2.8.2
WOUNDED-BUD INOCULATION OF USTILAGO SCITAMINEA: EFFECTS ON THE SUGARCANE SMUT PROGRESS CURVE
146 WOUNDED-BUD INOCULATION OF USTILAGO SCITAMINEA: EFFECTS ON THE SUGARCANE SMUT PROGRESS CURVE

L AMORIM1, A BERGAMIN FILHO1 and B HAU2

1 Departamento de Fitopatologia, ESALQ, 13418-900 Piracicaba, SP, Brazil; 2 Institut fĀr Pflanzenkrankheiten und Pflanzenschutz, UniversitĄt Hannover, 30419 Hannover, Germany

Background and objectives
Sugarcane smut can be responsible for as much as 100% damage in susceptible varieties. The selection of sugarcane varieties resistant to smut is one of the main objectives of sugarcane breeding programmes in Brazil since this is the only way to control the disease. The most common inoculation methods to select for resistance to smut are immersion of wounded or non-wounded buds in a suspension of teliospores of Ustilago scitaminea. The technique that uses wounded buds presents results with less variability than the technique that uses non-wounded buds but it has the inconvenience of eliminating the resistance to infection present in the sugarcane bud. The effects of wounded-bud inoculation on the smut progress curve was assessed in this study.

Materials and methods
The experiment was carried out in Brazil, with NA56-79 and SP70-1143 sugarcane varieties, respectively, susceptible and resistant to U. ;scitaminea. These varieties were submitted to two treatments: (1) inoculation with wounded buds, and (2) control, without inoculation. Assessments of disease incidence and plant tillering were made fortnightly, beginning with sugarcane sprouting, during four consecutive years. Yield was assessed each year at harvest. A generalization of the Gompertz model with five parameters [1] was fitted to the data of smut incidence.

Results and conclusions
The smut-resistant variety SP70-1143, presented a higher incidence of the disease than the smut-susceptible variety NA56-79 in the first year (plant-cane stage), when wounded buds were inoculated. The asymptotic value of the annual progress curves of disease estimated by the growth model decreased between the stages of the development of the crop in the SP70-1143 variety. Thus, there was recovery of the resistance previously eliminated by the wounded-bud inoculation at the plant-cane stage. In the susceptible variety, there was an increase in the incidence of the disease from the plant-cane stage to subsequent ratoons. The apparent rate of infection, which was variable in time, was not related to the disease severity in different plant treatments. The tillering curves were similar in inoculated and in uninoculated plants. The average number of tillers in inoculated plants was not different from the average number of tillers in the uninoculated plants for both varieties. The yield of NA56-79 variety was significantly lower in the inoculated plants than in uninoculated plants compared with no difference in yield between inoculated and uninoculated plants of SP70-1143 variety. The wounded-bud inoculation caused elimination of resistance in the SP70-1143 variety in the plant-cane stage and also caused death of stools. The elimination of potentially useful sugarcane varieties, as a result of such aggressive artificial inoculation, can be costly to Brazilian breeding programmes and would not necessarily lead to varieties with increased resistance to smut.

References
1. Hau B, Amorim L, Bergamin Filho A. 1993. Phytopathology 83, 928-32.