Lesson 4 - Plant diseases
Plant diseases are alterations to the growth and development of plants caused by microscopic, non-animal organisms. The main causes of plant disease are viruses and viroids, bacteria, and fungi. Plant disease can pass largely unnoticed or can cause severe or catastrophic crop losses. Historically plant diseases have caused major economic and/or human life losses. Two famous examples are late blight of potato, which lead to the Irish Famine of the mid 19th Century (students of this event will know that there were other factors at play here), and the introduction of grapevine powdery and downy mildews to Europe in the late 19th Century, which almost destroyed the wine grape industry globally.
Because there are so many plant disease organisms, so many different host plants, and so many different environments in which crops are grown, it is not possible to describe all the disease risks that you might encounter in your food growing journey. The notes below provide an overview but ultimately the best approach is to familiarize yourself with the crops that you grow or want to grow, and with the potential disease risks that they face (there may be none, but it’s worth checking). In many cases, particular combinations of plant, disease organism and environmental conditions will lead to a disease pressure makes it virtually impossible to grow that crop. Knowing when to quit is important. An example is downy mildew of grapevine which makes the cultivation of Vitis vinfera virtually impossible in coastal subtropical and tropical regions. Other grape species must be grown instead.
Viruses are extremely small particles of DNA or RNA with a protein coat. Viroids do not have the protein coat. Once introduced into the host plant, the virus/viroid hijacks the cell’s metabolic processes to reproduce more virus particles. Virus disease can be fatal (exocortis and tristeza diseases in citrus for instance), or can lead to yield reductions, often without obvious visual signs (crops commonly infected this way include potato, garlic, strawberry, fruit trees, grapevines).
Viruses are often spread by sap-sucking insects of the Order Hemiptera. They are rarely spread by seed. Virus diseases tend to be most common in perennial crops that are asexually reproduced, virus in the mother plants being spread to subsequent generations.
Management of virus diseases in plants is generally not done at the individual grower level. Systems like plant quarantine aim to keep new virus diseases out of the country. Grower or government virus eradications schemes aim to provide growers with virus free planting stock. In perennial crop plants where viruses are known to be a concern, replacing planting stock from time to time with new virus free material is important. If you are looking to start a new perennial crop, investigate possible sources of virus free planting material, rather than relying on sharing other people’s older stock (this sharing sounds romantic but may not be common sense). With species or cultivars that are not commercially important, it may not be possible to source virus free planting stock.
Bacteria are typically single celled microorganisms. They cause disease by invading host tissues. Some will cause tissue destruction (e.g. soft rot diseases), some can cause blockages of vascular tissues causing wilting and death (e.g. bacterial gummosis of stone fruit, fire blight).
Management and control of bacterial diseases will be a combination of site selection, cultivar choice, hygiene to remove contaminated debris, and careful post-harvest storage.
A very large number of fungi can cause plant diseases. Fungi are typically multi-cellular microorganisms with long thread-like structures called hyphae. The whole organism is called the mycelium. At certain growth stages fruiting bodies will form on the hyphae and produce spores that are the source of new infections. Usually, the mycelium is not visible to the naked eye, but spore masses can be (e.g. the white dusty spore masses of powdery mildew). Fungal disease effects can include soft rots of fruit, wilt diseases, root rots, leaf and stem spots, and cankers.
The conditions under which fungal diseases thrive vary with the species. Some require warm, wet weather (downy mildew, brown rot of stone fruit), some cool wet weather (apple scab, botrytis grey rot), some mild, humid weather (some powdery mildews). Once again, researching your crop and understanding your environment will help you understand the issues relevant to you.
Management of fungal diseases usually takes a multi-pronged approach. Elements can include understanding your site to avoid obvious potential problems before they arise, use of disease tolerant cultivars where they exist, monitoring of crop condition during the growing season, hygiene (removal and destruction of contaminated plant material), crop spacing, pruning and training to improve air flow and drying, and the use of fungicidal chemicals.
Fungicides can be of two types. The most common are prophylactic or protectant types. These are applied to the crop before the onset of a fungal attack. Typically they are used to form a barrier on the surface of the plant that inhibits the germination of spores. In many cases this is done during the early part of a growing season and may have to be repeated several times to maintain cover. See the case study on powdery mildew below for an example of this. The other class of fungicides are capable of being applied post infection and can have a curative effect. There are fewer of these available for use. See the case study on phytophthora root rot for an example.
Powdery mildews
Many plants can be attacked by powdery mildew disease. This disease is categorized initially by the spread of hyphae over the leaf or stem surface, the hyphae drawing nutrients from cells under the epidermis of the leaf. Eventually the fungus enters a reproductive phase and masses of white powdery spores are produced giving the disorder its name. Powdery mildews will cause losses of vigour and productivity and can kills plant tissue. It is important to note that powdery mildew diseases of different crops are not the same organism and an outbreak of powdery mildew on one species may not indicate that the same thing is happening to a different species.
Powdery mildew is usually managed by cultivar selection, canopy management to ensure good airflow and low humidity, and using fungicides. In commercial horticulture a number of synthetic fungicides are used for powdery mildew management but smaller scale growers can often use simpler approaches. Sulphur (as dust, or wettable sulphur), milk (1:10 diluted with water), and potassium bicarbonate solutions have all been used as protectant sprays against powdery mildew. In many cases however, a grower will simply let the disease progress and remove the plants when they become too severely affected (one does eventually get sick of zucchini and the inevitable late season mildew collapse might be seen as a blessing).
Phytophthora root rot
The genus Phytophthora includes a large number of species of plant pathogens that can causes serious damage in susceptible species, primarily by infecting and destroying root tissue, leading to impaired growth and death. Many fruiting species are susceptible to phytophthora, including avocado, citrus, pineapples, stone fruit and apples. Phytophthora, while sharing many attributes with fungi, belong to a different class of microorganisms. They spread by spores, or by fragments of mycelium, and require free water and warm temperatures to infect root tissues.
Management of phytophthora, once an infestation has been confirmed by soil testing, can involve a multi-pronged approach. This can include application of strict hygiene protocols to ensure that infected plants are not planted and that infested soil is not carried into clean areas (on tools, machinery, or feet, or by erosion of infested and transport of infested soil by water), site selection to avoid wet soil areas, artificial drainage and/or soil mounding, increased soil organic matter content, mulching with organic materials, use of tolerant rootstocks where they exist, and the use of anti-phytophthora chemicals such as phosphorous acid and its derivatives which are capable of post-infection treatment of the disease. In citrus, where poor drainage leads to root rot problems, the rootstocks trifoliata and citrange can confer tolerance. Serious infestations of phytophthora will preclude the growth of susceptible species.
When you need to resort to using chemicals, it is really important to always use PPE. Don't skip this step. It is very important to consider your personal safety is when using chemicals. The most common way for chemicals to enter the body is by inhalation. Other ways are: skin absorption or dermal absorption. The effect from exposure to uncovered skin of the face, forearms and hands can be significant. Solvents in particular can penetrate the skin barrier very quickly. Chemicals will continue to be absorbed as long as you have contact with them. The larger the area of affected skin, the greater the seriousness of exposure. Remove contaminated clothing and wash skin as soon as possible. Eyes are very sensitive to many chemicals. Always wear protective glasses as spray drift or a splash or spill can cause great damage. Be aware of not touching your eyes with contaminated hands. Ingesting chemicals by accidentally swallowing chemicals when eating or smoking without washing your hands first. You can also accidentally swallow particles of dust and breathing in vapour that then is caught in respiratory secretions and swallowed.
Further learning
- Plant pests and diseases - Department of Agriculture, Fisheries and Forestry
- Chemical Training Courses - Aus Chem