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Information on disease incidence, damage and losses due to late blight

Late blight of potato (known as la candelilla or tizón tardío in Venezuela), caused by the pseudofungus Phytophthora infestans found in almost all areas where potatoes are grown, causing losses of up to 100% when infection occurs during critical stages of plant development and before tuberization (Maffei et al., 1998;García et al., 1995, 1998, 1999; León and Varela, 1995). (Mont.) de Bary, is the most important disease of the crop. It is found in almost all areas where potatoes are grown, causing losses of up to 100% when infection occurs during critical stages of plant development and before tuberization (Maffei et al 1998 García et al., 1995, 1998, 1999; León and Varela, 1995).

 

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1 Translated from Spanish. E.N Fernandez-Northcote (ed). 2002. Memorias del Taller Internacional Complementando la Resistencia al Tizón (Phytophthora infestans) en los Andes, Febrero 13–16, 2001, Cochabamba, Bolivia, GILB, Taller Latinoamérica 1. Centro Internacional de la Papa (CIP), Lima, Peru.
2 INIA, Mérida, Venezuela, Email: cianmh(at)cantv.net
3 International Potato Center (CIP), Lima, Peru; Version 2003.0.0; Data sources Hijmans 2001; FAO (2000 - 2002); Update by Research Informatics Unit (cip-riu(at)cgiar.org). This map does not imply any opinion whatsoever on the part of the International Potato Center concerning the legal status of any country or area, or its authorities, or the delimitation of its frontiers.

 

Fungicides used for late blight control

The fungicides used are indicated in Table 1.

 

 

In 1987, Alcalá and Ramírez tested chlorothalonil (Bravo 500) at several application frequencie to control late blight on the cultivar Kennebec in Las Cuibas, in the state of Lara. They obtained effective control with applications at 4–7day intervals.

In 1999, Mafei et al. evaluated five commercial products singly and in combination to control late blight on the cultivar Granola in Mucuchíes, in the state of Mérida. They obtained best results with mixtures based on chlorothalonil + cymoxanil-metiram.

In a study to evaluate control strategies for late blight, García and García (1999) found that Strategy 3 (Table 2) with sprays every 4–7 days depending on environmental conditions and a total of 7–8 applications per growing season reduced disease incidence from 90% to 35%, increased yields by 10 t/ha, and decreased the percentage of diseased tubers by 15% (Table 3).

The evaluation took place at two sites with intermediate to high levels of disease using selected seed tubers of the cutivar Granola produced by the farmers.

 

 

Potato producers apply combinations of four fungicides with 12–15 applications per growing season. Up to 15 are made under rainy conditions, when the risk of late blight is greatest, while under dry conditions up to 8 applications are made.

 

Producers spray their crops according to their practical experience, i.e. empirically. They consider the rainy season of June to September when there is high humidity and descending mists as favorable to late blight development and carry out routine preventive treatments every 7 days using mixtures of different commercial products.

 

There are no official recommendations: Only advice given by researchers and extension organizations.

 

Only rarely do farmers take into account the level of genetic resistance of the cultivars used for the application of fungicides. Few use resistant cultivars. Of these, some have a general knowledge of how these cultivars react in the field and how to manage them because they have taken part in field and acceptability trials for the adoption and release of such materials. Even so, they still make 3–4 applications as a security measure, while susceptible cultivars are given 12–15 applications in blight areas.

 

Above 2800 masl does not develop rapidly. They treat only for security reasons at least 6 times per cropping cycle. P. infestans is not endemic and farmers know by experience that the disease

 

Adaption to climatic conditions

Farmers do not make use of meteorological information. They decide whether or not to make applications based on the time of year, summer or winter. As indicated earlier, in the rainy season when there is greater risk of late blight due to favorable conditions for the development of the disease, routine applications are made at intervals of 2–7 days, with a total of 13–15 treatments per cropping cycle (three months). In the dry season, treatments are made every 8–15 days with a total of 6–8 treatments.

 

Farmers do not know the importance of meteorological information, and therefore are not interested in it.

 

Fungicide resistance

The intensive use of metalaxyl has provoked resistance to this fungicide in the pathogen in some important areas of potato production, such as Municipio Pueblo Llano in the state of Mérida, where metalaxyl is losing its ability to control the disease.

 

Rodríguez (1996) used sensitivity to the fungicide phenylamide as a marker in a study to characterize the population of the P. infestans in Venezuela.

 

In a study to evaluate chemical control strategies for late blight, García (1999) found that strategies where Ridomil was sprayed at emergence or intermediate developmental stages of the crop did not control the disease effectively.

 

Aponte and Jiménez (1990) reported increasing variation in the pathogen population in the state of Lara.

 

Integrated Management

Fungicides are the principal method potato producers use to control late blight. They apply different fungicides sequentially, often employing fungicides with the same active ingredient in an inadequate manner. For example applications begin when the disease has already appeared, consecutive treatments are made with a systemic fungicide without any restrictions as to the number of applications, and active ingredients not recommended for blight control are used.

 

Another control practice under adoption is disease avoidance. Planting is scheduled during the summer months to avoid conditions favorable to blight. However, planting is also dependent on the sale price of potatoes and when this is high farmers will plant anytime in the year.

 

Institutional set up

Information concerning fungicide use is principally obtained from vendors and advisors in agrochemical companies and suppliers.

 

Information about other management practices is obtained from INIA (the national agricultural research institute) and extension groups through extensionists working in the area.

 

Other relevant information

Studies of the epidemiology of the disease and of the characterization of the pathogen population are still underway in Venezuela, but practical strategies have been devised and areas of high, intermediate or low late blight incidence have been identified (Table 4).

 

Literature cited

• Alcalá, D. y Ramírez, M. 1987. Efecto de la aplicación de tres dosis y tres frecuencias de fungicida Bravo 500 (Clorotalonil) en el control de la candelilla tardía de la papa. pp. 53 en:Memorias X Seminario Nacional de Fitopatología. Maracay – Venezuela.
• Aponte, A. y Jiménez, B. 1990. Determinación de razas fisiológicas de papa en el estado Lara, Venezuela. Fitopatol. Venezuela 3:29–34.
• García, R. 1998. Informe del Proyecto Manejo Integrado de las Principales Enfermedades delCultivo Papa en Venezuela. FONAIAP – Mérida. Venezuela (Mimeografiado).
• García, R. 1999. Productores conocen bondades de la variedad Tibisay lograda por el CIAEMérida.Prensa Andina. Agosto – Mérida.
• García, R. y García, A. 1999. Evaluación de estrategias para el control químico del tizón tardíode la Papa en dos localidades del estado Mérida. pp. 75 en: Memorias II Congreso Venezolano de Ecología. SOVECOL 99.
• García, R., García, A. y Delgado, L. 1998. Evaluación de seis funguicidas y sus combinaciones en la efectividad de control de Venezuela. VI Simposio Internacional de Sanidad Vegetal de la Agricultura Tropical. San Clara – Cuba.
• García, R., León, E. y Meneses, L. 1995. Selección de clones de papa por resistencia a Candelilla tardía y Marchitez bacteriana. FONAIAP Divulga, 48: 38–40.
• León, R. y Varela, R. 1995. Caribay una nueva variedad nacional de papa. FONAIAP Divulga 48:23–25.
• Maffei M., Quintero, I. y García, R. 1999. Combinación de fungicidas protectivos y sistémicospara el control químico de la Candelilla tardía de la papa en Mucuchíes, estado Mérida, Venezuela. Rev.Fac. Agron (LUZ). 16:517–527.
• Rodríguez, D. 1996. Informe de Gestión. FONAIAP Trujillo. Venezuela. (Mimeografiado).
• Phytophthora infestans enPhytophthora infestans en tres localidades del estado Mérida,Phytophthora infestans (Mont.) de Bary,