Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible

damping-off.
    Chemical: Dust the seeds with Captan®. Avoid benomyl fungicide soil drench because it kills beneficial organisms.
Downy Mildew
    Identify: Sometimes called false mildew, downy mildew affects vegetative and flowering plants. Young, succulent foliage is a favorite starting place. Powdery mildew develops in temperatures below 76°F (26°C).
    It appears as whitish-yellow spots on top of leaves creating pale patches. Grayish mycelium spawn is on leaf undersides, opposite the pale patches. Downy mildew can spread very quickly, causing a lack of vigor and slow growth; leaves yellow, die back, and drop. The disease is in the plant system and grows outward. It is often fatal, spreads quickly, and can wipeout a crop. Avoid promoting this disease by not crowding plants. Keep temperatures above 76°F (26°C) and the humidity below 50 percent.

A small, white spot and the beginning of rot at the soil line are the first visual signs of damping-off.

Lack of oxygen caused by overwatering impairs root development along the stem and contributes to damping-off.

Powdery mildew, fuzzy white spots signify the disease already permeates the entire plant. Climate control will prevent powdery mildew.

Foliar spots started on this leaf after it became severely nitrogen-deficient
    Control: Cleanliness! Use sterile growing medium. Remove and destroy affected plants, not just foliage.
    Biological: Apply Serenade® (Bacillus subtilis). Bordeaux mixture is also somewhat effective.
Blight
    Identify: Blight is a general term that describes many plant diseases which are caused by fungus, most often a few weeks before harvest. Signs of blight include dark, blotchy spots on foliage, slow growth, sudden yellowing, wilting, and plant death. Most blights spread quickly through large areas of plants.
    Control: Cleanliness! Use fresh, sterile growing medium. Avoid excess nitrogen fertilization. Avoid blights by keeping plants healthy with the proper nutrient balance and good drainage to prevent nutrient buildup.
    Biological: Use Serenade® (Bacillus subtilis) against Brown Blight. Use Binab®, Bio-Fungus®, RootShield®, Supresivit®, Trichopel®, (Trichoderma harzianum) or SoilGuard® (Trichoderma virens). Use a Bordeaux mixture to stop fungal blights. Stopping blights in advanced stages is difficult; the best solution is to remove diseased plants and destroy them.
Foliar Spots and Fungi
    Identify: Leaf and stem fungi, including leaf spot, attack foliage. Brown, gray, black, or yellow to white spots or blotches develop on leaves and stems. Leaves and stems discolor and develop spots that impair plant fluid flow and other life processes. Spots expand over leaves causing them to yellow and drop. Growth is slowed, harvest prolonged, and in severe cases, death results. Leaf spot is the symptomatic name given to many diseases. These diseases may be caused by bacteria, fungus, and nematodes. Spots or lesions caused by fungi often develop different colors as fruiting bodies grow. Leaf spots are often caused by cold water that was sprayed on plants under a hot HID. Temperature stress causes the spots that often develop into a disease.
    Control: Cleanliness! Use fresh, sterile growing medium with each crop. Move HIDs away from the garden canopy about 30 minutes before spraying so plants won’t be too hot. Do not spray within four hours of turning the lights off as excess moisture sits on the foliage and fosters fungal growth. Do not wet foliage when watering, avoid overwatering., and lower grow room humidity to 50 percent or less. Check the humidity both day and night. Employ dry heat to raise the nighttime temperature to 5-10°F (3-6°C) below the daytime levels, and keep humidity more constant. Allow adequate spacing between plants to provide air circulation. Remove damaged foliage. Avoid excessive nitrogen application.
    Biological: Bordeaux mixture may help keep leaf spots in check, but it is often phytotoxic when applied regularly indoors.
    Sprays: Bordeaux mixture.
Fusarium Wilt
    Identify: Fusarium wilt is most common in warm grow rooms and greenhouses. Recirculating nutrient solutions above 75°F (24°C) creates perfect conditions for Fusarium. The water and nutrient solution carries this disease with it when contaminated. Fusarium starts as small spots on older, lower leaves. Interveinal leaf chlorosis appears swiftly. Leaf tips may curl before wilting and suddenly drying to a crisp. Portions of the plant or the entire plant will wilt. The