SHINE COMSTOCK~ L. DEAN^ - ISSCT

1 SHINE Jr, et a/. Proc: ISSCT , V& 25, 2005 COMPARISON OF FIVE ISOLATES OF SUGARCANE BROWN RUST AND DIFFERENTIAL REACTION ON SIX SUGARCANE CLONES JAMES M. SHINE ~r', JACK C. COMSTOCK~ and JACK L. DEAN^ 'sugar Cane Growers Cooperative of Florida, Post Ofice Box 666, Belle Glade, Florida 33430, USA org USDA-ARS Sugarcane Field Station, Canal Point, Florida 33438 3~eceased KEYWORDS: Puccinia melanocephala, Saccharurn spp., Rust Races, Single-Degree of Freedom Comparisons, AUDPC, Abstract THE OCCURRENCE of pathological races of sugarcane brown rust (Puccinia melanocephala H. and P. Sydow) in Florida was first reported in 1984. Cultivars that were resistant to the disease in the variety development program displayed susceptibility to the disease once expanded commercially.

2 A study was conducted over two years to determine the effect on yield of six cultivars of sugarcane to five isolates of the sugarcane rust pathogen suspected to be differing pathological races. Sugarcane plants were grown in pots and inoculated under conditions favouring disease development while preventing infection of non-inoculated control plants. Plant height, estimates of percent infection and whole-plant disease ratings were recorded bi-weekly for 4 months during the late winter and spring of 1991 and 1992. Final plant height and fresh weight were measured at the end of the experiment each year. Areas under the disease progress curves (AUDPC) for plant height, percent infection and disease rating were calculated. Analysis of variance and single-degree-of-freedom comparisons were used to interpret results for all response variables.

3 These analyses indicated the existence of four pathogenic races of the brown rust pathogen in this experiment. These data coupled with field observations lead to the hypothesis that pathogenic variants of P. melanocephala develop within the sugarcane production region and parallel the development of new commercial sugarcane cultivars. The diversification of commercial cultivars and identification of horizontal resistance mechanisms will reduce losses to sugarcane brown rust in the Florida sugarcane industry. Introduction There has been much argument over the existence of races of the causal agent of sugarcane brown rust, Puccina melanocephala H. and P. Sydow, in sugarcane production regions of the world. Sugarcane rust was first reported in the Americas in 1978 (Dean et al.)

4 , 1979). A report in 1984 presented data on one variety of sugarcane, CP 79-1 580. This variety was originally rated as resistant to the disease, and was later observed to be susceptible at some outfield test sites while appearing resistant at others (Dean and Purdy, 1984). A second observation of a change in the amount of disease observed on a cultivar was observed two years after the release of CP 78-1247 (Raid, 1989). The cultivar was widely planted soon after its release to commercial growers due to its superior agronomic performance in trials. Significant sugarcane rust was observed on the cultivar early in 1988 and yields at harvest were 40% of expected yield as determined from experiments conducted on the same farms three years prior (Raid et al., 199 1).

5 In yet another example, CP 72- 12 10 displayed resistance to rust when it was released in 1980 and, by 1986, it occupied more than 50% of the total acreage planted to sugarcane in Florida (Glaz and Coale, 1992). During the spring of 1987, rust was severe and widespread on this variety although not uniformly distributed over the entire south Florida production region. Losses on CP 72-1210 were estimated to be as high as 39% of its historical yield in rust-infected plant cane (Raid et al., 1991). The importance of CP 72- 1210 declined rapidly because of the effects of sugarcane rust. SHINE Jr, et a/. Proc. ISSCT , Vol. 25, 2005 Economic yield losses due to rust experienced by farmers growing CP 72-1210 stimulated diversification of cultivars planted in Florida.

6 Cultivars planted to replace CP 72- 12 10 were rated 'resistant' to rust at release. Since 1989, two of the so-called resistant vasieties, CP 74-2005 and CL 73-239 have shown a susceptible reaction to the disease in localised areas. Yield losses due to rust on planting of the formerly resistant CL 73-239 were estimated to range from 20-30%. The cultivar CP 74-2005 showed a highly susceptible reaction in localised aseas. None-the-less, the cultivars were accepted as resistant and are now considered susceptible. Conversely, CL 41-223 was grown on 10% of the planted acreage when rust was first observed in Florida (Glaz, 1981). Sugarcane rust was a major factor contributing to the decision to totally replace this cultivar. Since the removal of this cultivar fiom commercial production, plantings of this cultivar in small plots show little rust development relative to other susceptible varieties planted in the same area.

7 These observations have led pathologists and breeders in Florida to believe that there are pathogenic races of sugarcane rust present (Dean and Purdy, 1984; Raid, 1989). Changes in the distribution of rust races appear to correspond with changes in cultivars grown, generally increasing in importance with increasing extent of new cultivars. Discussions on the existence of pathological races of the sugarcane rust fungus have been argued since it was first reported in the Americas. Arguments presented by Whittle (1980) suggested that the term race should only be used in pathosystems involving vertical resistance where a gene-for-gene resistance relationship can be demonstrated; and that this is not probable in the sugarcane/sust pathosystem since sugarcane is a complex polyploid.

8 The existence of races must be evidenced by a significant differential reaction to the disease between two isolates and two clones and it is on this basis that we are claiming to demonstrate the existence of races regardless of the genetic basis. The existence of races bears importance in breeding strategies employed to develop host plant resistance. The objectives of this study were to: 1) determine whether or not there are pathogenic races of sugarcane rust in Florida; 2) identifjr, if possible, some of the pathogenic races, if they exist; and 3) determine if identification of pathogenic races is possible using traditional qualitative ratings of percent TVD leaf infection, overall severity and quantitative measures of plant height compared to non-inoculated control and final fresh weight of above ground plant pasts.

9 Materials and methods Rmt isolates Five isolates of sugarcane rust were collected from sources that were suspected to represent individual pathogenic races of P. melanocephala. The rust isolates were iCP72-1210, iCP78-1247, iB4362, iCL41-223, and iCL73-239. The isolates were named based on the cultivar from which they were collected and expected to be the best adapted. Collection and increase of isolates from single uredia was attempted, but abandoned 'since there were questions regarding the possibility of selecting a different genetic variant than that generally observed in the field. isolate iCL41-223 was collected by Mike lrey (United States Sugar Corporation) in 1984 fiom infected fields. The cultivar was grown on a declining number of acres at the time and suffered severe losses to the disease.

10 isolate iCP72-1210 was collected in the spring of 1990. This isolate was collected from an area where a number of varieties were being grown, some of which were known to be moderately susceptible to the disease. The disease was more intense on CP 72-1210, while expression of the disease on other susceptible cultivars was vely light. isolate iB4362 was collected from the USDA Sugarcane Field Station at Canal Point and was suspected to be different because other cultivars used in the experiment, with the exception of CP 72-1210, appeared resistant to it. It would be plausible for several pathogenic races to be present at Canal Point, since a large sugarcane germplasin collection is grown on the experiment station and conditions favour rust development year-round.