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Anyone engaged in horticultural and agricultural practices must possess an in-depth knowledge of all the different plant species, their growth, and the various pathogens that might harm them. Knowledge of healthy plant growth patterns is beneficial for identifying early signs of plant disease. The sooner you learn, the sooner you can act on the cure.
What is Plant Disease?
Pathogens are organisms that cause diseases in the hosts they occupy. In the case of plant diseases, the causative agents can be bacteria, fungi, and viruses. Sometimes plants are hit by more than one type of pathogen. Plant diseases result in plants losing their ability to carry out everyday physiological processes required for their survival. Without timely intervention, diseased plants might die. Diseases can be infectious or non-infectious. Infectious diseases pose a more significant threat as the infection spreads to many plants, destroying entire crop plantations.
Causes of Plant Disease
Plants are susceptible to a variety of factors that may lead to diseases. Three main factors are instrumental in the occurrence of plant diseases. These are; the environment, host, and pathogen. Here, the host is our susceptible plant, and the pathogen is our biotic agent that attacks our host. The environment also plays a key role; if the environmental conditions are harsh and there is a shortage of essential nutrients, it will automatically make the plants weak and unable to fight off pathogens. It is important to note that most pathogens are plant-specific, and you have to use different methods to protect different plants.
The three leading causes of plant disease are:
1. Abiotic Factors
These are non-living or inanimate factors. The lack or excess of environmental factors hugely influences plant diseases. The lack of certain nutrients such as zinc, molybdenum, and calcium can cause plant diseases. An excessive amount of carbon dioxide and sulfur dioxide can cause a build-up of toxic materials in the plant body. In addition to this, light, humidity, soil moisture, soil compaction, salt, and aeration also control the occurrence of plant disease—abiotic factors cause non-infectious diseases.
2. Mesobiotic Factors
These include viruses and viroids; they are neither completely living nor non-living. Outside the host plant, they behave like particles. When inside the host, they replicate and spread disease. Viruses and viroids spread through vectors or infected seeds.
3. Biotic Factors
These are living organisms that cause infectious diseases. The pathogens in this class may be unicellular or multicellular. Some of the leading biotic causes of plant diseases are bacteria, fungi, FLOs (fungal-like organisms), phytoplasmas, nematodes, and other parasitic plants.
Common Plant Diseases
1. Black Spot
It is a fungal disease that occurs in flowering plants as well as fruits. This disease does not completely kill the plant. Instead, it weakens plants and reduces their immunity. Roses are very susceptible to this plant fungus – it turns the leaves yellow before they wilt and fall off. It starts as small, smudged black spots which spread throughout the foliage.
Solution: Avoid splashing leaves with water, make sure the plants are exposed to sunlight for some time, practice mulching beneath plants. Growing resistant plants and cutting off affected parts as soon as you identify the disease are other viable solutions.
2. Bacterial Blight (Bacterial Canker)
This disease, caused by bacteria, starts as large yellow spots on leaves and fruits, eventually developing into sunken brown and black bacterial leaf spots. Overly affected areas fall off. This disease spreads rampantly through the summer months, and rain aids in the spread. The primary cause is cool, wet weather.
Solution: There are no resistant species, so the best you can do to protect your plants is to be vigilant for early signs of the disease and immediately prune off affected areas to minimize spread. Pruning should be done in warm weather to prevent the infection from spreading. Also, make sure to keep some distance between plants. Copper-based fungicide sprays twice a year also work as antibiotic prophylaxis.
3. Cucumber Mosaic Virus (CMV)
This disease attacks nearly 1200 species of plants ranging from vegetable plants to ornamental species. CMV is transmitted by aphids and spreads very quickly in warm climates. Symptoms and signs of disease progression differ from species to species. Some common characteristics of CMV are; leaves becoming puckered and brittle, fruits becoming bitter, leaves developing yellow and dark green patches.
Solution: CMV-resistant species are the best options. Preventing the growth of aphids and covering young plants to hold off aphid infestation is helpful.
4. Rust in Plants
Causative agents are fungi. Rust in plants is convenient to identify as they exhibit rusty spots on the leaves and the stems and progress to dark orange and black spots. Many species of plants are easily affected by rust; corn, beans, roses, tomatoes, etc.
Solution: Fungicides are very helpful in preventing rust. Spraying plants with copper or sulfur-based pesticides after you notice signs of rust is the most common way to stop it from spreading. Another method involves cutting away affected parts.
5. Powdery Mildew Disease
It is a fungal disease that is highly host-specific. The powdery mildew disease presents itself as a white powdery residue on leaves and shoots. Warm days and cool nights are conditions for powder-like mildew growth.
Solution: Heavily infected parts of the plants must be destroyed so that fungal spores do not spread. If the mildew has just started sprouting, then fungicidal sprays work. One should place plants in areas with good air circulation and ample sunlight.
6. Verticillium Wilt
It is a deadly fungal disease that affects several plants, including vegetables, deciduous trees, shrubs, and berries. The soil-born fungus invades through the roots and spreads upwards through the stem, thus destroying the plant’s vascular system. Signs of this disease are wilted stems, yellow leaves, and stunted growth.
Solution: Fungicides do not work as there is no cure. The only way to salvage your plant is to prune away affected parts, even roots. While pruning, remember to sterilize the pruned parts to avoid spreading.
7. Snow Mold
Snow mold is a fungal disease that thrives in cold and moist conditions and is found beneath the snow. Grasses appear tan due to mold threads.
Solution: Matted grass should be raked up frequently, and grass should be cut short before the arrival of harsh winter months.
8. Apple Scab
A common apple disease where leaves turn yellow, fruits have circular brown and black marks, and some leaves have small green spots with uneven borders.
Solution: Plant resistant varieties, prune mature trees, and use sulfur-based fungicides for susceptible apple varieties.
9. Peach Leaf Curl
A fungal disease that affects varieties of peach plants, especially during heavy rainfall. Leaves are affected before trees get a growth spurt. The thick, curled buds are infected and reddish in appearance.
Solution: Some varieties of peach plants are resistant and tolerant. It would be best if you planted peach trees in areas away from direct contact with rain. Spray all the trees with lie sulfur right before the growth spurt.
10. Black Knot
Black Knot is a fungal disease that causes warts on the stems and branches of the plant. The extensions of branches turn into black knots in the later stages.
Solution: Resistant varieties are helpful. It is vital to prune knotty twigs and branches before February and spray with a sulfur-lime solution.
11. Spider Mites
Spider mites appear as white webbing in the underside of the leaves of the affected plant. Warm spaces attract spider mites.
Solution: Immediately prune affected leaves and isolate the plant. Gently wipe the plant with rubbing alcohol or soap solution.
Fluffy white insects invade the leaves of plants that are overwatered and are planted in soil with high nitrogen content.
Solution: The bugs can be removed by wiping with rubbing alcohol or spraying insecticide.
These are very small, beady bugs found on leaves and stems. The bugs may be green or yellow. The bugs are attracted to warm climates and high nutrient concentrations in soil (e.g., nitrogen)
Solution: Gently wipe the leaves and stem with rubbing alcohol or a soap solution.
Tips to Prevent Plant Diseases
Here are some rules to follow to prevent the invasion of plant pathogens:
- Do some research and plant disease-resistant varieties.
- Test your soil for pathogens and other conditions that might invite pathogens. Choose the right kind of plants for your soil.
- Study plant disease mechanisms and be aware of environmental conditions which might cause disease.
- Be very observant of your garden and isolate diseased plant parts as soon as you notice them.
- Test your soil for pH (acidity and basicity) and spend some time making the soil suitable for your plants.
- Do not randomly add plants without knowing how they interact with each other. One should keep plants susceptible to similar diseases far apart.
- Spray plants with mild preventive low concentration fungicidal solutions.
- A clean garden is of utmost importance. Always rotate crops, sanitize all tools, and remove infected plants.
- Plants need the maximum nutrition for their survival. Care for them by providing fertilizer and proper watering. Certain plants need nutritional supplements and adequate aeration for optimal growth.
- Mulching is an essential practice for disease prevention. You can use organic mulches to cool the soil and keep it amply hydrated.
How to Dispose of The Plants That Have Caught Diseases
Infected plants have to be disposed of very systematically. Otherwise, stubborn diseases will inevitably return and create havoc. Here are some of the best plant disposal methods:
- Burial: Bury rotting plant parts in a one-foot-deep hole. Make sure the burial area is far from the main planting area.
- Compost: Plants and parts infected by less deadly diseases are easily compostable, e.g., powdery mildew.
- Burning: Bonfires are a classic plant disposal method, but this is only applicable for dry woody materials, such as branches, twigs, and old leaves. Be careful to set up your bonfire in a non-windy area where the fire won’t spread.
- Household Trash: Small plant parts such as bulbs, twigs, branches, stems, knotty roots, and infected fruits and flowers can be dumped in the daily trash.
Staying aware of the different plant diseases is crucial to enable you to manage a beautiful garden. By efficiently treating such diseases as per the solutions provided in this article, you can ensure healthy plants and prevent any diseases in the future.