Pests and diseases

On account of their selective breeding, many vegetable varieties have an inherently low resistance and tolerance against insect pests and disease. As a consequnce, they can be challenging to raise according to strict organic principles. When discussing vegetable gardening in public, I’ll routinely advise beginners that they should expect to lose a certain percentage of their plants to insects and disease each season. Most individuals will accept this. Those who don’t might be comforted to know that the skills and tenacity developed in raising organic vegetables can also be applied to more robust species such as herbs, native shrubs, and flowers.

Since the early sixties, I’ve avoided those chemical products designed for controlling insect pests and plant disease in the garden. Despite adherence to an organic philosophy, some of my gardening colleagues occassionally purchase a chemical preparation which claims to control or eliminate problems caused by plant disease or insect infestation. When describing their reasons, they often grow defensive, as though I were preparing to condemn them for some kind of criminal behaviour. This is silly. It’s not about scoring or losing points. I’m opposed to fanaticism of any sort and always attempt to remain broad minded in my approach to organics. Don’t get into the habit of trying to be more “organic” than your friends and neighbours. I’ve seen people become discouraged by pursuing what appears to be the latest trend in gardening. It’s much easier to raise healthy organic vegetables when you ignore the competitive nonsense which appears to float around these days.

I try to avoid chemical products because I’m convinced they provide incomplete and short term solutions. Consider the average insecticide spray, for example. Despite advertising claims about selectivity, the majority of products kill indiscriminately. They upset the natural balance between nuisance insects and the predatory organisms which control their numbers within a complex ecosystem. Choose the right chemical and it’s possible to exterminate the aphid population which may have attacked your pumpkin vines. It’s also more than likely that the vast web of insect life within your garden will be similarly affected by the chemicals used to control the aphids. Does that really matter? Well yes, actually it does. The lifecycle of an aphid is approximately one week, so within a fortnight you should expect to see them return. Unfortunately, many of the aphid’s natural predators are more complex organisms with lifecycles in the order of several months. They take considerably longer to return. By knocking out their natural predators for several months or more, the chemicals provide the window of opportunity for local aphid populations to quickly reestablish themselves. Ultimately, the short term chemical treatments fail to provide permanent solutions, and are actually more likely to aggravate the original problem.

There is no certain remedy for alleviating problems caused by nuisance insects or disease pathogens. The organic grower depends upon establishing an environment which supports a range of protective resources which in turn strengthens the internal capabilities which enable most plants to maintain health and resist attack. While it’s possible to enhance your local growing conditions with minor interventions such as additional watering, and soil improvement, there are many components which cannot be altered without considerable effort or expense. From a practical perspective, it’s important to preference those varieties of vegetable which can comfortably adapt to the local growing conditions. The likelihood of encountering serious pest and disease problems is significantly decreased when organic vegetables are raised according to suitable climatic and seasonal conditions.

The most important principle of organic gardenining is based upon taking care of the soil’s needs which provides the foundation for a complex ecosystem, including the plants we grow for food. Maintaining healthy soils with a plentiful store of nutrients and sufficient moisture retention will provide your vegetables their best opportunity to grow healthy and strong. Any slowing of plant growth due to lack of nutrients, moisture, or sunlight will lower natural immunity and create opportunities for disease or insect pests to establish themselves.

I purchase my seeds from a specialist supplier who stocks many of the traditional vegetable types which are becoming increasingly difficult to source. As a consequence, I have limited first hand knowledge of the contemporary varieties which are less susceptible to certain forms of disease. In recent years, many organic growers have experienced positive outcomes with new generations of disease resistant tomatoes, onions, and lettuce. These are mostly hybrids with improved vigour and superior crop yields. Unfortunately they are more expensive to purchase, and, unlike their non-hybrid cousins, cannot be satisfactorily grown from collected seed.

People sometimes ask me how I manage to correctly identify the various pests and diseases which attack their vegetables, and other types of plant. I have to admit this amount of faith in my abilities is somewhat overstated. Okay, I’ve gained a decent amount of experience over the years and I’m fairly comfortable diagnosing the common and everyday symptoms of insect and disease pests. Like everyone else, I get it wrong on occassions, and sometimes I’m just plain perplexed. A few months ago I was asked to identify some weevils which were found amidst a crop of sweetcorn. They were smaller than the common brown variety and brightly colourful with streaks of iridescent blue across their back. I’d never previously encountered this variety so had no insight into their larval development or lifecycle. We ended up forwarding our collected specimens to an entomologist based at the zoological gardens. Several weeks later, I received an urgent telephone call from a quarantine official. He introduced himelf as the coordinator of a task force established to trace the unexplained arrival of these insects. Originating from elevated regions of Mexico, and Central America, this foreign weevil was considered a biosecurity risk.

The most accurate warning signs for identification or diagnosis of particular disease and insect problems will vary depending on a garden’s physical location, climate, and soil characteristics. One of the best studies for newcomers is to visit local gardeners who can share some of their own experience and perhaps offer suggestions about the early identification and control of frequently encountered diseases and insect pests. Keeping a regular garden notebook or journal is generally worth the effort. Try to include detailed notes about the visible signs of any problem along with the effectiveness of any control methods attempted. Most of the important issues related to gardening are cyclical or components of a larger scheme. With the help of careful observations undertaken over several growing seasons, the previously unexplained should begin to make sense. At the very least, good records will reduce gardening mistakes along with repetition of ineffective treatments.

Quick tip

Until recently, organic growers were frequently discouraged by the inconsistent availability and efficacy of products manufactured for controlling pests. Those dedicated to low impact and ecologically sustainable management options were occassionally compelled to develop their own recipes or depend on backyard scientists with good intentions. The industry responded with the gradual emergence of manufacturers keen to develop new products based on extensive research and development.

Operating since 1990, Organic Crop Protectants supply a range of specialty horticultural products to Australian commercial growers and recreational gardeners. General Manager, Gary Leeson describes their customer base as a combination of organic and conventional growers. “Our organic customers are very aware of the environmental and health implications as well as the social implications of organics versus large scale corporate farming. For conventional growers, the focus is often directed toward the practical benefits which can be quickly realised. By reducing exposure to certain harmful chemicals, an organically certified product can often improve the occupational health and safety profile of their operations. Many growers are also interested in control measures which avoid widespread negative effects on beneficial insect populations. This is particularly true for those involved with greenhouse food production systems”.

Commercial realities are another factor driving the current popularity of low impact controls. Primary producers are competing for overseas markets which now demand more sensitive screening levels for a wider range of chemical residues.

Serious minded professional and recreational gardeners are also looking for opportunities to reduce their dependence on harmful chemicals. Gary Leeson’s convinced that most consumers now want to use safer sprays. His optimism is restrained by the fact that many consumers are blind to what makes a good garden spray. “There are so many pest control products in the garden section that consumers are completely confused about what is a safe and effective spray and what isn’t. Many of the agrochemical companies that purchase mainstream advertising are continuing to push broad spectrum insecticides. Even the word ‘organic’ has been stolen and misappropriated”. When consumers require certainty about a product’s credentials they can request the safety data sheets and check whether it is registered organic by a reputable certifying authority.