Downy mildew on Argyranthemum
During recent inspections, PHSI inspectors have made a number of findings of downy mildew on Argyranthemum. Although the disease is of no quarantine significance (because it is already established in Britain), growers may be interested to note the current incidence of the disease. There is no obligation to notify findings to us.
Background
The downy mildew (Peronopsora radii) is a recognised fungal pathogen of Argyranthemum frutescens (marguerites). It has previously been recorded in the UK on this host but more commonly on other members of the Compositae (Achillea, Anthemis, Atrtemesia, Chamomilla, Chrysanthemum, and Matricaria species). The fungus was first described in Germany on Matricaria perforata in 1863, first reported in the UK in 1888 on Chrysanthemum segetum in Moray, Scotland and on cultivated chrysanthemums in an unheated glass house in Sussex in 1960. The organism can infect both wild and cultivated plants.
Symptoms
In the recent findings the mildew has been causing damage to the youngest foliage resulting in blackening of the leaves and dieback (see pictures). On the underside of the leaves there is greyish to dark brown down or felt of fungal growth. Previous reports of P. radii on Argyranthemum frutescens describe damage to flowers which are smaller than those on healthy ones, asymmetric with ray petals bent in various ways and directions. The fungus may also sporulate on flowers. P. radii can be separated from other downy mildews on Compositae (e.g. Paraperonospora leptosperma) by microscopic examination.
Life cycle
Under conditions of high humidity the fungus produces asexual spores that may be spread from plant to plant by water splash or over longer distances on air currents. The fungus may also produce thick-walled sexual spores that can survive in plant debris for long periods. The fungus may also be spread in infected cuttings before symptom expression and possibly within consignments of seed. The fungus is known to attack wild Compositae which could act as a source of infection. Leaf wetness is essential for spore germination and infection to take place.
As with all downy mildews reducing humidity and leaf wetness will help in managing the disease.
Page last modified:
20 Nov 2006
Page published: 27 May 2004