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 in most sunflower growing countries. The objectives of this study were (1) to investigate whether resistance to Phomopsis differs between the leaf and stem tissue and (2) to examine the genetic variation for resistance to Phomopsis in sunflower inbreds. A set of 16 inbred lines was tested using artificial leaf infection with mycelial explants. The lines were screened for their reaction on leaf and stem in four environments to a French Phomopsis isolate and in two environments to a Yugoslavian isolate. Significant genetic variance and mostly high genetic ratios (analogous to heritability) were found 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 the petiole-stem passage. No correlation existed between resistance in the leaf and stem, suggesting that both resistance factors are inherited independently. Both isolates resulted in similar resistance rankings of the inbreds and no difference in their aggressiveness was detected with regard to leaf resistance. Screening for Phomopsis consisting of several steps is recommended, beginning with petiole score or fungal growth to select the most susceptible plants in the early testing, and measuring stem lesion in further generations of breeding.