In a similar way to dealing with pests, if you grow fruit or veggies of any description it is likely that you will have to deal with disease in those plants at some stage. Dealing with diseases is different to dealing with pests in that diseases tend to be very specific to each plant and while some general rules can be formulated about preventing disease and dealing with the various causes of disease it is important to research and make sure the disease is correctly diagnosed before acting. It is not possible to cover all the possibilities in a short article like this one, so this is more an overview to help you gain an understanding of plant disease so you can then move on to further research.
Having said all that my experience with disease in backyard grown vegetables is fairly limited and while we have had the odd outbreak of blossom end rot in tomatoes mostly our disease profile is restricted to mildew of cucumber and zucchini and the odd occurrence of damping off. It’s good to keep an eye out on what is going on but don’t obsess about infected veggies, life’s too short.
The Causes of Disease in Plants
In general terms plant disease is caused by viruses, bacteria or fungi and the most common cause of disease in your veggie crop is likely to be a fungus. Some plant diseases or disorders can also be caused by a lack of one or more nutrient elements or by excess of a particular nutrient to the point where they become toxic to the plant, usually due to incorrect soil pH. These nutritional disorders are touched on in the article on keeping up fertility organically by adding nutrients while this article will focus more on the biological causes of disease.
The Organic Approach (Prevention Rather than Cure)
Rather than go for short term bandaids in the form of disease control chemicals like fungicides (no matter how enviro-friendly) we need to look at our backyard crops as being part of our backyard ecosystem and then try to keep the ecosystem in balance. We can also be smart and avoid disease problems before they get to the point where a chemical control is necessary by using some or all of the following strategies.
- Observe your back yard veggie crops regularly for signs of disease and if you do, diagnose as best you can or get advice from other gardeners, books or the ‘net and once the disease is diagnosed act accordingly. By identifying and acting quickly a minor irritation can be prevented from becoming a major headache.
- Start with a healthy soil – healthy soil means healthy plants and healthy plants means they are less likely to become a target for diseases. If you follow organic principles and use organic manures and fertilisers, if you mulch well, use green manures and maintain soil organic matter and avoid cultivating and exposing your soil to the sun you are much more likely to have healthy soil. Check and adjust your soil pH if necessary as well, as soil that is too acid or alkaline can be rough on the microbes in your soil as well as your fruit and veggies.
- Mulching – As mentioned above mulching will help improve soil health but aside from this mulch will prevent soil and spores being splashed up onto the plant during rain and reduce the likelihood of infection, particularly fungal infection, by this route.
- Don’t overcrowd your plants - allow room around them for good airflow although in my experience if you have healthy soil and are growing organically your veggies can be planted much more closely together without causing a problem than if your growing using chemical techniques. When planting the usual stuff in our backyard beds we generally use about 30cm spacing and that causes no problems, allowing separation of the plants and airflow when they are young and vulnerable but making good use of space when they are grown.
- Plant resistant varieties – The varieties of vegetables available today is very small in comparison to years gone by, even in plant nurseries the variety of seedlings is only slightly better than the stuff we can get in the supermarket. Fortunately there are specialised seed suppliers like Eden Seeds, Phoenix seeds, Greenpatch seeds and Green Harvest who can supply a broader number of varieties and some will be more resistant to than others to the diseases prevalent in your area.
- Keep your growing area clean and tidy – dying plants, rotting fruit and other organic material lying around and provide a place for disease to build up or even over-winter. Diseased plants should be removed from your site in the green waste bin or burned as the organisms may re-infect your veggies even if composted if unless your compost system is really hot. Don’t take the chance.
- Hygiene – Maintain your hygiene levels when raising seeds, potting on and planting out. Regularly wash all your flats, punnets, labels, trowels etc in a disinfectant solution like Dettol or one of the quaternary ammonium disinfectants to prevent a build up of disease organisms and get your seedlings off to a good start. Exposing your equipment to the sun will help to as the disease organisms generally don’t like the ultraviolet light (they have never heard of sunscreen I guess....).
- Practice crop rotation –by not planting the same family of veggie in the same plot two years running which not only helps maintain fertility in the soil it prevents a build up of crop specific diseases in the soil.
- Avoid overhead watering when possible – because this raises the humidity and can cause fungal problems when the leaves stay wet for an extended period of time. If you water in the morning so that any excess water is dried off by the sun the likelihood of this causing a problem is reduced.
- Don’t water your veggies with greywater – apart from the possibility of passing on human pathogens to the crops you will later eat, you may also pass on plant pathogens. Use greywater for watering fruit trees and apply direct to the soil or subsurface.
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Plant at the correct time of year for the seed being grown – forcing a plant to grow outside its normal season can reduce its vigour and cause it to become prey to disease as well as pests.
Disease Transmission Routes
You may say, with justification, that your backyard crops are disease free and so they should remain disease free unless a disease is introduced from outside, but there are a number of ways that diseases can be transmitted onto your property which you need to be aware of –
Infection from plant material brought in – this may be commercial seedlings or plant bought in by mail order or from surrounding nurseries or the likes of Bunnings. In general they should have enough of a handle on hygiene for infection due to this source to be possible but unlikely, however plant material provided by friends, neighbours or other backyard growers may be a different matter.
Infection from your neighbours – windblown spores can be blown in from your neighbours if they are growing veggies or other host plants. Spores like those produced by the powdery mildews are released in warm dry weather and may travel considerable distances on the wind.
Infection due to contaminated soil – again this may be soil brought in with purchased or donated seedlings but may also be carried into your backyard on dirt clinging to boots, garden tools such as spades or garden forks or even wheelbarrows that have been used off site.
Infection due to debris – imperfectly rotted compost or vegetable trash left around your backyard can act as a source of infection, particularly for fungal diseases.
Infection from seed – some diseases are able to infect the seed and in that way be transmitted to the next generation of vegetables. Again this is unlikely with commercially obtained seed but underscores the need to only save your own seed from only healthy plants and to make sure that any seed donated by friends is from healthy stock. If in doubt, don’t use the suspect seed.
Infection by insects – while insect pests cause their own havoc, the small sucking insects like aphids and thrips can spread disease, particularly viruses between plants that are anywhere within their flying range.
General Symptoms of Disease
This is not a comprehensive list of symptoms but a rough guide for some things to look for if your fruit and veggies are failing to thrive.
Virus – Mosaic patterns of the foliage of the plant along with malformed or yellow foliage and stunted growth with the plant possibly assuming a strange shape’
Bacteria – soft foul smelling rot, black or brown spots or patches on leaves which may shrivel and die
Fungus – fall into four main groups; root and stem rots, mildews, rusts and leaf spots.
Root and stem rots – Cause rotting of the stems, roots and a condition called collar rot, they attack the conducting tissues of the plant resulting in wilting and eventually collapse of the plant.
Mildews – cause a white or ash grey powdery film over the surface of the leaf, usually older leaves which eventually wither and die.
Rusts – cause orange or red pustules on leaves or stems.
Leaf spots – as the name suggests result in black spots on the leaves of several types of vegetables.
Control Options
Virus – There are no real control options for virus infected plants but to “rogue” them ie pull out and remove them preferably by burning to destroy the virus. Do NOT compost them.
Bacteria – a double strength garlic spray may improve matters, Bordeaux mixture can be used on perennials
Fungus – seaweed foliar spray; milk, garlic and chilli spray, urine (use full strength and fresh!); chamomile tea; chive tea, sulphur or lime sulphur spray.
