Common Bunt of Wheat and Genetic Variability within Tilletia spp. in Syria

Mayada Alaa Kayali
General Commission for Scientific Agriculture Research-
Aleppo Research Center
Faculty of Agriculture, Aleppo University


Wheat is infected by many diseases that vary in severity and incidence  according to prevalent climate conditions in cultivation areas. Common bunt (CB) caused by Tilletia caries and T. foetida, remains a serious and dangerous wheat disease in Syria. It affects most types of durum and bread wheat cultivated in the world, causing losses in grain yield. Given the importance of this strategic crop and the relative lack of studies on this disease in Syria, particularly in terms of prevalence, incidence, pathotypes, and genetic resistance, this study aimed to: (1) determine the occurrence of common bunt at major wheat growing areas in Syria; (2) study the pathogen variability (Tilletia caries and T. foetida) according to climatic variation in Syria;  (3) characterize pathogen races and resistant genes according to reaction on differential varieties under Syrian conditions; (4)  Molecular Characterization of T. caries and T. foetida races using the AFLP tool; and (5) evaluate and select durum and bread wheat accessions for resistance to common bunt.
Field surveys were carried out in main wheat growing governorates during 2006 and 2007, covering 220 and 151 durum, 119 and 80 bread wheat fields, respectively. The results revealed presence of significant differences in disease incidence in the surveyed fields. The highest infection rate was recorded in first season at Idleb on durum (14.5%) and bread wheat (32.2%), whereas the lowest infection rate was obtained in Deraa (3.8 and 8.8% respectively). In the second season, a highest infection rate was recorded in Aleppo 15.5 and 25.1% on durum and bread wheat, respectively, while the lowest infection was recorded  in Hassakeh, on durum wheat (3.7%). The results also showed  that both pathogens  (T. caries and T. foetida) were found on wheat at all surveyed  provinces, with variation in host-preference. Mean frequencies of  T. caries and T. foetida  teliospores during 2006 and 2007 in durum were (84.6, 15.4 and 88.8, 11.2%), where that relationship was approximately 8:1 and 6:1, respectively, and (19.4, 80.6 % and 16.7, 83.3 %) in bread wheat and a relationship of 1:4 and 1:5, respectively.
Pathogenicity tests of 29 combinations (mixtures) of  T .caries and T. foetida (1:1 w:w) teliospores, collected from the different  surveyed  fields, were conducted  under artificial infection, on four durum and four bread wheat cultivars with different reaction types against the disease: very susceptible (VS), susceptible (S), moderate resistant (MR), and resistant. The results revealed presence of variations in percentages of infected plants according to inoculum combinations, that ranged between 10 – 43.6% on the VS durum cultivar, and 26 – 80% on the VS bread wheat cultivar. Accordingly, the combination pathogenicity of both pathogens was divided into 3 levels: weak, moderate and highly virulent.
Reaction of tested differential varieties against 27 bunt isolates (15 T .caries and   12 T. foetida), collected in 2007. Four Isolates (A5, A6, A9 and A10) overcome the resistant genes Bt0 and Bt7, and were designated as race T-1. Also, seven isolates (H11, H12, H13, H14, K20, K21 and K22) defeated the resistant genes Bt0, Bt2 and Bt3, and were designated as race T-11; two other isolates (H15 and Da16) defeated resistant genes Bt7, Bt1 and Bt0, and were called race L-4. Finally, five isolates (A3, A4, A5, A7 and A8) defeated the resistant genes Bt0, Bt1, Bt2 and Bt3, and were designated as race L-9.
As far as the rest of isolates, they were not similar in their interaction with known universal differential varieties, and are therefore considered as new races  and were given new codes. Isolates that broke down the resistance genes Bt0, Bt2, Bt3, Bt4, Bt6, Bt7, Bt13, Bt14, Bt15 and Bp, named as races L-13 and T-17; isolates that defeated  resistance genes Bt0, Bt7, Bt13, Bt14, Bt15 and Bp, were named races L-14 and T-18, and the isolate that broke down the resistance genes Bt13, Bt14 and Bt15, were named race L-15.
Collected bunt isolates during 2007 are divided, according to their pathogenicity into three groups:
Group I= weakly virulent: including 7 isolates (H15, K21, K23, K24, K25, K26 and K27).
Group II= moderately virulent: including 12 isolates (A1, A3, A4, A9, A10, H12, H14, Da16, Da18 Da19, K20 and K22).
Group III= highly virulent: includes 8 isolates (A2, A5, A6, A7, A8, H11, H13 and Da17).
Statistical analysis showed positive significant correlation between the average height reduction of wheat plant and virulence degree of isolate, where r value increased proportionally with isolate virulence  (r = +0.79, +0.96 and +0.77 for groups I, II and III, respectively).
The cluster scheme of pathogenicity  determines the degree of kinship between tested isolates, which were distributed in two groups, from different geographical areas:
Group I included 12 isolates, 4 moderately virulent and 8 highly virulent (A2, A4, A5, A6, A7, A8, H11, H12, H13, H14, Da17 and Da18).
Group II: included 15 isolates, 7 weakly virulent and 8 moderately virulent (A1, A3, A9, A10, H15, Da16, Da19, K20, K21, K22, K23, K24, K25, K26 and K27).
The AFLP technique showed low genetic variation of the fungal pathogen between sites (15%), but was high within the same site (85%).
SYSTAT cluster analysis jointly with PCA analysis determined the genetic relationship between the studied isolates; it divided the isolates of both pathogens into four main groups. The results show isolate similarity despite the geographical divergence (Hama / Deraa, Tunisia / Hassakeh, and Tunisia / Deraa isolates). This may be due to the transfer of infected seed from one region to another. However, other isolates collected from one site appeared in different groups.
Seed of 104 durum and 102 bread wheat genotypes were screened in 2006/07 and 2007/08 seasons, under artificial inoculation using a mixture of teliospores of both Tilletia caries and T. foetida under  field conditions. Significant variations in reaction types were found in infected plants. In the first season, 12 durum and 4 bread wheat genotypes showed resistant reaction (R) to the disease, and  23 durum and 4 bread genotypes reacted moderately resistant (MR). Only 8 tested durum genotypes were resistant in the second season, meanwhile 24 tested durum and 10 bread wheat genotypes proved moderately resistant (MR). The resistant genotypes could be used in breeding programs for increasing resistance of high yielding-wheat cultivars.