The spread of Plasmopara helianthi Novot in relation to slope.
A study of Plasmopara helianthi spread in a sunflower crop was carried out following Van der Plank's method. In a highly infected sloping field the spread of the parasite preferentially followed the slope lines. With few exceptions, all slopewise sequences of data are significant by the Van der Plank's test, while sequences orthogonal to the slope are never significant. Tillage and surface water are likely to be the main causes affecting the spread of the pathogen.