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Everything you need to know about how to diagnose, control and treat plant diseases. Learn about:- 1. Diagnosis of Plant Diseases 2. Disease Control 3. Pest-Resistant Crops 4. Chemical Control
1. Diagnosis of Plant Diseases:
To correctly diagnose plant disease problems, follow a few basic steps. View the plant and its environment from various perspectives. The most obvious place to look first is up close. Use a hand lens if necessary.
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You want to find all of the symptoms. Look for symptoms on leaves, stems, roots, flowers and fruits. Cut open a branch or stem to look for vascular problems. Vascular problems show as discolouration of vascular tissue, leaf or stem wilting and sudden wilting of a section or a total plant.
Stand back and look at the overall picture. Consider the total environment: weather, soil, stage of development for plant and pathogens, cultural practices and condition of other plants in the area. A plant growing in the wrong location may be stressed. Consider pesticide applications, recent construction or digging, and weather conditions.
Time:
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Determine when the symptoms became apparent. The onset of a problem may be due to a cultural practice, the seasonal appearance of a disease or insect, or a weather-related event. Remember that long-term stress is slow to appear, taking a year or more at times.
Is the problem spreading? This may indicate it is a pathogen. Are plants of other species affected? Diseases are usually species-specific. Problems caused by environmental factors do not spread, although the symptoms may become more severe.
Remember that there is usually no single cause. There may be a primary cause; however, it may be associated with cultural or environmental conditions. Just as there is probably no single cause, there is usually no single symptom.
Search for all of the symptoms. Orderly thinking and good questions are the key to accurate diagnoses. When in doubt about a diagnosis, turn to agents, state specialists or the Pest and Plant Disease Clinic for assistance or a second opinion.
Non-living or abiotic agents can indirectly result in plant problems. Additionally, several factors in the plant’s environment can produce disease-like symptoms: weather extremes, high winds, high or low temperatures, nutrient deficiencies, physical damage and poor cultural conditions. Frost often damages buds and leaves in early spring. Hail can cause leaf spotting or holes.
Drought and high winds result in wilting and in extreme cases, browning and curling. Air, water and soil pollution affect plant health and can produce disease-like symptoms. Soil imbalance, resulting from construction or other dumping, or misapplied garden chemicals can cause damage and disease-like symptoms.
Dumping of household, automotive and industrial chemicals can also produce plant damage. Plant disease can result from a combination of abiotic agents and biotic agents. Plants may be initially placed under stress by non-living agents. This creates susceptibility in plants for attack by living agents. Drought may damage roots which then are more likely to be infected by fungal diseases.
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Random distribution of symptoms on injured plants is usually caused by a biotic factor, such as infectious disease pathogens or an insect/animal. Uniform patterns are generally associated with abiotic or non-infectious agents like pesticides, fertilizers, environmental or site stress and mechanical damage.
Plant specimens that are to be diagnosed should be taken from the area where symptoms are showing on living tissue. Dead plants are often invaded by secondary pathogens which may hide the original problem. Collect several representative samples showing various stages of disease development.
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A generous sampling will assist in diagnosis. If possible, collect the entire plant, including roots. Wrap the specimens in dry paper. Do not moisten them or seal them in plastic wrap or plastic bags. Never mix different specimens in a single bag. A fresh sample is required. Complete the diagnostic form as thoroughly as possible. This will result in better diagnosis.
2. Disease Control:
By the time disease symptoms appear disease pathogens are inside the plant and usually beyond control. Therefore, it is important to prevent penetration of pathogens.
Consider the following methods of disease control:
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Avoid certain diseases through choice of appropriate site and planting time, purchase of disease-free stock and cultural practices that do not favour disease infection. Insect control can be important for controlling spread of viruses, mycoplasmas, bacteria and fungi. Mulching can help control diseases by preventing the contact of foliage with soil.
Exclude insect vectors with row covers. Don’t bring diseased plants into the garden.
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3. Use Resistant or Tolerant Plants:
Appropriate selection of species or cultivars will avoid or minimize problems. Remove all diseased plants, including alternate hosts. Immediate removal of diseased plants or plant parts reduces the chance of the disease spreading.
Selective pruning and careful sanitizing of pruning equipment can prevent spread of disease. Rotate crops to avoid soil-borne diseases. Use best cultural practices. Follow recommended cultural practices.
3. Pest-Resistant Crops
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One of the mainstays of integrated pest management is the use of crop varieties that are resistant or tolerant to insect pests and diseases. A resistant variety may be less preferred by the insect pest, adversely affect its normal development and survival, or the plant may tolerate the damage without an economic loss in yield or quality.
Disease-resistant vegetables are widely used, whereas insect-resistant varieties are less common but nonetheless important. Examples include varieties of wheat which have tough stems that prevent development of the Hessian fly and cucurbits (squash, cucumbers, melons) that have lower concentrations of feeding stimulants (cucurbitacins) for cucumber beetles.
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Commercial corn lines with increased concentration of a chemical called DIMBOA display resistance to European corn borer, a major pest throughout the world. In the case of cabbage, the only reliable method of controlling onion thrips is through the use of resistant varieties.
Advantages of this tactic include ease of use, compatibility with other integrated pest management tactics, low cost, and cumulative impact on the pest (each subsequent generation of the pest is further reduced) with minimal environmental impact.
The development of resistant or pest-tolerant plant varieties, however, may require considerable time and money, and resistance is not necessarily permanent. Just as insect populations have developed resistance to insecticides; populations of insects have developed that are now able to damage plant varieties that were previously resistant.
4. Chemical Control
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If all other integrated pest management tactics are unable to keep an insect pest population below an economic threshold, then use of an insecticide to control the pest and prevent economic loss is justified. In most cropping systems, insecticides are still the principal means of controlling pests once the economic threshold has been reached.
They can be relatively cheap and are easy to apply, fast-acting, and in most instances can be relied on to control the pest(s). Because insecticides can be formulated as liquids, powders, aerosols, dusts, granules, baits, and slow-release forms, they are very versatile.
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Insecticides are classified in several ways, and it is important to be familiar with these classifications so that the choice of an insecticide is based on more than simply how well it controls the pest. When classified by method of application (site of encounter by insect), insecticides are referred to as stomach poisons (those that must be ingested), contact poisons, or fumigants. The most precise method of classifying insecticides is by their active ingredient (toxicant).
According to this method the major classes of insecticides are the organophosphates, chlorinated hydrocarbons, carbamates, and pyrethroids. Others in this classification system include the biologicals (or microbials), botanicals, oils, and fumigants.
Insecticides may be divided into two broad categories:
1. Conventional or chemical.
2. Biorational.
In this guide we define conventional or chemical insecticides as those having a broad spectrum of activity and being more detrimental to natural enemies. In contrast, insecticides those are more selective because they are most effective against insects with certain feeding habits, at certain life stages, or within certain taxonomic groups, are referred to as “bio-rational” pesticides. These are also known as “least toxic” pesticides.
Because the bio-rational are generally less toxic and more selective, they are generally less harmful to natural enemies and the environment. Bio-rational insecticides include the microbial-based insecticides such as the Bacillus thuringiensis products, chemicals such as pheromones that modify insect behaviour, insect growth regulators, and insecticidal soaps.
The majority of insecticides fall into the chemical category because they are typically more effective and can usually be used to control several pests. This provides the economic justification needed for the research and development of such products. In contrast, the more specialized market of the bio-rational makes their long-term economic return less favourable.
Despite the advantages of conventional insecticides, the problems associated with their use have been well documented. These include the resurgence of pest populations after decimation of the natural enemies, development of insecticide-resistant populations, and negative impacts on non-target organisms within and outside the crop system. One of the more serious problems is the development of resistance. Many insect pest species now possess resistance to some or several types of insecticides, and few chemical control options exist for these pests.
If all other integrated pest management tactics are unable to keep an insect pest population below an economic threshold, then use of an insecticide to control the pest and prevent economic loss is justified. In most cropping systems, insecticides are still the principal means of controlling pests once the economic threshold has been reached. They can be relatively cheap and are easy to apply, fast-acting, and in most instances can be relied on to control the pest(s).
Because insecticides can be formulated as liquids, powders, aerosols, dusts, granules, baits, and slow-release forms, they are very versatile. To compare pesticide use patterns, the dose, formulation or per cent active ingredient of the product and the frequency of application must also be considered and are used to calculate the EIQ Field Use Rating.
Pesticides with a lower EIQ Field Use Rating have a lower environmental impact. The EIQ concept and techniques are evolving, but this, or a similar rating, will provide information for appropriate pest management decisions in the future.
Natural enemies are generally more adversely affected by chemical insecticides than the target pest. Because predators and parasitoids must search for their prey, they generally are very mobile and spend a considerable amount of time moving across plant tissue.
This increases the likelihood that they will contact the insecticide. When an insecticide is applied, ideally only the target pest(s) should be affected. The goal is to maximize pest mortality while minimizing harm to natural enemies.
The following factors, some of which are used when determining the EIQ, influence the impact of insecticide applications on natural enemies:
1. Spectrum of Activity:
Insecticides toxic to most insects (broad-spectrum) will more adversely affect natural enemies than materials that are narrow-spectrum or selective for specific insect species or life stages. Most insecticides in use today have a broad spectrum of activity.
2. Residual (Half-Life) Activity of Insecticide:
Insecticides that remain toxic to pests for a long time and remain on the treated surface will have a similar effect on natural enemies.
3. Coverage and Formulation of Insecticide:
Full coverage sprays will generally have a greater impact on natural enemies than directed sprays, systemic, or bait formulations. Spot or edge treatments directed at localized pest infestations or to a specific plant surface most often occupied by the pest will be less detrimental than those applied to the entire field or plant.
4. Dosage and Frequency of Application:
Higher rates and repeated applications will have a more detrimental impact on natural enemies.
4. Timing of Application:
Applying insecticides when natural enemies are not abundant or are less susceptible, such as when immature are encased in host eggs, can be helpful.
5. Susceptibility of Natural Enemy to Insecticide:
Some natural enemies are inherently more resistant to insecticides than others, and some populations of natural enemies have been selected either naturally or through the efforts of researchers to possess higher levels of resistance.
For example – strains of predaceous mites, rove beetles, lacewings, and species of parasitoids have been selected for increased levels of resistance to insecticides, thereby increasing the likelihood that some will survive insecticide treatments directed at pests.
Some of these pesticide-tolerant strains are commercially available. Applications of some fungicides directed at controlling plant diseases can also reduce the incidence of fungal diseases of insects. In general, little can be done about this incompatibility except to use the fungicide only as needed.
In addition to killing natural enemies directly, insecticides may also have sub-lethal effects on insect behaviour, reproductive capabilities, egg hatch, rate of development, feeding rate, and life span. Fungicides and herbicides may also have lethal and sub-lethal effects on natural enemies.
5. Cultural Control
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There are many agricultural practices that make the environment less favourable to insect pests. Examples include cultivation of alternate hosts (e.g., weeds), crop rotation, selection of planting sites, trap crops, and adjusting the timing of planting or harvest.
Crop rotation, for example – is highly recommended for management of Colorado potato beetle. The beetles overwinter in or near potato fields and they require potato or related plants for food when they emerge in the spring.
With cool temperatures and no suitable food, the beetles will only crawl and be unable to fly. Planting potatoes well away from the previous year’s crop prevents access to needed food and the beetles will starve. The severity and incidence of many plant diseases can also be minimized by crop rotation, and selection of the planting site may affect the severity of insect infestations.
Trap crops are planted to attract and hold pest insects where they can be managed more efficiently and prevent or reduce their movement onto valuable crops. Early planted potatoes can act as a trap crop for Colorado potato beetles emerging in the spring. Since the early potatoes are the only food source available, the beetles will congregate on these plants where they can more easily be controlled.
Adjusting the timing of planting or harvest is another cultural control technique. The earlier planted processing tomatoes grown in the western United States are far less likely to be infested by the tomato fruit worm than those planted later in the season. It is also important to use pest-free transplants. Some vegetable crop transplants can be infested with insect pests, and growers using these transplants are put at a considerable disadvantage.
6. Control of Fungal Diseases:
Decay fungi are living organisms which send minute threads called “hyphae” through damp wood, taking their food from the wood as they grow. Gradually, the wood is decomposed and its strength is lost. Such damage is often inconspicuous until its final stages, and in a few instances homeowners have suddenly found floors breaking through or doors falling from their hinges due to wood rot.
When previously dry wood is placed in contact with moist soil, or in a location where it is subject to condensation (such as unventilated crawl space), it is likely that wood decay problems will occur.
Rain leaks, faulty plumbing and leaky downspouts also are common sources of moisture. In some instances, water can be transported to the site of decay through strands or “rhizomorphs” of the decay fungi. Water-transporting strands may extend for thirty or more feet across brick, concrete or similar materials.
The wood decay fungus, Serpula lacrimans, has been known to transport water up three stories to an area where decay is occurring. Poria incrassata is also capable of transporting water long distances.
However, these fungi are exceptions to the rule. Most wood-rotting fungi must have a direct supply of water at the site of decay. Thus the term “dry-rot,” sometimes applied to decay in wood structures, is erroneous.
Repair of Decayed Buildings:
First determine the source of moisture and remove it. If adequate ventilation and soil drainage are provided and all contacts of untreated wood with the soil or moist concrete or masonry are broken, decayed wood will dry out and further decay will be stopped.
When making replacements cut out at least one foot beyond the rotten area. Avoid placing new lumber in contact with old, decayed wood. Replacement lumber should be treated before installation. Remodel to provide more ventilation and better design rather than simply replacing decayed lumber.
Control of Nuisance and Detrimental Molds (Fungi) in Mulches and Composts:
Mulches and composts are often used to improve soils and plant health and to control weeds. They improve drainage as they decompose even though the ability of the soil to hold moisture is increased.
They lower soil temperature in the summer and insulate roots from cold in winter conditions. Eventually, they mineralize, release nutrients for plants, and leave humic substances as residues. Their beneficial side effects gradually disappear unless more mulch or compost is applied.
Generally, these organic materials inhibit undesirable microorganisms such as soil borne pathogens that cause diseases of plants. They also stimulate the activity of many types of beneficial microorganisms, including mycorrhizal fungi. Occasionally, however, microorganisms (primarily fungi) in mulches and composts can become a nuisance and even cause certain diseases of plants.
Whether a mulch or a compost provides beneficial or detrimental effects is largely determined by the type of organic matter from which it was produced and the degree to which it was decomposed and treated before its application in the landscape.
The temperature, pH, and moisture content of the products just before application also have an effect. The severity of nuisance fungi can be minimized if appropriate steps are taken in time.
Mulch Type and Fresh versus Composted Mulch:
Wood products from some trees are more resistant to decay than others and, therefore, cause fewer problems. Bark chips (nuggets) from large mature pine or other softwood trees such as cypress trees contain mostly lignin (dark material in bark), wax and protected cellulose that resist decay.
On the other hand, wood wastes from these same tree species, but ground as young trees, rot quite readily because the cellulose in such bark and wood products is not yet protected from decomposition by lignin waxes or tannins.
Hardwood tree bark (oak, maple, etc.), even from large trees, contains a large concentration of cellulose that is not protected from rotting. Therefore, hardwood bark mulches, like ground wood from almost all tree species, rot readily and cause most of the nuisance mold problems in the landscape. The finer the product is ground, the more severe the problem can be! These materials are low in nitrogen content.
The fine particles (less than 3/4″ diameter) in such mulches cause nitrogen immobilization in soil. The microflora that decomposes the wood particles takes up the nitrogen required for growth of plants.
The result is that the plant becomes starved for nitrogen. Some mulch producers screen all particles smaller than 3/8″ out of high-wood- content or hardwood-bark mulches, which avoids most of the nitrogen immobilization problem.
The best way to avoid all these problems and bring about beneficial effects by mulching is to add nitrogen to woody and hardwood bark products followed by composting to lower the carbon to nitrogen ratio.
Blending of grass clippings with wood wastes before composting is one way to achieve this. Addition of poultry manure or urea to supply 1.2 lbs. available nitrogen per cubic yard of material satisfies the nitrogen need also. Some landscapers add 10-15 per cent by volume composted sewage sludge to hardwood bark or wood wastes, and this makes an ideal product that has performed very well in landscapes.
These amended products should be composted at least six weeks. This process kills plant pathogens, eggs of insect pests, and produces a nitrified product that releases plant nutrients rather than ties up nitrogen. The microorganisms that have colonized these products reduce the potential for growth of nuisance fungi and provide control of many plant diseases.
Temperature, Moisture Content and pH:
Landscapers often apply quality mulch products from high temperature piles (140-160 degrees F) directly into the landscape. The temperature of the mulch is high because of heat produced by growth of microorganisms, known as thermophiles, in storage piles during the composting process. These microorganisms die soon after the mulch cools to 50-80oF after it has been placed around homes.
Because they require high temperatures to survive, they cannot grow and compete with soil microorganisms at the low temperature of mulches in the landscape. The sudden temperature drop that often occurs after mulch is applied creates what is known as a “biological vacuum.” It also can occur during bagging of products at producers of mulches and particularly during dry seasons.
Mesophiles (low temperature soil microorganisms) rapidly colonize such mulches. If the mulch is dry, or dries out to moisture content below 34 per cent during the first day after it is applied (mulches are dusty below this moisture content), fungi become the primary colonizers. This sets the stage for problems later, and the problem becomes most severe in mulches that are applied too deep in the landscape.
After prolonged heavy rains, the dry material colonized by fungi eventually becomes wet. Dry products stored in bags may also become moist when water produced as a result of microbial activity accumulates along the inner surface in bags.
Bacteria then rapidly colonize the fungal white mass to induce the formation of fruiting structures by the fungi. The nuisance toad stools and other fruiting structures appear a few days later. Mold problems occur also when dry products are bagged or applied to dry soils.
Dry composts removed from high-temperature piles occasionally cause mushroom problems in bags and also in soils at nurseries. These moldy products inhibit plant growth in field soils as well as in potting mixes.
They also cause wettability problems if dry conditions persist in the soil for a few weeks to give fungi a chance to become the dominant colonizers. Plants do not grow well in such moldy soils.
These problems can be reduced by soaking the high-temperature products with water as they are applied in the landscape or bagged. The high-moisture organic matter then becomes rapidly colonized by bacteria during the first few days.
These bacteria compete with fungi to reduce the potential for the development of major mold problems. This strategy has been successfully applied over the past decade to hardwood as well as softwood composts and mulches. It has controlled nuisance problems caused by many fungi in various parts of the United States and abroad.
The pH or acidity of the mulch is another important factor. Sour mulches that give off acrid odors may range in pH from as low as 2.5 up to 4.8. Highly acidic mulches are toxic to most plants and promote the growth of fungi.
Bacteria that inhibit fungal growth cannot colonize mulches when the pH is lower than 5.2. The low pH and fungal problems are avoided if the raw material is nitrified and composted.
In summary, water applied at the right time during composting, storage, and mulching can solve most of the fungal nuisance problems. It is best to maintain water content higher than 40 per cent on a total weight basis.
Again this allows bacteria as well as fungi to colonize the organic matter, and it sets up competition for nuisance molds. The moisture content of most organic products actually can be raised above 50 per cent and not present excessive weight problems during transport.
What to do once the Problems Occur?
Sometimes very little can be done to control nuisance fungi other than to spade the mulch into the surface soil layer followed by soaking with water. Another option is to remove the mulch, place it in a heap after thorough wetting to allow for self-heating to occur (110- 140 degrees F). This will kill nuisance fungi. If fresh dry mulch is placed on top of mulch colonized by nuisance fungi, the problems may occur again the following year or even earlier.
The best control strategy for homeowners and landscapers is to purchase composted products low in wood content. Fresh, finely ground woody products should be avoided for many reasons unless composted first. Coarse fresh woody products are much less likely to cause problems unless applied too deep. It is important to soak all mulches immediately after they have been applied.
Generally, mulches should not be applied to a depth greater than two inches. Mulches and composts applied in this manner provide many types of beneficial effects rather than nuisance problems, or worse, plant diseases. Sour mulches should be avoided altogether.
Artificial Inoculation:
This question is becoming more and more common as more companies have recently begun commercializing mycorrhizal inoculum. Artificial mycorrhizal inoculation may benefit plants only when there is no natural and appropriate mycorrhizal inoculum in the soil, or the inoculum level is low, or species present are less efficient at aiding the plant host than those being introduced. This situation is most commonly found in highly impoverished soils that do not support natural fungal growth, for example in the case of mine spoils.
In principle, highly compacted, organic nutrient-poor soils in urban environments (often the result of mineral subsoil horizons having been turned into the top layers by construction activities) may also be prime candidates for artificial mycorrhizal inoculation.
However, success of the treatment can never be guaranteed for two main reasons:
(i) The urban soil may already contain a complement of mycorrhizal fungi, however depleted, that out-compete the newly introduced species. Competition among mycorrhizal fungi is well documented and a common occurrence in natural soils, for example in reforestation operations. It is particularly important for ectomycorrhizal fungi;
(ii) The soil organic matter content may not be adequate. The presence of organic matter can greatly enhance the development of mycorrhizas.
Due to the specificity properties of the different mycorrhizal associations described above, it is likely that mycorrhizal inoculation will be more successful, in terms of actual establishment of the inoculated species, with endomycorrhizal fungal species, even though the main ectomycorrhizal inoculum being commercialized is a “generalise among specialists.”
At present, there is very limited, unbiased scientific evidence that artificial mycorrhizal inoculum in the landscape makes plant establishment more successful or that the inoculated plants grow better over time.
In fact, the available evidence is rather inconsistent. In part this is due to the fact that commercial preparations may not be very viable or not viable at all by the time they reach the consumer, or become not viable once they are in the consumer’s hands.
But perhaps the biggest factor is that soil conditions in the landscape may be too variable to be able to make generalizations about the usefulness of artificial inoculation with the few selected species available commercially.
Furthermore, any short-term positive effects observed on plant growth following mycorrhizal inoculation may simply be the result of the fertilizer components that are often present in the commercial preparations.
The bottom line is that artificial inoculation may have either a positive or neutral effect on plant growth and health. In all cases, however, it is very unlikely that artificial inoculation is detrimental to the plant.
In other words, the treatment may or may not work, but would not be injurious. Current research, conducted in the author’s and other researchers’ laboratories, suggests that organic soil amendments, particularly the addition of composted mulches, greatly enhances the mycorrhizal status of landscape plants, even without the addition of artificial inoculum. Improvement of soil conditions may therefore be the most important factor in mycorrhizal development in the urban landscape.
7. Control
of Virus Diseases:
A direct attack on virus diseases by the chemical methods so extensively used against parasitic fungi is out of the question. Viruses cannot be reached by these means and, at the most, control of their insect vectors may be undertaken when the economic conditions permit. Practical control of the aphis vector of strawberry yellow edge and crinkle has been achieved in some cases in England by the use of vapourised nicotine.
In commercial crops of the biennial drug plant, henbane a considerable reduction of the vector, Myzus persicae, was effected by weekly spraying with nicotine and soft soap during the first two months of vegetation in the first year. This gave a 3° per cent increase in yield in the third crop, harvested during May of the second year.
Rouging for the elimination of virus-diseased plants has had a considerable vogue, especially in crops propagated vegetatively such as potato, sugarcane strawberries, bananas, and the like; it is still practised extensively where the crop is valuable and the amount of infection moderate. Its principal use nowadays, however, is in the maintenance of ‘foundation’ or nursery stocks of disease-free plants which can be used for the planting of the main field crop.
Where insect vectors can be kept down fairly readily and the plants are under observation closely enough for early infections to be recognised, as in glasshouse cultivations, rouging is of much value, as it is also, obviously, where the virus is seed-borne or the crop is propagated vegetatively and a healthy parentage is, therefore, essential.
Where the infection arises anew each year in an annual crop, success in reducing the source of infection by the destruction of ‘volunteer’ plants or out-of-season plantings of the crop, which may serve to carry-over the disease between successive main crops, has sometimes been considerable.
Much benefit resulted from eradicating-volunteer cotton in the Sudan to reduce early attacks of leaf curl. In Southern Rhodesia a notice under the Tobacco Pest Suppression Act, 1933, prescribed the 1st August as the date by which destruction of all growing tobacco plants is to be completed as a control measure against tobacco leaf curl; this close season appears to cause the vector to die out.
In California the enforcement of a celery-free period in districts where western celery mosaic causes severe damage has been effective in raising the yield, which had fallen from over 1,000 half-crates per acre in 1930 to 31 in 1934, to an average of 1,100 in 1939 and 1,000 in 1940.
The growing of maize in the neighbourhood of sugarcane fields has been discouraged for similar reasons as it is susceptible to sugarcane mosaic and a favourite food plant of the insect vectors; this ban was enforced by legislative action in certain areas in Barbados in 1927.
Sometimes the eradication of weed hosts of the virus or of its insect vector has given good results. Destruction of the Russian thistle, Salsola kali-tenuifolia, on which the beet curly-top vector, Eutettix tenellus, and the virus are largely maintained before passing to the crop, has effectively reduced sugar-beet curly top in California. Excellent results in controlling the yellowish-green mosaic of cucumbers in the United States, where it has – many hosts, have followed the eradication of certain weeds, especially perennials.
In England, where, however, this mosaic is less common in glass-houses than the mild or green-mottle mosaic which seems to be confined to the Cucurbitaceae, the houses in which it occurs should be kept free from susceptible flower crops and the common bryony should not remain near cucumbers or marrows.
With the great prevalence of the spotted wilt virus amongst ornamentals, keeping the latter out of houses containing tomato seedlings is most important, while glass-house control in tomatoes and flower-crops alike must include elimination of the thrips vector from an infected house before another susceptible crop is grown; thrips control out of doors, however, is difficult.
The complete eradication of these diseases by destruction of all infected plants is being effectively carried out by large-scale campaign Saffiongst perennial plants. It was early advocated for the control of peach yellows and rosette in the United States and later was recommended against sandalwood spike, together with the establishment of a sandal wood- free belt round the infected area in South India. Recently, powers have been taken in the Gold Coast to remove plants such as cacao from declared areas in order to check the spread of diseases, such as the swollen shoot virus disease of cacao, to uninfected areas.
The object is to establish zones free from all cacao around the infected areas. The Psorosis Act of 1927 provides for the destruction, without compensation, of any citrus tree infected by Psorosis within the Union of South Africa, together with quarantine measures against new introductions of the disease; as infection may remain latent in the tree for 20 to 30 years and seldom shows before the trees are 9 or 10 years old, it will be long before the effectiveness of the measure can be judged. In 1939-40, however, only 1,468 affected trees were found in a total of 513,123 inspected.
When peach mosaic was first seen in Colorado in 1934 and California a year later, eradication was promptly undertaken; in Colorado only 3,100 fresh cases were found in 1937, compared with 9,835 in 1936 and 30,467 in 1935, while in the four years from 1936 to 1939, 89,355 cases were found in California, 63,651 of which were removed.
The attempt to eradicate the phony peach disease from the south-eastern States is more formidable; during 1936 over 2,000,000 abandoned and 3,500,000 ‘escaped’ peach trees were removed in II States by a force of 2,000 men employed under the Emergency Relief measures, while altogether over 21,000,000 trees were inspected in 20 States and 146,072 of the 156,977 found infected were removed. In New Zealand an attempt is being made to eradicate ‘onion yellow dwarf’ disease which was found to be confined to a small area in 1939.
In the opinion of many this remains the most hopeful line of attack on them. Control by ‘vaccination’ with a weak strain of the virus concerned, so as to induce immunity from more virulent strains, has been regarded as practicable by some. No satisfactory case of its use under field conditions seems to have been reported as yet and much further testing is necessary before the many risks associated with the use of such ‘live’ vaccines can be discounted.
A considerable measure of success has sometimes followed the adjustment of agronomic practices so as to reduce the prospect of infection. The most effective has been the alteration of the date of planting the crop so that it is too far advanced for serious injury by the time the insect vectors become numerous; this has worked well on occasion against the curly top of sugar-beet.
In Tanganyika, experiments show that cassava planted in June is less injured by mosaic than if planted at other times, for the crop then passes the main period of its growth when the probability of infection is at its lowest. Similarly in parts of Nigeria the native tobacco plantings escape serious injury from leaf curl because the vector has become scarce by the time the seedlings are transplanted.
Late sown oats and other cereals in Siberia are not severely injured by the Zakooklivanie or ‘pupation’ virus disease, because the vector, Delphacodes striatella, which over-winters in the larval stage on grass weeds about the cereal fields, migrates in the winged form too early to be able to dwarf the plants.
Infection by the groundnut rosette virus in Africa has been found to be considerably lessened by dense planting and also where grass mulching of the soil is practised; in the Gambia earlier sowings are less damaged than those that give young plants when the vector, Aphis laburni, is most active in July and August. Possibly, as with the vectors of leaf roll of potatoes, the activity of this aphis is reduced by high humidity.
The chief measure employed for the control of virus diseases of the potato and some other crops in the British Isles and elsewhere is the inspection and certification of planting stock. For this purpose an elaborate organisation has been built up by various Government Agricultural Departments. In areas in the British Isles where potato seed production is important, the inspection covers purity to type, as well as the amount of virus infection.
In Scotland, for instance, over 50,000 acres grown for seed are inspected annually and certificates granted by the Department of Agriculture of different designations according to purity and the percentage of virus diseases found – ‘Stock Seed’ must be 99.5 per cent true to type and practically free from virus symptoms except negligible mottle; T.S., is 99.5 per cent pure and with not more than 1 per cent total mild mosaic, severe mosaic, leaf roll, and ‘wildings’, and comprises varieties immune from wart disease, the corresponding nonimmune grade being designated ‘N.I.(A)’; ‘T.S.(H)’ and ‘N.I.(H)’ are 99.5 percent pure and with not more than 3 percent severe mosaic, leaf roll, and ‘wildings’.
These are all regarded in England as Class I Scotch seed; Class I Irish, and Class I Special English and Welsh stocks are also recognised, the latter from certain limited areas only. Recently certification has been extended to cover English crops which, though ordinarily grown for consumption, can, under certain circumstances, serve as sources of good seed; such crops are eligible only if grown from Class I seed and at least 5° yards from any crop not similarly grown.
In Canada and Holland also there are important seed potato industries and a very elaborate certifying organistation exists; in 1935 the acreage certified in Canada was 83,537. Many other countries have adopted similar measures.
Methods for the same purpose in the United States vary to some extent from State to State but are kept in general consonance by periodical conferences at which Canada is represented. Considerable use is made of the tuber unit method of establishing clean stock for planting the main crop – a portion of each tuber bearing an eye is tested under glass before planting time and only if it gives a healthy shoot is the rest of the tuber used.
The resulting seed plot is grown in isolation and carefully rogued to eliminate any virus diseases that may appear. Naturally, localities are selected for seed production where the risk of spread of virus diseases is low. By this method in commercial practice cases have been recorded in New York State of the reduction of virus diseases from 2.6 per cent in 1924 to 0.4 per cent in 1932 and in Quebec of an increase in yield from 287.7 bushels per acre in 1928 to 417 in 1934.
The outstanding importance of the ecological conditions controlling the insect vectors of these diseases is sufficiently domesticated by the fact that there are areas in Ireland and elsewhere in which potato varieties have been maintained free from degeneration for many years, Early Rose potatoes have been grown on bog-land in central Ireland since 1867 without change of seed and without running out, and almost forgotten varieties such as Pink Eyes and Lumpers on the western seaboard for much longer periods. In these areas the insect vectors are rare and the cool moist climate is adverse to their dispersal.
The bunchy-top disease of bananas in New South Wales and Queensland has been brought under fairly effective control by a series of measures directed to the eradication of diseased plants and the prevention of spread.
All plantations are registered and inspected diseased plants are removed after treatment by prescribed methods to destroy the vectors present on them, disease areas are quarantined so that no banana material may be moved from them, in the areas must free from weeds for no an 6 feet from each banana plant, all banana plants in other than registered plantations must be destroyed, and every care must be taken in the plantations to recognise the early symptoms of infection. Permission to sell planting material must be obtained, and is granted only to growers of disease-free stocks. In certain areas free from the disease no planting material from the infected areas is allowed ingress.
These stringent measures, applied under legislative authority in 1927 and varied from time to time, have been effective in restoring the New South Wales banana-growing industry-probably infected from Fiji in 1913 and which had fallen between 1922 and 1925 to a seventh of its previous production, back to far more than the old figures by 1935 and rank amongst the best examples of plant disease control by administrative action based on sound scientific investigation.
Similar methods are applicable to other virus diseases in advanced countries, especially in so far as they relate to the inspection and certification of nurseries supplying planting material.
ADVERTISEMENTS:
In New York State, for instance, all raspberry nursery stock offered for sale must be regularly inspected, rogued when necessary, and certified reasonably free from virus diseases. A test of the same procedure in England has broken down owing to the prevalence of carrier varieties and others with a low expression of symptoms.
From this survey it will appear that the question whether the viruses are, animal, vegetable, or mineral’ is largely of academic interest only. As agents of disease they behave sufficiently like the bacteria to allow of the application of the same considerations to the study of their behaviour and control.
In particular, the problems of their origin, dissemination, and prevention, which at one time seemed so difficult as to lead many plant pathologists almost to despair of their solution, have yielded to intensive research sufficiently to give good grounds for the hope that the practical outcome will eventually equal that achieved in the fight against parasitic fungi and bacteria.
8. Treat
ment:
Treat plants with recommended chemicals to prevent infection. Some treatments will stop the spread of the pathogen.
Chemical Treatment of Plant Disease:
To use fungicides and bactericides effectively, follow these 4 steps:
1. Be sure that the diagnosis is correct.
2. Choose a recommended control. Remember, a fungicide controls fungal diseases and bactericides only control bacteria. Read the product label and follow the instructions precisely. Make sure the plant you will be treating is listed on the label.
3. Apply the control correctly following all label instructions. To be effective, fungicidal sprays must be applied properly. They work by creating a barrier that pathogens cannot penetrate.
4. Good coverage is important. Give special attention to the underside of leaves, especially lower leaves. The barrier’s effectiveness is affected by how well the spray spreads and sticks. Many fungicides have Spreader-Stickers added to the product. If you are not sure about coverage, observe the pattern spray deposits.
5. Apply at the recommended time. In many cases, fungicides are a protectant and must be applied just before the pathogen is able to infect the plant. As the plant grows, additional sprays may be necessary to protect the new growth.
Fungicides are developed so that they degrade fairly rapidly. They are affected by rain, sun, and microbial activity. Excessive rainfall or growth will require shorter intervals between spraying. New shoots should be treated. Product label guidelines usually indicate a range of 7 to 14 days.
Know exactly what disease pathogen or combination of pathogens and abiotic agents are causing a plant problem. Fungicides and bactericides are effective against specific diseases under specific conditions. In the past, general fungicides were used. Many pathogens developed resistance to these general pesticides. Use common sense in analysing a situation. If an expensive or sentimental plant is experiencing problems, seek the help of professionals at the Pest and Plant Diagnostic Clinic.
