Fungi: Friend or Foe?
Truffle hunters love them, podiatrists hate them, mycologists are fascinated, and gardeners may have mixed feelings.
It is estimated that there are over 1.5 million species of fungi on earth. Of these, only around 100,000 have been identified so far. We have long known and used mushrooms, truffles and yeasts in the kitchen and brewery. Just over 100 years ago, fungi came to the fore in healthcare, as they are used to make antibiotics.
There’s a downside to fungi too. Mould in between the bathroom tiles is never welcome. Gardeners and farmers battle mildews and fungal attacks on their crops. The discovery of powdery mildew on your favourite rose bush or pumpkin vine is understandably frustrating for the home gardener.
But these are the annoying relatives in a family of solid useful citizens who toil away relentlessly to improve your soil, and thus your entire garden. Fungi are critical in the decomposition of organic material, breaking materials down into their basic components. And then they go further and aid the plants in re-absorbing those nutrients.
It’s only recently that we have begun to study mycorrhizae, those fungi with a symbiotic relationship with plant roots.
Arbuscular Mycorrhizal Fungi
Different fungi have different roles in the soil. Some are involved in various processes of decomposition, while others make nutrients available to plants. Mycorrhizal fungi – and specifically arbuscular mycorrhizae – are those fungi that both benefit soil structure and help plants to absorb water and nutrients (N, P, K etc). In return, they take carbohydrates manufactured by the plants to support their own survival.
To fully understand arbuscular mycorrhizal fungi it’s useful to know a little about the meaning of the terms used in its name. ‘Arbuscular’ refers to the tree-like branching of the fungi’s mycelia that forms a network of fine strands (or hyphae). The term ‘mycorrhizal’ can be broken down into two parts: ‘myco’, which refers to the fungus itself, and ‘rhiza’, which means root. ‘Mycorrhizal’ means that the fungi has a symbiotic relationship with plant roots; a relationship that benefits both the fungi and the plant.
Improving Soil Structure and Preventing Erosion
In 1996 Sara Wright and colleagues described arbuscular mycorrhizal fungi secreting a material referred to as glomalin. This was a highly stable protein-based material which appeared to both hold soil particles together and to increase the storage of carbon and nitrogen in the soil. This material, in combination with the widespread networks of mycorrhizal mycelia, appears to significantly improve soil structure and can help to prevent erosion. So this is one easy to understand benefit of fungi. But the truly fascinating and intriguing aspect of arbuscular mycorrhizae is in their dynamic and complex relationship with plants and their root systems.
Making Nutrients and Water Available to Plants
The hyphae of arbuscular mycorrhizae can penetrate between the surface cells of a plant’s roots, joining the two organisms and allowing a two-way flow of nutrients and chemicals without harming either the fungi or the plant. While plant roots can go some distance (metres) in search of water and necessary nutrients, fungi will venture much further afield. What’s more, fungi can quickly respond to changes in the environment, giving the host plant a surprising adaptability to changing conditions, and a far greater reach than its own roots provide.
Not all plants have these relationships (for example, canola and other brassicas, beets and spinach do not), and if these are grown in the same soil continuously, the levels of arbuscular mycorrhizae will decline.
Encouraging Beneficial Fungi in Your Soil
Rotating crops and planting green manures which form a symbiotic relationship with arbuscular mycorrhizae will maintain viable fungal populations in your soil. Arbuscular mycorrhizae appear to mainly reproduce via spores, which can remain viable in the soil for 5 years, so digging over your garden bed may break up existing fungal networks but likely won’t prevent the recolonisation of crops which benefit from these fungi.
Do Fungi and Host Plants Communicate?
Fascinating research has begun on this complex relationship between fungi and plants, with indications showing the relationship also involves ‘communications’ about pest attacks and moving nutrients from areas of plenty to areas of scarcity. For example, the hyphae of arbuscular mycorrhizae are known to spread more densely into areas with low phosphorus: the host plants, when stressed with low phosphorus conditions, will release an exudate from their roots which promotes the growth of the fungal hyphae. These hyphae can greatly increase the uptake of phosphorus, benefiting the plant, which in turn photosynthesises and makes carbohydrates and other carbons available to the fungi. A true symbiotic relationship.
We have barely touched the surface of what fungi are capable of, but we do know that they can be of great benefit in the garden. So don’t slosh around the fungicide carelessly and without thought.
Remember – most fungi are your friend.
References
First image: www.freepik.com
Arbuscular Mycorrhizas - SA | Fact Sheets | soilquality.org.au
Merlin Sheldrake. Entangled Life: how Fungi Make Our Worlds, Change Our Minds and Shape Our Futures. Published by Penguin. 28 September 2021.
Wright et al., 1996; Wright and Upadhyaya, 1996
Tomato Brown Rugose Fruit Virus (ToBRFV)
Tomato Brown Rugose Fruit Virus (ToBRFV) is a plant virus that causes severe crop losses (up to 75%) in tomatoes, but also in peppers (capsicums and chillies). As with many other plant viruses, Integrated Pest Management techniques have been largely unsuccessful in addressing ToBRFV. This means best practice in horticulture isn’t helping and there are no known sprays or treatments to limit the disease. For the moment, Australia’s best defence is to eradicate any outbreaks, and use extensive quarantining.
Primary causes of transmission are contaminated seeds, and physical transmission, including pollinating insects. It is a robust, persistent virus, surviving away from its host for a long time (months) without loss of virulence. The industry is looking at developing new strains of tomatoes that are resistant to the virus. At this stage all cultivars of tomatoes bred before 2022 are susceptible to ToBRFV.
This highly infectious virus has rapidly spread from the Middle East (first known location in 2014), across Europe, China, North America, and parts of central and southern America. Until 2024, Australia had been able to isolate itself from this virus, however three cases have now been confirmed in South Australia. There has been a swift and comprehensive response to this, but it is sensible for the home gardening community to be made aware of this disease and what to look for.
Identifying the disease
The first hint that you may have an issue will probably show up in young leaves. These will show mild to severe mosaic symptoms (see photo) with deep green bulges/blisters, yellowing veins, deformed, narrow leaves, and smaller than expected leaves. The entire plant may be stunted, failing to thrive. The severity of the symptoms can vary depending on time of infection, environmental conditions and how susceptible that cultivar is.
Once the fruit starts to form – inspect the calyx (the outer green structure that surrounds the petals where the fruit attaches to the stem), it can look to be drying out and going brown – especially at the tips. Fewer fruits are formed.
Fruits fail to ripen. They are pale, streaky, or with spotty yellow blemishes (see photo above). These can develop further into brown spots and then necrotic lesions (hard brown spots and flecks). Fruit can sometimes wrinkle and will often feel rough to the touch. Tomatoes can show these symptoms early on, or they can start off looking normal and develop these symptoms later.
In peppers, leaves will show puckering and mottling, plants are stunted, and fruits show wrinkled yellow to brown patches followed by necrotic spots and can be deformed.
What to do?
A number of different viruses affect tomato and other Solanaceae crops in Australia and some have similar symptoms to ToBRFV, so correct identification is important. A useful fact sheet about crop viruses, how they spread and management options (which usually involve destroying infected plants) can be found here.
Given the highly contagious nature of this disease, if you are suspicious that your plants have ToBRFV, don’t bring them into your local nursery for identification and potentially spread the virus around, call the Pest Plant Hotline on 1800 084 881 and follow their advice. If they ask for photos – send good quality, high resolution pictures. This virus has the potential to wreak havoc on our tomato and pepper growing industries, as well as home gardens, community garden plots etc. Remember it is incredibly contagious; if you are concerned, don’t go to other gardens until you get the all clear.
Eliminate any potential weedy host plants, like the nightshade family.
It is always good practice to regularly disinfect your gardening tools. Keep in mind that this virus lasts a long time off the host plant, so disinfecting practices are important. Use a solution that is 1 part liquid pool chlorine (this is usually in a 12.5% solution) to 100 parts water (a 1% solution).