Cytological Observations of the Infection Process by Phomopsis helianthi (Munt.-Cvet) in Leaves of Sunflower

The infection process of Phomopsis helianthi and the speci®c degradation of infected tissue were studied in detail using light and transmission electron microscopy. In comparison with other vascular pathogens, the infection and degradation process was in some aspects di€erent. The favourite tissue for the pathogen to grow 2in was the phloem. Parenchymatic cells in and around vascular bundles were extremely sensitive to infection long before hyphae arrived, probably due to a toxin. In the parenchymatic cells the ®rst changes were visible at the chloroplasts where electron-dense material accu- mulated in the thylakoid space. The chloroplast stroma changed contrast and later the whole cytoplasm also appeared electron dense. In the vascular bundles, ®rst the phloem was destroyed and then hyphae invaded the adjacent mesophyll, the cambium, and ®nally the vessel elements. In particular, the compact mesophyll of the midvein was severely a€ected. Vessel elements were lined with electron-dense material and some were ®lled with ¯occulent material. Severe wall destruction indi- cated the action of a complete set of cell wall- degrading enzymes before hyphae entered the tissue; it always started at the innermost wall layer. Wall degradation in vascular tissue and adjacent paren- chyma with intercellular spaces was di€erent. Before the degradation of the protoplasts started, the cell walls were completely metabolized and only the secondary walls of the vessels resisted for longer. There were no host±cell reactions visible that could be interpreted as a defence reaction.