buds. Eggs hatch throughout August and the subsequent active stages complete several generations on the fruit trees. Winged forms appear in November and begin to move away to their summer hosts.

Any damage caused by aphids prior to November, therefore, is likely to have originated from the overwintering eggs. Intermediate oil (2% at 2000 litres/ha) controls aphids by smothering the eggs before they hatch, and was presumed to work only if applied very early in August.

However, a trial using intermediate oil which was applied on 28 August 1989, gave excellent control of green peach aphid on nectarines, leafcurl plum aphid on plums and cherry aphid on cherry.

This suggests that oil is effective against the active stages of aphids as well as the eggs and oil applications may be carried out at any time during August. Aphids can also be controlled later in the season with pyrethrum, but this is expensive compared with oil (currently $12.13/100 litres dilute mix, compared with $3.38/100 litres for intermediate oil).

Thrips

New Zealand flower thrips damage nectarine fruitlets in the spring when they feed on pollen, nectar and the fruitlet. A trial at Clyde in 1991 tested two natural products for their effect on thrips and aphids. Four applications were made at 10-14 day intervals to 3-tree plots of nectarine ‘Fantasia’, with each treatment randomized and replicated 4 times. Applications were made 4 times at 10-14 day intervals from ‘pink’ (13.9.91) to 20 days after ‘petal fall’ (19.10.91). Insect damage was assessed prior to thinning on 80 fruits from each of the central trees in each plot. Ryania (Ryan 50, Dunhill Chemical Co., California, USA) was as effective as chlorpyrifos against thrips, but not against aphids (Table 2). In contrast, pyrethrum was as effective as chlorpyrifos against aphids, but not against thrips.

Table 2: Number of export grade fruit in sample of 80 fruits on 15 - 18 November 1991 (assessed separately for thrips and aphid damage). Numbers followed by the same letter in the same column were not significantly different at the 5% level.

Unfortunately there has been a delay in the registration of ryania in New Zealand and its future here is uncertain. We are currently seeking alternative, organically acceptable treatments for thrips.

Thrips are also a problem on export stonefruit at harvest time and are of quarantine significance in many export markets. Recent research has shown that thrips can be controlled by a postharvest treatment using hot water (50^oC for 2 minutes). Stonefruit tolerate this treatment well, but unfortunately, so do leafrollers (McLaren et al. 1993; Jones and Waddell 1994). Non-chemical postharvest treatments for the control of quarantine pests have the potential to allow organic producers to export fresh produce, but it is difficult to find treatments which are tolerated by all stonefruit, while being effective against all quarantine pests.

Oriental fruit moth

Oriental fruit moth is a pest of stonefruit in the North Island and will probably require control measures on organic orchards. This pest has been successfully controlled in Australia (Vickers 1990) and more recently North America (Kirsch 1989; Willett and Watson 1989) with mating disruption. Although this technique has not been tested against oriental fruit moth in New Zealand it is a proven method available to organic growers. Oriental fruit moth was accidentally introduced to Auckland in the 1970’s and has become established in Hawkes Bay and possibly further south. As far as we are aware, it has not established in the South Island but, in the absence of any quarantine measures to prevent its spread, it is only a matter of time before this happens. Transport of infested fruit or nursery stock from the North Island is likely to be the main route for its arrival.

Leafroller

Several different species of leafroller attack stonefruit crops, causing damage to the foliage and fruits, reducing the packout and storage life of the fruit. Four species are endemic to New Zealand (Planotortrix octo Dugdale, Planotortrix excessana (Walker), Ctenopseustis obliquana (Walker), Ctenopseustis herana (Felder and Rogenhofer)) but lightbrown apple moth, Epiphyas postvittana (Walker) is of Australian origin. All leafrollers vary in their distribution by district (Dugdale 1990; Foster et al. 1991; Clearwater et al. 1991) and host plant range. Several methods of control are available but at this stage, none can be relied on to produce results equivalent to conventional insecticide programmes. Options available to the organic grower include mating disruption, biological or botanical insecticides and hand-grading, assisted by a range of natural enemies.

In 1994, a commercial grower in Central Otago collaborated in a trial of mating disruption of P.octo, C. obliquana and E. postvittana over a 2.6 ha block of apricots using Shin-Etsu dispensers (Shin-Etsu Chemical Co., Japan). Although leafroller infestation on the fruit was higher than in the insecticide-treated apricots in a neighbouring block, the grower was able to hand grade out the 1% damaged fruit from the disruption-treatment and meet export standards for Australia. This demonstrated that mating disruption could keep leafroller levels low (especially if they were low to start with) and secondly, hand grading is a feasible option for stonefruit which need to meet quarantine standards.

The mating disruption technique involved the installation of 1000 Shin Etsu dispensers per hectare for light brown apple moth and 1000 dispensers of a common pheromone blend for the native leafrollers, P.octo and C. obliquana (Suckling et al. 1996 this volume). Dispensers release the pheromones over several months, making it difficult for males to locate females. In the absence of mating, infestation is reduced, except on the edges of the block where some immigration of mated females may occur. There are some major limitations on the choice of suitable sites for this technique. The blocks of trees need to be large (>3ha), with trees of uniform size, with few gaps, sheltered and on a flat, low-lying site. Unfortunately, stonefruit blocks are often planted on slopes or terraces with the rows orientated for maximum air drainage down the hill during frosts. The same air drainage carries the pheromone down the hill, thus reducing the benefit of this technique.

Two biological/botanical insecticides are available for leafroller control, Bacillus thuringiensis and pyrethrum. At this stage B. thuringiensis is not registered for use on stonefruit in New Zealand but it is overseas. Trials are needed to demonstrate its efficacy on stonefruit before it can be registered. Pyrethrum shows promise but may be too expensive. Summer oil is reported to cause some mortality of both the egg and larval stages of leafrollers (Tomkins and Thomson 1992; Thomson et al. 1993).

Leafroller control remains an area of difficulty for organic stonefruit producers. In our experience, leafrollers can be responsible for up to 16% loss of crop in unsprayed orchards, even when 28% of the larvae are parasitized by Dolichogenidea tasmanica (Cameron).

Armoured Scales

Scale insects, particularly San Jose scale Quadraspidiotus perniciosus (Comstock) and oystershell scale Quadraspidiotus ostreaeformis (Curtis) can be controlled by the use of intermediate oil (2%) at early bud movement (McLaren 1989; Wearing unpublished). This needs to be applied annually and can be achieved at the same time as aphid control.

Conclusion

Three protectants, copper, sulphur and oil, can combat many of the diseases and pests of stonefruit, when used at the correct time. These materials, together with mating disruption for oriental fruit moth, will allow growers to meet local market or processor’s requirements for fruit quality. Problems remain with the control of leafrollers on all crops and with thrips on nectarines in the spring. There may remain problems with the use of sulphur on some crops, making brown rot control impossible; selection of cultivars with low disease susceptibility is a priority for this group and is being treated as such by HortResearch. Export of fresh fruit of sulphur-tolerant crops may be possible by the use of non-chemical postharvest treatments for thrips combined with mating disruption and handgrading for leafroller.

References

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