Pests and Diseases

"Insects and diseases are the symptoms of a failing crop, not the cause of it". -- William Albrecht

If the approach taken by most agricultural scientists and books written by them were a guide, then pests and diseases are the result of a deficiency of pesticides and fungicides. Of course this is not so. Pests and diseases are almost always the result of plant stress. These stresses include:

Not a direct stress, but also important, is the decimation of predators caused by inappropriate pesticide use.

All of these factors are at least partially under the growers' control. If the stresses are avoided, or diminished, then many plant pests and diseases either simply do not occur, or fall below levels that justify control. The question then arises of the economic viability of avoiding plant stress versus using pesticides and fungicides. All of the stresses listed above decrease crop yields, not just through crop loss caused by the pests and diseases they encourage, but more directly.

Let's take spider mites as an example. You will probably have noticed that they are much worse in periods of hot, dry weather. Plants under stress from water deficiency are what spider mites demand. Of course a plant that is suffering from water deficiency is also not going to yield as well as it would were it supplied with adequate levels of water. Is it more economical to allow crops to suffer water deficiency, reducing yields and use miticide, or to supply more water, increasing yields and eliminating the cost of the miticide?

One way to supply more water without the necessity of additional irrigation or rainfall is to improve the water-holding capacity of the soil and also the infiltration rate of water falling onto it. Increasing the humus level will accomplish this. Humus is important in utilising water to its utmost. Water that runs off is not just wasted, but also carries topsoil and nutrients away from the crop.

Another common pest, one that is almost ubiquitous, is the aphid. These little suckers probably cause more damage than any other insect. The first thing to consider is their nutritional needs. Aphids cannot digest complete protein; they require free amino acids (the building blocks that plants use to manufacture protein). Excessive amounts of water-soluble nitrogenous fertiliser create the condition of high levels of free amino acids in plant sap, effectively a dinner invitation to aphids. Conversely, feeding protein to plants reduces the level of free amino acids and minimises the attractiveness of plants to aphids.

Many insect problems are caused by monoculture; that is the growing of vast areas of a single crop. In a polyculture, such as a natural ecosystem, insects have the problem of finding the next plant to feed on. Not only is it likely to be some distance away, its odour, essential for insects to find it, is masked by the odours of all the other plants in the insect's vicinity. Not only that, some of those other plants harbour predators on the insect, so it is more likely to be consumed in a polyculture than in a monoculture.

Insecticides, natural or synthetic, are a poor answer to the problem of excessive insect pests. This is because insect predators necessarily reproduce more slowly than their prey. If it were otherwise, then they would eat themselves into starvation. Most insecticides kill pest and predator alike, so unless they are used continuously, they give pests an edge over predators. Unfortunately, continuous use is not just expensive, it leads to pesticide resistance. Then, when a pest outbreak occurs, there is one less insecticide in the arsenal.

Some predatory insects can be encouraged by providing attractive food sources. For instance, hoverflies whose larvae consume aphids, are attracted to flowering umbelliferous plants whose nectar they consume. Traditionally, Britain's hedgerows provided habitat for many predators on insects. It is no coincidence that the decline of hedgerows in Britain has been accompanied by dramatically increasing pest problems. Many Australian farmers have discovered the virtues of leaving some bush to provide a predator reservoir, or reintroducing bush to their farms where similar problems are occurring.

Many birds are avid consumers of insects and insect larvae. In the New England Tableland, there has been an interesting study of a species of bird that consumes grass grubs. It is the female that consumes the grubs, while her male counterpart consumes the nectar of flowering gums. The females will not feed more than 150 metres or so from the males, so the maximum distance of pasture from trees needs to be no more than this distance for natural grub control. Growing belts of trees and shrubs on farms has other benefits apart from pest control. They keep groundwater under control and reduce evaporation of rainfall by reducing wind speed. Stock chilled by wind need to consume more feed to keep warm, so windbreaks can provide increased productivity. The shade from hot summer sun they provide reduces heat stress.

Insects that appear to be pests at first glance can also be seen in a quite different light. Japanese agricultural researcher, Masonabu Fukuoka, was trialling a pesticide to control a stem borer that afflicts rice. Much to his surprise, the first trial showed a yield decrease in the paddy treated to control the stem borer. A repeat trial also showed that killing the pest decreased rice yield. He came to the conclusion that plant density was the issue. The stem borer thinned the rice plants to produce a higher yield than when they were too crowded. The funds for this research came from a pesticide manufacturer who forbade publication of this interesting result. After all, it would have reduced sales of their products! Fukuoka, having drawn a number of conclusions from his years of agricultural research, took up organic farming and put his ideas into practise.

When we look at a natural ecosystem, which by definition is devoid of pesticide and fungicide inputs, we see very little pestilence and disease. Note that there is not a complete absence, just a very low background level. Of course, such a system is not very productive from the human economic viewpoint, which is why we developed farming. What organic farmers are attempting to do is bring the control mechanisms in the natural system into our more productive farming systems. The problem here is that the more we improve productivity, the further removed from the natural ecosystem we get. Maintaining the mechanisms of the natural ecosystem alongside improved productivity requires considerable effort and expertise.

Ironically, peasant populations the world over have achieved this, yet we have been trained to perceive peasants as ignorant. Miguel Altieri, who coined the term agroecology, took a group of botanists and a group of peasants into a Central American forest. Each group was required to identify as many different plants as they could. The peasants won by a country mile.

We referred in the previous chapter to the oxygen/ethylene cycle and its effects on soil biology. It is a feedback mechanism for maintaining the balance between aerobic and anaerobic micro-organisms. Its existence was discovered in natural ecosystems and appears to be what the best organic practises can achieve.

Ethylene is a gas produced by ripening fruit and decomposing vegetables. When we wrap tomatoes to ripen them, we are capturing the ethylene and preventing its escape, thus accelerating the ripening process. Ripe bananas are prolific producers of ethylene, so this is why we put a banana in a bag with tomatoes to accelerate their ripening.

Disease organisms are organisms that decompose organic matter and can be looked at from two differing viewpoints. When they are attacking our living food crops they are a problem. When they are decomposing crop residues, they are converting them into food for the next generation of plants. What is it about our current agricultural practises that allows what are usually benign organisms to run out of control? What keeps them in check under natural conditions and in organic farming? Let's look at what happens to organic matter under the systems of organic and conventional production.

Plants consist of mainly carbohydrate (starches, sugars, cellulose) and proteins. When plant matter is incorporated in the soil, it is decomposed by the soil micro-organisms. In the presence of oxygen, the carbohydrates are decomposed by fungi to generate carbon dioxide and water. The carbon dioxide displaces oxygen. These fungi are just as happy without oxygen, but now decompose the carbohydrate to alcohol and carbon dioxide. Under this condition, anaerobic bacteria come into the picture and decompose the carbohydrate to methane and ethylene. The ethylene suppresses the aerobic bacteria so they consume less oxygen. Consequently, oxygen levels increase, suppressing the anaerobic bacteria and the ethylene level then decreases. This allows the aerobic bacteria to revive and they transform the alcohol to acetic acid which dissolves nutrients from the silt. Proteins are decomposed to generate the free amino acids they require and some is converted to ammonia. Other aerobic bacteria convert ammonia to nitrate which is absorbed by plant roots. In the process these aerobic bacteria also convert oxygen to carbon dioxide. Plants convert carbon dioxide and water to carbohydrate, liberating oxygen. The plants then die to begin the cycle once more.

This is a grossly simplified view of what happens; there are over 2,000 different species of interacting micro-organisms in a healthy soil. However, it is enough to give us some insight into what we can do to ensure these processes occur and what happens when our farming practises interfere to create undesirable consequences. It illustrates the principle of the dynamics of a functioning ecosystem. Each micro-organism has a different purpose and also provides the checks and balances to maintain the system. As gardeners and farmers, we must either provide conditions that allow these processes to occur, or accept the consequences of hindering them.

What we call disease organisms are part of this ecosystem. They only become a problem when they are allowed to predominate over organisms that in a natural ecosystem keep them in check. Our gardening practises: tillage, fertilisers, pesticides, herbicides and fungicides, all affect the system. Nitrate fertilisers suppress ethylene production, the feedback mechanism for keeping fungal "diseases" in check. Many fungicides kill bacteria, and as we have seen, bacteria are an essential part of the soil ecosystem. The speed of gas diffusion is a function of soil structure. Insufficient air in the soil is a stimulant to the anaerobic organisms and suppressant of the aerobes. Excessive aeration leads to the rapid depletion of organic matter, the food source of micro-organisms. Herbicides are implicated in the chemical lock-up of trace elements needed by plants and micro-organisms for the formation of essential enzymes.

Does this mean we are advocating the immediate cessation of all synthetic inputs? Not at all! The establishment of a healthy soil ecosystem requires time and effort, which is a cost. The consequent reduced need for the supposedly necessary external inputs is a cost reduction. The difference between the two may be a profit, or a loss. For ecologically acceptable farming to be viable, a profit is essential. For a fortunate few farmers, the profit need not be monetary, but a sense of wellbeing engendered by not using toxic, or potentially toxic chemicals. The majority of farmers caught in the financial squeeze between high input costs and low returns must trial these techniques carefully to assess their economic viability.

A further factor is the changes in external economic conditions. The origin of our current farm economy woes was the demand for abundant and cheap food. Having succeeded in supplying that demand, we now find that requirements are changing. The consumer is expecting abundant cheap food without the chemical inputs. It has not yet occurred to them that they could be requiring a decrease in the standard of living for farmers in order to maintain their own. We need to inform them of these and other issues vital to the wellbeing of farming and bring them into the decision-making process. In some European countries, where the negative impact of farm chemicals is more pressing, governments are subsidising the farm conversion process, or requiring the cost of damage caused by agricultural chemicals be included in the purchase price.

Another factor to take into account is the small, but growing number of consumers who are aware of the problems of agriculture and many of them are sympathetic to farmers' needs. They have shown a willingness to pay significant premiums for organically grown produce. Ian McLaughlin, when he was shadow minister for primary industry, has called for cooperation between farmers and the public in solving farmland degradation. Revegetation in the form of trees on farms is a cost most farmers can't meet unaided. McLaughlin's suggestion is that farmers donate the 10-15% of the farm that need to be in trees and the public provide the trees and labour. The farmer benefits from improved productivity and reduced land degradation. The public benefits in improved landscape, water quality and reduced costs of production.

In any event, while a wholesale overnight change is impossible, small incremental changes are not only possible, but highly desirable. What works well on one farm does not necessarily work well on another. What may have a negative impact on profit in one location may have a positive impact at another. By proceeding slowly and sharing our experiences, we can expect to develop agricultural systems that are better and more organic than those predominating now, but they will not necessarily be identical to what we currently call organic. It would be a foolish person indeed who declared that current organic farming practise is a panacea for all our agricultural problems. After all, as we discussed in the early part of this book, our pre-industrial agricultural practises were just as capable of massive land degradation as our currently much maligned conventional agriculture. It's just a lot quicker with tractors than slaves. And it's worth noting that nature, unassisted, takes geological ages to repair the damage we can cause. If we expect to continue supporting a large human population on planet earth, we have a lot of hard decisions to make over the next decade, or two.

While the mechanisms of pestilence and disease as we currently understand them appear complex, the solutions to them, generally speaking, are not. While we cannot create the diversity in farm ecosystems that occur in natural ones, any move to increase diversity will help. An example from the Lockyer Valley in Queensland will illustrate. Broccoli growers adopted a number of strategies to reduce their pesticide inputs. One was the growing of a row of canola every few metres among the broccoli. The canola harbours a predator on one of the target pests and coincidentally provided some wind shelter, since it is taller growing than the broccoli. Another strategy was not growing broccoli when the market was flooded and prices so low that it wasn't really profitable to produce. This discontinuity created a feeding problem for the pests and reduced overall numbers. Dipel (Bacillus Thuringiensis) was adopted for some caterpillar control. This is a living organism, so it has the capacity to breed in the environment and infect subsequent generations of the target pest. Since the bacterial toxin is highly specific to caterpillars, only the target organism is killed. The last strategy was to rotate among a group of chemically unrelated pesticides to reduce the problems caused by target pests developing immunity to the spray, an invariable consequence of using a single pesticide continuously.

This illustrates a number of organic principles:

  1. Any increase in bio-diversity is likely to help reduce the problem
  2. There are incidental benefits to the adopted strategies other than the main goal
  3. Crop rotation provides pest control benefits
  4. There are biological alternatives to chemical pesticides

Specific Disease Control Methods

As has already been indicated, organic methods are rarely single-shot. Nearly always, a number of strategies are adopted. One of the simplest ways to reduce fungal disease on leaves is to ensure that adequate sunlight and air movement occur in a crop. Most fungi thrive where there is high humidity and shade. Soil fungi are more troublesome where there is inadequate humus in the soil and poor drainage.

Another strategy almost universally adopted by organic growers is varietal selection. The more cynical organic producers believe that many modern crop varieties are promoted because of their dependence on synthetic inputs. While older varieties yield less under a conventional regime, they can outperform modern varieties in an organic context without the expensive necessity for spraying.

Nearly all fungal diseases are controlled by the stimulation of bacterial activity. The bacteria appear to be competitors for the same ecological niches as fungi. Sclerotinia, botrytis, phytopthera, mildews and apple scab have all been controlled by applications of fish emulsion and a liquid extract made from compost. Increasing the pH of the leaf surface prevents spores of some fungal diseases from germinating. Examples of the use of this technique include control of botrytis and apple scab with applicationsof a 3% solution of sodium silicate, or a saturated solution of calcium hydroxide (Limil). Also organically acceptable are most of the copper sprays, such as Bordeaux and Burgundy mixtures, sulphur, lime sulphur and sodium bicarbonate (baking soda). Where seed rotting is a problem, potassium permanganate (Chondy's Crystals) is used as a seed dressing. Damping-off of seedlings is generally controlled by lightly dusting the soil surface with sifted wood ashes, or hydrated lime. Covering seeds with sand rather than seed raising mix also helps by improving drainage around the stem where the infection occurs. Mildews can be controlled with phosphorous acid.

Many diseases are a response to unbalanced plant nutrition. The emphasis on providing for the plants' nutritional requirements mitigates against most fungal diseases being a problem for the organic grower.

Research is currently under way to develop biological controls for a number of pest and disease problems. While this is laudable for its potential to reduce the level of synthetic pesticide use, this research is of more use to the users of these chemicals than to farmers whose management precludes their necessity.

Storage Diseases

One aspect of organic production that is remarked upon with some frequency is the claim for longer shelf-life of organic produce. Opponents of organic production say that because organic produce is not protected with chemicals, it is more subject to bacterial and fungal contamination. Therefore, they say, organic produce is more hazardous to the health of the consumer than the chemical residues in conventionally grown produce. This is not borne out by scientific research.

Production Method and Storage Loss

  Conventional Organic
Potatoes 24.5% 16.5%
Beetroot 59.8% 30.4%
Carrots 45.5% 34.5%

It is easy to see from results like these that yield could be lower in the paddock, but more produce be saleable at the all important market end of the production process.

My friend Ted Sloane was an agricultural extension officer in New Zealand when he decided to take up farming. He decided to put his conventional agricultural training to practical use by growing kiwifruit. The results in terms of yield were extremely gratifying; they were the best in the district. Unfortunately, the keeping quality of the fruit was poor and losses in storage were over 20%. Consequently, his income was well below the district average.

It was fortuitous that one day when Ted was burying the recently deceased domestic cat that he noticed the prolific number of earthworms in the home garden in contrast to their complete absence in the orchard. It was then that Ted decided to replace his conventional fertiliser program with organic fertiliser. He chose a liquid fish product that was available locally. The earthworms proliferated and the wind-drifts of leaves that previously banked up against the windbreaks for many months were rapidly consumed by the improved soil biology. Ted managed to reduce his spray program from thirteen, or more per season down to two, or three. As a consequence of this, Ted went on to develop his own fish fertiliser and become a manufacturer. Despite solving the kiwifruit growing problem, he was unable to control the ever-decreasing price he received for the fruit.

Dr Mike Walker of Watercress Valley Herbs trialled a range of fertiliser programs on parsley. Not only was the fully organic patch yielding better than the fully chemical, but the storage life of the organic was way ahead. From his customers' point of view, it was more economical to purchase longer storing herbs at a higher price less frequently than to pay less and have to buy more frequently.

Specific Pest Control Methods

Here again the organic grower has a multiple strategy of defence. The first line is to create as ecologically diverse an environment as possible. The few remaining pest problems can then be controlled by relatively innocuous materials. Aphids are controlled by soft soap (potassium stearate, Clensil), or garlic sprays, caterpillars by Bacillus thuringiensis (Dipel), mites with potassium permanganate (Chondy's Crystals) or salt solution, slugs and snails with metaldehyde baits (protected from consumption by birds, or other non-target animals) and codling moth by pheremone traps. Neem is starting to take off as an effective non-residual broad spectrum insecticide with pest-repellent and fungicidal properties.

The traditional organic broad spectrum insecticide, pyrethrum can be used against a wide variety of insect pests, including pear and cherry slug. Commercially, pyrethrum is almost always mixed with the synergist piperonyl butoxide. The organic standards demand that pyrethrum be used without this additive as it is a suspected carcinogen. Its inclusion appears to be to give faster knockdown of the pest, rather than increasing its kill rate. Other traditional broad spectrum natural materials include derris, rotenone and ryania.

One pest control method of note that is remarkably effective is making a spray from the target pest and spraying the crop. Caterpillars, slugs, or whatever, are finely minced in a food blender, strained and diluted. The required application rate per hectare is extremely low (around 1 kg of insects will treat 30 Ha). The theories as to why this works abound, but to the best of my knowledge no work has been conducted to ascertain which is correct. They include spread of disease from the few organisms infected through the whole population, interference with breeding patterns due to spreading the pests' pheremones onto all the plants in an area and repulsion due to the odour of deceased organisms of the same type.

Before predators brought the slugs under control in my market garden, I used a similar technique. Hand-picked slugs were killed by dehydration in dry sugar and the resultant slimy mess fermented for a few days in a warm place. The resultant even slimier mess was strained, diluted and sprinkled throughout the market garden area (approximately 0.5 ha). The slug population dropped to a tolerable level in a matter of a week or so and returned only briefly three years later. A repeat application saw no necessity for further control during a period of ten years. The effect also appeared spread beyond the area treated.

Much work is being conducted on alternative methods of pest control and most is in the field of biological control. Predators and diseases are being bred for many of the more recalcitrant pests. While this is commendable, it is important to realise that they are generally more expensive than chemical controls and often no more effective than providing a biologically diverse environment that produces its own predators and other checks on pest proliferation.

A very new method involves saturating the environment with pheremones, the chemicals that insects use to find each other for the purposes of reproduction. As biotechnology increases its efficiency, we will likely see the day when it is economical to spray a paddock with a pheremone to dramatically reduce the rate at which specific pests can reproduce. A compelling benefit of this approach is that it is highly specific to the target pest.

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Jonathan Sturm 2003 - 2011

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