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.8

Control measures-Continued.

.82

Fungicides Continued.

.824 Sulphur (other than spraying).
.825 Mercuric preparations.

.8251 Corrosive sublimate.

Organic mercury compounds.

Miscellaneous.

.8252

.8259

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PLANT DISEASES

GENERAL

Nature of Plant Diseases

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Disease Defined. In speaking of plant diseases one author has said, Health is a state in which each organ performs its own function and acts in harmony with the others; and disease, in the broader sense of the word, consists of any departure from this state." Plant diseases belong to two distinct groups: (1) Parasitic and (2) nonparasitic. These two groups differ as to cause and therefore require different methods of control. (1), (2), (3), (4), (9).1

Parasitic Diseases. Due to the attack of organisms which live on the higher plants and produce varying degrees of disease and disturbances, oftentimes finally resulting in the death of the plant or of some part of it. Most of the troubles in this group are caused by parasitic fungi or the related group, bacteria. In addition to these, there is an important group of infectious diseases, known as the virus troubles, with which no organisms have yet been definitely associated in a causal relation. Nemas, or microscopic animal parasites, and parasitic and epiphytic lichens belong in this group. Phytoptus and other mites, gall-producing and many puncturing insects cause disturbances which are essentially diseases. In Europe these are commonly classed as diseases and often studied by plant pathologists, but in this country it is customary to class all insect and mite injuries along with the other insect depredations on plants, and this whole group of pests is studied by entomologists. Insects often distribute bacteria, fungi, and infectious viruses, sometimes actually inoculating plants, and are thus instrumental in disseminating diseases which they do not directly produce.

Nonparasitic Diseases.-Sometimes are spoken of also as physiological or environmental diseases. They are the direct effect, sometimes cumulative, of unfavorable extremes of temperature and moisture, both in the soil and in the air, of light and of poisonous or unfavorable gases in the air and soil, of unbalanced chemical constituents of the soil, including actual poisons, and of unfavorable mechanical conditions of the soil, including nonaeration. Even unbalanced proportions of plant foods in the soil, which in proper relation might be beneficial, may produce disease. These environmental factors, which are capable of producing disease independently, also affect profoundly the severity of attack of parasitic diseases. Diseases of this group often produce definite symptoms and sometimes definite lesions by which they can be identified fairly accurately, usually more by external characters than by results of microscopic examination. They often are difficult to recognize in the laboratory, especially the milder cases, and always are easier to diagnose in the field. On the other hand, they often are difficult or impossible to diagnose in the absence of full knowledge of all the environmental factors, past and present.

1 Figures refer to bibliography at end of this section.

U.S. D. A.-10-1-26

Losses Resulting from Discase.-Some plant diseases cause serious reductions of yield, often completely destroying the crop in an individual field or orchard. Other diseases not only cut production, but may cause blemishing of the fruit, either making it unmarketable or throwing it into a low grade. Nailhead spot of tomatoes, apple scab, and potato scab are examples. Still others, as brown rot of peaches, potato late blight, and sweetpotato black rot, not only reduce the yield but so affect the harvested product as to cause decay in storage and transit. To wastage caused by this type of disease must be added destruction by still another group of diseases, which, although of little or no importance in the field, may be responsible for extensive losses during the marketing period. For example, Rhizopus rot of sweet potato, though not recognized as a field disease, may be very destructive after harvest.

Control.-Prevention plays an important part in control of parasitic diseases. At present spraying with fungicides constitutes an important method of attack, though use of proper cultural methods, disease-free seed, disinfection, eradication of host plants, and breeding of resistant sorts play important parts. Quarantine methods also have an important function in keeping out or preventing the spread of this type of disease. For these troubles the removal, eradication, or correction by various methods of the unfavorable physical factors constitute the principal methods of control. When there is any doubt as to the identity of a disease, representative material should be collected, prepared for mailing, and sent either to the State experiment station or to the Bureau of Plant Industry, United States Department of Agriculture, Washington, D. C.

ALKALOIDAL AND MEDICINAL PLANTS

Alkaloidal Plants

Tobacco (10)-Black shank (Phytophthora nicotiana). -Found in Florida and Georgia in seed beds and fields. Primarily a stalk disease. Affects leaves and roots and also causes wilting and death.

Control: Avoid Infested fields or seed beds. Transplant only healthy plants. Remove and destroy earliest infections. Promise of control by use of resistant strains.

Black root rot (Thielavia basicola).-General. Stunting of plants, blackened and deficient root system. Favored by cool soils. Occurs in seed beds and fields.

Control: Avoid infested soils or use resistant strains. Disinfect soil for seed beds.

Black rot (Aspergillus niger).-Found in fermenting tobacco. Causes decay and blackens the leaf.

Control: Do not pack tobacco too moist. fermentation if necessary.

Black fire (Bacterium angulatum).-General. leaf spot, usually without halo. dark brown or almost black.

Use bulk

Angular

Lesion light colored to

Control: Seed-bed sanitation. Avoid transplanting dis eased seedlings to field.

Blue mold (Peronospora hyoscyami).-Florida. Large. angular blotches on leaves and mildew on under surface. Control: Seed-bed sanitation. Destroy infected plants and spray beds with Bordeaux mixture (2-2-50).

Broom rape (Orobanche ramosa).-Most prevalent in Kentucky. Damage usually small, if any. Yellowish, compact growth of parasite on roots usually extending above ground.

Control: Avoid infested soils. Rotate crops, using immune plants.

Brown root rot.-Cause not determined. Occurs particularly in Connecticut Valley. Causes plants to become stunted and roots brown, decayed, and deficient.

Control: Satisfactory control not fully worked out. Probably dependent on system of rotation used.

80

Damping off (Pythium debaryanum, Rhizoctonia lani, and other organisms). Most prevalent in the North., Found in plant beds. Causes decay of stems, often entering the leaves, usually forming a slimy rot of plants in round areas.

Control: Soil sterilization. Thin sowing. the beds.

Frenching-General.

Ventilate

Noninfectious. Cause not determined. A chlorotic appearance with increasing thickness of leaf in early stages. An abnormally high number of narrow leaves is most characteristic symptom. Control: Avoid planting on areas known to yield high percentages of disease.

Frog-eye (Cercospora

nicotiana).-Southern

States,

Circular leaf spot with grayish-white center and darkbrown border. Generally on bottom leaves only. Control: Not satisfactorily worked out.

Fusarium wilt (Fusarium oxysporum var. nicotiana). Southern States. Wilting of plant, vascular ring discolored. No ooze on pressure of stem.

Control: Avoid infested soils. Varieties differ in resistance to disease..

12623--27- -51

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U. S. D. A.-10—1-26

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