Resistance in the leaf and stem of sunflower after infection with two isolates of Phomopsis

Phomopsis helianthi (sexual stage Diaporthe helianthi) is a major disease i n most sunflower growing countries. The objectives of this study were (1) t o investigate whether resistance to Phomopsis differs between the leaf and stem tissue and (2) to examine the genetic variation for resistance to Phom opsis in sunflower inbreds. A set of 16 inbred lines was tested using artif icial leaf infection with mycelial explants. The lines were screened for th eir reaction on leaf and stem in four environments to a French Phomopsis is olate and in two environments to a Yugoslavian isolate. Significant genetic variance and mostly high genetic ratios (analogous to heritability) were f ound for all resistance traits. While fungal growth reflected resistance to Phomopsis in the leaf, petiole score was associated with leaf length. Stem lesion proved to be the most suitable trait for evaluating resistance in t he petiole-stem passage. No correlation existed between resistance in the l eaf and stem, suggesting that both resistance factors are inherited indepen dently. Both isolates resulted in similar resistance rankings of the inbred s and no difference in their aggressiveness was detected with regard to lea f resistance. Screening for Phomopsis consisting of several steps is recomm ended, beginning with petiole score or fungal growth to select the most sus ceptible plants in the early testing, and measuring stem lesion in further generations of breeding.