Foliar Spots and Fungi

Marijuana Horticulture

by Jorge Cervantes

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 t 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 5-10F below the daytime levels, and keep humidity more constant. Allow adequate spacing between plants to provide air circulation. Remove damaged foliage. Avid 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.