Powdery Mildew and Botrytis (Bud Rot / Gray Mold): Not Fun Fungi
Powdery mildew and botrytis are terrible and tenacious destroyers of crops that plague many gardens. While both are fungal in nature and ruinous for your garden, each present differently on plants and infect plants in different ways.
These two fungal pests can creep up on even the most seasoned gardener, and once infection begins, it can be very difficult to stop and remove while saving your plants. There are however, many ways the aware gardener can both prevent and fight powdery mildew and botrytis.
To combat the issue with confidence, the first step is understanding the enemy.
What is Powdery Mildew?
Powdery mildew is a topical, biotrophic (feeding on living tissue) fungal disease. Just about every kind of plant in the world can be affected, and there are many different strains of powdery mildew that often affect only certain types of plants. Easily recognisable by the white splotchy patches that look like someone spilled flour or talcum powder, powdery mildew (or P.M. as gardeners often call it) is most often seen on the upper side of leaves, but can affect more or less any part of the plant.
Powdery Mildew grows on warm, dry plants, but is very fond of high humidity in the air. Infection occurs when spores land on a dry leaf and begin spreading mycelium across the surface of the plant. The fungi feed by sending roots down just into the very top layer of cells, the epidermis of the plant. They can grow in this way without water or humidity, but higher humidity in the air allows them to produce spores, which leads to spreading of the fungus that can overwhelm a plant.
Young, succulent plants are most susceptible to infection from powdery mildew. Because the fungus grows only on the surface of the plant, the more healthy and nutrient-rich a leaf’s surface is, the more vulnerable it is to becoming infected.
Although powdery mildew is an unsightly, unwelcome disease in any garden, it is only a surface blight, and will not kill your plants. Advanced infections do have the potential to hinder the quality of your harvest, so getting rid of the infection is always the aim, just not in a life or death sense.
What is Botrytis (Bud Rot)?
Botrytis, bud rot or gray mold, is an invasive, necrotrophic (feeding on dead tissue) fungal disease. While it is most commonly associated with infecting grapes (an infection is often called bunch rot for this reason), but can be hosted by a wide variety of plants. Unlike powdery mildew, botrytis infiltrates a plant and spreads systemically, invading the living tissue. Infected fruits or buds may be seen to lose their color in early stages, becoming pale and mushy. As the fungus rots the tissue, a mycelium coating of fuzzy gray growth will often appear around the infected tissue, spreading readily to any host sites touching or in close proximity. The end result is the fully necrotized site falling off the plant as a hardened mummy.
Because botrytis is systemic and gets into the vascular system of the plant, once an infection occurs, the entire plant is compromised. If you see gray mold on one sight, simply cutting away that singular sight is not a guarantee of removal. Botrytis can stay dormant in a pant until conditions for colonization and growth are met. It can even infect seeds of the plant and carry on the infection to the next generation!
Wounds or rotten tissue on a plant serve as the colonization grounds for botrytis, which feeds on the rot, then moves into the living tissue. The fruit or buds of the plant are the ultimate target of the fungus, which may remain dormant inside the plant until the fruit or bud becomes dense and sugary enough to support fungal growth.
Although wet, humid conditions are the most ideal for this fungus to grow, these conditions don’t need to be present for botrytis to infect a plant. Any rot or wound on a plant can serve as an entry point for botrytis, and apart from such environmental hazards as birds, bugs, and weather, wounds and rot that commonly lead to botrytis infection are caused by none other than powdery mildew itself.
Prevent Stress & Outbreak With Environmental Control
Although these two fungal diseases attack plants in different ways, they can be controlled in much the same way.
Powdery mildew will not grow on wet leaves, but it will very readily propagate in humid, poorly circulated environments such as occur when leaves are closely layered, trapping the moist, stagnant air between the leaves. Humidity and dense growth are choice conditions for botrytis as well, so a careful pruning of excess foliage and overlapping branches is highly recommended.
Both types of fungus will overwinter in leaf layers and plant detritus, so removing these from your garden will further decrease your risk of infection.
Watering methods are also important to consider. Especially in greenhouse grows, which can become very humid, watering your plants from above can cause massive spore dispersal. If you water your plants by hand, keep your hose or watering can close to the soil. Drip lines usually prove to be the safest watering method to prevent spreading spores.
Air flow is also a crucial element to control for powdery mildew, botrytis, and a host of other hazards your plants could potentially face. Make sure your canopy is not blocking air flow to lower leaves and blossoms, and keep a fan running to ensure there is no stagnant air anywhere.
If you do see signs of either infection, remove the infected part of the plant immediately and destroy it. Don’t compost it- you’ll only create an environment for the fungus to build up and lie dormant, waiting to infect the next plant it can reach.
If You’re Infected, How Do You Treat it?
There are some preemptive measures that can be taken to mitigate the risk of powdery mildew and botrytis. Most of these measures can also serve to mitigate the infection should one arise.
Lactobacillus is a family of bacteria that, as the name suggests, are bacteria that produce lactic acid. They do this by breaking down carbohydrates. For centuries, farming communities from Asia to Central America have used lactobacillus solutions (often called LAB) to keep fungal infections out of gardens.
When a LAB solution is added to the soil or leaves, it is a very effective preventative measure against most fungal infections, including powdery mildew and botrytis. Recent studies have shown very adept capabilities of lactobacillus-created acids in destroying botrytis spores specifically.
The traditional solution can easily be made with food items in your kitchen. You know that milky water that is left over when you soak rice or corn? Save that, and let it sit out for a night. When you come back to it, you should see a layer of solid material floating on top of a cloudy liquid. Strain out the solids, and add the liquid to whole milk in a 1:10 mixture (1 part cloudy water to 10 parts whole milk). Cover the container and let it ferment in room temperature for a week. You should end up with curds and whey. Toss the cheesy bits or feed them to animals, and use the liquid. Dilute it with FILTERED water (if you don’t filter your water, the chlorine and other heavy metals in your water can kill the bacteria, which is most effective at a pH of around 4.0). Pour this in the soil around your plants or spray it on the leaves.
Burning sulfur is a technique you can use to kill powdery mildew or any spores in the open air. Be aware that this technique creates an airborne toxin, so don’t be around while sulfur is burning. It also won’t be effective against botrytis that is already infecting plants. Because of the environmental hazard, burning sulfur is only recommended for greenhouse grows, not in-house or tent gardens.
Available at many garden supply stores, sulfur burners vaporize sulfur so that it fills the air in your greenhouse. For your first use, let the sulfur burn for about 12 hours, then turn the burner off. Wait a few days, then run the burner again for a couple hours. Wait another few days and burn for a couple hours. Repeat these short burns every few days until the fungus is killed.
Remember, don’t be in your greenhouse while the sulfur is burning, as sulfur is very toxic to breathe in, and it will coat your lungs. Speaking of coating, you will also find that the sulfur will coat every surface in your greenhouse. This residue can be easily wiped away with a sponge or wet cloth, but because of this coating, burning sulfur is really not recommended during flowering or fruiting stages of your plants.
Another bacterium in the bacilli class is bacillus subtilis. A soil-borne bacteria that can inoculate plants even at the seedling stage, bacillus subtilis can be found in nearly every type of plant, as well as the digestive systems of animals who eat plants (including humans). Bacillus subtilis has been shown in recent studies to be very effective as a systemic antifungal in plants. Adding bacillus subtilis to your soil can be very beneficial in preventing and aggressively attacking fungal infections.
Bacillus Subtilis is found in a number of organic gardening products, including our very own Earth Dust! The bacteria works mostly because it is very competitive in growing, and as the bacteria develops, it releases enzymes which kill off the harmful bacteria and create an environment in which the fungus cannot grow. Since bacillus subtilis lives in the soil and inoculates living plants, it is very effective as both a preventative and a reactive against powdery mildew and botrytis.
~Neem Seed Oil~
The oil extracted from the seeds of the neem tree have long been hailed as one of the greatest antifungal and anti-pest amendments you can add to your soil, or topical application to spray on your leaves. Several studies by universities and laboratories the world over have shown how effective neem seed oil is, especially as a preventative against pests both insectine and fungal.
Add neem seed oil to your soil when you are doing your initial cook to build an inoculation environment for your plant to be transplanted into, or spray a dilution of neem seed oil on the leaves as your plants grow.
Avoid Bro Science and Concentrate on Grow Science
The stigma of powdery mildew in the grow community has given rise to a massive amount of information on the internet. Much of it is spread by growers who are offering their own insights and experiences, which brings them to their own sonclusions and reasoning. While experience is certainly respectable, it does not make one infallible. Beware of all the bro science out there.
Powdery mildew is not systemic. It has no ability to move into the vascular system of a plant, and if somehow spores of powdery mildew did find their way into a plant, they would not survive in the anaerobic, moisture-rich atmosphere. Remember, powdery mildew needs DRY leaves and plant material to grow. The white powdery spotting that gives the mildew its name does not always present itself on an infected plant. Sometimes, all you see are slightly browned leaves or loss of color. Sometimes, it’s difficult to see any visual evidence at all of the infection, and cuttings are taken into other gardens where the infection spreads. This does not mean the infection was systemic, only that it was not presenting in the powdery fashion that makes it so easy to spot.
Always get your information from a reliable source. When we research for our blogs, we look up scholarly articles from universities, scientific journals, and research institutes. Be careful!
Keeping a Fungus Free, Healthy Garden
While powdery mildew and botrytis are pesky problems, they are not unavoidable, nor are they necessarily a death sentence for your garden. As we have seen, there are several readily available, organic options to prevent and combat these unfriendly fungi.
Most important to remember is your environmental conditions. As long as your plants have room to breathe, without layers of leaves overlapping and creating stagnant air, as long as you keep fans going to circulate that air and as long as you work on keeping your humidity in check, you should be safe from powdery mildew and botrytis.
The chemical treatments available range from the simple inoculants to environmental toxins, and it’s up to you which one you choose to prevent or treat infections. If leaves, buds or fruits are severely infected and not responding to treatment, be prepared to cut away the affected area. It’s smart to wrap the area in a plastic bag before cutting and removing, so you don’t release spores into the air. And always destroy or throw away the infected material instead of composting it.
Fungal disease is no joke, but there is no reason to despair. Keep a level head, prepare and grow your garden in an open, airy environment. If either of these fungal upsets rear their ugly heads, you now know that you can recover with aplomb.
We hope that this blog has been informative and helpful to you. What have you learned today? Is there anything we missed? Are there any topics you would like to see us cover in our blogs? Drop a comment below and get the conversation going!