Disease can strike marijuana plants at any stage. Most diseases that affect marijuana fall into two broad categories: fungal and bacterial. The spores and bacteria that cause plant diseases are ubiquitous. A garden’s susceptibility to disease is often traceable to environmental imbalances in temperature, moisture, light conditions, airflow and pH, among others.
Fungus grows when it encounters the right levels of moisture, temperature (the range varies by species), acidic conditions and a reliable food source.
Bacteria are much more likely to invade when the environment has been compromised. Conditions such as oxygen deprivation make their attack more successful.
Once disease hits, it is important to act quickly to restore balance to the environment. However, prevention, by way of a balanced environment for the plants, is the best solution.
Fungal spores often enter a grow space on air currents. A fine dust filter in the air intake system captures these spores and reduces the chance of fungal infections. Another option is a UVC lamp in the intake duct. The light from these lamps kills microbes and destroys spores. The two options can be combined.
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Algae are plantlike, usually microscopic organisms. Some are unicellular and others form multicelled organisms. They conduct photosynthesis like larger plants but lack stems, roots and leaves.
Algae occasionally grow in hydroponic and aeroponic systems.
Algae thrive in environments that provide warm temperature, light and nutrients. The water temperature 68-72°F (22°C), which is best for maximum plant growth, is, unfortunately, a good temperature for algae.
When light shines on nutrient-rich water, algae are almost guaranteed to grow.
Algae can grow anywhere in your system. It is commonly found inside tubes, in nutrient reservoirs, in buckets, on exposed rockwool cube surfaces and in DWC (deep water culture) buckets—mainly if the containers are not opaque and allow light in).
All containers should be opaque.
Some algae attach to surfaces such as tubes and reservoir surfaces as well as roots. They form a green film that looks and feels either velvety or slimy. When it covers the roots, it starves them of oxygen. The film also competes for nutrients and can clog system lines, motors and sprayers in aeroponic systems.
Algae photosynthesize during the day, using CO2 dissolved in the water and releasing oxygen (O2). However, during the dark period algae use oxygen dissolved in the water and release CO2. This depletes the water of oxygen, which the roots require to maintain health.
The best way to prevent algae is to deprive it of light. This is a safe, nonchemical method of prevention. Use black tubing, rather than clear or translucent, to prevent light from coming into the lines. Use a black or opaque reservoir to hold the water/nutrient solution. Make sure the cover is opaque and light-tight. Potting containers should be opaque.
Everything should be light-proof. Rock-wool cubes should also be covered. You can use landscape sun-block fabric, white-black plastic (white side up) or plastic rock-wool cube covers made for the purpose.
The best method of algae control is by excluding light. This should be the main emphasis in systems with algae problems. If you already have algae growing, clean the system and replace or cover light- transmitting tubing and reservoirs with opaque materials. Panda plastic (white-black flexible polyethylene plastic) can be used to cover systems to ensure opacity. It can be used indoors or outdoors.
If algae are still a problem, rest assured that there is a light leak in the system. Algae need light.
Green algae
Top and right photo by Fred Morledge
Gray mold, Botrytis cinerea, is found almost everywhere and can cause disease on most plants, including marijuana. It can cause damping off and stem canker but is most serious when it causes bud rot.
Gray mold is one of the most common fungal diseases that attack marijuana.
Although gray mold can infect seeds pre-germination, it is much more commonly encountered causing moldy growth of the flowers and foliage much later in development.
The fungus can germinate only on wet plant tissue when the temperature is between 55-70°F (13-21°C). This often happens in dry weather as dew accumulates on the leaves. Once it starts growing, it can tolerate a wide range of humidity and temperatures, but high humidity and cool temperatures help it thrive. Lowering the humidity stops its continued growth.
Gray mold, like most other fungi, enters and easily infects any part of a plant that is wounded, damaged from pests and pruning or beginning to die. Thus it is very important to sanitize pruning equipment between cuts.
Cuts and lesions are a normal part of plant life, so all plants are subject to attack when conditions are favorable to the mold. Unhealthy or shaded areas of plants or crevices in buds are ideal conditions for the mold. Spores travel mostly via wind and rain and even in tap water, but they can be brought into grow rooms on clothing and pets.
Shaded areas of the plant that do not get a lot of light are usually first infected. Then the disease spreads quickly through growth and spores.
Gray mold does the greatest amount of damage during flowering. It attacks the flowering tops, leaves and stalks. Seedlings and seeds can also be infected and killed.
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Gray mold starts out whitish like powdery mildew but then darkens to a smoky gray or brown color. It has a fuzzy appearance and light to dark brown rot forms in the damaged tissue.
Leaves and buds break down from being suffocated by the mold. In higher humidity, the gray-yellow mold leaves a brown slimy substance on the leaves and turns the bud to rot, especially when the tissue is dense late in flowering. When the mold dries, it leaves dark brown areas of infection. Dense buds are most susceptible because the moisture is trapped in the crevices. Sometimes only part of the bud is infected.
Stems with unhealed breaks can be infected with B. cinerea causing stem cankers, which then affect the rest of the plant by depriving it of nutrients and water.
Indoors, avoid conditions favorable to gray mold by controlling humidity and temperature. Keep humidity under 50%. Water when your grow lights are on or during the day. Remove and discard dead or dying plant tissue. Make sure no moisture is on leaves and buds when the lights go out. Gray mold indoors is usually caused by high humidity.
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Outdoors, you can’t control the weather but you can prevent bud rot. Bacillus subtilis is a bacterium that attacks B. cinerea. It can be sprayed on the plant as a preventive.
Mold
Potassium bicarbonate (KHCO3) and pH Up change the pH of the environment from acidic to alkaline. Alkaline environments inhibit the germination of molds and fungi, including gray mold.
Sulfur is often used as a spray or vapor to control fungi.
Neem oil and sesame oil form a barrier and inhibit mold germination.
If you see gray mold or stem wounds, apply a fungicide. The mold is particularly difficult to remove from plants in late flowering.
Removing the mold from living bud to prevent its spread may do more harm than good unless done carefully. The very act of fiddling with them may help transfer the mold to new sites. Sterilize tools by dipping them in alcohol or hydrogen peroxide after curing a bud.
Leaf Septoria, or yellow leaf spot infection is one of the most common types of leaf spot diseases, but it mostly affects outdoor plants. It is primarily spread by “wind, water and walking,” so in those rare instances that an indoor grow is affected it’s generally because spores blew in from a very near outside source or were carried inside by shoes worn on infected soil. The disease isn’t usually fatal to infected cannabis plants but it does radically reduce yields because of its disruption of leaf photosynthesis.
Two closely related fungi, Septoria cannabis and S. neocannabina, cause yellow leaf spot. Yellow leaf spot is a wet and cloudy warm weather infection. Warm water and rain trigger the release of spores from the storage structures so it usually appears mid to late season. Infections occur when the temperature is in the 60’s, but the fungi grow faster and are more destructive as the temperature rises, with its ideal temperature just below 80° F (25° C).
Septoria Conidia spend winter in a dormant state on residual plant material in the topsoil. Infection generally presents in early spring on juvenile plants and seedlings as spots on the upper side of the lower leaves, but has also been observed on stems and top fan leaves. It takes roughly a week for symptoms to appear after inoculation. While the infection cycle begins in spring, summer is when the disease spreads aggressively; if left unchecked, the wet, relatively warm rains will spread the conidia rapidly.
Spots begin as small round discolorations but usually expand into oblong, angular shapes, sometimes with a distinct reddish-brown border. These spots eventually desiccate and flake away from the plant, leaving jagged voids in the leaf. In extreme cases the entire leaf may curl up and wither away. The leaf damage caused by septoria infection undermines photosynthesis, which stunts growth and reduces yields of flowers, fiber and seeds.
There are other “leaf spot” infections that affect cannabis but they are relatively easy to distinguish by their presentation on the plant and obviously different when viewed under magnification. If the spots are lighter in color and do not dry up and leave holes, then it is more likely to be White Leaf Spot. Alternately, if the spots are darker and remain relatively small this may indicate a Brown Leaf Spot infection, while spots mostly presenting on the bottom of the leaf might suggest Olive Spot. Any lingering doubt can be easily erased with a glance through a microscope.
Plants are not likely to come in contact with the spores unless they are trekked in or there are infected outdoor plants nearby.
Septoria fungi are specific but ubiquitous. Only two species attack Cannabis. As a result, plants are more likely to be infected if there is marijuana, or perhaps hops, growing outdoors near the garden.
The spores overwinter on fallen infected marijuana leaves and debris and spread in three ways: wind, water and walking, either by humans and animals.
Infections are most likely to occur in gardens and spaces where marijuana has been grown before. To lessen the likelihood of infection remove all residue from the previous garden and then spray the area with a fungicide. Clean and decontaminate all tools after working with infected plants. If possible plant the garden in a different space each year.
As with all fungal plant diseases the cornerstone of prevention is proper sanitation after harvest: All residual plant matter (stalks, roots, etc.) should be shredded to uncover any pathogens or pests hiding inside, then burned, buried or composted at a separate location. All tools and materials used (and all clothing worn) during cultivation should be disinfected and/or sterilized. In a greenhouse setting, all surfaces should be thoroughly cleaned and sterilized. If possible, move the location of your grow each season.
Pruning is the key to controlling septoria. When possible remove infected limbs and treat all wounds with fungicide to prevent further spread. If necessary, you can remove whole diseased plants from otherwise uninfected fields: Quickly pull a trash bag over the infected plant and cinch it at the base of the stalk before removing it from the soil to minimize the chance of spreading the infection.
Powdery mildew Photo by Kristen Angelo
Powdery mildew is a common disease caused by several species of fungi known to infect cannabis that share hosts with many other plants (Golovinomyces cichoracearum, for example). Spores are ubiquitous and can travel extremely far distances on air currents, and infection is essentially invisible until the organism establishes, producing spores and a white mass that lowers the rate of photosynthesis. Powdery mildew can very quickly re-colonize the same plant or a nearby host that is suitable, making prevention an important aspect of management. Higher humidity brought on by moisture can facilitate development, but high amounts of standing water on foliage can have the opposite effect. The races that attack hops also attack marijuana.
Mildew spores are found everywhere. Powdery mildew is a common problem for both indoor and outdoor growers whenever the temperature and humidity fall into its favored range.
Mildew spores are ubiquitous and endemic. In areas where marijuana or hops is being grown, wind and air ventilation are the main vectors. Another major factor is contaminated cuttings. However, clothing, pets and outdoor animals can also deliver spores to the garden.
Spores can remain dormant until environmental factors, which include a suitable host, adequate humidity, moderate temperatures, low light intensity and acidity, trigger them.
Powdery mildew is most likely to attack young leaves, up to two or three weeks old. The infection spreads over the plant and to other plants in the garden. It affects buds, stems, stalks and leaves.
The first signs of an infection are raised humps on the upper leaf surfaces. Plant leaves look like they’ve been dusted with flour or confectionary sugar. At first it might appear on just a small portion of the leaf in an irregular circle pattern. It quickly spreads and soon the entire leaf is covered as if it had been powdered.
Infected plants prematurely yellow, brown and eventually die. If untreated, black specks can arise in the white powdery mildew. Buds have a stale, moist smell and are coated with a white powdery-looking mildew substance that can’t be removed. Powdery mildew hinders photosynthesis, causing your harvest to cripple to little or no yield.
Infected cannabis buds and leaves are not acceptable for smoking.
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Quarantine all new plants in a separate area where they can’t infect other plants.
Powdery mildew
Filter incoming air to prevent spores from entering the room in the airstream.
A germicidal UVC light like the ones used in food handling kills powdery mildew spores that are airborne.
Ionizers and ozone generators precipitate and inactivate spores, lessening the chances of infection.
As with other fungi, restrict humidity and spore production by not watering or foliar spraying at night or with lights off.
Water plants when lights first come on or with at least five hours of remaining light time.
Keep humidity in check; anything over 50% may trigger problems.
To minimize plant-to-plant infection, keep plants spaced apart to allow for maximum airflow in between plants.
To prevent outbreaks, you can use the following Ampelomyces quisqualis (beneficial fungi) products on a regular basis:
Powdery mildew in vegetative growth is easier to treat than in the later stages of flowering. If your plants get infected during flowering, especially far into the flowering stage, the buds will eventually become infected.
Removing infected leaves from the grow room is critical. Carefully remove them without knocking spores into the air. Place a bag over infected leaves and tie it shut; then remove the leaves. Use a fungicide on wounded stems from which you’ve removed the leaves.
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The following products will work for indoor and outdoor plants:
Every plant must have a healthy root system. Pathogens can attack and damage the roots of one plant, then rapidly infect other plants in the garden. Fusarium, Verticillium and Pythium are common and destructive root aggressors known to all growers, whether soil gardeners or hydroponicists.
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Fusarium
Fusarium is a complex of many different fungal species (two common species are Fusarium oxysporum and F. solani), which are able to reside in the soil for long periods. One of the more common species encountered is a pathovar, or special strain, that affects cannabis in particular (Fusarium oxysporum f. sp. cannabis). This strain was discovered several decades ago and was eventually considered inappropriate as a biocontrol for illegal Cannabis crops. Fusarium root rot is a common disease of many plants. It often quickly damages roots before symptoms are obvious and control can be administered effectively.
Different Fusarium species produce somewhat different diseases in cannabis. These diseases, primarily Fusarium wilt and Fusarium root rot, present different symptoms but respond to similar prevention and control methods.
Fusarium is not commonly found in marijuana gardens. However, residual spores are more common in soil where hemp was once grown or where it still grows as a weed. Growers using hydroponic systems or sterile or pasteurized planting mix seldom see it. Fusarium infections are most common in warm weather, but overall the frequency varies. Because Fusarium is soil-based and species-specific, it may be extremely common in one area but rare in another where the soil is not infected.
Fusarium spores can remain dormant in soil for years, and infected soil is nearly impossible to disinfect. The fungus can also spread from seed taken from infected plants.
Damage from Fusarium wilt is most evident on the leaves and stems. Fusarium root rot affects the roots first, then the disease works its way up the stem.
Fusarium wilt initially appears as small, dark irregular spots on lower leaves, which quickly become chlorotic (yellow-brown). Leaf tips curl upward, and wilted leaves dry and hang on plants without falling off. Stems turn yellow, then brown, and finally collapse. Roots show no outward symptoms if the diseased plant is pulled up.
Fusarium root rot begins below the soil line, turning the roots rotten and necrotic and giving them a characteristic red color. The first visible symptom usually appears as the rot works its way up the stem, producing a red-brown discoloration at the soil line. This discoloration may progress to swelling and the stem may split open. The plant soon begins to wilt, then collapses as the decay spreads up the stalk.
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Fusarium
In both wilt and root rot, the fungus spreads through plant cells and clogs the xylem vessels, inhibiting water and nutrient transport. This vascular clogging inside the plants causes the external symptoms of wilt and collapse. Infected plants usually die. Fusarium survives in the plant debris, so infected plant debris should not be buried, composted or placed on uninfected soil.
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If an outdoor cannabis crop falls prey to Fusarium, that patch of ground can no longer be used to grow cannabis at all, although other plants will do fine there. Likewise, seeds produced by infected plants should not be used. The pathogen stays dormant on the seed and attacks the plant when the seedling emerges, causing damping off and likely killing it.
Avoid planting marijuana in the same ground for many years in a row. Even though none of the plants show symptoms, multiple successive plantings can cause the fungus to build up in the soil until it reaches destructive levels.
Certain soil types have been found to be less conducive to the growth of Fusarium than others. Clay soils have fungistatic properties due to their high pH. Loamy soils with healthy and diverse plant growth often harbor native microbes that suppress Fusarium. These soils do not stop the fungus, but they slow it down and may be helpful in combination with other preventive measures.
Properly aged compost, and tea made from compost, help protect plants from all sorts of fungal infections.
Container gardening is one of the best ways to avoid Fusarium, because it gives the gardener complete control over the soil. If an area that would otherwise be ideal (weather, sun, etc.) is known to be infected with Fusarium, then containers allow you to take advantage of the site’s strong points while avoiding the disease issues. To prevent Fusarium infections, use sterilized or pasteurized soil mixes and new pots, and do not overfertilize. Make sure the soil drains.
Mycorrhizae (beneficial fungi) help improve plants’ disease resistance.
Streptomyces griseoviridis, Bacillus pumilus and Bacillus subtilis (all beneficial bacteria) or Gliocladium (beneficial fungus) can be applied as pretreatments for seeds, as a soil drench or as a foliar spray.
Make sure the soil pH doesn’t get too low. Neutralize acidic soil with dolomite lime or greensand. Fertilizers enhanced with potassium and calcium can help fight off and prevent Fusarium, while excess nitrogen and phosphorous may make the disease worse.
The only truly effective control is the removal and destruction of infected plants. After removing any affected plants, use hydrogen peroxide (H2O2) to clean all the tools that touched those plants before using them again.
Verticillium wilt is caused by soil-borne fungi that are common in many soils, and it attacks hundreds of herbaceous and woody plant species, including cannabis.
Verticillium wilt is a disease caused by fungi in the Verticillium group (e.g., Verticillium dahliae and Verticillium albo-atrum), which infect the leaves and roots of over 400 plant species and cause yellowing and other discoloration by blocking the water channels in the host. Small black protrusions can sometimes be a sign of an intermediate or advanced infection, usually found on dying tissue. Infected plants can be asymptomatic for a short time before the fungi start to cause wilting, which itself is not a symptom unique to Verticillium wilt, so observe caution both when evaluating the disease and after confirming its presence for treatment. Particularly moist-growing substrates encourage many kinds of pathogenic growth, including those that cause Verticillium wilt.
Verticillium wilt starts by attacking stressed roots, then proceeds to affect the leaves as well.
The lower leaves turn yellow along the margins and between the veins before turning a gray-brown and wilting. The stem turns brown near the soil line; symptoms can resemble Fusarium wilt. Once the roots have been affected, Verticillium wilt spreads through the xylem, which exhibits a brownish discoloration. The vascular system plugs up, reducing the flow of water through the roots and causing wilting.
Stressed roots
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Sterile planting mix and hydroponic growing systems prevent Verticillium infection. However, soil in the growing containers may carry the fungus. Many soil bacteria and fungi help to suppress Verticillium, so if sterile soil isn’t an option, then amending your soil with alfalfa meal or aged compost may offer some protection. Keep the planting beds well drained, as excess moisture in the root zone greatly increases the risk of Verticillium.
Properly aged compost and compost tea help protect plants from stresses.
There is no chemical control available. The best thing to do is to pasteurize the soil. A fertilizer that is low in nitrogen and high in potassium may help. Some possible biocontrols include:
Pythium species belong to a phylum (formerly known as kingdom) of organisms called oomycota, which have traits similar to both fungi and other single-celled organisms that allow them to actively move to infect hosts. Another oomycota causes downy mildew. Unlike fungi, they do not build cell wall of chitin but of cellulose. Pythium infections can be present inside seed and affect seedling germination, or they can affect post-seedling emergence, one of several soil pathogens that cause the phenomenon of “damping off.” Continuously moist or waterlogged substrate encourages growth and infection rates of this organism, which can quickly spread to nearby plants, especially if they share the same substrate.
Pythium is a destructive parasitic root disease. Under favorable conditions, Pythium multiplies very rapidly and releases microscopic mobile spores with tails that swim using a whiplash tail to search for roots to infect. It attacks mainly seeds and seedlings, which have little resistance to disease. Larger plants can be treated and saved if they are identified early, although your crop will be stunted.
Pythium is a common problem in field, container and hydroponic cultivation.
Pythium exists everywhere in the plant’s environment. It is present in your growing area, no matter how clean it is. Sanitation still helps: Pythium is often described as a “secondary infection,” because it attacks plants that have already been weakened by stress: disease, damage, pests, nutrient deficiencies or poor growing conditions. Providing a healthy growing environment helps to prevent infection.
The best conditions for Pythium development include high moisture levels in the soil, high humidity and a temperature between 70-85°F (20-30°C). Lack of oxygen (O) hydroponic nutrient solutions also helps Pythium take hold. An abundance of O lessens the chances of infection.
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Pythium generally affects the entire plant but mainly the roots and foliage. Young plants and seedlings in both soil fields and hydroponic gardens are the most susceptible.
Look for an overall yellowing of the foliage, sometimes accompanied by browning leaf edges. The plant appears wilted or stunted. The roots become discolored, soft and watery. As the infection advances, the outer part of the roots comes off, exposing a stringy inner core. In soil gardens and containers, the roots will not be visible, but Pythium often advances from the roots to the “crown” of the plant, just above the soil line, causing the same browning and softening that it produces in the roots.
Pythium moves through soil or water to the plant roots where it germinates before entering the roots. Once in the roots, it spreads through the tissue and produces resting spores, which further germinate and can infect the roots indirectly.
The best prevention is keeping the plants healthy and free of other pests that might give Pythium an opening. Use well-drained, pasteurized soil or soilless mixes in containers and avoid overfertilization and overwatering.
Water with high O levels helps keep the roots healthy and resist microorganism onslaughts.
Address pests, other diseases and nutrient deficiencies promptly, as these stresses make plants more susceptible to Pythium.
Properly aged compost, and tea made from compost, help protect plants from all sorts of fungal infections.
Use mychorrhizzae and trichoderma in planting mixes and rock wool. These fungi protect the roots and are antagonistic to pathogens.
Keep fungus gnats in check, as they can carry Pythium spores into containers with pasteurized soil. Note that one of the preventive measures for fungus gnats is to let the soil dry to a depth of about 1 inch (2-3 cm) between waterings, which reduces Pythium growth. A protective cover over the container or block prevents gnat larvae from borrowing in.
Prevention is especially important in hydroponic systems. Once Pythium infects these systems, it spreads through the water too quickly to stop. Keep your system as clean as possible to avoid infection.
An option for very thorough cleaning is to treat the water with hydrogen peroxide (H2O2) at each nutrient solution change. UVC water disinfection systems kill water-borne spores. However, these methods kill beneficial organisms in the nutrient solution as well as pathogens like Pythium. The value of this trade-off varies from one system to the next, but it is worthwhile for some growers.
Biocontrols such as those listed below are effective if applied before the plant shows any symptoms. Note that disinfection measures such as peroxide and UVC in hydroponic systems will also kill off these biocontrol agents.
Pythium presents very similar symptoms to Phytophthora root rot in plants other than cannabis. However, while Phytophthora is a common problem in many other plants, no species is known to infect cannabis. Therefore, marijuana gardeners should verify that any control they use is listed for use against Pythium. Controls that are specific for Phytophthora are not effective.
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Damping off is a condition rather than a specific disease: the rotting of seedlings underground, at the soil line or at the crown. It is caused by several seed- and soil-borne fungi, especially Rhizoctonia and Pythium. Damping off mainly affects soil growers and generally kills any seedling it affects.
Damping off is a common problem. It is often blamed on bad seeds, as it may destroy the seedlings even before they emerge from the growing medium.
Damping off occurs in warm, nitrogen-rich soil that stays wet for long periods. Even hydroponic systems can get damping off when the growing media become too saturated and the roots do not have a plentiful supply of oxygen. Oxygenating the water helps prevent infection.
The infection begins below the soil line and affects seedlings with up to eight sets of true leaves or three leaf nodes. Leaves, roots and stems can be affected.
Damping off begins as a yellowish-brown discoloration on the lower portion of the stalk. Stems have brown lesions and eventually a dark reddish-brown sunken canker. Cankers form mostly on the stems, between nodes. The third, fourth and fifth nodes are affected most frequently. As the disease progresses, the lower part of the stem becomes soft and brown. Eventually, the underdeveloped plant simply falls over.
The wilting may resemble root rot when the plant first starts to wilt and the leaves turn yellow, but stem canker has not yet appeared. The key difference from root rot is that in damping off the roots are not affected.
In the later stages of damping off in vegetative seedlings, the leaves droop and turn yellow. As lesions extend up the plant, it begins to wilt due to vascular damage (see the entry on Pythium for more information about this).
Damping off can easily be mistaken for excessive fertilization (nutrient burn), high levels of salts in the soil, nutrient solution buildup in hydroponic systems, excessive heat or cold or excessive or insufficient soil moisture. A key diagnostic sign is the brown discoloration near the soil line.
The best preventives for damping off all focus on keeping the soil from getting too wet. Although it is always possible to overwater, these measures greatly reduce damping off. Use a porous well-draining soil with large enough particles to have air spaces between them. Soggy, muddy or mucky soil or planting mixes have particles that are very fine. They hold too much water so the roots are deprived of oxygen.
Use a planting mix that includes a generous amount of larger-size particles. This creates a well-draining mix without excess water and with sufficient air for the roots.
Let the soil surface dry before watering. Use a moisture meter to test soil moisture. Testing the soil near the edge of the pot is less likely to disturb root development. The deeper the container, the more likely that lower levels have a higher moisture level than the top of the container. Make sure to use a planting mix with large-enough particles to drain well.
Place the seeds no deeper than 0.25 inch. Soil is wetter farther down.
Use sterile soil or planting mix to minimize chance of an infection.
Apply a fungicide to seeds before planting to minimize post-emergence damping off. Don’t transplant seedlings outside until they have several sets of leaves. Younger plants don’t have a robust resistance to disease.
Properly aged compost, and tea made from compost, help protect plants from all sorts of fungal infections.
Keep the grow room and tools clean, and keep the plants free of pests. This helps to prevent damping off just as it helps against other fungal diseases.
There is very little chance of saving seedlings that are struck by damping off soon after they germinate. Vegetative-stage plants with a few sets of leaves have a better chance of fighting it off with the help of a fungicide.
Catching the problem early makes your growing life easier. As with other fungal diseases, remove infected areas of the plant entirely and treat the resulting wounds with hydrogen peroxide (H2O2). If the stem canker becomes severe, then foliar feeding is a must to maintain the plants’ vigor, strength and stamina for the fight.