by Ed Rosenthal
CONTROL
+ 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
+ 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 thrives in moist soil that is rich in clay or otherwise poorly drained.
+ 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, it spreads through the xylem, which exhibits a brownish discoloration. The vascular system becomes plugged up, which reduces the flow of water through the roots and causes the wilting.
PREVENTION
+ Sterile planting mix or hydroponic growing systems prevents Verticillium infection. However any amount of 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 tea made from compost, help protect plants from all sorts of fungal infections.
+ Regular use of a 1% hydrogen peroxide solution kills many soil organisms.
CONTROL
+ There is no chemical control available. The best thing to do is pasteurize the soil. A fertilizer that is low in nitrogen and high in potassium may help. Some possible biocontrols include:
+ Bacillus subtilis (FZB24 strain)
+ Trichoderma fungus
PYTHIUM
Pythium is a destructive parasitic root fungus. Under favorable conditions Pythium multiplies very rapidly and releases microscopic spores that infect the roots and deprive the plant of food. 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° and 85° F (20 to 30° C). Lack of oxygen in hydroponic nutrient solutions also helps Pythium take hold.
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.
PREVENTION
+ 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 over-fertilization and over-watering. 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.
+ 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.
+ Prevention is especially important in hydroponic systems. Once Pythium infects these systems it may spread 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 bio-control agents.
+ Bacillus subtilis (beneficial bacteria)
+ Gliocladium (beneficial fungus)
+ Pseudomonas (beneficial bacteria)
+ Streptomyces griseoviridis (beneficial bacteria)
CONTROLS
+ 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.
+ Copper
+ Clove oil
+ Coriander oil
+ Quaternary amines
+ Sesame oil
+ Trichoderma (beneficial fungi)
STEM ROTS—DAMPING OFF
Damping off is a condition rather than a specific disease: the rotting of seedlings either underground, at the soil line, or at the crown. It is caused by several seed- and soil-born 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 grow medium.
Damping off occurs in warm, nitrogen-rich soil that stays wet for long periods. Even hydroponics systems can get damping off when the growing media become too saturated.
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 if the plant was over watered. As lesions extend up the plant, it begins to wilt due to vascular damage (see the entry on Pythium for more information on 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.
PREVENTION
+ The best preventives for damping off all focus on keeping your soil from getting too wet. Although it is always possible to over-water, these measures greatly reduce damping off.
+ Use a planting mix that includes a generous amount of perlite or vermiculite. This gives the excess water a chance to drain.
+ Make sure the soil surface is dry before watering. Use a moisture meter or a “finger test” to test soil moisture. Testing the soil near the edge of the pot is less likely to disturb root development.
+ Place the seeds no deeper than ¼ inch (6 mm). Soil is wetter further down.
+ Apply a fungicide to the seeds before planting to minimize post-emergence damping off. Don’t transplant seedlings outside until they have several sets of leaves, as younger plants don’t yet have a robust root system or resistance to disease.
+ Properly aged compost, and tea made from compost, help protect plants from all sorts of fungal infections.
+ Nitrogen-rich soils hinder root growth and aggravate damping off. Soil-less planting mixtures drain well and deter fungus growth. Using sterile soil helps greatly.
+ 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.
+ Bacillus subtilis (beneficial bacteria)
+ Gliocladium (beneficial fungus)
+ Pseudomonas (beneficial bacteria)
+ Streptomyces griseoviridis (beneficial bacteria)
+ Control
+ 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 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.
+ Copper
+ Clove oil
+ Coriander oil
+ Quaternary amines
+ Sesame oil
+ Trichoderma (beneficial fungi)
ABOUT THE AUTHOR
Ed Rosenthal has been cultivating and studying marijuana for 40 years. His “Ask Ed” marijuana advice column has been relied on by generations of marijuana growers and enthusiasts. His most recent edition of Marijuana Grower’s Handbook has revolutionized the field of marijuana cultivation with techniques and findings that were previously unknown to the general public or other so-called marijuana grow experts. Additionally, Rosenthal is well known as a marijuana legalization activist and his 2003 trial for medical marijuana cultivation is regarded as a major victory for the legalization movement.
MARIJUANA GROWER’S HANDBOOK
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