Tuber blight is a major economic problem in parts of North America (Dorrance and Inglis 1998; Glass, Johnson et al. 2001) and Europe (Schepers and van Soesbergen 1995; Bain and Moeller 1999).  Curiously, tuber blight does not appear to be a major problem in all production areas.  Minimal blight occurs in the Toluca Valley of Central Mexico where foliage blight is severe (Torres and Garcia E 1992), and tuber infection is infrequent in many highland tropical locations (Oyarzún, Garzón et al. 2003).  The exact reasons for this are not well known but some hypotheses can be formed based on what is known of factors affecting the development of tuber blight.

Mycelium of P. infestans infecting tuber sprouts.

The problem of tuber blight predates the relatively recent international spread of oospores and is thought to develop from sporangia and zoospores (Glass, Johnson et al. 2001) coming from foliage infection.  There appears to be little evidence that oospores present in the soil are significant sources of tuber infection (Flier, Kessel et al. 2002).   Infection generally occurs at eyes, lenticels or through wounds (Lacey 1967; Adams 1975).  Most researchers agree that sporangia wash down through soil and infect tubers directly or indirectly via zoospores (Andrivon 1995).  There appear to be few studies on sporangia transport in soils, but one done in a sandy loam demonstrated that 0.6 cm of water could carry sporangia to a depth of 40 cm (Dubey and Stevenson 1996).  Lacey (Lacey 1967) demonstrated the important role of channels within the soil and especially those formed by the stem.  He found more infection near the stolen end of the tubers and attributed this to water running down the stem.  Recent studies (Dubey and Stevenson 1996) indicate that sporangia arrive to the vicinity of tubers through various means in the soil and do not necessarily require channels.  These researchers found that mulching, high mounding and chemical treatment of soil could reduce but did not eliminate tuber infection when foliage infection was present.  Workers in the tropics, who have studied the phenomenon of reduced tuber blight, have focused on soil biology and soil chemistry as factors limiting tuber blight (Torres 1995; Garzón and Forbes 1999; Oyarzún, Garzón et al. 2003).  Biological (bacteria, actinomycetes and fungi) and chemical factors have been implicated in inhibition of P. infestans in soil (Lacey 1965; Andrivon 1994; Garzón and Forbes 1999) and are related to suppressiveness in soils of other Phythophthoras (Andrivon 1995).  Acid soils and high aluminum availability in soils, commonly found in the tropics, have been shown to inhibit P. infestans in soil (Andrivon 1995) and may therefore play a role in reduced tuber blight reported in acidic tropical soils (Oyarzún, Garzón et al. 2003).

Internal tuber blight caused by P. infestans.

Although tuber blight is known to be a major problem in the temperate zone, we have been unable to find good quantitative information for levels of tuber blight occurring in farmers’ fields.  Generally, qualitative terms (severe, bad) are used to describe tuber blight epidemics.  Quantitative data are available for some experiments, especially fungicide trials where tuber blight was one of the variables measured.   Under experimental conditions in Northern Ireland, incidence of tuber blight is frequently over 10% and can surpass 50% (Cooke and Little 1995; Cooke, Little et al. 1999).  Incidence of blight on tubers at intermediate depth in untreated plots in the US was over 60% one year and near 20% the second year (Dubey and Stevenson 1996).

The most commonly used means of protecting tubers against infection is by applying fungicides to foliage.  In theory, this approach can reduce tuber blight in three ways: i) reducing sporulation; ii) reducing viability of spores on leaves, and iii) when repeated applications cause residues to form in the soil that can inhibit formation or motility of zoospores (Schepers and van Soesbergen 1995), as zoospores are generally thought to be the principle infecting agents causing tuber blight (Sato 1979).  As may be expected, not all fungicides are equally effective at controlling tuber blight via foliar application (Schepers and van Soesbergen 1995).  Organic potato growers rely on other approaches to manage tuber blight including early removal or destruction of foliage prior to harvest, deep mounding (hilling up) and harvest under dry conditions (soil and air).  Potatoes should be stored dry and frequently monitored.  Potato genotypes differ for resistance to tuber blight (Wastie, Caligari et al. 1987), but the level of resistance currently available is not sufficient to protect plants from tuber blight. Resistance mechanisms apparently differ for foliage and tuber blight and in parts of Europe the pathogen population varies in its aggressiveness on tubers (Flier, van den Bosch et al. 2003).

Literature Cited

Adams, M. J. (1975). "Potato tuber lenticels: susceptibility to infection by Erwinia carotovora var. atroseptica and Phytophthora infestans." Annals of Applied Biology 79(3): 275-282.

Andrivon, D. (1994). "Dynamics of the survival and infectivity to potato tubers of sporangia of Phytophthora infestans in three different soils." Soil Biology and Biochemistry 26(8): 945-952.

Andrivon, D. (1995). "Biology, ecology, and epidemiology of the potato late blight pathogen Phytophthora infestans in soil." Phytopathology 85(10): 1053-1056.

Bain, R. A. and K. Moeller (1999). Factors influencing potato tuber infection by Phytophthora infestans. Third Workshop of a European Network for Development of an Integrated Control strategy of Potato Late Blight. Uppsala, Sweden. PAV-Special Report no. 5.: 210-227.

Cooke, L. R. and G. Little (1995). "Evaluation of fluazinam for the control of potato late blight. Tests of Agrochemicals and Cultivars." Supplement to Annals of Applied Biology 16: 28-29.

Cooke, L. R., G. Little, et al. (1999). Use of fluazinam in potato late blight control: sensitivity and field performance. Third Workshop of a European Network for Development of an Integrated Control strategy of Potato Late Blight. Uppsala, Sweden. PAV-Special Report no. 5.: 237-246.

Dorrance, A. E. and D. A. Inglis (1998). "Assessment of laboratory methods for evaluating potato tubers for resistance to late blight." Plant Disease 82(4): 442-446.

Dubey, T. and W. R. Stevenson (1996). "Factors affecting the movement and viability of sporangia of Phytophthora infestans in soil." Phytopathology 86 (11 SUPPL): S61-S62.

Flier, W. G., G. J. T. Kessel, et al. (2002). The impact of oospores of Phytophthora infestans on late blight epidemics. Late Blight: Managing the Global Threat, Hamburg, Germany, International Potato Center.

Flier, W. G., G. B. M. van den Bosch, et al. (2003). "Stability of partial resistance in potato cultivars exposed to aggressive strains of Phytophthora infestans." Plant Pathology 52: 326-337.

Garzón, C. D. and G. A. Forbes (1999). Suppression of Phytophthora infestans in six Ecuadorian Soils. Late Blight: A Threat to Global Food Security, Quito, Ecuador, International Potato Center.

Glass, J. R., K. B. Johnson, et al. (2001). "Assessment of barriers to prevent the development of potato tuber blight caused by Phytophthora infestans." Plant Disease 85(5): 521-528.

Lacey, J. (1965). "The infectivity of soils containing Phytophthora infestans." Annals of Applied Biology 56: 363-380.

Lacey, J. (1967). "The role of water in the spread of Phytophthora infestans in the potato crop." Annals of Applied Biology 59: 245-255.

Lacey, J. (1967). "Susceptibility of potato tuber infection by P. infestans." Annals of Applied Biology 59: 257.

Oyarzún, P. J., C. D. Garzón, et al. (2003). "Incidence of Potato Tuber Blight in Ecuador." American Potato Journal (submitted).

Sato, N. (1979). "Effect of soil temperature on the field infection of potato tubers by Phytophthora infestans." Phytopathology 69(9): 989-993.

Schepers, H. T. A. M. and M. A. T. van Soesbergen (1995). Factors affecting the occurrence and control of tuber blight. Phytophthora infestans 150. L. J. Dowley, E. Bannon, L. R. Cooke, T. Keane and E. O'Sullivan. Dublin, Ireland, Boole Press Ltd.: 171-176.

Torres, J. M. (1995). Supresividad de suelo al ataque de Phytophthora infestans (Mont) de Bary a tubérculos de papa. XVII Reunión de la Asociación Latinoamericana de la Papa (ALAP), Mérida, Venezuela.

Torres, J. M. and R. Garcia E (1992). "Supresividad del suelo al ataque de Phytophthora infestans (Mont) de Bary en tubérculos de papa en Toluca, México." Fitopatologia (Peru) 27(2): 61.

Wastie, R. L., P. D. S. Caligari, et al. (1987). "A glasshouse progeny test for resistance to tuber blight (Phytophthora infestans)." Potato-Research 30(4): 533-538.