Plant diseases

Diseases of plants are classified according to the agents which cause them or according to the symptoms which mark their existence. Such ailments as winter injury, frost injury, and sunscald, which may befall any plant, are not referred to, but rather those plant sicknesses which are caused by pathogens such as fungi, bacteria, and viruses and are named for their herald symptoms, such as blight, leaf spot, rust, and root knot.

Nothing short of long, intensive training can make a plant pathologist of the home gardener. This discussion is intended merely to give him sufficient knowledge to realize the problem and to seek aid intelligently from professional diagnosticians on matters which he cannot solve himself. The information will be helpful to him in sending diseased specimens to county agricultural agents or state experiment stations and applying remedies which they prescribe.

Specimens for examination

Often when a plant does not look well, appears wilted, shows spotted leaves and drooping flowers, no visible pathogen, either insect or fungus, can be detected by the gardener. Such plants should be observed carefully and parts that appear sick, wilted, spotted, discoloured, wrinkled, or blighted should be sent to the county agent or state station for identification of the ailment.

Moisture chamber tests

Frequently the pathologist himself will not be able to identify the sickness merely by looking at the specimen. If he cannot see spore formation or a fungus or note the characteristics of some bacterium or virus he will resort to mechanical means of identification. His most valuable apparatus is a bell jar under which he places the specimen. This jar is a controlled moisture chamber, and with its controlled heat and Other factors becomes an ideal place for the development of the disease by promoting the formation of spores, spore cases, or other indications of the pathogen.

The plant doctor's job

Naturally the pathologist's microscope is of great importance to him in studying the cross section of the leaf or stem to note the internal conditions, such as whether the conducting tissue is filled with fungus growth. The microscope also reveals any fructification of a fungus growth that may be present on the specimen after a few days in the chamber.

Spots, scabs, discolorations, or extraneous growths often are so characteristic that the pathologist can quickly identify the disease without further need of test and research. Hence your sick plant must be considered as a patient in need of a doctor. The county agent or state scientist serves as physician to your plant. Often he will recommend some medication or treatment, and when the disease is severe he may advise a quarantine in the form of complete eradication of all your plants of the species of the ailing one, in order to help prevent the scourge from spreading to nearby plants or to other gardens in your neighbourhood. In this every gardener, whether amateur or professional, should always cooperate, since it may achieve the salvation of necessary crops and the complete eradication of a serious plant disease in your district.


Among the most common ailments of ornamental plants are the powdery mildews, fungus growths which develop a dirty white coating on foliage. They generally occur toward the end of summer, weaken the plants through their parasitic action, and cause the leaves to yellow and drop prematurely. As foliage ages and growth vigor declines with the advancing season, the infection finds favourable internal conditions for development, augmented by high humidity, fog, and cooler evenings. Mildew develops especially in damp, shady spots and in sunny places in which a wall or hedge impedes circulation of air.

Dusting sulphur, applied sufficiently early, is an effective counter measure. It should be put on in sufficient quantity to keep the leaves coated with it. Dusting sulphur not only prevents spread but checks the development of mildew which already has started. At the end of the season, be sure toy burn infected parts and fallen leaves.


Rust occurs on a considerable variety of plants, caused by several fungi. It appears on leaves and stems in small, reddish, rusty-looking pustules which turn black, and causes the leaves to become yellow and to fall. Dusting sulphur is prescribed for many garden plants, and where the disease is troublesome rust-resistant varieties should be obtained if available.

Spot diseases

Spot diseases, with spots on foliage and stems, are common ills, caused by fungi, bacteria, and conditions in which the plants grow. The dead spots frequently have white centres and dark, purplish edges. Bordeaux mixture is recommended against many fungi causing leaf spot. It should be applied as a preventive measure rather than after the spots are observed; once they appear the disease becomes difficult to curb. If the disease is known to exist in your neighbourhood, apply Bordeaux mixture to your healthy plants; it is a wise precaution against possible infection.

Root knot

Root knot, affecting a large list of plants, results from nematodes or eelworms which cause galls or irregular swellings on the roots. It is especially troublesome in sandy soils. Root knots may be brought in on the roots of seedlings and may remain in the soil to damage future plantings. Be sure to buy plants that are healthy and insect-free, with clean roots free of knots and swellings.


Anthracnose is a fungus disease which shows up in round leaf spots with pinkish centres and reddish-brown edges. It may live over the winter in seeds, leaves and twigs, for which no successful treatment has been found. Generally anthracnose may be controlled by spraying plants with Bordeaux mixture, applying it before the disease is indicated on your plants if it is known to exist in your district.


Mosaic is a virus disease and may be carried in seeds of diseased plants and spread by insects, especially plant lice and leafhoppers. The leaves become mottled, crinkled and dwarfed, and in severe cases plants are stunted. No effective treatment is known, and the best control measures consist in weed control, removing diseased plants, and controlling aphids and leafhoppers which spread the disease.


Blight covers a considerable catalogue of bacterial and fungus diseases causing large irregular splotches noting the destruction of plant tissues so marked. There are diseases known as late blight, early blight, bacterial blight, and so on. Spraying with Bordeaux mixture is sometimes an effective control.


Wilt-the name describes the effect of these diseases caused by bacteria and fungi which live in the soil, enter and multiply in the water vessels of plants, cutting down the water supply, causing plants to wilt, curl up, turn yellow, dry out, and die. The success of fungicidal measures is not assured but many wilt-resistant varieties of plants are available.

In addition there are numerous other diseases named for their symptoms, often lying entirely within the province of the pathologist for diagnosis and determination of treatment.

Among them are cankers; especially in woody plants, causing tissues to die and eventually to crack open; scorch, in which leaves are shrivelled by the sun; damping off, found principally in seedlings and caused by several fungi which kill root and stem tissues near the surface of the ground, with the result that the plants fail over.

Rots are caused chiefly by bacteria but sometimes by other organisms. They cause living plant cells to decompose, Galls are swellings caused frequently by insects laying their eggs within leaf or stem tissues.

Chlorosis is caused by viruses or by lack of mineral or other substances, resulting in the leaves turning yellow. Witches' brooms are caused by various fungi; the afflicted plant grows extra shoots so wildly and extensively that it takes on the weird and scraggly appearance of the brooms on which witches rode in fairy tales.

Following are some of the diseases that attack specific plants over extensive areas:

  • Aster Rust is frequent east of the Great. Plains; fusarium wilt andyellows are general in the North.
  • Azalea A gall disease in the Atlantic Coast and Gulf states; rust flower spot, Atlantic Coast states.
  • Barberry Rust caused by the funguspuccinia graminis attacks common barberry wherever the host grows and is the commongrain rust.
  • Boxwood Suffers from the leaf blight east of the Mississippi River.
  • Carnation A rust and stem rot are general.
  • Chrysanthemum Powdery mildew and rust are general; a leaf spot disease is general east of the Great Plains.
  • Clematis Powdery mildew an leaf pot disease are freq in widely scattered states; a rust is general.
  • Dahlia Powdery mildew is general.
  • Delphinium Black spot, general.
  • Dogwood Powdery mildew and a leaf spot disease are general; twig blight is frequent in the Northeast.
  • Gladiolus Stem rot widely distributed; leaf spot general wherever the host is grown.
  • Hibiscus Root knotprevalent in the South.
  • Honeysuckle Powdery mildew frequent in northern half of United States.
  • Hydrangea Rust frequent east of Mississippi River.
  • Iris A leaf spot widespread in northern two-thirds of United States; a rust is frequent on the Pacific Coast and in the blueflag iris, throughout the Northeast. Lilac Powdery mildew general.
  • Mignonette Leaf spot widespread east of Great Plains.
  • Mountainlaurel Leaf spot in Atlantic Coast states.
  • Peony Botrytis blight widespread; leaf mold general.
  • Phlox Powdery mildew general.
  • Privet Anthracnose widespread east of Great Plains.
  • Rhododendron A bud blight on Pacific Coast; a leaf sot in Connecticut, New York, New Jersey, Pennsylvania, Dela- ware, Indiana, and Washington; a hypertrophy in Pennsylvania, North Carolina, Florida,Indiana, Iowa, Washington, and Oregon.
  • Rudbeckia A rustdisease from Mississippi River to the Rockies.
  • Snapdragon Rust, general; anthracnose, general east of Mississippi River.
  • Snow-on-the-Mountain Rustdiseases in theMississippi Valley and Missouri Valley.
  • Spirea Powdery mildew frequent in New England, Ohio, Missouri, New York, Connecticut, Washington and Wyoming.
  • Sunflower Powdery mildew, rust, general; leaf spot, widespread. Sweetpea Powdery mildew, rust, general, certain rust diseases widespread.
  • Tulip Botrytis blight general.
  • Viburnum Powdery mildew widespread.
  • Virginia Creeper Black rot, leaf spot and powdery mildew general east of Great Plains.

The above list by no means represents all the plants affected by disease or even all the diseases affecting these specific plants. It presents as general only diseases reported from a large number of states in which the host plant grows or as widespread those diseases reported from a large number of states in different parts of the country.

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