6.106
DISTRIBUTION OF WINTER WHEAT DISEASES IN LATVIA

B BANKINA

Department of Plant Protection, Latvian University of Agriculture, Liela 2, Jelgava, Latvia LV 3001

Background and objectives
Winter wheat is one of the most important cereals in Latvia. Diseases of wheat are causing important yield losses, especially Erysiphe graminis, Septoria spp. and Puccinia spp. There is an increasing need for data on disease development to be transferable, so that comparisons may be made between seasons, locations and investigators. In the period 1995-97, original and adapted (from the Nordic states) methods of registration and estimations for cereal crop diseases were developed, evaluated and approved by the Department of Plant Protection of the Latvian University of Agriculture and the Department of Forecasts and Diagnostics of the State Plant Protection Station [1].

Materials and methods
Distribution and intensity of winter wheat diseases were registered in 15 fields in 1996 and in 22 fields in 1997, in the different regions of Latvia. Assessments were carried out each week from the start of tillering to full ripening. Data on development of diseases have been used to build databases in order to research warning systems and thresholds for fungicide applications.

Results and conclusions
The first symptoms of powdery mildew (E. graminis) and Septoria leaf blotch (Septoria tritici) were registered only in some fields at the time of shooting in 1996 and 1997. Distribution and intensity of diseases were not high at this time. Assessment of disease distribution is very important at the time of ear formation, because it is the recommended time for application of pesticides. E. graminis was observed in most of the fields, but distribution was not important - only 4-50% of plants were infected, and intensity (infected area of leaves) was only 3-10% in 1996. At this time the distribution of Septoria spp. was observed. Symptoms of this disease were noted in all fields, distribution was 5-80%, and intensity 1-13%. Puccinia striiformis and Puccinia recondita were observed in only one field, and intensity was very low. E. graminis, S. tritici and S. nodorum were also observed in 1997, although distribution and intensity were lower. Conditions for development of Septoria spp. were not favourable in this spring, because only 5 rainy days were recorded at the time of stem elongation. Drechslera tritici-repentis was observed in 1997; distribution of this disease was 2-80%. Until 1997 this disease was seldom observed, but in 1997 damage caused by this disease was higher than damage of Septoria spp. At the time of ripening, high levels of distribution of E. graminis were observed (100% infected plants in half of the fields studied), and also of Septoria spp. (distribution ca 100% on the leaves in all fields, and infected ears in half of the fields studied in 1996). At this time all fields were infected by S. tritici and S. nodorum (distribution ca 100%), and half of the fields were infected by D. tritici-repentis in 1997. Speedy development of these diseases was observed at the time of ear formation and flowering, because of heavy rainfall (200% of normal precipitation) [2]. Other diseases were not observed in 1997.

Disease distribution was mostly dependent on weather conditions. But disease development was also influenced by growing conditions and other reasons. The collection of a large amount of data is required in order to understand the interacting factors that may be affecting disease development - this will be researched in the future and a database built up.

References
1. Turka I, 1997. Jelgava: LUA, 24 61 (in Latvian).
2. Hansen JG, Secher BJM, Jorgensen LN, Welling B, 1994. Plant Pathology 43, 183-189.