A meteorite fell to earth in Mexico around 65 million years ago. It plunged the earth into pitch black for about four years. There was no light for plants and they died off in great numbers. This meant that there was no more food for the animals and they also died off on a large scale. It is widely believed that this was responsible for the decline of the dinosaurs. It was only fungi, which do not require light to grow, that did well and grew in abundance on the remains of the dead plants and animals.
Grey mold (botrytis), mildew or pythium: everyone has at least heard of these at some time. They aren’t just diseases that can attack plants, they are also all fungi. What are fungi exactly? And do all fungi make plants sick?
What is a fungus?
We can try to classify the organisms that live on our planet in groups. How many groups needed to do this has still to be agreed upon by scientists. In practice, it seems that living organisms simply don’t fit into slots perfectly since there are always exceptions to established criteria and new insights into the criteria itself. The classification is currently being reviewed again. There are, however, three groups that have been established for a long time. These groups, or better said, kingdoms, are animals, plants and fungi. Other organisms such as bacteria still have no definitive categorization.
The fact that fungi are a separate group means that they also have specific characteristics. Plants and fungi were previously categorized in the same group. The big difference between plants and fungi is that fungi have no chlorophyll. A further difference is the cell wall of fungi, which, in contrast to that of plants, is made from chitin. Nutrients are absorbed through the cell wall once they have been broken down by the enzymes that they secrete.
Fungi can provide for their nutritional needs in different ways. There are fungi that have cohabitation (symbiotic) contacts with other organisms. There are many that live together with trees, for instance. Their reproductive organs are well known; they are the mushrooms in the woods. The fungi get sugars from the trees in exchange for certain nutrients.
Another important group is what is known as the saprophytes. These are our planet’s waste disposal system. They live on dead material, such as leaves, that have fallen. Then there are the parasites that live at the expense of other organisms. These fungi are by far the most responsible for the majority of diseases encountered in agricultural crops. Finally there is the group of predators, which is the least well-known group of fungi.
Predators hunt other organisms. There is a type of fungi that hunts nematodes and strikes by using traps. After the trap is sprung, the fungus grows into the nematode and digests it. Another, somewhat better known ex-ample is the Trichoderma fungi family. These fungi attack what are known as damping-off diseases which are parasitic fungi that target germinated seeds or very young plants in particular. Trichoderma occurs naturally in coconut fiber among other things. Coconut fiber is generally steamed which kills the fungi; however, coconut fiber that hasn’t been steamed is also available.
Parasitic fungi in plants above ground level
A dreaded fungal disease in horticulture is caused by the grey fungi (Botrytis sp.). The grey fungus is parasitic but can also live as a saprophyte. It certainly isn’t choosy and has a lot of agricultural crops on its menu. A well-known example of this is the strawberry where it can often be seen as a grey fluff. A pink rot (Trichothecium roseum) infection can sometimes resemble a grey fungi infection very closely. It is only under a microscope that the differences become clear.
A white powder is sometimes found on plants’ leaves. This is mildew which is a different parasitic fungus. If the fungus is on the upper side of the leaf, then it is genuine mildew but if it is on the underside it is what is known as false mildew. This is often confused by many growers.
Mildew can be caused by different fungi and is generally plant specific. Mildew on cucumber is caused by Sphaerotheca fusia but by the Uncinula necator fungi on grapes. False mildew is caused by Pseudoperonospora cubensis and Plasmopora viticola respectively. High humidity is generally ideal for fungus growth. In contrast with most other fungi spores of the genuine mildew only germinate well on dry leaves.
The microscope provides the solution: (centre) Botrytis cinerea and (right) Trichothecium roseum. Photo left: Ministry of Agriculture and Lands, Canada, photo centre: CABI Bioscience and photo right: University of Adelaide
How an infection develops
If the conditions on the leaf are ideal, and this includes the correct humidity and temperature, a spore that lands on the leaf surface can germinate. A filament grows from the spore which enters the plant via a wound or stoma and then swells up.
Once the filament is inside it swells up again and these swellings are used to extract nutrients from the plant cells. Following this, more filaments with swellings are formed. In addition, it forms reproductive organs, which distribute new spores into the air without sex, or asexually. This is how the infection cycle begins.
Attacks below ground level
Parasitic fungi do not just attack parts of plants that are above ground. Fungi also attack from inside the growing medium. These notorious villains are grouped under the ‘damping-off’ name and have imaginative names such as Pythium, Rhizoctonia, Fusarium and Phytophthora. The grey fungi and a number of others also belong to this group. It is these fungi that often have the ‘crib deaths’ of plants that have just rooted and the demise of newly germinated seeds on their conscience.
A number of tips in the battle against damping-off diseases
- Spores that are on seeds can be killed by soaking the seeds that are to be germinated in a solution of bleach (one part bleach to nine parts water) for two minutes. After this rinse well preferably using water that has been boiled and then cooled.
- Place seeds that are to be germinated on the surface of the growing medium and then cover them with a thin layer (three or four times the thickness of a seed) of an inorganic medium such as perlite.
- Do not use soil from outside for indoor cultivation.
- Do not use garden tools inside that are also used outside.
- Clean gardening tools and pots well after use and disinfect them with a bleach or alcohol solution.
- With a protected crop, ensure that there is constant ventilation that isn’t linked to the lighting timer.
- Avoid giving too much water and allow the medium to dry out a little before giving more water.
- Do not use rainwater.
Short summary
Fungi are classified into their own kingdom as are plants and animals. We actually only know the fungi that we encounter and this is a very small proportion of those that exist. Unfortunately, a lot of the fungi that we know are pathogenic for our plants so it is necessary to protect our plants against them. You should ensure that they get a good start by preventing damping-off diseases. Also, make sure that later there is sufficient ventilation and avoid excessive humidity. Doing this will hamper the distribution of fungi spores.
Keep in the back of your mind the fact that insufficient humidity can also cause problems, encouraging spider mites for example.
