Occurrence of necrotrophic leaf pathogens in wheat and their relation to symptom development in Hungary (2000-2002)

Summa r y 1879-2720 leaf samples from 8-13 stations of Hungary were collected in March, April, May and June 2000-2002. Drechslera tritici-repentis, Septoria tritici, Stagonospora nodorum and Bipolaris sorokiniana were found in the leaf samples. The occurrence of necrotrophic pathogens was highest (10,79%) in 2001 and lowest (2,63%) in 2002. The occurrence and rate of the necrotrophic pathogens changed significantly among years and locations. The resistance of cultivars based on natural infection could not be properly evaluated, as the level of the epidemic was mostly low and contradictory among locations. Additionally forty four cultivars were tested in Szeged after winter wheat in protected and unprotected environment. Among the necrotrophic pathogens, the dominant pathogens were D. tritici-repentis (2001 and 2002) and S. nodorum (2001). The differences in variety resistance were significant. The biotrophic (leaf rust, yellow rust and powdery mildew) as well as the mentioned necrotrophic pathogens caused significant yield decrease in both years. According to values of correlation coefficients, the influence of biotrophic pathogens was greater on the yield.


INTRODUCTION
The infections by D. tritici-repentis, S. tritici, S. nodorum and B. sorokiniana significantly increased due to the poor economic situation, increasing monocultural and minimum tillage practices in Hungary at the late 90s. Medium epidemic caused by S. tritici what was observed in 1996 and 1999 in the Szeged nurseries. In 1996, Pál Békési recorded in Kecskemét a medium level of epidemic. The two data series often showed significant differences for the same cultivars.
D. tritici-repentis was described first in Hungary by A p o n y i et al. (1988). The data basis is poor; mostly data of monocultural production were reported (B al o g h et al., 1991; R á t a i and P e c z e 1997). Severe epidemics occurred only under these conditions, in the nurseries where the previous crop was canola or peas, only sporadic symptoms occurred that not allowed differentiation in resistance. Under these conditions only 1-2 cultivars showed significant infection like GK Mura (R ö j t ö k m u z s a j and Tá p l á n s z e n t k e r e s z t 1999). A consequent resistance testing and breeding work has not been made until now (F a r k a s 2000).
For these diseases, especially for D. tritici-repentis the identification of the disease by leaf symptoms is often problematic as another saprophytic pathogens like B. sorokiniana, Fusarium leaf spot, Alternaria, etc. may cause similar symptoms. Among them the identification of S. tritici is the less problematic.
To clarify the identification problems, describe the symptoms more precisely and characterize the resistance of cultivars we started a new project: The aims of this work: to receive data about the change of occurrence of the necrotrophic pathogens, studying resistance of the winter wheat cultivars at natural infection, studying resistance and yield reaction of the winter wheat cultivars after previous winter wheat.
We scored the leaf spots, powdery mildew (Blumeria graminis), leaf rust (Puccinia triticina) and yellow rust (Puccinia striiformis). Leaf samples were collected from each cultivar and identified by microscope for D. tritici-repentis, S. tritici, S. nodorum and B. sorokiniana, other pathogens were not found. After harvesting we measured the yield and the results were evaluated by two-way ANOVA.

1.) Change of occurrence of necrotrophic pathogens:
Epidemiologically the four diseases have different patterns. The D. tritici-repentis and the S. nodorum was the highest in June and the S. tritici in March and April and B. sorokiniana was similar in each month, but the measure of the occurrence was not higher than 20 % as an average of three years (Table 1). These data are supported also by the yearly occurrence data. Presence of necrotrophic pathogens was lowest in 2002. We observed significant differences in composition and occurrence of pathogens among the locations and years. Natural leaf spot infection of 39 cultivars was observed on each location. Disease differences could evaluate only in some places, but the severity was mostly low to evaluate cultivar differences and even differences in cultivar behavior were observed among locations. For these reasons, we believe that artificial inoculation methods should be applied to evaluate resistance deviations.
2.) Testing of winter wheat cultivars resistance against necrotrophic pathogens after winter wheat as fore crop 2a.) Level of epidemic severity of different diseases: Leaf rust epidemic was heavy; the powdery mildew epidemic was medium in both years. We observed a medium yellow rust epidemic in 2001. Level of leaf spots epidemic was different in both years (Table 2).  (CIMMYT, 1976) Leaf samples from 32 cultivars were collected and identified for the following necrotrophic pathogens in the leaf samples (Table 3). The occurrence of necrotrophic pathogens changed significantly even in the same location between years.

2b.) Evaluation of yield reactions
The above diseases caused significant yield decrease related to the protected control in both years (Figure 1).