Gray Mold

Marijuana Horticulture

by Jorge Cervantes

Identify

Gray mold is the most common fungus that attacks indoor plants and flourishes in moist temperate climates common to many grow rooms. Botrytis damage is compounded by humid (above 50 percent) climates. It starts within the bud and is difficult to see at the onset – grayish-whitish or bluish-green in color – Botrytis appears hair-like and similar to laundry lint in moist climates. As the disease progresses, the foliage turns somewhat slimy. Damage can also appear as dark, brownish spots on buds in less humid environments. Dry to the touch, Botrytis-affected area often crumbles when rubbed. Gray mold attacks countless other crops, and airborne spores are present virtually everywhere. While most commonly found attacking dense and swelling flower buds, it also attacks stems, eaves, and seeds, causes damping off, and decomposes dry, stored bud. It is also transmitted via seeds.

Damage

Watch for single leaves that mysteriously dry out on the buds. They could be the telltale signs of a Botrytis attack inside the bud. Constant observation, especially during the last two weeks before harvest, is necessary t keep this disease out of the garden. Flower buds are quickly reduced t slime in cool, humid conditions or unsmokable powder in warm, dry rooms. Botrytis can destroy an entire crop in seven to ten days if left unchecked. Stem damage – Botrytis starts on stems and not buds – is less common indoors. First, stems turn yellow and cankerous growths develop. The damage causes growth above the wound to wilt and can cause stems to fold over. Transported by air, contaminated hands, and tools, gray mold spreads very quickly indoors, infecting an entire grow room in less than a week when conditions are right.

Control

Minimize Botrytis attack incidence with low humidity (50 percent or less), ample air circulation, and ventilation. Grow varieties that do not produce heavy, tightly packed buds that provide a perfect place for this fungus to flourish. Col (below 70F), moist climates with humidity above 50 percent are perfect for rampant gray mold growth. Remove dead leaf stems, petioles, from stalks when removing damaged eaves t avoid Botrytis outbreaks, which is often harbored by dead, rotting foliage. Increase ventilation and keep humidity below 60 percent, and keep the grow room clean! Use fresh, sterile growing medium for each crop.

Cultural and physical control

As soon as Botrytis symptoms appear, use alcohol sterilized pruners to remove Botrytis infected buds at least one inch below the infected area. Some growers amputate two to four inches below damage to ensure removal. Do not let the bud r anything that touches it contaminate other buds and foliage. Remove from the garden and destroy. Wash you hands and tools after removing. Increase temperature to 80F and lower humidity to below 50 percent. Excessive nitrogen and phosphorus levels make foliage tender, so Botrytis can get a foothold. Make sure pH is around 6 to facilitate calcium uptake. Low light levels also encourage weak growth and gray mold attack. Avid heavy crowding of plants and keep the light levels bright. Botrytis needs UV light to complete its life cycle; without UV light it cannot live. Some varieties seldom fall victim to gray mold. Many crosses are more resistant to gray mold than pure indica varieties. harvest when resin glands are still translucent. Once glands turn amber, threat of gray mold increases substantially.

Biological

Spray plants with Gliocadium roseum and Trichderma species. Prevent damping off with a soil application of Gliocladium and Trichoderma species. Hemp Diseases and Pests suggests to experiment with the yeasts Pichia guilliermondii and Candida oleophila or the bacterium Pseudomonas syringae,

Sprays

Bordeaux mixture keeps early stages of Botrytis in check as long as it is present on the foliage. Preventive spraying is advised if in high risk area, but spraying buds near harvest time is not advised. Seeds are protected from Botrytis with a coating of Captan. Check with your local nursery for product recommendations.