There are a number of soil-borne diseases caused by specific organisms that can survive for years in the garden. But they are not likely to attack plants well grown in Gardener's Loam in a plot open to sun and air circulation. However, it is well to keep an eye open for them-particularly the following:
ON FLOWERING PLANTS
Stem Rust: Caused by a Phytophthera organism, it girdles the stem at ground level, after which the plant wilts and dies. Rust can be controlled in greenhouse soils only by steam sterilization of the soil. Outdoors it is not too common, which is fortunate, for there is no practical control. The classic snapdragon rust does not invade the soil; it is best controlled by growing rust-resistant varieties.
Phyllosticta Blight: Like the preceding, it girdles stems at ground level if plants are young. On older plants, cream-colored dots enlarge to dark brown or black-zoned patches. Spraying with Phaltan every 10 days during the growing season will control it. It occurs largely on outdoor snaps.
Botrytis Blight: Caused by Botrytis cinerea,* an aggressive fungus which attacks hundreds of different species of plants. It is the cause of "tulip fire" and browned, shriveled peony buds, to name just two of the familiar injuries (see later entries in this chapter). Burn infected plants. Spray with a modern fungicide.
Fusarium Wilt: This is one of the "classic" diseases (so called by plant pathologists), well known, widely distributed and difficult to control. It will survive for years and infest asters, chrysanthemums and many other plant species. Seedlings damp off and at this stage the infection is hard to tell from Rhizoctonia. Mature plants may wilt suddenly. There are so-called wilt-resistant varieties but, in my experience, these are of little value in the Middle West and South because soil temperatures go too high. In California and other sections where nights are cool, wilt-resistant varieties do well.
Rhizoctonia solani: This is a confusing fungus because it has many forms, often called by different names in the perfect and imperfect stages. It is one of the major causes of damping-off of seedlings. Seeds may be attacked before they sprout. I have, for example, seen a row of mixed sweet peas', in which Rhizoctonia killed off all white-seeded forms but left black-seeded varieties intact
* Vintners in Germany and France, far from fearing Botrytis cinerea as a disease, welcome its presence on their ripe grapes. Infested grapes turn a hoary gray as the fungus creeps across the berries. Before they are harvested, they lose more than half their moisture and begin to shrivel. Vineyard owners prize these moldy berries above all the rest of their harvest. For some strange reason, although the mycelium penetrates the skin, it does not change the flavor. It condenses the juice until instead of the usual 15 to 25 per cent of sugar, the juice has become so sweet that readings of 55 to 60 per cent sugar are not unusual.