Most serious diseases of crop plants appear on a few plants in an area year after year, spread rapidly, and are difficult to cure after they have begun to develop. Therefore, almost all control methods are aimed at protecting plants from becoming diseased rather than at curing them after they have become diseased. Few infectious plant diseases can be controlled satisfactorily in the field by therapeutic means. The various control methods can be classified as regulatory, cultural, biological, physical, and chemical, depending on the nature of the agents employed. Regulatory control measures aim at excluding a pathogen from a host or from a certain geographic area. Most cultural control methods aim at helping plants avoid contact with a pathogen, creating environmental conditions unfavorable to the pathogen or avoiding favorable ones, and eradicating or reducing the amount of a pathogen in a plant, a field, or an area. Most biological and some cultural control methods aim at improving the resistance of the host or favoring microorganisms antagonistic to the pathogen. A new type of biological control involves the transfer of genetic material (DNA) into plants and the generation of transgenic plants that exhibit resistance to a certain disease(s). Finally, physical and chemical methods aim at protecting the plants from pathogen inoculum that has arrived, or is likely to arrive, or curing an infection that is already in progress
Cultural Methods of Disease Management
Host plants infected by or suspected of harboring the pathogen may have to be removed and burned. This eliminates the pathogen and prevents greater losses from the spread of the pathogen to additional plants. In some crops, e.g. potatoes, pathogens of all types may overwinter in infected tubers that are left in the field affecting successive crop. Eradication of such volunteer plants helps greatly to reduce the inoculum of these pathogens. Some other pathogens require two alternate hosts to complete their full life cycles. For example, Puccinia graminis tritici requires wheat and barberry. In this case, eradication of the wild or economically less important alternate host interrupts the life cycle of the pathogen and leads to control of the disease.
Soil borne pathogens that infect plants of one or a few species or even families of plants can sometimes be reduced in the soil by planting, for 3 or 4 years, crops belonging to species or families not attacked by the particular pathogen. Satisfactory control through crop rotation is possible with pathogens that are soil invaders, i.e., survive only on living plants or only as long as the host residue persists as a substrate for their saprophytic existence. When the pathogen is a soil inhabitant, however, i.e., produces long-lived spores or can live as a saprophyte for more than 5 or 6 years, crop rotation becomes less effective or impractical. In the latter cases, crop rotation can still reduce populations of the pathogen in the soil (e.g., Verticillium) and appreciable yields from the susceptible crop can be obtained every third or fourth year of the rotation.
Sanitation consists of all activities aimed at eliminating or reducing the amount of inoculum present in a plant, a field, or a warehouse and at preventing the spread of the pathogen to other healthy plants and plant products. Thus, plowing under infected plants after harvest, such as leftover infected fruit, stems, tubers, or leaves, helps cover the inoculum with soil and speeds up its disintegration (rotting) and concurrent destruction of most pathogens carried in or on them. Similarly, removing infected leaves of house or garden plants helps remove or reduce the inoculum. Pruning infected plants or infected or dead branches and then removing infected fruit and any other plant debris that may harbor the pathogen help reduce the inoculum and do not allow the pathogen to grow into still healthy parts of the tree.Such actions reduce the amount of disease that will develop later.
Creating Conditions unfavorable to the pathogen
Stored products should be aerated properly to hasten the drying of their surfaces and inhibit germination and infection by any fungal or bacterial pathogens present on them. Similarly, spacing plants properly in the field or greenhouse prevents the creation of high-humidity conditions on plant surfaces and inhibits infection by certain pathogens, such as Botrytis and Peronospora tabacina. Good soil drainage also reduces the number and activity of certain oomycete pathogens (e.g.,Pythium) and nematodes and may result in significant disease control. The appropriate choice of fertilizers or soil amendments may also lead to changes in the soil pH, which may unfavorably influence the development of the pathogen. Flooding fields for long periods or dry fallowing may also reduce the number of certain pathogens in the soil (e.g., Fusarium, Sclerotinia sclerotiorum, and nematodes) by inducing starvation, lack ofoxygen, or desiccation. In the production of many crops, particularly containerized nursery stock, using composted tree bark in the planting medium has resulted in the successful control of diseases caused by several soil borne pathogens, e.g., Phytophthora,Pythium, and Thielaviopsis root rots, Rhizoctoniadamping-off and crown rot, Fusarium wilt, andsome nematode diseases of several crops,
Polyethylene Traps and Mulches
Many plant viruses, such as cucumber mosaic virus, are brought into crops, such as peppers, by airborne aphid vectors. When vertical, sticky, yellow polyethylene sheets are erected along the edges of susceptible crops, a considerable number of aphids are attracted to and stick to the plastic. This is done primarily to trap and monitor incoming insects, but to some extent it also reduces the amount of virus inoculum reaching the crop. However, if reflectant aluminum or black, whitish-gray,or colored polyethylene sheets are used as mulches between the plants or rows in the field, incoming aphids,thrips, and possibly other insect vectors are repelled and misled away from the field. As a result, fewer virus carrying vectors land on the plants and fewer plants become infected with the virus. Reflectant mulches, however, cease to function as soon as the crop canopy covers them.
Biological control of Plant Diseases
Biological control of pathogens, i.e., the total or partial destruction of pathogen populations by other organisms, occurs routinely in nature. There are, for example, several diseases in which the pathogen cannot develop in certain areas either because the soil, called suppressive soil, contains microorganisms antagonistic to the pathogen or because the plant that is attacked by a pathogen has also been inoculated naturally with antagonistic microorganisms before or after the pathogen attack. Sometimes, the antagonistic microorganisms may consist of a virulent strains of the same pathogen that destroy or inhibit the development of the pathogen, as happens in hypo virulence and cross protection. Agriculturalists have increased their efforts to take advantage of such natural biological antagonisms and to develop strategies by which biological control can be used effectively against several plant diseases. Biological antagonisms, although subject to numerous ecological limitations, are expected to become an important part of the control measures employed against many more diseases.
Several soil borne pathogens, such as Fusarium oxysporum (the cause of vascular wilts), Gaeumannomyces graminis (the cause of take-all of wheat), Phytophthora cinnamomi (the cause of root rots of many fruit and forest trees), Pythium spp. (a cause of damping-off), and Heterodera avenae (the oat cyst nematode), develop well and cause severe diseases in some soils, known as conducive soils, whereas they develop much less and cause much milder diseases in other soils, known as suppressive soils. The mechanisms by which soils are suppressive to different pathogens are not always clear but may involve biotic and/or abiotic factors and may vary with the pathogen. In most cases, however, it appears that they operate primarily by the presence in such soils of one or several microorganisms antagonistic to the pathogen. Such antagonists, through the antibiotics they produce, through lytic enzymes, through competition for food, or through direct parasitizing of the pathogen, do not allow the pathogen to reach high enough populations to cause severe disease. Numerous kinds of antagonistic microorganisms have been found to increase in suppressive soils; most commonly, however, pathogen and disease suppression has been shown to be caused by fungi, such as Trichoderma,Penicillium, and Sporidesmium, or by bacteria of the genera Pseudomonas, Bacillus, and Streptomyces.Suppressive soil added to conducive soil can reduce the amount of disease by introducing microorganismsantagonistic to the pathogen. For example, soil amended with soil containing a strain of a Streptomyces species antagonistic to Streptomyces scabies, the cause of potato scab, resulted in potato tubers significantly free from potato scab.
Antagonistic Microorganisms-Soilborne Pathogens
The mycelium and resting spores (oospores) or sclerotia of several phytopathogenic soil oomycetes and fungi such as Pythium, Phytophthora, Rhizoctonia, Sclerotinia,and Sclerotium are invaded and parasitized (mycoparasitism)or are lysed (mycolysis) by several fungi, which as a rule are not pathogenic to plants. Several non-plant pathogenic oomycetes and fungi, including some chytridiomycetes and hyphomycetes, and some pseudomonad and actinomycetous bacteria infect the resting spores of several plant pathogenic fungi. Among the most common myco-parasitic fungi are Trichodermasp., mainly T. harzianum. The latter fungus has been shown to parasitize mycelia of Rhizoctonia and Sclerotium, to inhibit the growth of many oomycetes such as Pythium, Phytophthora, and other fungi, e.g., Fusarium and Heterobasidion (Fomes), and to reduce the diseases caused by most of these pathogens. Other common mycoparasitic fungi are Laetisaria arvalis (Corticium sp.), a mycoparasite and antagonist of Rhizoctonia and Pythium; also, Sporidesmium sclerotivorum, Gliocladium virens, and Coniothyrium minitants, all destructive parasites and antagonists of Sclerotinia sclerotiorum and all effectively controlling several of the Sclerotinia diseases; and Talaromyces flavus, which parasitizes Verticillium and controls Verticillium wilt of eggplant. Also, some Pythium species parasitize species of Phytophthora and other species of Pythium. Several yeasts, e.g., Pichia gulliermondii, also parasitize and inhibit the growth of plant pathogenic fungi such as Botrytis and Penicillium
.In addition to fungi, bacteria of the genera Bacillus, Enterobacter, Pseudomonas, and Pantoea have been shown to parasitize and/or inhibit the pathogenic oomycetes Phytophthora sp., Pythium sp, and the fungi Fusarium Sclerotium ceptivorum, and Gaeumannomyces tritici; the mycophagous nematode Aphelenchus avenae parasitizes Rhizoctonia and Fusarium;and the amoeba Vampyrella parasitizes the pathogenic fungi Cochliobolus sativus and Gaeumannomyces graminis. Plant pathogenic nematodes are also parasitized by other microorganisms. For example, Meloidogyne javanica and Pratylenchus sp. nematodes are parasitized by the bacterium Pasteuria (Bacillus) penetrans. Cysts of the soybean cyst nematode Heterodera glycines are parasitized by the fungus Verticillium lecanii; the root-knot nematode.
Control through Trap Plants
If a few rows of rye, corn, or other tall plants are planted around a field of beans, peppers, or squash, many of the incoming aphids carrying viruses that attack the beans, peppers, and squash will first stop and feed on the peripheral taller rows of rye or corn. Because most of the aphid-borne viruses are non persistent in the aphid, many of the aphids lose the bean-, pepper-, or squash infecting viruses by the time they move onto these crops. In this way, trap crops reduce the amount of inoculum that reaches a crop. Trap plants are also used against nematodes, although in a different way. Some plants that are not actually susceptible to certain sedentary plant-parasitic nematodes produce exudates that stimulate eggs of these nematodes to hatch. The juveniles enter these plants but are unable to develop into adults and eventually they die. Such plants are also called trap crops. By using trap crops in a crop rotation program, growers can reduce the nematode population in the soil. For example, Crotalaria plants trap the juveniles of the root-knot nematode Meloidogyne spp. and black nightshade plants (Solanum nigrum) reduce the populations of the golden nematode Heterodera rostochiensis. Similar results can be obtained by planting highly susceptible plants, which after infection by the nematodes are destroyed (plowed under) before the nematodes reach maturity and begin to reproduce. Unfortunately, trap plants have not given a sufficient degree of disease control to offset the expense and risk involved with their use. Therefore, they have been little used in the practical control of nematode diseases of plants.
The physical agents used most commonly in controlling plant diseases are temperature (high or low), dry air, unfavorable light wavelengths, and various types of radiation. With some crops, cultivation in glass or plastic greenhouses provides physical barriers to pathogens and their vectors and in that way protects the crop from some diseases. Similarly, plastic or net covering of row crops may protect the crop from infection by preventing pathogens or vectors from reaching the plants.
Control by Heat Treatment
Soil Sterilization by Heat
Soil can be sterilized in greenhouses, and sometimes in seed beds and cold frames, by the heat carried in live or aerated steam or hot water. The soil is steam sterilized either in special containers (soil sterilizers), into which steam is supplied under pressure, or on the greenhouse benches, in which case steam is piped into and is allowed to diffuse through the soil. At about 50°C, nematodes, some oomycetes, and other water molds are killed, whereas most plant pathogenic fungi and bacteria, along with some worms, slugs, and centipedes, are usually killed at temperatures between 60 and 72°C. At about 82°C, most weeds, the rest of the plant pathogenic bacteria, most plant viruses in plant debris, and most insects are killed Heat-tolerant weed seeds and some plant viruses, such as tobacco mosaic virus (TMV), are killed at or near the boiling point, i.e., between 95 and 100°C. Generally, soil sterilization is completed when the temperature in the coldest part of the soil has remained for at least 30 minutes at 82°C or above, at which temperature almost all plant pathogens in the soil are killed. Heat sterilization of soil can also be achieved by heat produced electrically rather than supplied by steam or hot water. It is important to note, however, that excessively high or prolonged high temperatures should be avoided during soil sterilization.
When clear polyethylene is placed over moist soil during sunny summer days, the temperature at the top 5 centimeters of soil may reach as high as 52°C compared to a maximum of 37°C in un mulched soil. If sunny weather continues for several days or weeks, the increased soil temperature from solar heat, known as solarization, inactivates (kills) many soil borne pathogen fungi, nematodes, and bacteria near the soil surface, thereby reducing the inoculum and the potential for disease
Hot-Water Treatment of Propagative Organs
Hot-water treatment of certain seeds, bulbs, and nursery stock is used to kill any pathogens with which they are infected or which may be present inside seed coats, bulb scales, and so on, or which may be present in external surfaces or wounds. In some diseases, seed treatment with hot water was for many years the only means of control, as in the loose smut of cereals, in which the fungus overwinters as mycelium inside the seed where it could not be reached by chemicals. Similarly, treatment of bulbs and nursery stock with hot water frees them from nematodes that may be present within them, such as Ditylenchus dipsaci in bulbs ofvarious ornamentals and Radolpholus similis in citrus rootstocks. The effectiveness of the method is based on the fact that dormant plant organs can withstand higher temperatures than those their respective pathogens can survive for a given time.
The temperature of the hot water used and the duration of the treatment vary with the different host–pathogen combinations. Thus, in the loose smut of wheat the seed is kept in hot water at 52°Cfor 11 minutes, whereas bulbs treated for D. dipsaci are kept at 43°C for 3 hours. It has been reported that a short (15 seconds) treatment of melon fruit with hot (59 ± 1°C) water rinse and brushes resulted in a significant reduction of fruit decay while maintaining fruit quality after prolonged storage. Treated fruit had less soil, dust, and fungal spores at its surface while many of its natural openings in the epidermis were partially or entirely sealed.
Hot-Air Treatment of Storage Organs
Treatment of certain storage organs with warm air(curing) removes excess moisture from their surfaces and hastens the healing of wounds, thus preventing their infection by certain weak pathogens. For example, keeping sweet potatoes at 28 to 32°C for 2 weeks helps the wounds to heal and prevents infection by Rhizopus and by soft-rotting bacteria. Also, hot-air curing of harvested ears of corn, tobacco leaves, and so on removes most moisture from them and protects them from attack by fungal and bacterial saprophytes. Similarly, dry heat treatment of barley seed at 72°C for 7 to 10 days eliminates the leaf streak- and black chaff-causing bacterium Xanthomonas campestris pv. translucens from the seed with negligible reduction of seed germination.
Disease Control by Radiation
Various types of electromagnetic radiation, such as UV light, X rays, and g rays, as well as particulate radiation, such as a particles and b particles, have been studied for their ability to control postharvest diseases of fruits and vegetables by killing the pathogens present on them. Some satisfactory results were obtained in experimental studies using g rays to control postharvest infections of peaches, strawberries, and tomatoes by some of their fungal pathogens. Unfortunately, with many of these diseases the dosage of radiation required to kill the pathogen may also injure the plant tissues on which the pathogens exist.
Disease Control by Refrigeration
Refrigeration is probably the most widely used and the most effective method of controlling postharvest diseases of fleshy plant products. Although low temperatures at or slightly above the freezing point do not kill any of the pathogens that may be on or in the plant tissues, they do inhibit or greatly retard the growth and activities of all such pathogens, thereby reducing the spread of existing infections and the initiation of new ones. Most perishable fruits and vegetables should be refrigerated as soon as possible after harvest, transported in refrigerated vehicles, and kept refrigerated until they are used by the consumer. Regular refrigeration of especially succulent fruits and vegetables is sometimes preceded by a quick hydro cooling or air cooling of these products, aimed at removing the excess heat carried in them from the field as quickly as possible to prevent the development of any new or latent infections.
Drying Stored Grains and Fruit
All grains, legumes, and nuts carry with them a variety and number of fungi and bacteria that can cause decay of these organs in the presence of sufficient moisture. Such decay, however, can be avoided if seeds and nuts are harvested when properly mature and then are allowed to dry in the air or are treated with heated air until the moisture content is reduced sufficiently (to about 12% moisture) before storage. Subsequently, they are stored under conditions of ventilation that do not allow buildup of moisture to levels (about 12%) that would allow storage fungi to become activated. Fleshy fruits, such as peaches and strawberries, should be harvested later in the day, after the dew is gone, to ensure that the fruit does not carry surface moisture with it during storage and transit, which could result in decay of the fruit by fungi and bacteria. Many fruits can also be stored dry for a long time and can be kept free of disease if they are dried sufficiently before storage and if moisture is kept below a certain level during storage. For example, grapes, plums, dates, and figs can be dried in the sun or through warm air treatment to produce raisins, prunes, and dried dates and figs, respectively, that are generally unaffected by fungi and bacteria as long as they are kept dry
Chemical pesticides are generally used to protect plant surfaces from infection or to eradicate a pathogen that has already infected a plant. A few chemical treatments, however, are aimed at eradicating or greatly reducing the inoculum before it comes in contact with the plant. They include soil treatments (such as fumigation), disinfestations of warehouses, sanitation of handling equipment, and control of insect vectors of pathogens.
Soil Treatment with Chemicals
Soil to be planted with vegetables, strawberries, ornamentals ,trees, or other high-value crops, such as tobacco, is frequently treated with chemicals for contro lprimarily of nematodes but occasionally also of soil borne fungi, such as Fusarium and Verticillium, weeds, and bacteria. Certain fungicides are applied to the soil as dusts, liquid drenches, or granules to control damping-off, seedling blights, crown and root rots, and other diseases. In fields where irrigation is possible, the fungicide is sometimes applied with the irrigation water, particularly in sprinkler irrigation. Fungicides used for soil treatments include metalaxyl, diazoben, penta-chloro-nitro-benzene (PCNB), captan, and chloroneb, although the last two are used primarily as seed treatments. Most soil treatments, however, are aimed at controlling nematodes, and the materials used are volatile gases or produce volatile gases (fumigants) that penetrate the soil throughout (fumigate). Some nematicides, however, are not volatile but, instead, dissolve in soil water and are then distributed through the soil.
The most promising method of controlling nematodes and certain other soil borne pathogens and pests in the field has been through the use of chemicals usually called fumigants. Some of them, including chloropicrin, methyl bromide, dazomet, and metam sodium, either volatilize as they are applied to the soil or decompose into gases in the soil. These materials are general purpose preplant fumigants; they are effective against a wide range of soil microorganisms, including nematodes, many fungi, insects, certain bacteria, and weeds. Contact nematicides, such as fensulfothion, carbofuran,ethoprop, and aldicarb, are of low volatility, are effective against nematodes and insects, and can be applied before and after planting of many crops that are tolerant to these chemicals.
Use of Resistant Varieties
The use of resistant varieties is the least expensive, easiest, safest, and one of the most effective means of controlling plant diseases in crops. Cultivation of resistant varieties not only eliminates losses from disease, but also eliminates expenses for sprays and other methods of disease control and avoids the addition of toxic chemicals to the environment that would otherwise be used to control plant diseases. Moreover, for many diseases, such as those caused by vascular pathogens and viruses, that often cannot be controlled adequately by other means, and for others, such as cereal rusts, powdery mildews, and root rots, that in most countries are economically impractical to control in other ways, the use of resistant varieties provides a means of producing acceptable yields without any pesticides Varieties of crops resistant to some of the most important or most difficult to control diseases are made available to growers by federal and state experiment stations and by commercial seed companies.
Quarantines and Inspections
When plant pathogens are introduced into an area in which host plants have been growing in the absence of the pathogen, such introduced pathogens may cause much more catastrophic epidemics than the existing endemic pathogens. This happens because plants that develop in the absence of a pathogen have no opportunity to select resistance factors specific against the pathogen and are, therefore, unprotected and extremely vulnerable to attack. Also, no microorganisms antagonistic or competing with the pathogen are likely to be present, while, on the other hand, the pathogen finds a large amount of available susceptible tissue on which it can feast and multiply unchecked. Plant quarantines are already credited for the interception of numerous foreign plant pathogens and, thereby, with saving the country’s plant world from potentially catastrophic diseases.
Plant quarantines are considerably less than foolproof, however, because pathogens may be introduced in the form of spores or eggs on unsuspected carriers, and latent infections of seeds and other plant propagative organs may exist even after treatment. Various steps taken by plant quarantine stations, such as growing plants under observation for certain times before they are released to the importer, repeated serological tests of seed lots (mostly through ELISA), nucleic acid tests involving DNA probes and polymerase chain reaction (PCR) amplification of specific pathogen DNA sequences, and inspection of imported nursery stock in the grower’s premises, tend to reduce the chances of introduction of harmful pathogens.
Several voluntary or compulsory inspection systems are in effect in various states in which appreciable amounts of nursery stock and potato seed tubers are produced. Growers interested in producing and selling disease-free plants submit to a voluntary inspection or indexing of their crop in the field and in storage by the state regulatory agency, experiment station personnel, or others. If, after certain procedures recommended by the inspecting agency are carried out, the plant material is found to be free of certain, usually virus, diseases, the inspecting agency issues a certificate indicating that the plants are free from these specific diseases, and the grower may then advertise and sell the plant material as disease free — at least from the diseases for which it was tested.