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The symptoms of late blight on potatoes and tomatoes may vary, depending on the age of the lesion and the immediate preceding environment (previous 12 h). Very young lesions on potato or tomato foliage appear as irregularly shaped, small (2-10 mm) lesions with or without a small surrounding area of collapsed but still green tissue. Lesions later turn brown. Older lesions are larger and assume a circular appearance unless delimited by the leaflet margin. They are usually not delimited by the veins. Older lesions are typically surrounded by a zone of collapsed tissue that is not yet necrotic. The non-necrotic tissue may also appear somewhat chlorotic. If there are many lesions on a single leaflet, the entire leaf can turn chlorotic.
Sporulation may be evident on the collapsed tissue and on the outermost portions of the necrotic areas of a lesion if it has been in a saturated atmosphere (100% RH) for more than 7 or 8 h. The length of time required for sporulation is dependent on temperature and host resistance. On resistant cultivars, sporulation might not appear until some hours after it would appear on a susceptible cultivar. Optimal temperature for sporulation is usually regarded to be 15-20°C. Temperatures above or below this range will reduce the rate of pathogen growth and thus extend the time required for sporulation. Under optimal conditions for sporulation it is easily visible as a noticeable fuzzy white growth on lesion margins. Sporulation occurs from lesions whether they are on leaflets or on stems.
When the immediately preceding conditions have been dry, there is no sporulation and the lesions may appear dried up with no remnants of sporulation.
Blighting of the entire plant (even entire fields) occurs during moist warm periods. Patches of infected plants have a characteristic odour.
Infected potato tubers exhibit wet and dry rots. On tomato fruits, lesions are firm, large, irregular, brownish-green blotches; the lesion surface has a greasy, rough appearance.
Both reduction in the amount of initial inoculum and suppression of pathogen growth rates are important in the suppression of late blight of potatoes and tomatoes. At this point, there is no biological control of known efficacy for use in suppressing late blight. Several fungicides have been shown to have a curative effect in tuber-borne P. infestans (Inglis et al., 1999). Thiophanate-methyl plus mancozeb applied to blighted potato seed pieces reduced the amount of surface colonized by P. infestans and when planted had higher emergence in two locations.
Reduce Initial Inoculum
For potatoes, it is important to plant healthy seed tubers, so that the pathogen is not imported with seed tubers. Other sources of inoculum in a growing region should be eliminated. These include any place where infected potato tubers might reside: piles of cull potatoes, or unharvested potato tubers that survive from one season to the next. Because sporangia of P. infestans can be dispersed aerially, late blight is a 'communal' disease. It is important that all growers in a production region collaborate to eliminate sources of inoculum. If this doesn't happen, a few fields with infected plants can jeopardize production in an entire region.
Tuber infections can be limited by increasing the depth of the soil barrier that protects the tubers. This can be achieved by constructing deep hills over the tubers to lessen the probability of tuber infections. Theoretically, tuber infections can be prevented by systemic fungicides when the pathogen is sensitive to those fungicides. For example metalaxyl was very effective at preventing tuber infections caused by metalaxyl-sensitive strains of P. infestans (Fry et al., 1979). However metalaxyl has almost no effect against strains that are insensitive.
Limit Pathogen Growth Rates
Late-blight-resistant cultivars and periodic application of fungicides limit pathogen growth rates. Both are effective and can be used together. In some agroecosystems, cultivars with very high levels of resistance are available and these alone are sufficient to suppress late blight. In other locations, such highly resistant cultivars are not available, and fungicides are also required. Note that some fungal strains are insensitive to some fungicides such as metalaxyl.
Many 'forecasting' schemes have been developed to improve the efficiency with which fungicides are used (Van Everdingen, 1926; Beaumont, 1947; de Weille, 1964; Krause and Massie, 1975; Connell et al., 1991). Most of these schemes identify when the first applications of protectant fungicide should be made in each season; such schemes operate in areas of defined seasons and not in the highland tropics where planting can occur throughout the year. In general, such schemes have successfully identified when periodic protectant fungicide applications should commence. Some forecasting schemes have also attempted to identify the subsequent frequency of fungicide applications. The efficacy of this approach is less certain.
If a hot spot of late blight appears in a field, growers should destroy that section of the field as rapidly as possible and perhaps also increase the frequency of fungicide application in surrounding areas.
Late blight of potatoes or tomatoes can be a devastating disease with dramatic and disastrous economic consequences. It is known as the most devastating disease of potatoes and one of the most devastating plant diseases of any crop. When conditions favour pathogen development and there are no steps taken to suppress the disease, late blight can completely destroy the above-ground parts of plants (stems, leaves, tomato fruits) and can also affect potato tubers. The disease is a very serious economic threat in the vast majority of potato-production systems, and in many tomato-production systems. In locations where disease pressure is high, protectant fungicides may need to be applied as frequently as twice per week.
The Irish potato famine provides a grim indication of the destructive potential of P. infestans. The disease was first detected in Ireland in 1845 and for the succeeding several years it devastated the potato crops (Bourke, 1993). More than a million Irish died from starvation and at least another million emigrated. The population of Ireland declined steadily after 1845 from a high of about 8.5 million to just over half that by the end of the nineteenth century.
The Global Late Blight Initiative
The International Potato Center, known by its Spanish acronym, CIP, has made an attempt at estimating global losses due to potato late blight. The information that follows can be found on the CIP web page.
The acceptance of the late-blight-resistant cultivars to be developed under the global late blight initiative (GILB) is consistent with the economic benefits to be derived by reducing fungicide inputs and crop damage in developing countries. For the purpose of projecting potential returns on investment, CIP has conservatively estimated current losses from late blight at approximately 15% of annual production in developing countries. Assuming that a third of global potato production now occurs in developing countries (FAO/CIP, 1995), an equivalent or proportional share of these losses must occur there as well. Thus, 15% of one-third of 275 million tons (annual global potato production) is equivalent to 13.75 million tons. According to the CGIAR Technical Advisory Committee, a reasonable estimate of global producer prices for potatoes is US$200 per ton. Hence, the annual economic value of crop losses from late blight in developing countries is said to total $2.75 billion. In 1976, late-blight-related crop losses occurred despite increases in fungicide use. At the time, estimated global fungicide costs exceeded $1 billion annually. Again, for the purpose of the GILB, CIP conservatively estimates current fungicide costs to be on the order of $100 million annually in developing countries, that is, an average of $15 per hectare. Hence, crop losses (production forgone) and increased costs (expenses incurred) for late blight begin to approach the $3 billion mark. It should be noted that these figures represent only current losses.
Late blight is a major problem throughout Northern and Eastern Europe. In Poland it causes significant losses (Piekarczyk and Babilas, 1986) that have been estimated at about 22% (Pietkiewicz, 1991). In Romania, in experiments conducted in 1982, there were losses of about 40% on the susceptible cultivar Bintje and losses of between 6 and 30% on other cultivars (Cupsa et al., 1983). In 1990 late blight was reported as increasing in importance as potato acreage increased (Bicici and Cinar, 1990).
The severity of late blight was recently reported to be increasing in Hungary, but quantitative data were not given. The increased severity was associated with the presence of the A2 mating type, as well as increased specific virulence and fungicide resistance of the pathogen population (Bakonyi and Ersek, 1997).
In Denmark, yield losses were simulated for pesticide-free agricultural systems. Losses due to potato late blight were among the highest of all crop systems considered, indicating the potential destructiveness of this disease in that country (Jorgensen et al., 1999).
Fungicide use comprises one of the simplest ways to estimate economic loss attributed to a disease and therefore is frequently used in developing countries. A survey done in Eastern African countries in the early 1990s (Kalyebara, 1994) demonstrated that fungicide use in this part of the world was highly variable, even among neighboring countries. In Zaire, fungicides were almost never used, in Burundi somewhat more and in Rwanda, most farmers sprayed at least 3 times. In Kenya, commercial farmers sprayed 5-7 times; smaller semi-commercial farmers spray 3-5 times. The most common product was Dithane, but Ridomil was used to some extent. Although production losses were not estimated in this survey one can probably assume to some degree an inverse relationship between fungicide use and losses; in those countries where fungicides were used more, production losses would be less. Workers in those parts of the world and even recent simulation efforts indicate that fungicides are generally poorly utilized, and frequently under-utilized in sub-Saharan Africa (Hijmans et al., 2000).
This view is consistent with a study undertaken in Burundi in 1989-90, where 32,000 t of potatoes were produced on 10,000 ha of land. Late blight was the most important disease causing yield losses up to 40% (Higiro and Danial, 1994).
Late blight is also important in western Africa. Yield reductions were found to vary between 25 and 71% in experiments done in Cameroon in 1990 and 1991. These data reflect comparisons of fungicide-treated and untreated plots under experimental conditions. Disease was more severe in Bansoa than it was in Dschang (Fontem and Aighewi, 1993).
Experiments conducted in India demonstrated loss potential of 39 and 37% due to late blight in two consecutive years. Two sprays of fungicide were able to control disease in these years (Rao and Veeresh, 1989). Bisht (Bisht et al., 1997) estimated that yield losses can be much higher, about 65%, in higher altitudes of India.
Yield losses in Bhutan were estimated as between 20 and 90%, but this was for a complex of disease including late blight, early blight (Alternaria solani), wart (Synchytrium endobioticum), black scurf (Rhizoctonia solani), brown rot (Ralstonia solanacearum), blackleg (Erwinia carotovora subsp. atroseptica) and potato virus Y (Shrestha et al., 1986).
Late blight was apparently introduced recently in Pakistan (Khan et al., 1985). At that time (1980s) it caused complete crop loss in some cases, presumably due in part to lack of control measures. Workers in the region indicate that late blight is still an important problem (CIP, unpublished data).
Fungicides cost Andean farmers a lot of money. A recent survey estimated costs of about $150.00 per ha for a single season (Ortiz et al., 1999). This survey, however, was done in a particularly dry period and probably underestimates costs. The cost of spraying a field 15 times, which is not uncommon during rainy periords, would be closer to $600.00, depending on the type of fungicides used.
Based also on farm surveys, Andrade and Revelo (1994) estimated losses in Ecuador in 1992 to be about US$ 2.4 million. They added that 1992 was a dry year and that this figure would double in a wet year. Ecuador is probably a good indicator for fungicide use in other Andean countries.
The economic consequences were estimated for one epidemic occurring in 1995 in the Columbia Basin of the state of Washington in the USA. The mean number of fungicide applications per field varied from 5.1 to 12.3, depending on cultivar. Total per acre expenses (application costs plus fungicide material) ranged from $106.77 to $226.85, depending on cultivar and location. Approximately 28% of the crop was chemically desiccated before harvest as a disease management practice for the first time in 1995, resulting in an additional mean cost of $34.48/acre or $1.3 million for the region. Harvested yields were 4 to 6% less than in 1994. The total cost of managing late blight in the Columbia Basin in 1995 is estimated to have approached $30 million (Johnson et al., 1997).
Avoiding Crop Losses
Avoiding disease can be an effective disease management strategy. Farmers in the Andes plant susceptible potatoes at high altitudes where low temperatures reduce late blight pressure (Thurston, 1994). However, this strategy is frequently used in such a way that farmers trade off yield potential for decreased risk of disease. One survey in Ecuador estimated that between 30 and 40% of potato production in the province of Cotopaxi (central Ecuador) was done in the dry season to avoid late blight (Instituto Nacional de Investigación Agropecuaria, unpublished data). Yields in the dry season are considerably lower. Similarly, much of the potato production in the highlands of Ethiopia occurs during a period known as the "short rains". Yields are low during this period because of limited water supply, but the risk of losses due to blight is also reduced. Overall production in this country could be increased by the introduction of potato cultivars with resistance to late blight that could be planted in the main rainy season.
Late Blight on other Hosts
Late blight can be a devastating disease of tomato. Although tomato is generally an intensively-cultivated crop and farmers therefore justify the expense of fungicides, several factors make late blight a particularly difficult problem. First, there is very little resistance available in commercial tomato cultivars (Oyarzun et al., 1998), which means that with favourable weather conditions it is difficult to manage the disease even with fungicides. Second, unlike potato, the edible portion of tomato is directly exposed to fungicide applications. This complicates management practices near harvest time. Finally, pathogen populations from tomato and potato appear to be separate and adapted only to one host (Oyarzun et al., 1998). This means that tomato workers can not readily apply information gathered on the pathogen population from potato. Late blight is the most important disease of tomato in some developing countries.
P. infestans (or a very closely related species) is a pathogen of other cultivated hosts that only occur in specific regions of the world. One host is tree tomato (Solanum betaceum), which is economically important in certain parts of the Andes. Late blight of tree tomato appears to be important in specific climatic zones. In one valley in Ecuador known as San Jose de Minas, tree tomato production was abandoned because of late blight. Late blight of pear melon (S. muricatum) is also a limiting factor for producers in the Andes. The pathogen populations of tree tomato and pear melon are also host adapted and do not pose a threat to potato. Both tree tomato and pear melon are cultivated outside of the Andean region but it is not known if blight attacks these crops in other parts of the world.