Submitted to: Journal Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 15, 1998
Publication Date: N/A
Interpretive Summary: Rosette is a blackberry disease that seriously limits blackberry production in the Southeastern United States. A breeding program was started at Poplarville, Miss. to develop rosette resistant varieties capable of supporting a pick-your-own blackberry industry. 'Humble', a variety selected from the wild in Texas, was used as the source of rosette resistance. Through crossing and selection, two resistant plants with acceptable blackberry traits were identified. One selection was thornless. These plants have been crossed with other selections to improve yield and quality of fruit from rosette resistant plants. Progress in the breeding program is slowed by the inability to inoculate plants to test for disease resistance. All selections must be made in the field with natural rosette infection.
Technical Abstract: Rosette, incited by Cercosporella rubi (Wint) Plakidas, is a serious disease of most erect blackberries (Rubus spp.) and probably the most limiting factor in blackberry production in the southern United States. The tetraploid cultivar Humble has been reported variously as immune or tolerant to rosette. A breeding program was initiated at Poplarville, Miss. with the objective of breeding thornless 4X cultivars with rosette resistance derived from 'Humble'. A productive rosette resistant plant (MSUS29) was identified from the cross 'Humble' X 'Brazos'. An elite thornless selection (MSUS119) was identified from crosses with the thornless cultivar Navaho. The most recent crosses were made between selections with 'Humble' as the source of rosette resistance and 'Arapaho' or MSUS119 as the source of thornlessness. It is a challenge to select rosette resistant plants because we have not been able to develop a reliable screening procedure using artificial inoculation with C. rubi and all seedlings are screened in the field using natural infection. To determine the propensity of 'Humble', 'Brazos', and 'Rosborough' to transmit rosette resistance, we studied its heritability (h2) in blackberry. Only 'Humble' transmitted enough resistance to be usable as a parent. Although the h2 of resistance (0.48) was fairly high, low variability among parents other than 'Humble' would suggest little progress from mass selection in these cultivars. We evaluated the effect of rosette on ripening date, yield, and berry weight. Berries ripened about the same time and weighed about the same when grown at rosette-free or disease endemic locations. A significant clone X location interaction for yield was caused by reduced fruit set on plants where rosette was endemic.