Cereal Rusts and Powdery Mildews Bulletin

Introduction

Wheat leaf rust (caused by Puccinia recondita f. sp. tritici) is one of the most important diseases of wheat in Bulgaria. It occurs annually throughout the growing areas and in some epidemic years could cause serious yield losses. Except the traditional use of fungicides, the protection against it is mainly based on the application of resistance. This approach is highly preferable because of its economic importance and biological safety. Unfortunately, the resistance genes, used in the selection, lose their effectiveness after commercial growing for some years, due to pathogen adaptation and changes in virulence. This is the main reason that regular virulence surveys are needed to develop successful resistance breeding strategies.
Analyses of Puccinia recondita f. sp. tritici physiologic specialization in Bulgaria have been conducted since 1930 (Dodov, 1931; Daskalov, 1974). They were carried out using the international differential set of Mains and Jackson (1926). In 1977 Gospodinova (1980) started to analyze the virulence frequency using the near isogenic lines of Thatcher (NILs). Because of the reduced effectiveness of the standard differential set and the change in virulence differential concept in Europe (COST 817), since the 1998 virulence survey has been conducted using adifferential set of 15 NILs and The North American System of Nomenclature in addition (Todorova and Kiryakova, 2000).

The aim of this study was to analyze Puccinia recondita f. sp. tritici population in Bulgaria in 1999.

Materials and Methods

Leaf rust samples were collected in 1999 from different cultivars in commercial and experimental fields in several regions. Single pustule isolates were obtained from each sample and multiplied on the susceptible cultivar Michigan Amber and 148 isolates were analyzed.

International standard differential set (Mains and Jackson, 1926) and The North American System of Nomenclature (Long and Kolmer, 1989) were used to determine the races and Prt code. According to this UN standard races were also identified (Basile, 1957).

Pathotypes were determined on the basis of 15 NILs with the following resistance genes: Lr1, Lr2a, Lr2b, Lr2c, Lr3, Lr9, Lr11, Lr15, Lr17, Lr19, Lr21, Lr23, Lr24, Lr26 and Lr28. They were recommended by the members of European Union COST Action 817 'Airborne pathogens on cereals'. The triplet code was identified in the order of gene number. The values 1, 2 and 4 were given for the first, second and third differential line, respectively, in case of susceptibility.

Seedling stage inoculations were carried out using standard methods (Mikhailova and Kvitko, 1970; Johnston and Browder, 1966). Scoring was carried out eight to ten days after inoculation. Infection types of 0 to 2+ were classified as resistant and those of 3 to 4 as susceptible.

Results and Discussion

One hundred isolates from the total number of 148 were tested on the standard differential set (Mains and Jackson, 1926), revealing five different races: 57, 77, 149, 167 and 184 (Table 1). The most common race was 167 (64%), followed by race 77 (31%). The other three races were presented by single isolates. Race 167 become prevalent in 1992 in Bulgaria (Todorova, 1994) and is still dominant in the pathogen population. Race 149 was identified in the period 1973-1980 and later again in 1998. Race 184 was detected in Bulgaria in 1974, 1976 and in the period 1982-1985 (Gospodinova, 1980; Gospodinova, 1987; Todorova and Kiryakova, 2000). Race 57 was first identified in Bulgaria by Mitoff (Mitoff, 1953) and again in 1983. These three races appear and disappear regularly in the pathogen population but always in a very low level.

Data from the analyze using The North American System of Nomenclature are presented on Table 2. This method provides considerably larger differential ability as 22 pathotypes were identified. Most widespread was pathotype FHT (39%), followed by FKT (13%) and THT (8%). The rest of the pathotypes were present at lower level, as some of them were determined by single isolates. Pathotype FHT was the dominant pathotype in 1998 too but its proportion was higher (51,97%) (Todorova and Kiryakova, 2000). Pathotypes FKT and THT increased from 4% to 13% and from 3% to 8% respectively. Compared to the prevoius year some new pathotypes were identified: CKP, FHK, MGT, TGT, TCT, TJT and KGK but 12 others were absent. According to the UN races, UN race 3 was dominant again (61%). UN race 13 was represented by 6 isolates (pathotypes TKT and THT) in 1998. In 1999 21 isolates were identified to this race and they composed 14% of the pathogen population. UN race 10 was not detected at all. UN race 6 decreased to 5% and UN race 2 remained at 10%.

A very great diversity was established when pathotypes were estimated using the 15 NILs and the triplet code. 49 different pathotypes were identified within 148 isolates (Table3).

This method has a very high differential ability but is more difficult to compare the results from the different years. Most of the pathotypes were different in 1998 and 1999. Pathotype 03766 took the highest part in the pathogen population (27%), followed by 13766 (6%) and 02767, 03767, 43766 and 43767 (5%). The frequency of the other pathotypes was less then 4%. The most common pathotypes in 1998 were also established in 1999. These were 13564, 13565, 03566, 03567, 53560, 53565 and 73766. In 1998 four pathotypes composed 24% of the population as each of them took 6%. No pathotype was as predominant as pathotype 03766 in 1999 which represented 1/3 of the pathogen composition. It is possible that the reason for this difference could be that in 1998 only 51 isolates were tested using this method, whereas in 1999 the number of tested isolates was 148. The variation within the other 2/3 of the population was great and quite different in the two compared years. Similar variation between years was also found in France, Italy and Hungary as well as in the USA (Mesterhazy et al., 2000).

References

Basile R, 1957. A diagnostic key for the identification of physiologic races of Puccinia rubigo-vera tritici grouped according to a unified numeration scheme. Plant Disease Reporter 41, 508-511.

Daskalov Chr, 1974. Biologische spezialisierung des weizenrostes im zusammenhang mit der zuchtung resistenten sorten . Untersuchungen uber den graunen rost (Puccinia triticina Erikss) in Bulgarien. Ausgewahlte Werke, Sofia, 12-24.

Dodov D, 1931. Physiological races of brown rust of wheat (Puccinia triticina Erikss) in Bulgaria. Zemedelska Misal 2, 91-104

Gospodinova El, 1980. Racial and genetic differentiation of wheat broun rust (Puccinia recondita Rob. ex Desm. f. sp. tritici Erikss) in Bulgaria for the period 1977-1978. Plant Science vol. XVII, 4, 158-164.

Gospodinova El, 1987. Racial and genetic differentiation of Puccinia recondita f. sp. tritici in Bulgaria during the period 1982 to 1984. Soil Science, Agrochemistry and Plant Protection , vol. XXII, 3, 76-82.

Johnston C, Browder LE, 1966. Seventh revision of the physiologic races of Puccinia recondita f. sp. tritici. Plant Disease Reporter 10, 756-760.

Long DL, Kolmer JA, 1989. A North American System of Nomenclature for Puccinia recondita f. sp. tritici. Phytopathology 79, 525-529.

Mains E, Jackson H, 1926. Physiologic specialization in the leaf rust on wheat, Puccinia tritici Erikss. Phytopathology 2, 89-128.

Mikhailova LA, Kvitko K, 1970. Laboratory methods of culture of the causal organism of leaf rust of wheat Puccinia recondita f. sp. tritici Rob. Ex Desm.,Mycology and Phytopathology 4, 269-273.

Mitoff N, 1953. Untersuchungen uber die Widerstandsfahigkeit einheimischer und auslandischer Weizen Sorten gegen Getreiderost in Zusammenhang mit Ihren Stadienentwicklung. Journal of the scientific-experimental Institutes of the Ministry of Agriculture 1, 156-166.

Todorova M, 1994. Racial and genetic specialization of Puccinia recondita f. sp. tritici in Bulgaria in 1992 and 1993. Plant science XXXI, 7-10, 9-12.

Todorova M, Kiryakova V, 2000. Physiologic specialization of Puccinia recondita f. sp. tritici in Bulgaria in 1998. International scientific conference '75 years university forestry education in Bulgaria', 15-16 June, Sofia. Proceedings, 127-132.