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Pests & diseases

Pests and diseases

Managing pests and diseases has always been a major challenge for apple and pear orchardists. Not only do these pests and diseases reduce quality of product for the consumer, but some such as codling moth can decimate production. Many of the main pests and diseases of apples and pears present in South Australia were introduced on planting material during early colonisation.

Most early pest and disease management revolved around physical management of trees and orchards. This included processes such as picking and disposing of pest infested fruit from trees, minimising weeds on the orchard floor, treating harvested fruit, and minimising movement of pests and diseases in second hand boxes.

To assist pest management, the Vine, Fruit and Vegetable Protection Act 1885 was introduced. This Act was subsequently updated in 1910, 1959 and replaced by the Fruit and Plant Protection Act in 1968. It was designed to prevent the introduction and provide for the destruction of certain insects and eradication of diseases. It was particularly important for preventing introduction of fruit fly.

Administration of this legislation was applied by a network of district inspectors located across the Adelaide Hills, Adelaide Plains and Northern districts. These were initially introduced in 1905 (although George Quinn provided some inspection services prior to this date) to ensure growers applied control measures prescribed under the Vine, Fruit and Vegetable Protection Act. By the 1920s these inspectors had taken on largely an advisory role.

Codling Moth – the Main Pest

The most persistent apple pest over the years has been codling moth. Control sprays began with lead arsenate, and went through DDT, malathion and other organo-phosphorous insecticides to azinphos methyl.

As in many apple-growing areas of the world, codling moth, Cydia pomonella, has been, and remains the key pest of most South Australian apple-growing areas. It has been a pest here since the 19th century and was presumably imported accidentally from interstate or overseas. In 1894 George Quinn was appointed Codling Moth Inspector. Quinn organised an area-wide management program for commercial and non-commercial apple trees in most parts of South Australia, aided by district inspectors. By the early 1900s Inspectors reported codling moth from all districts, including the Adelaide Hills, Adelaide Plains and South East.

For over 50 years, the main codling moth control methods were applications of lead arsenate to the surface of the developing fruit supplemented by trunk banding to trap overwintering larvae, bark scraping to remove larvae and removal of fallen fruit from the ground. These techniques appear to have been effective in commercial orchards as they were part of a coordinated program. Township and neglected trees were sources of reinfestation and a continuing irritation to commercial growers.

Trunk trapping band, Blackwood 1961

Hand spraying apples, Blackwood 1950

Spray residues on the surface of apples were a source of complaint by consumers. The industry addressed this problem by polishing apples before marketing. Even when the need for polishing was obviated by the curtailment of lead arsenate usage, polished and waxed apples continue to be regarded by consumers as the sign of a healthy apple.

Other Major Pests

Bryobia mite was another common pest but during the 1960s was replaced by the more insidious two spotted mite because broader spectrum insecticides knocked out predators and parasites that had kept the two spotted mite in check. Bryobia mite was easy to control with an oil spray. Heavy dormant oils like red oil or lighter semi dormant oils were effective. Later when two spotted mite was more of a problem requiring maybe a semi-dormant oil in the spring followed by sprays throughout the growing season of materials such as Kelthane(R) or Tedion(R), and now more efficient materials, bryobia mite doesn’t seem to be a problem.

Woolly aphid, Eriosoma lanigerum, was a persistent pest of apple trees that was not suppressed by lead arsenate. A parasitic wasp, Aphelinus mali, provided some control but trees were often debilitated by the aphid. Use of the aphid resistant rootstock Northern Spy dates back to 1907 (initially evaluated for resistance to Bitter Pit) when planting of Blackwood Experimental Orchard first started. By 1936, Blackwood Experimental Orchard was planted with aphid-resistant rootstocks imported from East Malling, England. The trend to dwarfing stocks has lowered tolerance to woolly aphis.

Scale insects such as oyster scale were a bit of a problem but fairly easy to control with dormant or semi-dormant oil sprays. In addition scale insects were partially controlled with lime sulphur sprays which were regularly applied to control black spot (up until about the mid 1960s).

Managing Chemical Resistance

After 1945, broad-spectrum insecticides largely replaced older methods of insect control in apple orchards. The newer insecticides were easier to use and mostly more effective. By 1947, DDT was registered for use in South Australian apple orchards. In a prescient observation, Harry Kemp, who did the initial trials noted that DDT killed predators and parasites and warned against indiscriminate use of this new insecticide. In 1952 Kemp reported that the hitherto uncommon two-spotted spider mite, Tetranychus urticae, appeared to have developed resistance to DDT and, in the absence of its predators, was increasing in numbers on apple leaves. By 1953, a DDT resistant strain of codling moth was reported.

For the following 4-5 decades, organophosphate insecticides were the main control option for pest control in commercial orchards. These broad spectrum insecticides suppressed susceptible pests; codling moth, light brown apple moth (Epiphyas postvittana), bryobia mite (Bryobia rubrioculus), and apple weevil (Otiorhynchus cribricollis). However, organophosphate resistance of two-spotted spider mites was recorded by Ted Fenner in 1962. Mites with induced resistance to organophosphates were more difficult and expensive to control than the primary pest, codling moth. A crisis was averted by adopting rational pest management methods, including reduced frequency of insecticide application and conservation of beneficial organisms. Integrated pest management (IPM) was rediscovered after a lapse of some 50 years.

Reintroduction of IPM

During the 1980s, a national program for managing imported insecticide-resistant predator mites was coordinated by Les Readshaw of CSIRO. By the 1990s many South Australian growers were using integrated predator conservation in their orchards, often with the help of monitoring services.

Following research by Richard Vickers of CSIRO, pheromone mating disruption is being used to replace codling moth sprays in many South Australian apple orchards. While this technique obviates the need for codling moth sprays and consequent two-spotted spider mites, pests previously suppressed by broad-spectrum insecticides are now requiring management.

Regular orchard inspections, monitoring populations of pests and diseases along with computer based prediction models are now used extensively to optimise introduction of biological control agents and apply required pest and disease management treatments. These techniques have greatly reduced reliance on agricultural chemicals.

Codlin moth pheromone lure, Lenswood 1990

Major Diseases of Apples

As far as diseases were concerned the main problem was Black Spot also known elsewhere as apple scab (Venturia inaequalis), for which sprays of lime sulphur were applied. This was not a very pleasant exercise and because of the frequency of sprays believed to be necessary. It was expensive and labour consuming.

Plant pathologist Dr Bill Moller pioneered the work on Black Spot control beginning in the early 1960’s, when he ran trials in the Adelaide Hills to determine when spore releases occurred in spring, and then how much leaf wetness was required to allow infections to become established on apple leaves. Dr Moller’s work showed that most spores were released in October, and years with above average October rainfall were always worst for Black Spot. In 1968 Dr Moller’s trials at the Lenswood Research Centre showed that control of Black Spot with the newly develped fungicide benomyl was superior to that achieved with other commercial fungicides. Benomyl was widely adopted by the industry as it also controlled powdery mildew, a major problem on Jonathans, a major variety at that time. However by 1974 a few growers in Lenswood reported that benomyl was no longer effective on Black Spot. This was shown to be due to fungicide resistant strains; the first world report of benomyl resistance with Black Spot.

When the leaf wetness recorder indicated conditions were suitable for Black Spot infection to occur, warnings via press and radio advised growers to apply a spray if they were not already protected with a cover spray, or to apply an eradicant spray within a certain period to eliminate infections that had already established. This system developed into a prediction service based on weather forecasts and worked very well. It did a lot to help growers understand the disease and how to control it.

This system has been significantly refined using a network of weather stations linked to computers and leaf wetness recorders to more accurately predict disease infection periods. This service is now provided to growers on a commercial basis from the Lenswood Cooperative.

Leaf wetness recorder – part of the Black Spot warning system, Lenswood 1971

Powdery mildew is no longer a significant problem as most of the apple cultivars grown in the Adelaide Hills are less susceptible than Jonathan. However most are highly susceptible to Black Spot.

Spray Charts and Advisory Services

Since the formation of the SA Department of Agriculture in 1905, the SA apple and pear industry was serviced by a network of regional advisers. These advisers prepared annual spraying and compatibility information for pests and diseases as well as nutrient deficiencies. This work was carried out in liaison with various specialist research officers, particularly entomologists and plant pathologists within the Department of Agriculture, CSIRO and Waite Institute.

Distributed via local packing sheds and Department of Agriculture officers, these annual spraying charts commenced in the mid 1950s and continued until 1988. These spraying charts contained comprehensive information about treatments for various apple and pear pests and diseases, application rates, timing and compatibility of different chemicals. The move from high volume tree spraying to concentrate spraying with air blast sprayers required a significant education program for growers during the 1950s. Additional information on concentrate rates were incorporated into the spray charts.

With the growing technical complexity of using IPM, pest and disease management information was presented in a more comprehensive hand book. The Pest and Disease Control Handbook: pome fruit, stone fruit and berry fruit was first published by the SA Dept of Agriculture in 1987–88.

Introduction of Air Blast Sprayers

Hand spraying fruit tress was a labour intensive process. It was generally done using hand held spraying wands with pressurized spray being provided from a mobile tank with petrol engine powered pump, or a network of pressurized spray supply lines in an orchard. Normally an orchard worker would be expected to hand spray 1 to 1.5 acres per day.

Post WW2, there was considerable movement of labour from agriculture to manufacturing industries. The shortages of labour was a critical problem for orchardists. Finding labour saving devices was a major focus for horticulture research during this period. Introduction of air blast sprayers and bulk handling of fruit were key labour saving opportunities identified for horticulture industries by Harry Kemp (Harry Kemp was the first Horticulture Research Officer appointed by the Dept of Agriculture in 1936, and continued as the Senior Horticulture Research Officer until 1954).

The SA Department of Agriculture played a lead role in introduction of these air blast sprayers to Australia from Canada. This included initial assessment, licensing of local manufacture, calibration and informing industry about air blast sprayer technology.

Rapid and wide scale adoption of air blast sprayers by orchardists and vignerons occurred through the mid 1950s.

The original imported Okanagan air blast orchard sprayer in use at Blackwood Experimental Orchard C1950.