Characteristics of wheat leaf diseases development in Latvia

The field observations were carried out in 1999–2004 on 29 fields in Latvia. Incidence and severities of wheat leaf diseases were determined. Tan pot caused by Drechslera tritici-repentis and Septoria leaf blotch, induced by Septoria tritici were the most harmful diseases at the time of research (incidence 10–100% and 1–100%, respectively). Also the incidence of powdery mildew, caused by Blomeria graminis was high (4–100%), while that of rusts (Puccinia tritici and P. striiformis) was very low. Changes in disease epidemics were determined and showed the differences between the analyzed diseases.


INTRODUCTION
Winter wheat is one of the most important cereal crops in Latvia.Diseases, especially leaf diseases, of wheat are causing important losses of the yield.Since the 1990 wheat has been grown under increasingly intensive management regimes in part of the farms.These changes in the cropping system have increased incidence and severity of the wheat diseases (B a n k i n a , 2002).Assessment of incidence and severity of each disease and epidemics analysis are the main tasks for the present.
Changes in the epidemics of disease are reflected by alterations in the disease progress curve.The most important parameters are rate of infection, shape of the curve and area under the disease progress curve (C a m p b e l l and M a d d e n , 1990).
The primary infections of winter wheat crops by leaf spot diseases -tan spot, caused by Drechslera tritici-repentis (teleomorph Pyrenophora tritici-repentis) and Septoria leaf blotch, brought about by Septoria tritici (teleomorph Mycosphaerella graminicola) are initiated in autumn by air-borne ascospores of the teleomorph stage, which originate from sources outside the crop.The subsequent disease development was commonly ascribed to the anamorph stage, which develops to give rise to splashborne conidia.(F r a n c l , 1998; H u n t e r et al., 1999).
Powdery mildew, caused by Blumeria graminis was observed every year, but it is harmful only in some cases under the conditions of Latvia.Development of powdery mildew depends mainly on wheat density, level of nitrogen and variety.Leaf spot diseases were recorded to a rather high level in the research period, especially tan spot.
The detailed studies of the life cycle, distribution and dynamics of development of wheat leaf diseases help to understand better the development and propagation of diseases what may improve control strategy.

MATERIAL AND METHODS
The field observations were carried out in different localities during 1999-2004.Development of diseases was investigated in areas without fungicide application.Different wheat varieties were included into the research program, total number of observation areas -29.
All agronomic requirements were noticed in the observation areas.Seed dressing, herbicides and high doses of nitrogen were applied in all the cases.
Assessments of diseases were made on the upper three leaves every week from the start of stem elongation (GS 31) to dough stage (GS 85).Incidence and severity of diseases were determined (expressed in %).
Causal agents of diseases were determined in a laboratory by investigation of pycnidias and morphology of conidiophores and conidia.Moist chambers were used for development of conidia.
Average severity and incidence were calculated for analyzing the economic importance of diseases.
The meteorological conditions were rather different during the experimental period.The season was extremely dry in 1999 restricting the development of diseases.
A similar situation was observed in 2002.Vegetation seasons of 2000, 2003 and particularly of 2001 and 2004 were favorable for the diseases development, with the wet and rainy summer stimulating the occurrence of leaf spot diseases.stage, which first arises from primary ascosporic lesions and develops to give rise to splash-borne pycnidiospores produced in pycnidia (H u n t e r et al.,1999, G l a d d e r s et al., 2001).The relative contribution of ascospores and conidia to the development of tan spot under the Latvian conditions needs further studies.LITERATURE B a n k i n a B ., 2002.Some aspects of epidemiology of fungal diseases observed on foliage of wheat in Latvia, 1998-2000.Petria Giornale di Patologia delle Piante, 12 (1/2): 193-197.B a n k i n a B ., 2003.Tan spot development peculiarities in Latvia.Plant Prot.Sci., 2: 381--383.C a m p b e l l C .L ., M a d d e n V. L ., 1990: Introduction to plant disease epidemiology.A John Wiley& Sons.INC New York.: 161-187.C o x D .J ., H o s f o r d R .M ., 1987: Resistant winter wheat compared at differing growth stages and leaf positions for tan spot severity.Plant Dis.71: 883-886 F r a n c l L .J ., 1998: Genesis and liberation on conidia of Pyrenophora tritici-repentis / Can.J. Plant Pathol.20: 387-393.G l a d d e r s P. , P a v e l e y N .D ., B a r r i e I .A ., H a r d w i c k N.V., H i m s M.J., L a n g t o n S ., T a y l o r M .C ., 2001.Agronomic and meteorological factors affecting the severity of leaf blotch caused by Mycosphaerella graminicola in commercial wheat crops in England.Ann.Appl.Biol., 138: 301-311 H u n t e r T. , C o k e r R .R ., R o y l e D .J ., 1999.The teleomorph stage, Mycosphaerella graminicola, in epidemics of septoria tritici blotch on winter wheat in the UK.Plant Pathology, 48: 51-57.W r i g h t K .H . , S u t t o n J .C , 1990: Inoculum of Pyrenophora tritici-repentis in relation to epidemics of tan spot of winter wheat in Ontario.Can.J. Plant Pathol., 12: 149-157.