16/11/2024
#6 Fundamental principles of plant disease control
Management of a plant disease means reduction in the amount of damage caused. It is estimated that the U.S. loses four billion dollars annually due to plant diseases. Complete control is rare, but profitable control, when the increased yield more than covers the cost of disease management, is quite possible. Commercial growers now average a return of four dollars for each dollar invested. The six fundamental principles of disease management are exclusion, eradication, protection, resistance, therapy, and avoidance of insect vectors and w**d hosts.
1. Exclusion means preventing the entrance and establishment of pathogens in uninfested crops in a particular area. It means using certified seed or plants, sorting bulbs before planting, discarding any that are doubtful, possibly treating seeds, tubers or corms before they are planted, and most especially, refusing obviously diseased specimens from dealers. For example, tare soil returned to trucks at sugarbeet dump stations should never be returned to production fields because of contamination by nematode and rhizomania diseases from other infested fields.
In order to prevent the import and spread of plant pathogens into the country or individual states, certain federal and state laws regulate the conditions under which certain crops may be grown and distributed between states and countries. Such regulatory control is applied by means of quarantines, inspections of plants in the field or warehouse, and occasionally by voluntary or compulsory eradication of certain host plants. Plant quarantines are carried out by experienced inspectors, stationed in all points of entry into the country, to stop persons or produce likely to introduce new pathogens. Similar quarantine regulations govern the interstate, and even the intrastate, sale of nursery stock, tubers, bulbs, seeds, and other propagative organs, especially of certain crops, such as potatoes and fruit trees. For example, a Michigan quarantine prohibits the entry of seed potatoes produced in regions infested with rhizomania disease of sugar beet unless accompanied by a certificate indicating the production field has tested free of the disease.
2. Eradication involves the elimination of a pathogen once it has become established on a plant or in a field. It can be accomplished by removal of diseased plants, or parts, as in roguing to control virus diseases or cutting off a cankered tree limb; by cultivating to keep down w**d hosts and deep ploughing or spading to bury diseased plant debris; by rotation of susceptible with nonsusceptible crops to starve out the pathogen; and by disinfection, usually by chemicals, sometimes by heat treatment. Spraying or dusting foliage with sulfur after mildew mycelium is present is eradication, and so is treating the soil with chloropicrin to kill nematodes and fungi. Soil treatment with various nematicides (Telone II, Temik 15G, Counter 15 and 20G) is useful to control sugar beet nematodes.
Tan spot, caused by the fungus Pyrenophora triticirepentis, is a major leaf spot disease of winter wheat in the Great Plains of North America. It has become an increasing problem in wheat cropping systems using conservation tillage. This disease can be managed by applying a three-year conservation tillage rotation system called ecofallow. Ecofallow is defined as crop rotation system of controlling w**ds and conserving soil moisture with minimum disturbance of crop residue. In this system, corn or sorghum is seeded directly into winter wheat stubble in a winter wheat-grain sorghum/corn-fallow rotation. The uniqueness of this system is that one crop is planted directly into the residue of a different crop rather than into the residue of the same crop. This crop rotation-fallow system effectively breaks disease cycles, such as tan spot, which involve pathogens that survive in crop residue.
3. Protection is the use of some protective barrier between the susceptible part of the suspect or host and the pathogen. In most cases this is a protective spray or dust applied to the plant in advance of the arrival of the fungus spores; sometimes it means killing insects or other inoculating agents; sometimes it means the er****on of a windbreak or other mechanical barrier.
Fungicidal sprays that act as protectants are used to control Cercospora leaf spot of sugar beet, especially in those fields where inoculum has carried over from the previous year. The principle of protective fungicides is to disrupt the natural sequence of infection. These fungicides act on the leaf surface to kill the newly germinated spores. Flowable sulfur is used as a protectant fungicide to control powdery mildew of sugar beet.
There is a long list of chemicals available in the literature that can be used in present-day protective spraying and dusting, along with eradicant chemicals. The commercially sold chemicals are provided with instructions or notes on compatibility and possibilities of injury. A commercial grower can do his plants irreparable harm instead of the good he intends if he doesn't follow the instructions supplied. Spraying is never to be undertaken lightly or thoughtlessly. Read all of the fine print on the label; be sure of the dosage and the safety of that particular chemical on the plant species to be protected.
4. Disease-resistant and tolerant varieties are the cheapest, easiest, and most efficient way to reduce disease losses. Varieties should be selected that possess resistance or tolerance to one or more disease organisms. For some diseases, such as the soilborne vascular wilts and the viruses, the use of resistant varieties is the only means of ensuring control. Certified seed of resistant varieties is available and sold commercially. The use of varieties of plants resistant to particular diseases has proved to be very effective, i.e., stem rust of wheat, rust of dry bean, and Rhizoctonia root rot of sugar beet. Most plant breeding is done for the development of varieties that produce greater yields of better quality. When such varieties become available, they are then tested for resistance against some of the most important pathogens present in the area where the variety is developed and where it is expected to be cultivated. If the variety is resistant to these pathogens for that area, it may be released to the growers for immediate production. If, however, it is susceptible to one or more of these pathogens, the variety is usually discarded, or sometimes it is released for production if the pathogen can be controlled by other means, e.g., chemical, but more often it is subjected to further breeding in an attempt to incorporate into the variety genes that would make it resistant to pathogens without changing any of its desirable characteristics.
There are degrees of resistance to certain diseases, some varieties being completely immune, others partially susceptible. Resistant varieties may become susceptible to new races of a pathogen, i.e., dry bean varieties Beryl and Olathe were resistant to rust races present at the time of their release, but are now susceptible to new rust races.
5. Therapy is used on individual plants and can't be used on a large scale. It is achieved by inoculating or treating the plant with something that will inactivate the pathogen. Chemotherapy is the use of chemicals to inactivate the pathogen, whereas heat is sometimes used to inactivate or inhibit virus development in infected plant tissues so that newly developing tissue may be obtained which is free of pathogen. Thermotherapy involves the exposure of diseased plants or parts of them to hot water or high air temperatures for different periods of time. Loose s**t of wheat is controlled by treating the seeds with hot water, but modern resistant varieties are a simpler method of control. Hot water treatment has been used to kill nematodes in bulbs, corms, tubers, and fleshy roots while they are in a dormant condition. Dormant chrysanthemum stools can be rid of foliar nematodes by submerging in water at 112°F (44°C) for 30 minutes.
6. Control of insect vectors and w**d hosts: certain insects, especially aphids, beetles and leafhoppers, are known to transmit viruses and mycoplasmas from infected plants to healthy plants. Perennial w**ds, including pokew**d, milkw**d, Johnson grass, and horse nettle, serve as overwintering reservoirs of some viruses. Curly top in sugarbeet is a leafhopper-transmissible viral disease, and w**ds play a significant role in its spread. Some of the important w**ds involved in the spread of curly top disease are certain species of Chenopodium, Russian thistle, Amaranthus, deadly nightshade, shepherd's pursed, and knotw**d.
In some cases, aphids feed on some of the early-appearing w**ds and then move to new crop plantings, thus introducing viruses which are then spread in secondary cycles within the planting. Sugarcane mosaic virus (SCMV), an aphid-transmitted virus in maize in the U.S. corn belt, is thought to spread from local w**d reservoirs. Spread of SCMV depends upon three conditions: the coincident presence of large numbers of aphids and moderate numbers of infected source plants; when moderate numbers of aphids coincide with large numbers of source plants; or when large numbers of both vectors and source plants coincide. Johnson grass is found to be an important source of primary inoculum for SCMV in several areas. Bean yellow mosaic virus (BYMV) is a common problem in bean growing areas. Forage legumes (red clover) are found to be the source of primary inoculum for aphids to carry BYMV into bean fields.
For lettuce mosaic virus, only 10 to 15 seconds of feeding are needed for an aphid to acquire the virus; then another 10 to 20 seconds on another plant suffices for the aphid to transmit the virus. Diseases in which the aphid-virus relationship is important include lettuce mosaic, celery mosaic, spinach blight, cucurbit mosaic, pea mosaic, sugar beet mosaic, and tomato mosaic. Several grassy w**d species host the wheat curl mite vector of wheat streak mosaic virus and build up inoculum for transmission into adjacent fields of wheat. Many broadleaf w**d species build up inoculum of nematodes, root rot fungi, and leaf spot fungi that attack sugar beets, dry beans, and corn. All growers should practice good control of insects and w**ds.
