Diseases and disease control

It is estimated that a majority of organic gardeners will be challenged by several outbreaks of plant disease during each growing season. This statement never fails to alarm a small percentage of faces in my audience. I quickly identify them as worry prone gardeners. Occasionally, they will approach me following a speaking engagement. Some of them possess an encyclopaedic knowledge of plant disease. They worry and fret over every perceived blemish. Patchy lawns, curled leaves, mottled bark, misshapen flower buds, and so on. After brief discussion, I appreciate their anxieties but cannot genuinely help them. I have also encountered the opposite reaction in gardeners. There are a minority of individuals who refuse to acknowledge serious problems when they occur. Surrounded by obvious and widespread symptoms, they somehow prefer to avoid any of the assertive actions required. While both behaviours can probably be attributed to some form of fear, I would personally prefer my closest neighbours to be the worry prone - as opposed to oblivious - variety of gardener.

While I have certainly chosen to describe two polar extremes of gardening behaviour, we are all capable of unnecessary panic, and neglect on occasions. At the present time, there are no magic bullets, nothing in the manner of a universal remedy which will safeguard our plants and gardens from the threat of disease organisms.

All we depend upon is resourceful intelligence and experience. Disease prevention and control can be simplified to compose four basic objectives.

• Disease risk analysis
• Identification of significant disease symptoms
• Strategies for prevention within an organic framework; and
• Control and treatment measures that can be safely applied within the integrated organic environment.

Let’s consider these in a little more detail.

The analysis of disease risk

Plant and soil borne diseases are classified by the biological status of their active pathogen. The commonest pathogens are the microscopic fungi, bacteria, viruses and nematodes. Most of these pathogens are opportunistic, surviving in small numbers until prevailing conditions enable them to rapidly multiply. Following this stage, it is usually possible to identify visible symptoms although these will vary according to the types of plant affected, the climate, soil condition, and the virulence and progression of the active pathogen. Experienced gardeners will also appreciate that the risk of encountering any particular disease will depend upon a similar combination of factors. In most situations the risk can be approximately determined by a careful assessment of the local and seasonal growing conditions, availability of quarantine protections, and the susceptibility of individual plant species.

For every organic gardening forum I attend, I always expect to learn at least one thing which totally surprises me. Inevitably, the majority of these surprises relate to the peculiarities of a particular geoclimatic situation. I’m constantly telling people there’s no substitute for accurate local knowledge. The most effective gardening strategies are based on specific as opposed to generalist sources of information. This is never more fully demonstrated than the practical aspects of disease risk assessment. Here’s my advice. Purchase a fifty cent notebook and visit your closest public library. Search for publications released by the official departments of agriculture and primary industries. Focus on the most recent reports which describe horticultural diseases and quarantine regulations. While influenced by the local impact of common plant diseases, they also provide some indication of recently introduced strains and those most likely to threaten commercial growers and native species. The next step is to visit local plant nurseries, garden centres, and plant breeders. Ask general questions about local growing conditions. Look carefully at the plants and ask relevant questions. The technical staff will often be trained to provide practical information about the identification of healthy as opposed to symptomatic plants.

If you haven’t already done so, it is time to join the local branch of the organic society. Many participants will be experienced gardeners prepared to share their knowledge on various plant diseases and the level of risk pertaining to these.

Identification of disease

Early detection of plant disease makes it easier to control the spread and thereby limit any negative effects within the garden. The ability to recognise early stages of disease depends upon some understanding of the normal appearance and life cycles of susceptible plants. Excellent information can be sourced from illustrated gardening encyclopaedias and specialist botanical publications. Establishing a routine of physically checking each plants condition is the best way to consolidate and apply the information sourced from books. Early to mid morning and late afternoons provide a softer light, ideal for inspecting foliage, particularly when the magnification of a hand lens is required. Any signs of abnormality should become apparent when compared with that of healthy growth and reproduction.

Prevention strategies 

Plant stress is always a significant risk factor for susceptibility to disease, so avoid overcrowding and preferably choose varieties which are well suited to local conditions. Where individual species are particularly vulnerable to certain diseases, it may be possible to obtain strains developed for their improved resistance. Alternatively, when these are unavailable, it may be wiser to choose plants from a different family. Healthy plants develop their own immunity which provides the best natural defence against most forms of disease. Optimum immunity requires superior genetics, steady growth, and a track history of survival.

It is important to choose and maintain healthy garden sites for the majority of plant species. Damp or stagnant areas can eventually provide refuge for troublesome plant diseases. It is surprising how many disease organisms are destroyed or greatly weakened by sufficient exposure to sunlight and clean circulating air. The lack of sunlight and circulating air appears to contribute to the general susceptibility of many indoor plants.

A soil which is periodically enriched with compost and other organic materials can help plants maintain a steady rate of growth. The excessive use of nitrogen-based fertilisers tends to encourage a sudden proliferation of foliage which may eventually reduce plant vigour and increase susceptibility to disease. Synthetic chemical fertilisers will also deplete the concentration of microorganisms which are essential for breaking complex soil nutrients to a form which can be used by plants. One of the newly emerging garden products are concentrated supplies of these healthy bacteria which can be applied directly to soils to encourage new generations of living organisms. This reflects the organic philosophy of building balanced ecosystems where complex life is intricately connected and ultimately dependent upon the microscopic soil organisms.

When introducing new plants, quarantine them for at least a couple of weeks before planting. To further reduce the risk of introducing disease, all seedlings, mature plants and soil products should be obtained from reputable organic suppliers.

Control and treatment

It is rarely possible to identify a single cause for any disease outbreak. Sudden changes in temperature, rainfall, or soil chemistry can cause microscopic pathogens to be activated from a dormant state. This may precipitate a localised outbreak of disease, but the consequences will vary according to the control and treatment measures applied. The best of these will reduce or eliminate the need to pursue chemical and other potentially harmful remedies.

One of the most effective control measures is to rapidly isolate and contain the originating source of any minor disease outbreak. The isolation of transmissible plant disease requires strict attention to garden hygiene. Many diseases have the potential to spread quickly when infected materials are carelessly handled or disposed. I own several gardening books which recommend deep burial although I generally prefer incinerating diseased and suspect plant materials. Pathogenic fungi spores and bacteria can be carried into healthy sections of garden through the action of wind, browsing animals, and on the footwear or clothing of humans. Wiping garden cutters and other frequently used tools with a sterilising alcohol helps to lower the possibility of transferring disease.

For most gardeners, soil borne diseases are particularly difficult to detect and isolate. Over many years, I had noted that mulched garden beds tended to have significantly fewer incidences of plant disease. It wasn’t until overhearing a comment from an organic strawberry grower that I discovered the likely reason for this. Mulching restricts disease transmission by preventing infectious spores splashing onto plants. It also provides a natural weed control which reduces the need for constant digging and soil disruption.

Until recently, organic certification authorities have allowed commercial producers to use simple fungicidal compounds based on copper, tin, and sulphur. These are effective when administered as powdered or gelatinous agents, controlling a spectrum of fungal disease across diverse plant species including tomatoes, cocoa, potatoes, and grapes. At present, there is a new wave of restrictions coinciding with greater regulation of the various organic industries and the contribution of international trade bodies. This has caused a considerable amount of resentment, particularly among growers attached to the organic wine and beverage industries.

According to my own evaluation of mainstream organic sentiment, there is an approximate division as to whether these substances should be used in horticultural production and general gardening pursuits. From the historical perspective, these preparations appear to have been used with remarkably few adverse effects. One of the arguments for restricting copper compounds is their potential toxicity and the risks associated with ingestion of treated materials. The other concern raised by the organic purists is their possible effects upon soil chemistry and the balance of microbial life forms. Longitudinal testing of vineyard soils in France, Canada, and the United States have, for example, revealed significantly elevated copper concentrations combined with deficiencies of zinc, selenium, and other trace minerals. In view of these findings, I have avoided the use of copper compounds for several years. I occasionally use sulphur dust on my tomatoes but am increasingly shifting toward the use of home made herbal extracts for fungicidal control.

Natural remedies against fungal diseases can be made at home from seaweed and various herbal extracts. I have discovered that infusions of nettle, spring onion, or chamomile are excellent for controlling troublesome mildew on squashes, cucumbers and other climbing vines. To ensure adequate potency, the nettles, spring onions, or chamomile flowers must be carefully harvested from high quality stock. I tend to harvest chamomile early in the spring but keep a reasonable store of the dried flowers for later in the season when fungal problems are on the rise. The flowers or leaves are soaked in boiling water for several minutes. After cooling, the liquid extracts should be diluted to a weak tea consistency. Strain these with a fine muslin filter prior to application. For best results, the strained liquids should be transferred sealed canisters and used within approximately two weeks. In addition to their gentle fungicidal properties, nettle and seaweed foliar sprays provide a range of beneficial plant nutrients.

For those of us living within reasonable distance to the coast, seaweed or kelp can be gathered in abundance after a storm. I know gardeners who fill their trailers several times each year. Unlike many of the chemical fungicides, kelp solutions are entirely safe and simple to prepare. I remove excess salt with a careful rinsing before soaking the long strands inside a plastic basin containing fresh water. After approximately two weeks the concentrated kelp solution should be diluted to the consistency of weak tea. Strain through muslin filters to remove any solid particles then transfer the remaining liquids to a sealed canister. There’s no reason why a kelp solution cannot be stored for several months, although I tend to prefer mixing new batches once the storage period extends beyond a couple of weeks.