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EAP Publication - 73
Know your fungus
Summary and conclusion
One of the challenges to implementing more ecological farming practices in potato production is how to reduce the use of synthetic chemical fungicides while protecting the crop against late blight.
Late blight of potatoes is caused by a fungus-like organism named Phytophthora infestans. The disease is common to the cool and humid regions of the world such as northeastern North America and western Europe, which are also among the best potato-growing regions. However, the fungus is adaptable to a wide range of climate and it is found wherever potatoes are grown.
Late blight is a disease that cannot be neglected. Although unusual, the fungus is capable of totally destroying an entire unprotected potato crop within one or two weeks under the right weather conditions.
At the same time, the toxicity of chemical fungicides, the persistence of their breakdown products, and the frequency with which they are sprayed poses health and environmental risks that are of concern. Particularly worrisome are the potential reproductive effects and increased cancer risk posed by some of the synthetics and their breakdown products.
Late blight is probably the most studied plant disease in the world. Research on the disease has been ongoing since the infamous Irish Famine it caused 150 years ago. Yet relatively few alternatives to synthetic chemical fungicides are available, and the fungicide resistant population is building up with the use of some chemicals.
However, with a good knowledge of the fungus lifecycle and of the conditions that favour its development, it is possible to design a prevention and control program of late blight that could help reduce the use of synthetic chemical fungicides in potato growing regions.
The first step to achieving a more ecological approach to protecting your potato crop from late blight is to understand the nature of the organism you are dealing with and its disease cycle (Figure 1). The blight fungus can be compared to a microscopic plant with its various parts. The mycelium is a combination of its leaves, stem and roots which grow on the leaves, stems and tubers and also on potato residues. The sporangiophore is the structure that bears the fruit of the fungus called sporangia, which can number in the millions. Each sporangia contains 8 zoospores, which are like seeds, except that they can swim.
Blight spreads rapidly in cool wet weather (temperature around 13?C and 100% R.H.) when sporangia release their zoospores. Under such conditions these "swimming seeds" spread rapidly throughout the field in rainwater and germinate, infecting the crop. The sporangia themselves can be carried by the wind and infect plants directly, usually when temperatures are milder (around 18°C).
The Lifecycle of the Late Blight Fungus
1. Sporangia, coming from cull piles, volunteer potato plants or infected seed, are carried by wind and rain to leaf surfaces. When the moisture and temperature conditions are right, infection occurs. Three to seven days later, the first symptoms appear.
2. The mycelium of the fungus invades plant cells and kills them, causing blight. It continues to grow and eventually emerges from the underside of the leaves. The fungus then grow sporangiophores which release more sporangia to be carried elsewhere in the field by wind and rain.
3. Tubers become infected when the fungus is washed off the leaves and the sporangia and zoospores are carried down into the soil with the water. Exposed tubers are infected more rapidly. Uninfected tubers can be infected during harvesting if they come in contact with sporangia in the soil as they are dug.
4. Blight pathogen overwinters on tubers, not in the soil so cull piles, volunteer plants and diseased seed can be a source of infection the following year. Blight must have living plant tissue on which to live.
In northeastern North America, the symptoms of late blight generally appear later in the growing season, after flowering. The decaying leaves of blight infected plants give of a musty-fishy odour. Symptoms of infection can be seen on potato plant foliage, stems, flower, and tubers.
The first symptoms are olive to dark green spots which appear on the upper surface of the leaves close to the leaf tip or leaf margins. They can be dry but will often look water-soaked. Under light, the margins of the leaf zone that have been infected will look watery and light green, indicating the continuing progression of the fungus. These areas spread as the disease progresses, turn brown and die. These symptoms first appear on the lower leaves of the plant except in very wet conditions.
When it is cool and humid, the underside of the leaves in blight-infected plants will be covered with a whitish or greyish mold-like fungal growth: the mycelium. In dry weather, blighted leaves will curl and shrivel and become dark and crisp. These symptoms should not be confused with those of early blight or grey mold. Symptoms of early blight are not confined to leaf edges and have a dart-board appearance. Grey mold will first show up on leaf tips and margins like late blight, but the effected areas will be covered with lots of greyish coloured mycelium and spore mats.
On the Stem
On the stem, infected parts will appear brown just below the growing point - sort of dark strips running along the length of the stem. Infected areas on the stem do not appear as markedly water-soaked as the leaves can. In extreme cases, infection of the stem will kill the leaves just above the point of infection.
Once blight is present on the plant, it can infect the tubers if rain leaches its spores into the soil. Symptoms on tubers will appear as purplish-brown sunken areas of irregular size and shape. These are often located in and around the eyes. A dry reddish brown rot spreads under the skin causing infected potatoes to eventually rot in storage.
Conditions that Favour Blight
Late blight progresses more rapidly under cool, moist conditions than hot dry weather since its spores require water and an appropriate temperature range to germinate. That's why in Canada's Maritimes, the risk of blight is greatest from the end of July to potato harvest in mid-September. However, if the conditions for blight are right for an extended periods of time earlier in the season there can be a high risk of late blight at that time.
Cool humid weather: When relative humidity is over 80%, the air temperature is between 10 and 18?C, and water is present on the leaves, the spores will germinate and produce mycelium (blight symptoms will appear). However, no reproduction will occur as long as temperatures stay cool.
Mild humid weather: When relative humidity is over 80%, the temperature is between 16 and 22?C, and there is water on the leaves, the spores germinate and infect the plant on which they are located. They then grow and reproduce to yield millions more spores.
Hot weather: When temperature is above 30?C and relative humidity is below 80% or water is absent from the leaves, the fungus growth is stopped and it cannot reproduce. However high temperatures alone will not kill the fungus. A combination of low light intensity and continuous high temperatures will even favour its spread. Warm temperatures and moist conditions will favour growth of the mycelium. Extended dry periods or rapid dehydration kills many sporangia.
Ideal Conditions for a Severe Disease Outbreak
Major blight attacks will happen when night temperatures are around 12-13?C and there is heavy dew or rain, followed by day-time temperatures between 16-24?C and the high humidity that comes with rain, fog, or a persistent dew.
Late Blight and Potato Yields
While late blight reduces potato yields (less green leaf area) its impact is not as big as generally thought, assuming prevention and control practices are being followed. In fact a worldwide study of late blight epidemics done in 1960 has come to the conclusion that yields were higher on blight years! The explanation is that years with more late blight were also years with more rainfall which produced higher yields!
In other words, potato yields are more affected by absence of rain than by the presence of the fungus. Nevertheless, if blight prevention and control are neglected, the potential for blight epidemics increases, threatening the crop's yield. Early defoliation resulting from a neglected blight attack will translate into a high percentage of small, unsaleable potatoes.
The First Line of Defense -- Cultural Controls
All aspects of potato growing from planting to harvesting and storage can have an impact on late blight. Cultural control methods should be the first line of defense against the disease.
Choosing the Right Site and Soil Conditions
The area where potatoes are grown should be well aerated so that the leaves and air within the canopy do not retain humidity.
- Avoid planting close to the woods. Windbreaks are not a problem as long as they are not a dense wall of spruce and fir.
- Make sure the field is free of low spots where water might accumulate. Orient the rows into the prevailing wind. Widen the spacing between rows to encourage good air circulation. A dense canopy results in cooler temperatures and higher humidity which favours the production of spores and the spread of blight.
- Make sure the land is well prepared to provide for optimal growth. Avoid growing potatoes in heavy soils that retain humidity. It is known that the threat of blight disappears faster in sandy acid soils than in other types of soils.
If at all possible, potatoes should be grown on sites well isolated from other potato fields or even gardens by several kilometers to reduce the risk of importing blight from neighbours.
Seed and Seeding
Avoid the temptation to save seed from your own crop. Use certified potato seeds, free of blight. Destroy any potato seed that shows signs of infection before filling the seeder. One way to recognize blighted tubers is that they often start sprouting before the other ones.
Another reason to use certified seeds is that the seed kept from a crop will often be infected with viruses unless you are located in a region where aphid and leafhopper populations are absent (cold regions). Virus-infected stands are more susceptible to late blight..
Planting date is a factor that can be considered in regions with a long growing season. In these regions, early planting with early to mid-season cultivars usually decreases disease development and harvesting can be done before the major blight period. Pre-sprouting the seeds under artificial light before planting will have the same effect as earlier seeding in a short growing season area. Pre-sprouting or "green sprouting" in small meshed bags, a long-standing practice in Holland, is becoming increasingly popular in Canada.
Choice of cultivars
Yield loss caused by late blight varies among potato cultivars. Generally, late cultivars are more tolerant because of their vigorous foliage growth. However, no variety is totally resistant to blight so some form of fungicide still must be used. Moreover, a cultivar may be resistant to only one or two races of blight.
Most of the old resistant cultivars have complete resistance to one or more races. This type of resistance often breakdowns however when a new race appears. Many of the more recent resistant cultivars are considered field-resistant meaning that they are partially resistant to all races of the pathogen. These can be managed with very few fungicides. There is also less chance of their resistance breaking down, should a new race appear. Resistance may be located more in the foliage or more in the tuber depending on the cultivar, although most blight-resistance breeding programs have traditionally aimed at foliage resistance.
To reduce the number of spray applications, it may actually be better to choose early maturing cultivars than late maturing ones. Late cultivars will need protective spraying until harvest. If you choose early cultivars, prefer second early cultivars that will bulk up before the foliage is effected.
CURRENT AMERICAN AND CANADIAN NAMED CULTIVARS RESISTANT TO LATE BLIGHT
AC Brador, Alamo, Atlantic, Avon, BelleIsle, Bison, Boone, Butte, Catoosa, Cherokee, Delus, Fundy, Island Sunshine, Kennebec, Keswick, Krantz, Merrimack, Nooksack, Ona, Onaway, Ontario, Pennchip, Plymouth, Pungo, Raritan, Redsen, Reliance, Rosa, Saco, Sebago, Sunrise, Tawa, Trent, York.
OLD AMERICAN AND CANADIAN CULTIVARS RESISTANT TO LATE BLIGHT
Ashworth, Clarose, Canso, Cayuga, Chenango, Cortland, Empire, Essex, Fillmore, Glenmeer, Harford, Madison, Menominee, Placid, President, Russet Sebago, Saranac, Snowdrift, Virgil.
Good sanitation is the most important preventative measure that can be taken against late blight, but to be effective all growers in an area must be vigilant. Potato culls are often the main source of blight spores, especially when dumped in a field corner or in a shallow pit. Culls need to be buried, burned, fed to animals or composted before spring planting.
When burying culls, make sure they are buried deep and covered with soil or plastic to block out any light that might cause them to grow. To burn culls, place them in thin layers over a combustible material. It is also possible to chop culls and spread them on the surface of a field in the fall so that they are exposed to freezing temperatures which will kill the spores. However, if you choose this last option, it is preferable to choose a field where you will not grow potatoes or tomatoes the following year, in case a generous and early snow blanket prevents thorough freezing at ground level.
In the spring, destroy volunteers that have survived winter in the fields. Keep potato weeds such as hairy nightshade under control as they too can harbour the disease.
Scouting and Selective Destruction
Develop the habit of walking your fields regularly to identify spots of infected plants. You probably already know those areas where humidity stays longer and therefore should be watched more closely. Some organic farmer are known to walk through such high risk areas twice a day.
If a particular area of a field is infected with late blight, the plants should be destroyed to reduce chance of the fungus reproducing and spreading its spores to uninfected areas. Diseased plants can be cut, bagged and disposed or burned. Do not add diseased plants to a compost or manure pile.
Rotation will diminish the risk of infection from volunteers and culls because the potatoes are grown in a field free of immediate sources of infection. However, blight spores can still come from the neighbour's fields on the wind. Tomatoes and other plants from the potato family are subject to late blight and should not be planted following or preceding potatoes.
Some studies indicate that too much nitrogen fertilizer (greater than 200 kg/ha) increases infection by late blight. The form of nitrogen applied is also important. Nitrogen in nitrate form is more favourable to blight. A very high concentration of potassium also increases susceptibility to blight while phosphorus applications do not.
Potatoes fertilized with just nitrogen or potassium fertilizers are more susceptible than those given full NPK fertilization. Studies have shown that certain micronutrients (especially Zinc, Copper, Molybdenum and Manganese) applied as foliar sprays or mixed with basic fertilizer are very effective in reducing late blight and in increasing tuber yield.
Sprinkle irrigation favours blight because it provides an essential element for blight development: free water sitting on the plants. Furrow and drip irrigation do not favour blight development as much but are seldom used in northeastern North America for potato production. These methods maintain a more humid soil but will not deposit water on the foliage.
It is important to hill up the potato rows as heavily as possible. Thick hills will help keep the spores from leaching through the soil and infecting the tubers. For the same reason it is advisable to plant deeper rather than shallower.
The contamination of tubers can be prevented during harvesting by proper top killing. If the foliage is not severely effected, killing the tops removes most of the risk of infection in storage. This is one of the oldest method of blight prevention and is usually done two to three weeks prior to harvesting.
Top killing can be done mechanically by cutting, stripping or pulling the stalks; or physically by flaming; although conventionally it is done with herbicides.
Mechanical methods are less effective as they may not kill all the tops leaving areas where blight will survive. Dessication of the tops will be faster with chopping rather than cutting only. If you are using a machine that cuts off the tops near ground level and shreds them into pieces, it is important to make sure it does not cut too close to the ground where it could damage the tubers. Vine-pulling is as good as top killing with herbicide to prevent tuber rot. The fungus is effectively killed when infected tops are destroyed by burning.
|BURN 'EM OR FREEZE'EM
Physical control of blight using very high or very low temperatures is an effective means of control. Flaming is becoming popular as a way to control potato beetles and weeds early in the season. Flaming can also be used late in the season to destroy potato vines. It will effectively kill blight spores and some second generation potato beetles as well.
Freezing temperatures are also lethal to blight spores. Letting the frost kill the vines can prevent the infection of tubers during harvesting.
Mechanical harvesters unfortunately promote the infection of tubers by late blight. They disperse the spores found in the soil, infecting tubers that would not otherwise come in contact with the disease. Damaged tubers also offer ideal points of entry for the disease.
Harvest in dry weather if possible. Spores will only infect tubers when wet. Harvest when vines are completely dead; not only the leaves, but the stems too. If foliage dies of blight naturally, wait at least fourteen days before harvesting to allow spores to die.
If you can wait for a hard frost, the frost will kill any spores on the soil surface and you can harvest fourteen days later. Make sure you harvest all potatoes. Harvest suspect, shaded or wet areas at the end.
Disinfect the potato house before bringing in a new crop, especially if there was a problem with blight the year before. Disinfect with 1 part bleach to 9 parts water solution, peroxide or other suitable disinfectants. When you notice diseased tubers, cull them. A hot water treatment may be used to disinfect tubers.
The Last Line of Defence - Spraying
Fungicidal sprays are a preventative measure to be used before late blight hits the crop. If the disease is already present when spraying begins the fungicide will help foster the creation of fungicide-resistant strains -- which are increasing in number. Even systemic fungicides which have some curative action should be used on a preventative basis.
The timing of spray applications depends on the efficacy of the agent used to protect against late blight, the susceptibility of the cultivar, the cultural status of the crop and the blight risk forecast. A spray application shortly before harvest will reduce the incidence of tuber rot.
When to Start Spraying
Fungicidal sprays have to be applied before the risk of blight becomes significant. In northeastern North America spraying usually begins in late July. In Europe, the first spray is traditionally done before the canopy closes on the rows when the plants are about 15 cm (6 inches) high. This is to protect the lower leaves which cannot be reached later when the foliage becomes too dense.
Late blight forecasts can help reduce unneccesary sprays by identifying the appropriate time for the first spray application and the frequency of spraying required. Forecast programs have been running in Canada since the early 1970s.
Relative humidity and temperature are monitored at the canopy level, and a computer model uses this information to forecast the risk of blight.
While regional forecast programs are primarily designed to increase the effectiveness of spraying, they generally help avoid one or two sprays (usually the early ones) as compared to following a schedule.
To further reduce the number of sprays, blight prediction software installed on your home computer and an on-farm weather station can help you determine accurately and rapidly when you should spray for your particular farm. This would require keeping track of temperature, rainfall and humidity in the field on a daily basis. Thus you would be working from your acutal on-farm conditions rather than more generalized regional information. Some farmers have found that the farm weather station and blight software pay for themselves quite rapidly with the savings in fungicide applications.
BLITECAST is a computer software available for individual farm use. Its forecasts are based on the cumulative rainfall in a 10-day period and average temperature over 5 days, as well as long-term cumulative values. It also considers the susceptibility of the cultivars being grown.
|A Rule of Thumb for Blight Forecasting
Many different forecasting systems, some of them simple, some of them complex, have been developed for late blight, beginning in the 1920's. One of the simplest ways to determine the timing of the first spray is to use the Beaumont rule: when the temperature stays above 10?C and the relative humidity above 75% for more than 48 consecutive hours then spraying should be carried out.
If you farm in a region where the disease pressure is high or if you are growing susceptible cultivars, then you probably spray according to a schedule. If this is the case then spraying at 7-10 day intervals is advisable. Plan to spray every ten days, but if moist conditions prevail, increase the frequency to every seven days. If you are growing very blight tolerant cultivars then, everything else being equal, spray every 10-12 days, if the cultivars are moderately tolerant to blight spray every 8-10 days and if they are susceptible to blight spray every 7 days.
Do not spray when it is wet or raining. Ideally spray before it rains.
Your sprayer should allow you to cover all of your potato acreage in two or three days (in case of persistent bad weather). Make sure the sprayer is well calibrated and that the nozzles are not clogged or providing low or unenven pressure. Low-volume sprays are usually as effective as diluted sprays.
To reduce damage to the crop, use a wide boom sprayer. You can also have rows of low-growing plants at intervals between the potato rows to allow access with less damage.
Newer spraying technologies such as electrostatic sprayers reduce the drift and drastically lower the amount of water required by charging the spray particles. This also gets more of the spray onto the growing leaves. Experience in England has demonstated that late blight was controlled at least as well when fungicides were applied electrostatically (4 to 9 litres/ha) as compared to hydraulically (200 litres/ha).
The significant amount of drift resulting from airplane applications makes this approach environmentally undesirable.
What to Spray
The least hazardous sprays to use are copper-based fungicides and compost extracts. Both are used by organic potato growers.
Copper-based fungicides were the only ones used from the end of the 19th century to the Second World War. Organic producers still rely on these non-synthetic type of fungicides to protect against late blight.
While some of these sprays like the Bordeaux mixture can be more phytotoxic to potatoes than newer synthetic fungicides their use does not create resistance in late blight. They also are more effective at staying on the plants when it rains. Copper is also superior to other sprays in protecting the tubers from infection towards the end of the season. However, the build-up of copper in soil is a concern, particularly on farms with poor crop rotation practices.
Copper-based fungicides have no effect on the fungus once it has infected the plant so they must be applied according to a rigid schedule. Copper oxychloride should be sprayed at 10-14 day intervals; copper sulphate + sulphur at 7-10 day intervals. Bordeaux mixture is sticky enough but other formulations should be applied with a wetting/sticking agent.
Copper-based fungicides come in different forms: copper oxychloride; Bordeaux mixtures; Burgundy mixtures; copper hydroxide (e.g. Kocide), etc. While commercial formulations are easier to handle, they also are more expensive.
|RECIPE FOR a 2% BORDEAUX MIXTURE
Different concentrations and proportions can be used to prepare a Bordeaux mixture. The most commonly used for potatoes is the 2% Bordeaux, a 2:1 mixture of copper sulphate and hydrated lime. Mix 10 lbs of copper sulphate and 5 lbs of hydrated lime in 100 gallons of water. (Metric: 1.75 kg of copper sulphate and 875 grams of hydrated lime in 100 litres of water).
There are three ways to make your own Bordeaux mixture.
Method 1: While the sprayer tank is being filled with water, keep the agitators turning, and sift in the copper sulphate. When the tank is nearly full (90%), sift in the hydrated lime (some people prefer to add the lime already diluted in a little water). Add enough water to fill the tank, keeping the agitators running during the whole mixing operation.
Method 2: Sift in the copper sulphate in half the volume of water in one container. Sift in the hydrated lime in the other half in another container. Pour the two solutions into the sprayer simultaneously, keeping the agitators turning.
Method 3: Same as method 2, but instead of pouring the two dilutions into a third one, you pour the copper sulphate solution in the lime solution (not the opposite!).
Methods 2 and 3 involve more manipulation but will produce a more stable mixture with finer particles that will not block the sprayer as much as in Method 1.
When spraying Bordeaux mixture, there should be enough water to wet the foliage but not enought that its drips off.
The pH of the Bordeaux mixture is critical: if the mixture is acid, its phytotoxicity is worse. Young plants in hot and dry weather are more susceptible to Bordeaux mixture phytotoxicity. The higher the pH, the finer the particles. The finer the particles, the better the coverage of the foliage.
|RECIPE FOR A 1% BURGUNDY MIXTURE
Burgundy mixture is easier to spray than Bordeaux mixture, having less tendency to clog the equipment. Burgundy mixture was popular in very humid regions like Ireland before the advent of synthetic fungicides. A 1% Burgundy mixture is made by mixing 4lbs of copper sulphate and 5 lbs of sodium bicarbonate in 40 gallons of water (metric:1kg copper sulphate, 1.25 kg sodium bicarbonate in 100 litres of water). Make sure you put the sodium bicarbonate in the copper sulfate and not the opposite. The opposite will result in an unstable product that will loose its effectiveness at high temperatures. 2% sprays are phytotoxic and do not give better control.
It has been demonstrated that watery compost extracts have anti-fungal properties. Once sprayed, compost "teas", as they are often called, actually result in a surface coating of live antagonistic bacteria on the foliage that can inhibit spore germination, compete with fungal pathogens or induce resistance against pathogens. Compost teas can provide similar control to chemicals for many plant diseases.
Results from using compost teas to protect against late blight have been inconsistent, with very good control being obtained in certain cases, and failure in others. The best results have been obtained using horse, dairy or goat manure compost with a 7-day "steeping" time.
|How to make compost tea (Brinton et al., 1996)
1. Preparation: mix fresh compost with tap water in a ratio of 1 to 5-10 by volume into a large clean tank or barrel well.
2. Steeping: keep at 15-25?C for 3 to 8 days stirring 2-3 times for 30 minutes.
3. Filtration: filter through a 200 mesh sieve (75 micron). Make sure you have the right spray rig nozzle to prevent clogging. Do not stir within 8 hours of filtering.
4. Spraying: apply with regular equipment. Spreader-sticker that are not toxic to microbes should be used (ex.: Nu-Film 17; Canola oil).
In Sweden, better results have been obtained against late blight by spraying with half the normal dose of fungicide once a week instead of the recommended dose every second week. However, this results in more traffic in the field and potentially more damage to the crop.
To help fight against the creation of more blight strains resistant to fungicides make sure you rotate your spraying between different types of fungicides. Systemics are particularly prone to induce resistance in blight. Try to limit their use to emergency situations such as when continuous rain makes it impossible to spraying with residual fungicides.
Up and Coming Products
Most new protective materials are still at the experimental stage. Canadian experiments with methionine-riboflavin mixture sprays indicate good disease growth inhibition. In Germany, research is being conducted on plant health enhancers that are active against late blight. Israeli research with various vegetable oil extracts has shown some induced resistance to late blight fungus.
Several research projects have been going on around the world on ways to induce resistance to late blight in potato plants, aimed at the development of a potato blight vaccine. Protection is obtained against the disease by injecting the stem or treating the lower leaves of a susceptible cultivar with components of the blight fungus, zoospores, etc. Resistance has also been induced with other materials than the blight fungus itself. Closely related fungi such as Phytophtora cryptogea (which does not cause blight in potatoes) has been used successfully. Experiments using salicylic acid (Aspirin) and fatty acids (especially arachidonic acid) have shown promise too.
Biological control involves using a living organism to compete with or prey on a particular pest. In the case of late blight, several soil fungi (Penicillium, Rhizoctonia and Trichoderma) have been found to inhibit its growth, while others seem to effectively compete with it (e.g. Aspergillus, Fusarium and Mucor). Cornell University researchers have developed a foliar spray composed of a common fungus, Fusarium proliferatum, which prevents blight infection when applied to foliage.
Just like the potato, the late blight fungus thrives in cool and humid conditions. It overwinters on tubers, releases spores from the growth of infected seeds, cull piles or volunteers, and spreads rapidly to growing vines under favorable temperatures and relative humidity. If neglected, the disease can seriously affect the quantity and quality of the crop. However there are a number of options that can help reduce the use of synthetic chemical fungicides.
The first lines of defence against late blight are preventative measures: well-aerated sites, wide row spacing, well-drained light soil, moderate and well-balanced fertilization, proper destruction of volunteers and cull piles, quality seed, high hilling, choice of cultivars, top-killing of vines two weeks prior to harvest and other measures.
Fungicides have to be sprayed before the disease is present. The number of sprays can be reduced somewhat with the use of blight forecasting, more so if the forecasts are made at the individual farm level. The use of synthetic fungicides should be rotated to reduce the rate at which resistant strains of blight are created. Organic potato growers use copper-based fungicides and/or compost teas to prevent blight. New types of more biologically-based sprays that are easier on the environment and safer to use are being researched and show promise.
Callbeck, L.C. 1968. Late Blight of Potatoes and Its Control. Publication 837. Canada Department of Agriculture. 11 pages.
Cox, A.E. and E.C. Large. 1960. Potato blight epidemics throughout the world. Agriculture Handbook No. 174. United States Department of Agriculture, Washington D.C.
Harrison, J.G. 1992. Effects of the aerial environment on late blight of potato-a review. Plant Pathology, 41:384-416.
Ingram, D.S. and P.H. Williams. 1991. Phythophtora infestans, the cause of late blight of potato. Advances in Plant Pathology, vol. 7. 273 pages.
Brinton, W.F., A. Tränkner and M. Droffner. 1996. Investigations into liquid compost extracts. Biocycle, November 1996: 68-70.
Pest Management Regulatory Agency. 1996. Integrated Management of Late Blight of Potatoes. Sustainable Pest Management Series Fact Sheet S96-02. 6 pages.
Platt, H.W. 1992. Potato Late Blight. Pp 93-123 In Chaube, H.S. et al. Plant Diseases of International Importance. Volume II. Diseases of Vegetables and Oilseed Crops. Prentice-Hall, Englewood Cliffs, New Jersey.
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