Soil-Borne Plant Diseases
At other times, it kills the seedling after it has formed its first true leaves, rotting it off at the soil line.

Best defense against Rhizoctonia is the use of a sterile growing medium, such as vermiculite, Per lite or sphagnum moss (florists often use steam-sterilized soil, but this is beyond the reach of most amateurs). Such treatment does not, however, completely avoid infection if tools and flats or pots are not sterilized or if they are set on dirty benches.

Seed treatment with commercially available chemicals such as Spergon, Panodrench, Arasan, Semesan and Cuprocide is so cheap that no gardener should ever plant untreated seed.

Other Damping-off Organisms: Many fungi that live in a soil on organic matter, such as saprophytes (living off dead matter), will turn parasitic when tender seedlings come in contact with them. As indicated, the answer lies in clean soil and chemical soil treatment.

Petal Blight: Here is another "classic" disease which causes tremendous damage, especially to azaleas. Several plant pathologists worked on it, but it was not until Dr. Cynthia Westcott discovered, in 1945, that Dithane-D-14 was a specific control, that it could be kept in check. Azalea petal blight is mentioned here because the resting stage survives on the soil under the plants. Here it can be killed by calcium cyanamide. Unfortunately, this is of little value except in protected, isolated gardens where a single plant is infected. The spores float for miles to infect blossoms, which turn a watery brown.

Wilt Diseases: Especially bad on carnations, wilt diseases occasionally do invade the garden if outdoor varieties are propagated in the same house as greenhouse carnations. The only answer is to destroy the plants and refrain from growing this flower for three years. In the greenhouse, an elaborate ritual of culturing will produce clean stock which can only be kept clean if hospital-like practices are followed.

Botrytis Blight: This disease is the bane of many plants, including some types of lilies; it often kills L. candidum (Madonna lily) bulbs in the soil. It is, however, spread above ground and comes within the scope of this soil disease discussion only because it moves down the stem and under the soil. Other lilies are less drastically affected. Spots are at first small purplish or brownish, fading to yellow. Leaf may finally collapse. Spraying with Bordeaux mixture is the old-time remedy. The newer Phaltan seems to work as well in many cases. When tulips are infected with Botrytis Blight (tulip fire), small yellow or brown dots enlarge and cover the entire plant, flower and all. Leaves and petals look as though scorched by fire. Infected bulbs may carry the disease. Imported Dutch bulbs are reasonably clean.