Evaluation of susceptibility to mosaic in Louisiana's sugarcane breeding program

Authors: RICE, JANCEE - LSU Agcenter; HOY, JEFFREY - LSU Agcenter; Hale, Anna; Todd, James; Grisham, Michael; KIMBENG, COLLINS - LSU Agcenter; PONTIF, MICHAEL - LSU Agcenter

Submitted to: American Society of Sugar Cane Technologists
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2019
Publication Date: 12/1/2019
Citation: Rice, J.L., Hoy, J.W., Hale, A.L., Todd, J.R., Grisham, M.P., Kimbeng, C.A., Pontif, M.J. 2019. Evaluation of susceptibility to mosaic in Louisiana's sugarcane breeding program. Journal of the American Society of Sugar Cane Technologists. 39:1-11.

Interpretive Summary: Mosaic is a viral disease of sugarcane known in Louisiana for historical losses it has caused to the sugar industry. Typically, the industry uses varieties that are resistant to the virus to minimize effects on the crop. Resistant varieties have been grown so long in the state that the incidence of the virus has decreased dramatically. With little natural infection pressure in the field, it has become difficult to assess new breeding material for resistance to the disease through observation in yield trials. This has led to an increase in susceptibility in newly-released varieties. Increasing susceptibility highlighted the need to perform artificial inoculation of varieties used for breeding purposes to determine the levels of resistance and susceptibility in the genepool. Two different groups of plants were inoculated for mosaic susceptibility testing. The first set, the commercial varieties, are high sucrose and low fiber varieties that are likely to be released to the industry. The second group tested are part of the USDA-ARS germplasm enhancement program, and included hybrids between wild relatives of sugarcane and commercial or near-commercial varieties. Susceptible parents were identified in both groups of plants, with more resistance being observed in the group from the germplasm enhancement program. This suggests there is an ongoing need for continued breeding with wild relatives of sugarcane. Results were verified using RNA-based testing, and visual ratings were found to be 97% accurate. Plants were inoculated multiple times, and the results were similar for 76% of the varieties. The repeatability results demonstrate the need to conduct multiple inoculations to accurately determine the reaction of varieties to the mosaic virus. Results from these tests will empower breeders to optimize crosses to maximize mosaic resistance in varieties released to the sugarcane industry.

Technical Abstract: Mosaic is a viral disease of sugarcane known in Louisiana for historical losses it has caused to the sugar industry. The disease can be caused by both Sugarcane mosaic virus (SCMV) and Sorghum mosaic virus (SrMV), with SrMV thought to be the current causal species in Louisiana. Mosaic is managed primarily through the cultivation of resistant cultivars. Successful breeding has led to an increase in resistant cultivars that has resulted in a decrease in disease pressure in commercial fields. Mosaic was detected in advanced selections of the Louisiana breeding program leading to uncertainty concerning levels of resistance in the commercial parent breeding populations. Mechanical inoculations were used to evaluate the level of susceptibility of the basic and commercial parent populations during 2017 and 2018. Greenhouse grown plants were inoculated and rated for resistance based on plant infection percentage intervals determined from assessment of visual mosaic symptoms. Mosaic susceptible parents were identified in both the basic and commercial parent populations. The basic parent population had a higher frequency of resistance suggesting it will continue to provide a source of resistance. The detection of virus infection by visual observation of mosaic symptoms was 97% accurate according to RT-PCR testing of symptomatic and asymptomatic leaves. The repeatability of resistant ratings for clones in different experiments was 68%. Repeatability of resistant and susceptible mosaic reactions considered together was 76%. The repeatability results demonstrate that multiple inoculations are needed to accurately determine clone mosaic reactions. The results from mechanical inoculations will allow for the identification of sources of susceptibility and informed crossing to continue to manage mosaic through host plant resistance.