The sunflower rust situation: Current races in the northern and central Great Plains, and resistance in oilseed and confection hybrids

Tom Gulya USDA-ARS Northern Crop Science Laboratory Fargo, ND 58105-5677 [email protected]

ABSTRACT

While rust has had only a sporadic impact on United States sunflower production recently, the disease still occurs with regularity on both cultivated and wild sunflower. Urediospore samples were collected from both wild and cultivated sunflower from 2002 to 2005 and identified to race in greenhouse tests, using a set of nine, internationally adopted differential lines. From 59 collections, a total of 25 races were identified, with three races, 100, 310 and 700, comprising 34%. The remaining 22 races only comprised 2 to 7% each. Race diversity was much greater on wilds (19 races) compared to cultivated (11 races). Rust collected from the Northern Great Plains was comprised of 13 races, compared to 18 races from the Central Great Plains. To effectively combat the greatest number of rust races, breeders will need to utilize more than one resistance gene. The combination of resistance from USDA-ARS lines HA-R4 and HA-R5 or HA-R3 and HA-R5 will control 98% of the rust races currently found in the United States. The diversity of races collected during this period is comparable to that found in the early 1990’s, when 20 races were identified from 42 samples collected in the Central Great Plains.

INTRODUCTION

Sunflower rust, caused by the fungus Puccinia helianthi, is periodically of economic importance to the U. S. sunflower crop. In the late 1980’s it was of considerable impact (Gulya, 1990) and then declined in significance as weather patterns produced cooler, wetter growing seasons in the 1990’s (Gulya et. al, 1997). Nonetheless, sunflower rust is present each year in some portion of the US sunflower production area (Gulya, 2002; 2003), and is always present on wild Helianthus annuus across its range in the Midwest. The constant presence of rust on wild H. annuus (Gulya and Viranyi, 1994) has led to the evolution of rust resistance within the wild species (Gulya and Brothers, 2000), and this has served as a valuable source of rust resistance genes for cultivated sunflower (Miller and Gulya, 2001).

The objective of this study was to summarize efforts over the past few years to identify rust races occurring on both cultivated and wild sunflowers. This information is necessary for breeders to know which resistance genes to select for. Additionally, we evaluated the rust reaction of a range of current commercial and experimental hybrids to determine what portion of the hybrid market is occupied by resistant hybrids. MATERIALS and METHODS

Data from the annual NSA-sponsored survey conducted during September, 2003 and September, 2005 were analyzed to calculate the incidence and severity of rust in six states (North Dakota, South Dakota, Minnesota, Kansas, Colorado and Texas). In each of the two years, surveyors were instructed to inspect two areas within each field. At each site, a strip of 25 plants were observed, and the average percent of the upper leaves covered by pustules was estimated, using computer generated diagrams available on extension bulletins (Gulya et. al, 1990). Rust samples were not collected during the surveys as most pustules in September contained teliospores rather than urediospores.

Samples of rust urediospores were collected from 39 wild Helianthus annuus populations and from 18 cultivated sunflower fields. Five collections were made in 2002, 12 in 2003, 34 in 2004, and six in 2005. The samples came from twelve states, including Alabama (1), California (8), Colorado (3), Georgia (2), Kansas (2), Minnesota (6), Nebraska (13), North Dakota (9), South Carolina (1), South Dakota (5), Texas (2) and Wyoming (4), plus one sample from Manitoba, Canada. Each urediospore sample was initially increased by inoculating a susceptible sunflower variety, and the freshly produced spores were used to inoculate a set of nine differentials (Gulya and Masirevic, 1996). These differentials were adopted by an ad hoc committee of the International Sunflower Association in 1987. The “nomenclature” of the rust races has changed from one of simple sequential numbers to the present system which uses a triplet code to produce a virulence formula (Gulya and Masirevic, 1996). The use of nine differentials, assuming independent inheritance of single, dominant genes, should allow for the identification of up to 29 , or 512 virulence patterns.

The last phase of the study consisted of evaluating commercial hybrids for their reaction to the current predominant races of rust and to the most virulent races. A total of 71 hybrids were tested in winter greenhouse experiments. Ten plants of each hybrid were inoculated with race 720, and another group of ten plants were inoculated with a mixture of races. Plants were inoculated at the V-2 stage (~ 3 wks after planting) and the rust reaction evaluated two wks later. Any hybrids with > 2% of their leaves covered by pustules were classified as susceptible. Since races 520 and 700 were the most common races found on cultivated sunflower, race 720 (which infects the same differentials as 520 and 700 combined) was used. The race mixture was intended to be a ‘777,’ which would infect all nine differentials, but upon examining the check differential lines at the end of the test, the virulence of the mixture was classified as a ‘766,’ which was still highly virulent, infecting seven of the nine differentials.

RESULTS

Rust incidence in 2003 and 2005, while not in epidemic proportions, was widespread in both years. In 2003 and 2005, rust was found in 44 and 49% of surveyed fields, respectively (Table 1). The average severity (% of leaf area covered, in affected fields) was 1.3% and 0.95% in 2003 and 2005, respectively. Based on elaborate yield loss studies done in Israel, yield losses can be expected when pustule coverage on the upper leaves exceeds 3% by area. Thus, in 2003 and 2005, 6% and 13%, respectively, of all U.S. sunflower fields would have been expected to have sustained some level of yield loss due to rust. States with the highest incidence and severity changed from year to year. In 2003, South Dakota had an average severity of 5%, with 26% of its fields expected to sustain yield losses, while in 2005, the severity dropped to 0.24% and no fields were expected to sustain a yield loss. The highest severity of rust consistently appears in the southern states (Colorado, Texas and Kansas) where environmental conditions are more favorable for rust infection and development, but since these areas contain less acreage than northern states, the impact of rust upon total U.S. production is lessened.

Table 1. Incidence and severity of rust on sunflower fields from data collected during the annual 2003 and 2005 NSA sponsored survey. Severity is the average percentage of leaf area covered by rust pustules. Fields with >3% rust pustule coverage on the upper leaves (last column) would sustain enough damage to cause a yield loss.

2003 Region # fields Incidence % severity, % fields with surveyed (% fields with average yield losses any rust) across affected fields U.S. Total 185 44 1.3 6 North Dakota 92 44 0.5 1 South Dakota 29 43 5.0 26 Minnesota 5 0 0 0 Colorado 13 31 2.7 15 Kansas 18 67 0.9 6 Texas 8 50 2.5 12 2005 U.S. Total 143 49 0.95 13 North Dakota 92 49 2.5 16 South Dakota 29 45 0.24 0 Minnesota 8 50 2 12 Colorado 5 71 2.1 29 Kansas 11 36 0.6 0 Texas 4 75 2.0 25

Race identification: A total of 59 rust samples were collected/received from 2002 to 2005 and races characterized in greenhouse tests. A total of 25 different races were identified (Fig. 1). Three races (100, 510 and 700) comprised 35% of the isolates. Conversely, 12 of the 25 races were identified only from a single sample, and thus each represented only 2% of the population. The differential lines HA-R3, HA-R4 and HA-R5 were the least frequently susceptible lines with these 59 rust samples (Fig. 2). Use of any one of these three lines as a source of rust resistance would make the resultant germplasm resistant to at least 90% of the rust races occurring during this period of time in the United States. 12

10

Cult % Wild %

8

6

% of Rust population 4

2

0 100 300 303 310 320 330 335 500 503 504 510 514 520 530 531 532 534 544 700 704 710 720 730 731 734

Rust Race

Figure 1. Frequency of 25 rust races isolated from wild Helianthus and cultivated sunflower during the period 2002 to 2005. Note that many races are found either only on wild or only on cultivated sunflowers.

100 90 80 70 60 50 40 30 20 % isolates controlled % 10 0 CM 29 CM 90 P386 HAR1 HAR2 HAR3 HAR4 HAR5 Rust Differential Lines

Figure 2. Percentage of United States rust isolates collected from 2002 to 2005 that are controlled by genes in the eight differential lines.

Regional race comparison: A total of 21 rust samples came from wild and cultivated sunflower in the Northern Great Plains (North Dakota, South Dakota, Minnesota, Manitoba), and was comprised of 13 races, with race 500, 520 and 700 making up 52% of the total. A total of 24 rust samples came from wild and cultivated sunflower in the Central Great Plains (Colorado, Wyoming, Nebraska, Kansas and Texas) and was comprised of 18 races, with race 310 being predominant at 21%. It appears that there is a regional difference in races between the Northern and Central Great Plains. This comparison, however, is confounded by the fact that a majority of the Northern Plains samples were from cultivated sunflower while most of the Central Great Plains samples originated from wild sunflower. Thus, the apparent differences in race composition may be due to regional differences or to the genetic differences between wild and cultivated sunflower.

Races from Wild versus Cultivated Sunflower: Thirty-nine rust samples came from wild sunflower and contained 19 races (Fig. 2), with race 100 and 310 comprising 35% of the races recovered from wild sunflower. Eighteen rust samples came from cultivated sunflower, and contained 12 races , with races 500, 520, and 700 comprising 45% of the total recovered from cultivated sunflower. These five races (100, 310, 500, 520 and 700) comprised 39% of the isolates recovered from both wild and cultivated sunflower. Not only was there a greater number of races isolated from wild sunflowers than cultivated, but, additionally, the “average virulence,” of races recovered from wild Helianthus, (estimated as the average number of differentials infected by those races) was 3.8 compared to 3.2 for races recovered from cultivated sunflower.

Rust resistance in commercial hybrids: Seventy-one commercial hybrids (entries in the 2005 Sclerotinia trials) were tested for rust resistance in greenhouse trials (some entries were not tested due to insufficient remnant seed). Rust severity in greenhouse trials was quite uniform and nearly all susceptible entries had in excess of 5% pustule coverage (Fig. 3), so there was no doubt about the resistance/susceptibility of the hybrids. Seven hybrids were immune or highly resistant to race 720, and five of these seven hybrids were immune or highly resistant to a mixture of races giving a race phenotype of 766. Most of the rust resistant entries were developed by Kaystar and Triumph, with Cropland Genetics having one hybrid with resistance in one test. None of the confection entries had resistance to either rust race.

Figure 3. Rust pustules on the underside of a sunflower leaf showing approximately 5 to 10% pustule coverage

R ace Race Hybrid 720 766 Hybrid 720 766 Advanta AP422 S S Legend Seeds LSF 126N nt nt Advanta AP425NS S S M onsanto 35-10N S nt nt Advanta AP431NS S S M onstano DKX001 S S Advanta AP461NS S S M onstano DKX002 S S Advanta AP534NS/CL S S M onstano DKX003 S S Advanta AP542CF S S M onstano DKX004 S S CHS 04 E xp 1 S S M onstano MH 4331B S S CHS 05 E xp 1 S S M ycogen 8H 350D M S S CHS 05 E xp 10 S S M ycogen 8N 270 S S CHS 05 E xp 6 S S M ycogen 8N 352 nt nt CHS 05 E xp 7 S S M ycogen 8N 510 nt nt CHS O5 Exp 11 S S M ycogen E84325 S S CHS RH 118 S S M ycogen E85419 S S CHS RH 318 nt S M ycogen E86525 S S C roplan 305 DM R nt nt ProSeed C L-43 S S C roplan 135 nt nt Proseed 9405 nt nt C roplan 2N S 004 R nt Proseed C 9011 nt nt C roplan 130 S S Red River 2214 S S C roplan 343 S S Red River 2215 S S C roplan 378 S S Red River 2216 S S Dahlgren 9518 S S S eeds2000 Bigfoot S S Dahlgren 9532 S S S eeds2000 Panther S S Dahlgren DO-4455 S S S eeds2000 Viper S S Dyna-Gro 91N 05 S S S eeds2000 X 4625 S S Dyna-Gro 93C 05 S S S eeds2000 X 4794 S S Dyna-Gro 93N 05 S S S eeds2000 X 5470 S S Dyna-Gro EX 92H06 S S S eeds2000 X 7434 S S Dyna-Gro EXP 93C 6 S S S eeds2000 X 7843 S S Dyna-Gro EXP 93N 6 S S S eeds2000 X 936 nt nt Dyna-Gro EXP 94F6 S S S eeds2000 X 9708 S S Garst 4682C L S S S eeds2000 X 978 S S Garst 4690 S S SigcoSun S S3638 S S H eaton MTH1 nt S SigcoSun S S3938 S S H eaton MTH2 nt nt Trium ph 620C L S S INRA XRQ x P S C 8 S nt Trium ph 757C S S Interstate 4540N S S S Trium ph 777C S S Interstate 4704N S S S Trium ph 820H O S S Interstate F10016 S S Trium ph TRX 2352C S S Interstate H ysun 525 S S Trium ph TRX 3241 S S Interstate H ysun 450 nt S Trium ph TRX 3249 R R Kaystar 9404 I I Trium ph TRX 4240 R S Kaystar 9501 R R Trium ph TRX 5441 R R Kaystar EX 2453 I I T able 2. Reaction of released and experimental sunflower hybrids in greenhouse trials to race 720 and a mixture of races giving the virulence of 766. (R=resistant; S=susceptible; I=immune; nt=not tested).

DISCUSSION White rust has not been a serious economic threat recently, it is present each year in the U.S. sunflower production area, and constitutes a bona fide threat as the spores are easily distributed across wide geographic areas. A wide diversity of rust races exists, both on cultivated sunflower and wild H. annuus. The later definitely acts as a reservoir of virulent races to infect cultivated sunflower, and serves as a breeding population for the development of new races. In an earlier study (Gulya and Viranyi, 1994), twenty rust races were recovered from 42 samples collected from wild H. annuus in the Central Great Plains, compared to the 19 races identified from wild Helianthus in this study. Thus, the great diversity of rust races found on wild sunflowers appears to be quite constant. These same wild H. annuus (and other Helianthus species), however, also are an excellent source of resistance genes for cultivated sunflower, and some of this resistance has already been identified (Gulya and Brothers, 2000) and transferred into released germplasm (Miller and Gulya, 2001).

ACKNOWLEDGEMENTS

The author is indebted to Scott Radi and Nikolay Balbyshev for their technical assistance, and to the many individuals who took the time to collect and mail us rust samples.

LITERATURE CITED

Gulya, T. J. 1988. International standardization of techniques and nomenclature for sunflower rust. Proc. Sunflower Workshop, January 25, 1988. Bismarck, ND.

Gulya, T. J. 1990. The sunflower rust situation in 1989. Proc. Natl. Sunflower Assoc. Res. Workshop, p. 106, January 8-9, 1990. Fargo, ND.

Gulya, T. R. Venette, J. R. Venette, and H. A. Lamey. Sunflower rust. North Dakota State University Ext. Service. Bull. 19PP. 4p. Available at: http://www.ext.nodak.edu/extpubs/plantsci/rowcrops/pp998.pdf

Gulya, T. 2002. Sunflower Crop Survey - 2002: Disease Assessment Across Eight States Proc. 24nd Sunflower Research Workshop. Fargo, ND. http://www.sunflowernsa.com/research/research-workshop/documents/102.PDF

Gulya, T. 2003. Sunflower disease incidence and distribution in Midwestern U.S. in 2003. Proc. 25nd Sunflower Research Workshop. Fargo, ND. http://www.sunflowernsa.com/research/research- workshop/documents/Gulya_Disease_Midwest_2003_04.PDF

Gulya, T. and M. Brothers. 2000. Rust resistance in wild Helianthus annuus and variation by geographic origin. Proc. 22nd Sunflower Research Workshop. Fargo, ND. January 18-19. p. 38- 41.

Gulya, T. J., and S. Masirevic. 1996. Inoculation and evaluation methods for sunflower rust. Proc.18th Sunflower Research Workshop. Fargo, ND. January 11-12. p. 31-38.

Gulya, T. J., and F. Viranyi. 1994. Virulent new races of sunflower rust (Puccinia helianthi) from the southern Great Plains. Proc. 16th Sunflower Research Workshop. Fargo, ND. January 13-14. p. 94-98.

Gulya, T. J., R. Lee, D. Jardine, and H. Schwartz. 1997. Central Great Plains sunflower rust situation in 1996: Race identification and hybrid evaluation for resistance. Proc. 19th Sunflower Research Workshop. Fargo, ND. January 9-10. p. 79-81.

Miller, J. F. and T. J. Gulya. 2001. Registration of three rust resistant sunflower germplasm populations. Crop Science 41:601.