Many times farmers think it is about buying whichever fungicides but never question themselves about what they are fighting, and which weapon would be perfect!
This is the same reason why almost 60% of farmers buy different inputs that do the same thing or spend o9 inputs whose diseases or pests their specific crops may never even face! Some farmers even ask why we create different spray programs for different crops.
Today I would like to say something about fungicides in particular.
Fungicides are pesticides that kill or prevent the growth of fungi and their spores. They can be used to control fungi that damage plants, including rusts, mildews, and blights.
They might also be used to control mold and mildew in other settings. Fungicides work in a variety of ways, but most of them damage fungal cell membranes or interfere with energy production within fungal cells.
Before we go down into fungicides, you know this;
• Often, plant diseases are transmitted when leaves are wet. Ground-level watering and good air circulation can be used to keep leaves dry. Overhead irrigation is the major spreader of fungal!
• Many fungicides remain on the surface of plant tissues and do not spread throughout the plant. Others penetrate the cuticle and circulate through plant tissues.
• Pruning tools, hoes, pangas, etc. can carry plant diseases from one plant to another. Learn about garden sanitation to prevent spreading fungal pathogens yourself.
• Although they can slow or stop the development of new symptoms, many fungicides are designed only to prevent disease. These are not highly effective after symptoms have developed.
The Two (2) Groups of Fungicides
Fungicides can be divided into two groups: protectant and systemic fungicides.
a) Protectant fungicides are contact materials that remain on the outside of the plant surface and kill fungal spores and hyphae upon contact, thereby preventing infection from occurring.
b) Systemic fungicides are absorbed by the plant cuticle and underlying tissues and can act by killing spores and hyphae as well as incipient infections where the fungus has penetrated the plant surface.
When they stop infections and prevent symptoms from developing they are called “curative.” However, symptoms that are already present will not be “cured” by the fungicide in question.
After symptoms appear, some fungicides can reduce or inhibit fungal sporulation: these are called “anti-sporulates.” The term “eradicant” is often used for products like lime sulfur, which kills fungal structures in woody plant tissues when applied as a dormant spray.
However, eradicants seldom eliminate all overwintering inoculum. Occasionally people use the term “eradicant” for very effective fungicides (e.g., Ridomil) that prevent current-season infections to the point that the disease appears to have been eradicated.
The term “translaminar” refers to the movement of a fungicide from one side of the leaf to the other, providing disease control on both sides of the leaf.
Systemic fungicides are systemic to different degrees, with some fungicides being locally systemic (they move only a short distance away from the spray droplet, others being more mobile in the plant (systemic) and able to move to the tip of the leaf or shoot and yet others being highly systemic and able to move throughout the plant including the roots.
Most systemic fungicides are highly effective against their target pathogens regardless of whether they are locally systemic or systemic. However, products that are more systemic tend to have longer post-infection activity because they penetrate deeper into the plant tissues and can catch more advanced infections.
In the latter case, the higher the rate used, the better the post-infection activity.
Both protectant and systemic fungicides are effective when applied before infection occurs, but only systemic fungicides have efficacy after the fungus has penetrated the plant (for a limited time, e.g., 24 to 72 hours, depending on the fungicide, disease, and rate used).
Since systemic fungicides are absorbed by plant tissues and get redistributed in the plant, they tend to be less susceptible to wash-off by rain compared to protectant fungicides, which remain on the outside of the plant.
A general rule of thumb that is often used is that one inch of rain removes about 50 percent of the protectant fungicide residue and over two inches of rain will remove most of the spray residue.
However, newer “sticky” formulations (e.g., super-grow) and fungicides applied with spreader-stickers may be more resistant to wash-off by rain. Also, fungicides and formulations differ a lot in their ability to adhere to plant surfaces.
Therefore, research is needed to describe the effect of rainfall on wash-off on specific products.
In addition, protectant fungicide residues naturally decrease over time due to weathering, such as degradation by sunlight (UV radiation), heat or microbial activity, and redistribution over the plant surface by rainfall, dew, or irrigation water.
Most protectant fungicides are good for about seven to 14 days of protection, and systemic fungicides for seven to 21 days depending on the product, the rate applied weather conditions, and disease pressure.
During the rainy season, it is better to use systemic(or a fungicide that has both curative and protective properties e.g metalaxyl and Mancozeb) than protectant fungicides or a mixture of the two since systemic fungicides are less sensitive to wash-off by rain.
Applying a mixture of systemic and protectant fungicides may be the best compromise. In addition, spreader-stickers(like Super gro) can enhance adherence to protectant fungicides, while penetrants may speed up the penetration of systemic fungicides.
Technological advances ensure that many newer fungicides and fungicide formulations have excellent adhesion or absorption properties.
Remember fungicides are pesticides that prevent, kill, mitigate or inhibit the growth of fungi on plants, but they are not effective against bacteria, nematodes, or viral diseases.
Classification of Fungicides
Fungicides can be classified based on:
(1) Based on Mobility in the Plant: Contact vs. mobile (types of systemics)
Contact fungicides also known as protectants are not absorbed by the plant and stick to plant surfaces. They provide a protective barrier that prevents the fungus from entering and damaging plant tissues.
Systemic products (also known as penetrants), are absorbed by the plant and can move from the site of application to other parts of the plant. Movement in the plant varies by fungicide, from moving to old and new tissues, new growth, moving from the top to the bottom of the leaf surface (translaminar).
(2) Preventive Vs Curative
Preventive fungicides work by preventing the fungus from getting into the plant. The preventive fungicide must come into direct contact with the fungus, and they have to be re-applied to new plant tissues a lot during wet seasons or if the product washes off.
Curative fungicides affect the fungus after infection. This means they can stop the disease after the infection has started or after the first symptoms are observed. Fungicides that can move in the plant can be both preventative and curative.
(3) Mode of Action
This refers to how fungicide affects fungus. Fungicides may work by damaging the cell membrane of the fungus, inhibiting an important process that the fungi, and pinpointing single or multiple processes in the fungus.
It’s important to incorporate different modes of action by mixture or by alternating products to maintain effectiveness and prevent fungicide resistance. Stay tuned for our article “what is fungicide resistance?”.
14 Ultimate Guide to Effective Use of Fungicide
For efficient and safe fungicide use, certain rules have to be followed:
1. The problem has to be diagnosed correctly
Before applying a fungicide, make sure that you know the cause of the disease and how often to apply the fungicide.
The timing of the fungicide application can enhance the effectiveness of the product and prevent additional sprays.
2. When ready to use the recommended fungicide for the particular problem your plant is facing, read the label and follow the instructions.
This will not only protect your plant, but it will also protect your health and the environment. Remember always to apply fungicides using the appropriate equipment at the recommended application rate.
Fungicide labels provide information on recommended use, ingredients, mode of action, and formulation of the product.
3. Remember that the best management strategy against plant diseases is by promoting plant health in the first place.
Before planting, make sure that soil, water, and light conditions are ideal for your plant.
Once the plants have been established, make sure to use the appropriate sanitation, fertilization, and pruning practices to enhance plant health.
That’s why during massive fungicide use, we also draft fertilizer schedules that support fighting these diseases.
4. Walking your fields can help uncover disease and prompt an immediate decision.
Seeing disease in the field can mean different things, though.
Some diseases like gray leaf spots often move up the canopy, while others may come in from outside the field.
However, a fungicide application can halt further infection.
5. Apply fungicides before the development of the disease. Although many fungicides have systemic (“kickback”) action they will not completely eradicate diseases after they have started.
By the time a single disease lesion is observed in the field, many more lesions too small to observe are already working at your crop. Most systemic fungicides move less than an inch toward the tip of the plant or may just move from the upper to the lower side of the leaf.
6. Use shorter spray intervals during weather conducive to plant disease. Each plant disease has its own “personality” and thus prefers different weather.
However, most plant diseases require leaf wetness. Therefore, during periods of rain and heavy dews, more frequent fungicide applications are a good idea. The normal range of spray applications is every 7 to 14 days.
7. Apply fungicides before a rain if possible. Water is necessary for most fungal spores to infect foliage and for the splash dispersal of spores. Therefore apply fungicides before rain if it appears that the fungicide will have a chance to dry before the rain.
Some fungicides list the rain fastness period on the label. It is not necessary to apply fungicides again after every rain. Most fungicides have a good sticker and will persist through rains pretty well. You can as well mix it with a sticker-like super grow.
8. Know when to alternate fungicides. Systemic fungicides, those with a single mode of action, if applied again and again in sequence, may cause the disease fungi to mutate into a form resistant to the fungicide.
Always alternate fungicide applications from one mode of action to another, this explains why when making a spray program we advise stocking up to 3 different types of fungicides depending on weather patterns of that season.
9. Timing of fungicide applications is more important than nozzle type and spray pressure. Studies have found that nozzle type and spray pressure don’t make as much difference as we once thought. But some nozzles may skip some parts of the plant, make sure the spray is thorough.
10. Some diseases cannot be managed by foliar sprays. Problems caused by soil-borne fungi or nematodes cannot be controlled with foliar fungicides. Examples of these types of problems would be the Fusarium wilt of watermelon or root-knot nematodes of tomatoes. Also, be certain that the problem you observe is a disease. No amount of fungicide will improve a problem caused by soil fertility. Contact us for soil sampling and testing.
11. Use copper products for bacterial diseases. For the most part, copper products are more effective against bacterial diseases than they are against fungal diseases.
12. Some diseases require specialized fungicides. Diseases, such as downy mildew and Phytophthora blight may require specialized fungicides. It may be wasteful to apply specialized fungicides all season long for diseases that are not a threat. For example, the downy mildew of cucurbits(pumpkins, melons, cucumber, etc.) usually does not arrive until late in the season.
13. Double-check the label for details. Rates may vary widely based on label changes and different formulations. While you are checking the rate, also make sure that the crop and disease are on the label. Can this fungicide be applied in the greenhouse?
14. Play it safe. Always adhere to the Post-Harvest Intervals, Re-Entry Intervals, and Worker Protection Standards listed on the label. No one wants an accident or lawsuit. Besides, the label is the law. Next time we shall look at other disease protection inputs!
Possible Reasons why Fungicide Applications Fail Sometimes
Fungicide-resistant fungi often come to mind when fungicides fail. There are other reasons for failure and they include:
1. Incorrect diagnosis. “Make sure you have fungal disease in the field,”
2. Improper sprayer rate and calibration. If you spray at a lower rate, you might certainly be increasing the chances of selecting a resistant fungal strain. Pay attention to the label.”
3. Improper application timing. Ideal application timing can differ, It depends on the weather and the amount of inoculum out there. It is a moving target.
Read Also: Agriculture and the Natural Environment
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