Cytological observations of the infection process by Phomopsis helianthi (Munt.-Cvet) in leaves of sunflower

The infection process of Phomopsis helianthi and the specific degradation o f infected tissue were studied in detail using light and transmission elect ron microscopy. In comparison with other vascular pathogens, the infection and degradation process was in some aspects different. The favourite tissue for the pathogen to grow in was the phloem. Parenchymatic cells in and aro und vascular bundles were extremely sensitive to infection long before hyph ae arrived, probably due to a toxin. In the parenchymatic cells the first c hanges were visible at the chloroplasts where electron-dense material accum ulated in the thylakoid space. The chloroplast stroma changed contrast and later the whole cytoplasm also appeared electron dense. In the vascular bun dles, first the phloem was destroyed and then hyphae invaded the adjacent m esophyll, the cambium, and finally the vessel elements. In particular, the compact mesophyll of the midvein was severely affected. Vessel elements wer e lined with electron-dense material and some were filled with flocculent m aterial. Severe wall destruction indicated the action of a complete set of cell wall-degrading enzymes before hyphae entered the tissue; it always sta rted at the innermost wall layer. Wall degradation in vascular tissue and a djacent parenchyma with intercellular spaces was different. Before the degr adation of the protoplasts started, the cell walls were completely metaboli zed 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.