Chapter 14. Soil-Borne Plant Diseases

I wish I could draw a clear-cut, one-solution picture of plant diseases; if only organocultists were right and all soil-borne diseases of plants—caused by bacteria, viruses and fungi—could be elim­inated simply by not using chemical (inorganic) fertilizers! Unfor­tunately, the answer is not that simple.

Feeding practices do have definite effects on many diseases— sometimes causing them and sometimes preventing them. In the U.S.D.A. Yearbook of Agriculture for 1953, Dr. George L. McNew cites a most interesting example of confused effects. He describes wheat grown on moderately fertile soil and fed with an extra supply of nitrogen. This special feeding helped the wheat escape seedling diseases but made it more susceptible to Pythium root rot; pro­tected it against "take all" disease but made the plants more subject to leaf rust and mildew. If, instead of an extra supply of nitrogen, extra phosphorus and potash had been used, a different series of effects and diseases would have resulted.

An application of manure to a poor soil in Arkansas might protect cotton against wilt by supplying nitrogen and potash, yet this same application of manure made on nitrogen-rich soil in the Egyptian Delta of the Nile would encourage wilt in cotton.

A Lot of Difference

A similar case of "different places-different diseases" is that of the use of sewerage sludge in California to cure a certain turf disease. But in my own work in Illinois, the snow mold disease was far more prevalent in turf fed with this same organic fertilizer.

Potato scab, a soil-borne disease, is much more severe in alkaline soils. It can be prevented by the use of fertilizers and soil treatments that bring pH so low that the scab organism cannot grow. The re­verse is true of wilt and club root diseases of cabbage, which is made worse by acid soil; the use of alkaline materials helps control these diseases. (For a more complete discussion of diseases, see Plant Disease Handbook by Cynthia Westcott, published by D. Van Nostrand Co.)

At the U.S.D.A.'s research station in Beltsville, Maryland, Drs. McClelland and Stuart proved that gladiolus diseases were much more severe when organic fertilizers were used than when only chemical plant foods were used. To cite an opposite, the use of or­ganic matter around pineapple plants in Hawaii saved them from injury by a soil fungus. The plowed-in organic matter (various waste products from sugar refining) stimulated organisms that used up excess nitrogen, thus preventing Pythium fungus from propagat­ing rapidly.

In the Diet

In all disease cases mentioned, the cause was found to be an imbalance of nutrition. It may have been too much nitrogen with too little phosphorus to balance it, or too little of all elements needed to produce a strong, healthy, disease-resistant plant, or a deficiency of some minor element. In some cases, it made little difference whether missing elements were supplied from chemical or organic sources. In others, existence of starches and sugars in organic matter provided food for fungi that caused a particular disease.

Again, an organic fertilizer might show better response because it is more complete, or contains an element missing from chemical plant foods. When no specific organism can be found that causes disease, full nutrition may be the remedy.

Gardener's Loam, with its complete supply of every element needed by plants, is the answer to many plant diseases.

FALL CLEANUP

Many garden experts talk about fall cleanup in much the same way that a dentist tells you to brush your teeth three times a day. He knows you won't take time to do so, but he's done his duty.

This cleanup recommendation has been echoed and re-echoed until it has lost most of its effect. In the past I have neglected fall cleanup, and four years out of five it made little difference in the amount of disease in my garden. In the fifth year, however, I was usually punished for my negligence, doubled and redoubled. The fact is that except for surface diseases which are carried by insects, such as aster yellows and virus diseases of some plants (carried by aphids and leaf hoppers from sources of infection outside your prop­erty), sanitation can prevent disease. The fall cleanup must not, however, be a perfunctory ritual. It calls for cutting off every stand­ing plant about a quarter of an inch below the surface, removing a spoonful of soil as well as the stems. This is tossed in a waiting wheelbarrow.

Fall is a good time to start new compost piles (see Chapter Eight), "seeding" each new pile with bacteria-rich leftover material from an old pile. This old compost forms the foundation on which fresh garden debris is laid to form the first layer of the pile, unless the autumn leaf crop has already been added. Be sure that any plant wastes that might contain disease spores or insect eggs are buried deeply in the pile: they should not be closer than 12 inches to any exposed surface, for they must be subjected to the heat of fermenta­tion. Add a good mixed fertilizer as well as some extra sugar or starch if possible (a good place to dispose of spoiled jellies, jams, wormy flour, and so on).

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.

* 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.

The French phrase for bunches infected in this way is pourriture noble, while in Germany the name for it is Edelfaule. Both phrases mean "noble rot." Wines made with these condensed grapes develop 15 to 17 per cent alcohol without fortification, yet retain natural sugars. These are the great Sauternes and Rhinewines that are so rich they are usually drunk as dessert wines. If you are fortunate enough to drink a Chateau dTTquem, Chateau La Tour Blanche or a Rhinewine labeled Spatlese, Auslese or Beerenauslese (perhaps even that rare Johannisberger Kabinett), bow your head to Botrytis cinerea as you sip 1

Fusariutn 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 1 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-ojJ 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. Spraying the shoots as they break through the ground with Fermate (four sprays, a week apart, starting when shoots are 3 inches tall) will usually give good control.

Basal Rot: A serious disease of narcissus: it is encouraged by nematodes that open a path for a Fusarium fungus. Buying bulbs from a reliable source is about the home gardener's only control.

VEGETABLES

Except for a condition known as curly top or shoe string top in tomatoes, few disease ills beset the plants in the home vegetable garden. If you are in cabbage yellows country, the resistant varieties will give you a crop in spite of the fungus. Commercial vegetable growers are plagued with a number of serious diseases, due to the concentration of vast numbers of plants of a single crop in a given area.

Clean seed is important. Buy seed in packets from a reliable source (avoid bulk seeds sold out of jars or bins). Treat with one of the commercial seed protectant chemicals before sowing.

If a good fall cleanup program is followed, clean seed sown in Gardener's Loam should give practically no trouble.

Chapter Digest

Several soil factors bear significantly on many of the most common plant diseases (caused by bacteria, fungi and viruses). The condition of the soil (pH, tilth, and so forth), and the kinds and amounts of fertilizers used, are among the major in-ground influences. Garden sanitation is an important preventive. A number of serious soil-borne plant diseases—their characteristics and some effective control measures—are discussed.

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