Many gardeners would like to know more about diseases and very often there seems to be some confusion between deficiencies, diseases and insect attacks.
As it’s a large topic, I decided to narrow it down and make a blog post about fungal diseases. To make this post more helpful, I have used photos from the web. Some show VERY CLEARLY the symptoms of each disease and some of them are added, to demonstrate the complexity of a diagnosis from a photo.
To start with, “A plant disease is defined as “anything that prevents a plant from performing to its maximum potential.” This definition is broad and includes abiotic and biotic plant diseases.
1. Abiotic diseases are caused by environmental conditions and not by living agents. They don’t spread from plant to plant, however many plants in a garden might show the same symptoms.
Examples of abiotic diseases / factors causing abiotic diseases are:
- Deficiencies (caused by low supply of nutrients or unsuitable soil type)
- Sun burn
- Soil compaction
- Ice and others
2. Biotic or infectious diseases are caused by living organisms, called plant pathogens when they infect plants. Pathogens can spread from plant to plant and may infect all types of plant tissue including leaves, shoots, stems, crowns, roots, tubers, fruit, seeds.
Some plant pathogens are:
- Fungal-like organisms
- Parasitic higher plants
To keep things simple, in this post, I will focus on fungi and fungal-like organisms, as they cause most of the infectious diseases on plants and also have more chances than the rest, to be treated.
3. Disease triangle: It is important to know that plants that are:
- Newly transplanted
- Poorly Fed
- Weakened or stressed by external factor (e.g. extreme heat in the summer)
Are much more prone to diseases. Why?
Because for a disease to occur in any plant system, there are three components that are absolutely necessary:
- a susceptible host plant (weak, young, hungry – just like humans, these plants have a lower “immune” system)
- a harmful pathogen
- a favorable environment
When these three components are present at the same time, a disease will occur if a susceptible host plant is in intimate association with a harmful plant pathogen under favorable environmental conditions.
A gardener should have a general idea about the pathogens classification, in order to be able to select the correct treatment for the relevant disease. As said above, infectious diseases are caused by 2 main categories of pathogens:
- Fungal like organisms
The categorization happens based on molecular data (DNA), anatomical characteristics (cell walls) and mode of reproduction. (Their taxonomy is constantly changing, especially due to the developments of the research based on DNA comparisons). It is important to know that both categories can be transferred to our plants through the air, water, soil, insects, birds or other plants.
5. Common diseases:
The most common diseases in Qatar are powdery mildew, downy mildew, leaf spots and blight.In this post, you will see many photos from all of these diseases on different plants.
Powdery mildew: It is caused by various fugal-like organisms, like Golovinomyces cichoracearum for sunflower or Podosphaera pannosa for roses. The clearest symptom (or sign) of the disease called “powdery mildew” is the white “powder” on TOP of the leaves.
Photos from the Hellenic Society of Phytiatry
Downy mildew: It is also caused by various fugal-like organisms, belonging to a family called “Peronosporaceae”, like Pseudoperonospora cubensis for cucumber or Plasmopara obducens for impatiens or Peronospora megasperma for pansy.The way to separate downy mildew from powdery mildew is by the powder (usually grey) UNDER the leaves. Initially you will see pale blotches on the upper leaf surfaces and corresponding patches of fuzzy grey growth on the underside.
Leaf spot: Can be caused by the fungi Alternaria (tomatoes) or Diplocarpon rosae on roses or Cercospora on mulberry.Black spot looks like circular black spots on leaves. It usually occurs on the upper sides of leaves, but can also develop on the undersides. The outer margins of the black circles are ragged or feathery and they are usually surrounded by a ring of yellow. Spots usually begin on the older leaves and move upward.
If you read the label of a fungicide, you will see that each fungicide is suitable for certain diseases, so it’s important to identify which disease you have in your garden. In many cases, the symptoms can be confusing and the only way to know 100% what disease your plants have, is by a lab test. And this is another reason that THE RANDOM USAGE OF PESTICIDES is NOT RECOMMENDED; unless you are really certain about the disease you have, it’s better to avoid useless applications of dangerous chemicals.
A. Organic control: Organic control is harmless, but its efficacy is generally lower than conventional pesticides.
The most common organic, human/animal/insect friendly treatments for most of the fungal (and fungal like) diseases available in Qatar, are:
• Neem oil (5-10 drops/1 lt of water – preferably mixed with a few drops of dishwashing liquid)
• Baking soda (1 tsp/1 lt of water)
• Mix of both (baking soda & neem oil)
If all of the above fails, the next option is using a Copper solution. It is slightly toxic, but it can work well if you apply at an early stage. It is recommended to wear protective clothing and mask when you spray it.
For any application, be cautious when the weather is hot and only spray very early in the morning, so that the droplets dry before the sun sees your plants.
B. Chemical control:
When it comes to chemical control, it is important to understand that fungicides are completely different products from insecticides. The first ones are used for infectious diseases (from fungi or fungal-like organisms), while the second ones, are used against insects. Following everything that has been mentioned before, regarding the classification and the identification of various diseases, I hope that it is clear that using any chemical fungicide off the shelf, without consulting a specialist, might not give you the desired result, as it might not be suitable for your case. (Apart from the obvious drawback of causing health problems, like cancer, or killing beneficial insects and damaging the environment).
Also, it is good to keep in mind that many diseases (like downy mildew or phytophthora blight) cannot be managed by fungicide applications alone; successful disease control is achieved only by a season-long effort to manage water and utilize other cultural practices, like aerating your plants, keeping them healthy, spraying preventively with neem oil etc.
In this post, I will use as an example of how bad the pesticides can be for our health, the active ingredient mancozeb, which has been used in agriculture for many years. In EU, mancozeb was recently classified as a category 1B reproductive toxicant and an endocrine disruptor and its usage is banned. I think that in Qatar it is still allowed, however it should be banned globally soon.
To close this long post, let me remind again that the first step for every plant problem is to identify if your plants suffer from water problems, a deficiency, an insect attack, a disease or anything else.
If you would like to know more, comment below or contact us.