The biology of shepherd's purse (Capsella bursa-pastoris) as effected by white rust (Albugo candida) and downy mildew (Peronospora parasitica) and their potential as sources for biological weed control: A dissertation in partial fulfilment of the requirements for the Degree of Bachelor of Horticultural Science with Honours in Plant Science at Lincoln University

Abstract

The objectives of this research were to study the biology of the Cruciferous weed shepherd's purse ( Capsella bursa-pastoris) as it grew though the summer/autumn period and to look at the incidence of infection on leaves, stems and seed pods by the fungal pathogens white rust (Albugo candida) and downy mildew (Peronospora parasitica) . The environment of the plants was changed by altering plant available water, humidity and species composition to try and favour the growth of the fungal pathogens. The disease incidence level would give an indication of whether these fungal pathogens have potential as biological herbicides for the control of this weed. Weed seed in seedbed after one year in fallow was allowed to germinate. There were four treatments applied to plots on March 24 1993 of fluazifop-P-butyl herbicide applied at 200 g a.i./100/ ha, irrigation of plots, irrigation of plots plus Fusilade herbicide, and the no treatment control plot. An experiment was also carried out to evaluate the germination requirements of fresh shepherd's purse seed at 5, 10, 15 °C in light and dark conditions. The plants were sampled over eight weeks to record the incidence of the pathogen on the leaves, stems and seed pods. The total level of disease that developed on all parts of the plants was highly significant (p < 0.01) over the eight week duration of the experiment. Removal of the grass weeds with fluazifop-P-butyl increased the incidence of downy mildew on seed pods. Irrigation and removal of grass weeds increased the incidence of downy mildew on shepherd's purse leaves. The density of shepherd's purse was decreased more by plant competition than by disease. The optimum germination conditions for germination of fresh shepherd's purse seed were 10 °C in light.