Symptoms and signs
Bacterial diseases can be grouped into four broad categories based on the extent of damage to plant tissue and the symptoms that they cause, which may include vascular wilt, necrosis, soft rot, and tumours. Vascular wilt results from the bacterial invasion of the plant’s vascular system. The subsequent multiplication and blockage prevents movement (translocation) of water and nutrients through the xylem of the host plant. Drooping, wilting, or death of the aerial plant structure may occur; examples include bacterial wilt of sweet corn, alfalfa, tobacco, tomato, and cucurbits (e.g., squash, pumpkin, and cucumber) and black rot of crucifers. Pathogens can cause necrosis by secreting a toxin (poison). Symptoms include formation of leaf spots, stem blights, or cankers. Soft rot diseases are caused by pathogens that secrete enzymes capable of decomposing cell wall structures, thereby destroying the texture of plant tissue—i.e., the plant tissue becomes macerated (soft and watery). Soft rots commonly occur on fleshy vegetables such as potato, carrot, eggplant, squash, and tomato. Tumour diseases are caused by bacteria that stimulate uncontrolled multiplication of plant cells, resulting in the formation of abnormally large structures.
Most bacteria produce one major symptom, but a few produce a range or combination of symptoms. In general, it is not particularly difficult to tell whether a plant is affected by a bacterial pathogen; however, identification of the causative agent at the species level requires isolation and characterization of the pathogen using numerous laboratory techniques ().
Transmission and infection
In order for a bacterium to produce a disease in a plant, the bacterium must first invade the plant tissue and multiply. Bacterial pathogens enter plants through wounds, principally produced by adverse weather conditions, humans, tools and machinery, insects, and nematodes, or through natural openings such as stomata, lenticels, hydathodes, nectar-producing glands, and leaf scars.
Most foliage invaders are spread from plant to plant by windblown rain or dust. Humans disseminate bacteria through cultivation, grafting, pruning, and transporting diseased plant material. Animals, including insects and mites, are other common transmission agents. Some bacteria, such as the causal agent of Stewart’s, or bacterial, wilt of corn (Erwinia stewartii), not only are spread by a flea beetle but also survive over winter in this insect.
When conditions are unfavourable for growth and multiplication, bacteria remain dormant on or inside plant tissue. Some, such as the crown gall bacterium, may survive for months or years in the soil.
Bacterial diseases are influenced greatly by temperature and moisture. Often, a difference of only a few degrees in temperature determines whether a bacterial disease will develop. In most cases, moisture as a water film on plant surfaces is essential for establishing an infection.
Control
In general, the diseases caused by bacteria are relatively difficult to control. This is partly attributable to the speed of invasion as bacteria enter natural openings or wounds directly. Direct introduction also enables them to escape the toxic effects of chemical protectants. Losses from bacterial diseases are reduced by the use of pathogen-free seed grown in arid regions. Examples of diseases controlled by this method include bacterial blights of beans and peas, black rot of crucifers, and bacterial spot and canker of tomato. Seed treatment with hot water at about 50 °C (120 °F) is also effective for crucifers, cucurbits, carrot, eggplant, pepper, and tomato. Bactericidal seed compounds control some bacterial diseases, such as angular leaf spot of cotton, gladiolus scab, and soft rot of ornamentals. Rotation with nonhost crops reduces losses caused by wilt of alfalfa, blights of beans and peas, black rot of crucifers, crown gall, and bacterial spot and canker of tomato. Eradication and exclusion of host plants has been useful against citrus canker, angular leaf spot of cotton, fire blight, and crown gall. Resistant varieties of crop plants have been developed to reduce losses from wilts of alfalfa, corn, and tobacco; angular leaf spot of cotton and tobacco; and bacterial pustule of soybeans, among others. Protective insecticidal sprays help control bacterial diseases, such as wilts of sweet corn and cucurbits and soft rot of iris. Protective bactericidal sprays, paints, or drenches containing copper or antibiotics are used against bacterial blights of beans and celery, fire blight, crown gall, blackleg of delphinium, and hazelnut and walnut blights. Finally, sanitary measures—i.e., clean plow down of crop refuse, destruction of volunteer plants and weeds, sterilization of pruning and grafting tools—as well as refraining from cultivating when foliage is wet, overhead watering and spraying of indoor plants, and late cutting or grazing of alfalfa and other crops, are useful in reducing the incidence of bacterial diseases.
The characteristics of several plant diseases caused by bacteria are summarized in the table.
| Some bacterial diseases of plants | ||||
| disease | causative agent | hosts | symptoms and signs | additional features |
| Granville wilt | Pseudomonas solanacearum | tobacco, tomato, potato, eggplant, pepper, and other plants | stunting, yellowing, and wilting of parts above ground; roots decay and become black or brown | occurs in most countries in temperate and semitropical zones; causes crop losses of hundreds of millions of dollars |
| fire blight | Erwinia amylovora | apple and pear | blossoms appear water-soaked and shrivel; spreads to leaves and stems, causing rapid dieback | first plant disease proved to be caused by a bacterium |
| wildfire of tobacco | Pseudomonas syringae | tobacco | yellowish green spots on leaves | wildfire of tobacco occurs worldwide; causes losses in seedlings and field plants |
| blight of beans | Xanthomonas campestris | beans (common blight) | yellowish green spots on leaves | most phytopathogenic xanthomonads and pseudomonads cause necrotic spots on green parts of susceptible hosts; may be localized or systemic |
| Pseudomonas syringae | beans (brown spot) | small water-soaked spots on lower side of leaves enlarge, coalesce, and become necrotic | ||
| soft rot | Erwinia carotovora | many fleshy-tissue fruits—e.g., cabbage, carrot, celery, onion | soft decay of fleshy tissues that become mushy and soft | occurs worldwide; causes major economic losses |
| crown gall | Agrobacterium tumefaciens | more than 100 genera of woody and herbaceous plants | initially a small enlargement of stems or roots usually at or near the soil line, increasing in size, becoming wrinkled, and turning brown to black | the conversion of a normal cell to one that produces excessive cell multiplication is caused by a plasmid (a small circular piece of DNA) carried by the pathogenic bacterium |
| aster yellows | Mycoplasma-like organism (MLO) | many vegetables, ornamentals, and weeds | chlorosis; dwarfing malformations | greatest losses suffered by carrots; transmission by leafhoppers |
| citrus stubborn disease | Spiroplasma citri (MLO) | citrus and stone fruits and vegetables | chlorosis, yellowing of leaves, shortened internodes, wilting | first MLO pathogen of plant disease cultured |
Diseases caused by fungi
Fungi cause the great majority, an estimated two-thirds, of infectious plant diseases. They include all white and true rusts, smuts, needle casts, leaf curls, mildew, sooty molds, and anthracnoses; most leaf, fruit, and flower spots; cankers; blights; scabs, root, stem, fruit, and wood rots; wilts; leaf, shoot, and bud galls; and many others. All economically important plants apparently are attacked by one or more fungi; often many different fungi may cause disease in one plant species.
General characteristics
The fungi represent an extremely large and diverse group of eukaryotic microorganisms. The cells, which contain a membrane-bound nucleus, are devoid of chlorophyll and have rigid cell walls. Fungi often have a plantlike vegetative body consisting of microscopic branching threadlike filaments of various lengths, called hyphae (singular hypha), some of which extend into the air while others penetrate the substrate on which they grow. The hyphae are arranged into a network called a mycelium. It is the mass of the mycelium that gives fungal growth its characteristic “cottony” or “fuzzy” appearance. Fungi reproduce by a variety of methods, both asexual and sexual. They produce many kinds of spores in very large numbers. For example, the colour of a moldy piece of bread is due to the colour of a massive number of microscopic mold spores.
Symptoms and signs
In general, a fungal infection can cause local or extensive necrosis. It can also inhibit normal growth (hypotrophy) or induce excessive abnormal growth (hypertrophy or hyperplasia) in a portion of or throughout an entire plant. Symptoms associated with necrosis include leaf spots, blight, scab, rots, damping-off, anthracnose, dieback, and canker. Symptoms associated with hyperplasia include clubroot, galls, warts, and leaf curls.
In some instances, the fungus infecting the plant may produce growth or structures on the plant, stems, or leaves such as masses of mycelium or aggregates of spores with a characteristic appearance. These developments are referred to as signs of infection, in contrast to symptoms, which refer specifically to the plant or plant tissue.
Transmission
Fungi are spread primarily by spores, which are produced in abundance. The spores can be carried and disseminated by wind currents, water (splashing and rain), soil (dust), insects, birds, and the remains of plants that once were infected. Vegetative fungal cells that exist in dead plant material also can be transmitted when they come in contact with a susceptible host. The survival of vegetative cells of plant pathogenic fungi in nature depends on climatic conditions, particularly temperature and moisture. Vegetative cells can survive temperatures from −5 to 45 °C (23 to 113 °F); fungal spores are considerably more resistant. The germination of spores, however, is favoured by mild temperatures and high humidity.
Control
Because many thousands of fungal species can infect a broad range of plants and because each fungal species has different characteristics, a variety of practices are available to control fungal diseases. The principal control measures include the use of disease-free seed and propagating stock, the destruction of all plant materials that may harbour pathogenic fungi, crop rotation, the development and use of resistant plant varieties, and the use of chemical and biological fungicides.
Several fungal diseases are characterized in the table.
| Some fungal diseases of plants | ||||
| disease | causative agent | hosts | symptoms and signs | additional features |
| late blight of potato | Phytophthora infestans | potato | water-soaked dark green to black or purplish lesions with pale green margins on lower leaves, white mildew at edge of lesions | responsible for Irish famine; caused starvation and death and mass migration of population |
| chestnut blight | Endothia parasitica | chestnut tree | yellowish to reddish brown patches appear on bark; lesions spread quickly and girdle twigs or limbs, which die | disease accidentally imported from Asia; first observed in New York in 1904 and rapidly spread across the United States, practically eliminating native American chestnuts |
| Dutch elm disease | Ceratocystis ulmi | elm tree | leaves wilt, turn dull green to yellow or brown, and drop off; branches die | the causative fungus is believed to have entered Europe from Asia during World War I and was later transported to the United States (1930) on elm burl logs imported for furniture veneer; elm bark beetles spread the pathogen in the United States |
| black stem rust of wheat | Puccinia graminis | wheat; many grasses | on wheat, rust-coloured pustules with spores, chlorosis of surrounding tissue, followed by development of black teliospores; on barberry, chlorosis and hypertrophy of infected tissue, orange spore masses | disease occurs wherever wheat is grown; in 1935 it destroyed about 60 percent of the total hard red spring wheat crop in Minnesota and South Dakota; fungus has a complex life cycle, partly on wheat and partly on the barberry plant; eradication of the barberry plant is an important control measure |
| coffee rust | Hemileia vastatrix | coffee | orange-yellow powdery spots on lower side of leaves; centres turn brown and leaves fall | most destructive disease of coffee; has caused devastating losses in all coffee-producing countries |
| white-pine blister rust | Cronartium ribicola | white pine tree | small, discoloured, spindle-shaped cankers surrounded by narrow band of yellow-orange bark; blisters exude secretion followed by bright orange pustules | one of the most important forest diseases in the United States; currant is the alternate host, and its eradication is an important control measure |
| corn smut | Ustilago maydis | corn | minute galls form on young corn seedlings; on older plants, large galls are produced on the silk of ears and on tassels, leaves, and stalks | occurs wherever corn is grown; may cause serious crop damage |
| loose smut | Ustilago nuda | barley, oats, wheat | infected heads are covered with masses of olive-green spores | worldwide occurrence; destroys kernels of the infected plant |
| downy mildew | many species of the family Peronosporaceae | many types of plants: grapes, grasses, vegetables, and others | yellow irregular spots appear on upper leaf surface; downy fungus growth appears on underside; leaves die | one of the first plant diseases controlled by a fungicide—i.e., Bordeaux mixture, a mixture of lime and copper sulfate used on grapes |
| powdery mildew | many species of the family Erysiphaceae | many types of plants: grasses, vegetables, shrubs, and trees | spots of powdery mildew growth that enlarge to cover leaves or other plant organs | one of the most common and widely spread plant diseases |
| apple scab | Venturia inaequalis | apple | small olive-coloured areas appear on young leaves, later turn black, and may coalesce; black circular spots appear on fruit | occurs almost everywhere apples are grown; infection reduces fruit size and quality |
| black spot of rose | Diplocarpon rosae | rose | large circular black lesions on leaves; leaves turn yellow and fall off | classified as an anthracnose, which affects leaves, stems, and fruits of many plants |
| anthracnose of grape | Elsinae ampelina | grape | (as above) | (as above) |
| nectria canker | Nectria galligena | apple and pear and many hardwood forest trees | initially small circular brown areas that enlarge and become depressed with raised edges; callus tissue produced around canker | one of the most important diseases of pear, apple, and hardwood forest trees |
| black knot of plum and cherry | Plowrightia morbosum | plum and cherry | small black knotty swellings on twigs and branches | occurs primarily in the eastern half of the United States and New Zealand |
| brown rot | Monilinia fructicola | stone fruits | brown spots on blossoms; twigs develop small sunken brown cankers; fruit develops brown spots that spread rapidly | worldwide occurrence; can cause heavy losses both in orchards and in shipment |
| soft rot | Rhizopus species | flowers, fruits, and vegetables with fleshy organs | tissues become soft with water-soaked appearance that often spreads rapidly, followed by development of fuzzy gray mycelium and black spores | infection develops most rapidly on ripe fruits with favourable conditions (moderate temperature and high humidity) |
| fusarium wilt of tomato | Fusarium oxysporum | tomatoes | leaves are bent down, growth is stunted, plant dies; dark streaks appear in vascular tissue | one of the most destructive diseases of tomato; entire fields can be destroyed |
| wilts of vegetables, flowers, and some trees | Verticillium species | cotton, potato, tomato, alfalfa, shade trees, and others | similar to fusarium wilts; develops primarily in seedlings that die shortly after infection; older plants also are attacked | worldwide distribution; the fungus infects hundreds of species of plants |
Diseases caused by nematodes
Nematodes parasitic on plants are active, slender, unsegmented roundworms (also called nemas or eelworms). The great majority cannot be seen with the unaided eye, because they are very small and translucent. Practically all adult forms fall within the range of 0.25 to 2 millimetres in length. About 1,200 species cause disease in plants. Probably every form of plant life is fed upon by at least one species of nematode. They usually live in soil and attack small roots, but some species inhabit and feed in bulbs, buds, stems, leaves, or flowers.
Mode of nematode attack
Nematodes parasitic on plants obtain food by sucking juices from them. Feeding is accomplished through a hollow, needlelike mouthpart called a spear or stylet. The nematode pushes the stylet into plant cells and injects a liquid containing enzymes, which digest plant cell contents. The liquefied contents are then sucked back into the nematode’s digestive tract through the stylet. Nematode feeding lowers natural resistance, reduces vigour and yield of plants, and affords easy entrance for wilt-producing or root rot-producing fungi or bacteria and other nematodes. Nematode-infested plants are weak and often appear to suffer from drought, excessive soil moisture, sunburn or frost, a mineral deficiency or imbalance, insect injury to roots or stems, or disease.
Common symptoms of nematode injury include stunting, loss of green colour and yellowing; dieback of twigs and shoots; slow general decline; wilting on hot, bright days; and lack of response to water and fertilizer. Feeder root systems are reduced; they may be stubby or excessively branched, often discoloured, and decayed. Winterkill of orchard trees, raspberries, strawberries, ornamentals, and other perennials is commonly associated with nematode infestations.
Root injury develops partly from the nematodes feeding on cells and partly from toxic salivary excretions of the parasite. Tissues often respond by producing either an enlargement or degeneration of cells; sometimes both occur.
Many nematodes are native and attack cultivated plants when their natural hosts are removed. Others have been introduced with seedling plants, bulbs, tubers, and particularly in soil balled around roots of infested nursery stock.
Nematodes may live part of the time free in soil around roots or in fallow gardens and fields. They tunnel inside plant tissues (endoparasites) or feed externally from the surface (ectoparasites) and may enter a plant through wounds or natural openings or by penetrating roots. All nematodes parasitic on plants require living plant tissues for reproduction. Nematodes are attracted to host roots by sensing either the heat given off by roots or the chemicals secreted by roots.
Most species require 20 to 60 days to complete a generation from egg through four larval stages to adult and back to egg. Some nematodes have only one generation a year but still produce several hundred offspring.
Soil populations and developmental rate of nematodes are affected by the length of the growing season; temperature; availability of water and nutrients; and moisture, type, texture, and structure of soil. Also important are populations of nematode-parasitic bacteria, viruses, some 50 different nematode-trapping fungi, protozoans, mites, flatworms, or other pests, and other nematodes. Toxic chemicals added to the soil or those secreted by plant roots; crop rotations and past cropping history; species, variety, age and nutrition of growing plants; and other factors are additional conditions that affect nematode populations.
Certain species live strictly in light, sandy soils; some build up high populations in muck soils; and a few seem to thrive in heavy soils. High populations and greater crop damage are much more common in light sandy soils than in heavy clay soils.
Many plant-infecting nematodes become inactive at temperatures between 5 and 15 °C (41 and 59 °F) and 30 and 40 °C (86 and 104 °F). The optimum for most is 20 to 30 °C (68 to 86 °F), but this varies greatly with the species, stage of development, activity, growth of the host, and other factors.
Nematodes may be found in plant tissues in large numbers. Hundreds of thousands may be present in infested roots or bulbs.
After a plant-infecting nematode has been accidentally introduced into a garden or field, several years pass before the population builds up sufficiently (i.e., up to several billion or more active nematodes per hectare) to cause conspicuous symptoms in a large number of plants. This is because nematodes move very slowly through soil—rarely more than 75 cm (29.5 inches) a year. Nematodes are easily spread, however, by moving infested soil, plant parts, or contaminated objects—e.g., tools and machinery, bags and other containers, running water, wind, clothing, shoes, animals, birds, and infested planting stock.
