Thanks ex-tropical cyclone Olga. What beautiful rain you’ve delivered, and just in the nick of time too. After great falls in December, January was a much drier than average month as those northerly winds and hot temperatures kicked back in. My garden was starting to look completely frazzled. But you’ve given us what we really needed – some respectable follow up rain. If the bureau’s right and we get further decent falls throughout February, we’ll be set up for a classic Downs autumn.
With extended periods of wet weather like we’ve had recently, there is a downside for gardeners. A warm, rainy week equals fungal disease. While some fungi are actually beneficial, there are those caused by pathogens that can wreak damage on plants that lack built in resistance. Sometimes the effects are superficial, affecting the gardener more than the plant. Rust on frangipani or canna leaves, for example. Sometimes though, fungal diseases can seriously damage a plant or its fruit, causing crop failures and in extreme cases, death.
Some of the more common fungal diseases you might encounter this week include: powdery mildew, a particular problem with cucurbits, roses and some perennials; blackspot, the major fungal problem with roses; brown rot, a major problem with stonefruit; and wilt, fungal diseases that particularly affect tomatoes. There are many others of course, and their causes vary.
Lots of fungal diseases manifest during, or shortly after, periods of wet weather. But some prefer dry conditions, others are spread by a “vector” such as insect pests, while others can reside in the soil and become a problem when waterlogging occurs. Phytophthora is an example of the latter, known colloquially as “die back”. Diagnosis of this, and many fungal diseases, is particularly problematic for amateur gardens and few people are prepared to send samples off to a laboratory for proper identification.
Fortunately, fungal diseases can often be effectively controlled without a specific diagnosis, and through a number of different means. As with the human body, the most effective way to deal with fungal problems is through prevention. This can take the form of traditional plant breeding, where nurserymen select and develop plant cultivars that have built-in resistance to specific fungi, or through good cultural practices on the part of the gardener.
A case can be made for having a reasonably tidy garden. By cleaning up fungus affected leaves of roses and fallen fruit, for example, the gardener can prevent outbreaks of fungal disease occurring in the first place. Another means of prevention is by controlling vector insects like aphids and beetles so that they are unable to transfer fungal disease from plant to plant. Increase air circulation around susceptible plants, an even better, practice building a healthy soil full of beneficial micro-organisms. This will help plants naturally resist disease.
Sometimes though, prevention is best achieved by spraying. Many of the fungal diseases affecting fruiting plants, particularly stonefruit and others in the rose (Rosaceae family), can be stopped in their tracks by applying an over wintering spray. Being an organic gardener, I recommend using products that are acceptable under the Australian Organic Standard and in pragmatic terms, this means using either a copper-based spray such as Bordeaux, or lime sulphur.
Neither is ideal, but of the two, lime sulphur is the preferred option. Copper is a heavy metal. This means that it can accumulate and persist in the soil, and research indicates that it’s particularly harmful to earthworms and soil-borne organisms. It is an acceptable input under the new Australian Certified Organic Standard, but it’s worth noting that the European Union has banned the use of copper sprays and other countries are heading in a similar direction. Both copper and lime must be used in accordance with the manufacturer’s directions as they can be harmful to plants when applied incorrectly.
For minor fungal infections, there are even “softer” options available. Milk sprays are effective in preventing powdery mildew on grapes, roses and cucurbits. Simply mix one part full cream milk to nine parts water (or 100ml milk topped up with water in a one litre container) and spray on the foliage of susceptible plants every week or two. Potassium bicarbonate (sold as EcoRose) is also effective in preventing a range of minor fungal diseases, and some gardeners get decent results by spraying foliage regularly with seaweed extract.
Above all, it’s worth remembering an old gardener’s rule of thumb: if a fungal disease has already presented itself, then it’s too late for prevention. Control the existing outbreak and resolve to get in earlier next year.
First published in The Chronicle 6th February, 2010.
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