Embryogenic patterns and parasitic habits of Helicotylenchus oleae and H. pseudorobustus.
Embryology and histological changes induced on their hosts by 2 Italian Helicotylenchus species (H. oleae and H. pseudorobustus) are discussed and illustrated. Similar embryogenic patterns were shown by the 2 species with 10-12 days elapsing between egg laying and fully formed juveniles in water-agar Petri dishes at 22-24°C. After egg-laying, development started rapidly and the multicellular stage was reached in about 16% of the total development time, while the vermiform juvenile appeared in 75% of the total development time. H. oleae was observed in a semiendoparasitic feeding position on olive feeder roots, inducing brown lesions adjacent to the nematode body and thickened cell walls of cortical cells that were fed upon. H. pseudorobustus also fed generally in a semiendoparasitic position in cortical parenchyma causing necrosis and disruption of a number of cortical cells. Both species induced a single modified "food-cell" with high metabolic activity and their feeding affected only epidermal and cortical cells with no evidence of vascular damage.