Susceptibility of sunfl ower hybrids (Helianthus annuus) to mid stem rot and broken stem caused by Sclerotinia sclerotiorum. Cien. Inv. Agr. 35(1):27-35.

In the sunflower, Sclerotinia sclerotiorum is the cause of stem rot and broken stems. Seventeen sunflower hybrids were evaluated in Balcarce, Argentina after mid-stem inoculation. The association between rot length and the rate of stem breakage was calculated. Sunflower hybrids showed considerable differences in the size of the rot lesion developed and in the relative number of broken stems, as determined 20 days after the beginning of flowering (daf). No variability was detected for broken stems, both 6 and 60 days after inoculation. Sunflower plants with greater mid stem rot length had a greater relative number of broken stems. The incidence of broken stems could have been enhanced by the mid-stem rot, especially in those genotypes with the lowest proportion of structural tissues in the stems. The degree of genetic determination suggested that environmental effects influence the length of symptoms less than that the proportion of broken stems determined at 20 daf. The results allowed us to propose a breeding program based on the selection of the most adequate genotypes given their level of resistance to these two characters. A phenotypic selection could be performed on individual plants based on the extent of rotting caused by S. sclerotiorum at the mid-stems. Then, the proportion of broken stems could be scored on F2-derived lines in F3 (F2:3 line). En el girasol ( Helianthus annuus L.), cultivo de gran interes agronomico, Sclerotinia sclerotiorum ataca la parte media del tallo produciendo la podredumbre media del tallo. En Balcarce, Argentina, se realizo un ensayo a campo donde se evaluo la respuesta de 17 hibridos de girasol a S. sclerotiorum . Se evaluo la relacion entre la lesion provocada por este patogeno y el quebrado del tallo enfermo. Los hibridos mostraron respuestas diferenciales al tamano de la podredumbre y en el numero relativo de tallos rotos, medido a los 20 dias de iniciada la fl oracion (dif). No hubo variabilidad de plantas quebradas a los 60 dif. Los hibridos con mayor longitud de lesion promedio mostraron mayor numero de plantas quebradas. El debilitamiento del eje caulinar, particularmente en genotipos con menor proporcion de tejidos de sosten, habria facilitado la rotura del tallo. El grado de determinacion genetica sugirio que la longitud de las lesiones estaria menos infl uenciada por factores ambientales que la proporcion de plantas quebradas a los 20 dif. Estos resultados permiten delinear un programa de mejoramiento, para incrementar el nivel de resistencia a esta enfermedad en girasol, basado en la seleccion de genotipos para ambos caracteres. Una seleccion fenotipica se podria realizar sobre plantas individuales, utilizando el caracter extension de la podredumbre en tallos. La proporcion de plantas quebradas se evaluaria, luego, en las familias F3 derivadas en F2 (lineas F2:3).