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TAN SPOT AND SPOT BLOTCH OF WHEAT IN PARAGUAY

L DE VIEDMA1 and M MOHAN KHOLI2

1Centro Regional de Investigacion Agricola - DIA/MAG, Km 16, Ruta VI, Itapua, Paraguay; 2CIMMYT Programa Regional, Andes 1365, Esc. 314 11.100 Montevideo, Uruguay

Background and objetives
Historically wheat has not been a traditional crop in Paraguay. With the significant increase in soybean production over the past two decades, the area of wheat during the winter has also increased. However, high temperatures and humidity are favourable for severe epidemics, especially rust, powdery mildew and foliar and head blight of wheat [1]. These diseases are limiting factors for stable wheat production in the country. Foliar blight is caused by spot blotch (Bipolaris sorokiniana) and tan spot (Drechslera tritici-repentis), two very important wheat diseases. Spot blotch, combined with other diseases, was responsible for a grain reduction of almost 70% in the mid 1970s, while tan spot has recently become important. Increase in tan spot prevalence is primarily associated with the increase in cropping area under zero tillage [2]. This paper discusses an integrated disease management strategy that has been developed to reduce losses caused by these diseases.

Results and discussion
The national germplasm development program has been successful in releasing commercial varieties including IAN 8, IAN 9, ltapua 35 and ltapua 40, that allow only moderate infection levels. Seeding these varieties every third or fourth year on oat or vetch stubble has significantly reduced infection levels; however, in a year of severe disease, chemical control is essential. Chemical control of black point in grain, caused by B. sorokiniana, has been successfully achieved using iprodione, guazatine and triadimenol fungicides at a rate of 200 g/kg seed [3].

Foliar applications of systemic fungicides such as tebuconazole, propiconazole, flutriafol, ciproconazole, fluzilazole and metconazole, applied between heading and grain filling stages, have achieved cost-effective disease control. Under severe disease levels, a double application of fungicide can result in a grain yield increase of 38-61%.

References
1. Kohli MM, Mann MM, Rajaram S, 1990. Proceedings of the International Conference: Wheat for the Nontraditional Warm Areas, pp. 96-112.
2. Viedma L, Kolhi MM, 1997. Abstracts, International Workshop on Helminthosporium Blights of Wheat. CIMMYT, El Batan, February 9-14, 1977.