Progress in Development of Solid-Stem Winter Wheat Cultivars for Management of Wheat Stem Sawfly.

Wheat stem sawfly (WSS) (Cephus cinctus Norton) is the most important biotic limitation to winter wheat production in Montana. Larval feeding in stems reduces grain weight by an average of 20%. Infested stems lodge, causing additional losses and increased costs at harvest. Existing insecticides and cultural management practices are ineffective. Stem solidness is the only known type of effective host plant resistance to WSS in wheat. Solid stems are filled with undifferentiated parenchyma cells (pith). Solidstemmed lines exhibited lower levels of infestation (reduced survival of eggs or larvae) and restrict larval movement (shorter tunnel length) relative to hollowstemmed cultivars. Phenotypic selection for stem solidness has been effective. From 2004 to 2009, stem solidness of wheat and stem cutting by WSS were evaluated in 10 NC Montana environments. Results indicate successful phenotypic selection for stem solidness improved stem solidness by about 15 solidness units on a 5 (hollow) to 25 (solid) evaluation scale and that each unit improvement in stem solidness reduced cutting by WSS by 1.5 to 2.0%. In 1996 and 2004, ‘Rampart’ and ‘Genou’ solid-stem HRW wheat cultivars were released and adopted rapidly by Montana producers. Our breeding project used two PCR markers to rapidly backcross stem solidness into adapted lines. A field study completed in 2008 indicated that these markers had significant effects on stem solidness; however the levels of stem solidness achieved via marker assisted selection were not adequate to provide reliable WSS cutting tolerance in target environments. At present, phenotypic selection for stem solidness and sawfly cutting tolerance will remain our selection strategy to develop new solid-stem cultivars. Development of sawfly-resistant winter wheat cultivars with adequate winter hardiness, yield potential, and quality is of highest priority.